An Arithmetic Riemann-Roch Theorem for Pointed Stable Curves

Home , Smooth completion

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Let ( , Σ, F ) be an arithmetic ring of Krull dimension at most 1 [22, O ∞ Def. 3.1.1]. This means that is an excellent, regular, Noetherian integral domain, Σ is a finite non-emptyO set of monomorphisms σ : ֒ C and Σ Σ O → F∞ : C C is a conjugate-linear involution of C-algebras such that the diagram → CΣ ||> δ || || || F ∞ BB O BB δ BB BB CΣ commutes. Here δ is induced by the set Σ. Define = Spec and let S O (π : ; σ1,...,σn) be a n-pointed stable curve of genus g, in the sense of KnudsenX → S and Mumford [36, Def. 1.1]. Assume that is regular. Write = σ ( ). To and we associate the complexX analytic spaces U X \ ∪j j S X U = (C), = (C). X∞ Xσ U∞ Uσ σG∈Σ σG∈Σ Notice that F acts on and . The stability hypothesis guarantees ∞ X∞ U∞ that every connected component of ∞ has a hyperbolic metric of constant curvature 1. The whole family is invariantU under the action of F . Dualiz- − ∞ ing we obtain an arakelovian –i.e. invariant under F∞– hermitian structure on ω (σ + ... + σ ). Contrary to the requirements of classical k·khyp X /S 1 n Arakelov theory [23], the metric hyp is not smooth, but has some mild singularities of logarithmic type.k·k Actually is a pre-log-log hermitian k·khyp metric in the sense of Burgos-Kramer-Kühn [8, Sec. 7]. Following loc. cit., there is a first arithmetic Chern class c1(ωX /S (σ1 + ... + σn)hyp) that lives 1 in a pre-log-log arithmetic Chow group CHpre( ). The authors define an intersection product b X

1 1 c · 2 CH ( ) Z CH ( ) CH ( ) pre X ⊗ pre X −→ pre X and a pushforward map c c c 2 1 π : CH ( ) CH ( ). ∗ pre X −→ S c 2 c 2 This paper is concerned with the class π∗(c1(ωX /S(σ1 + ... + σn)hyp) ). In their celebrated work [24], Gillet and Soulé–with deep contributions of Bismut– proved an arithmetic analogue ofb the Grothendieck-Riemann-Roch theorem. Their theorem deals with the push-forward of a smooth hermitian vector bundle by a proper and generically smooth morphism of arithmetic varieties. The associated relative complex tangent bundle is equipped with a smooth Kähler structure. With the notations above, if n = 0 and g 2, then ≥ the metric is smooth and the arithmetic Grothendieck-Riemann-Roch k·khyp theorem may be applied to ωX /S,hyp and the “hyperbolic” Kähler structure 1 2 on ∞. The result is a relation between π∗(c1(ωX /S,hyp) ) CH ( ) and theX class c (λ(ω ), ), where is the Quillen metric∈ correspondingS 1 X /S k·kQ k·kQ to our data. However, for n > 0 the singularities of preventc from b k·khyp applyingb the theorem of Gillet and Soulé. The present article focuses on the so far untreated case n > 0. We prove an arithmetic analogue of the Riemann-Roch theorem that relates 2 π∗(c1(ωX /S (σ1 + ... + σn)hyp) ) and c1(λ(ωX /S), Q). The Quillen type metric is defined by means of the Selbergk·k zeta function of the con- k·kQ nectedb components of ∞ (see Definitionb 2.2). In contrast with the result of Gillet and Soulé, ourU formula involves the first arithmetic Chern class of a new hermitian line bundle ψW . The corresponding invertible sheaf is the pull-back of the so called tautological psi line bundle on the moduli stack , by the classifying morphism . The underlying hermitian Mg,n S → Mg,n structure is dual to Wolpert’s renormalization of the hyperbolic metric [64, Def. 1] (see also Definition 2.1 below). The class c1(ψW ) reflects the appear- ance of the continuous spectrum in the spectral resolution of the hyperbolic laplacian. After the necessary normalizations andb definitions given in Section 2, the main theorem is stated as follows: Theorem A. Let g, n 0 be integers with 2g 2+ n > 0, ( , Σ, F ) an ≥ − O ∞ arithmetic ring of Krull dimension at most 1 and = Spec . Let (π : ; σ ,...,σ ) be a n-pointed stable curve of genusS g, withO regular.X For → S 1 n X every closed point ℘ denote by n℘ the number of singular points in the ∈ S 1 geometric fiber and put ∆ = [ n ℘] CH ( ). Then the identity X℘ X /S ℘ ℘ ∈ S 12 c (λ(ω ) ) ∆ + cP(ψ )= 1 X /S Q − X /S 1 W 2 π∗ c1(ωX /S (σ1 + ... + σn)hyp) b b+ c ( (C(g, n))) 1 O b1 holds in the arithmetic Chow group CH ( ). b S The theorem is deduced from the Mumford isomorphism on g,n (Theo- rem 3.10) and a metrized version thatc incorporates the appropriatMe hermitian

3 structures (Theorem 6.1).1 The techniques employed combine the geometry of the boundary of –through the so called clutching morphisms– and Mg+n,0 the behavior of the small eigenvalues of the hyperbolic laplacian on degenerating families of compact surfaces. By a theorem of Burger [7, Th. 1.1] we can replace the small eigenvalues by the lengths of the pinching geodesics. Then Wolpert’s pinching expansion of the family hyperbolic metric [63, Exp. 4.2] provides an expression of these lengths in terms of a local equation of the boundary divisor ∂ g+n,0. This gives a geometric manner to treat the small eigenvalues. AnotherM consequence of theorems 3.10 and 6.1 is a significant case of the local index theorem of Takhtajan-Zograf [56]–[57] (Theorem 6.8 below). Natural candidates to which Theorem A applies are provided by arithmetic models of modular curves, taking their cusps as sections. We focus on the curves X(Γ)/C, where Γ PSL2(Z) is a congruence subgroup of the kind Γ (p)orΓ (p). We assume that⊂ p 11 is a prime number. If Γ = Γ (p), 0 1 ≥ 0 we further suppose p 11 mod 12. This guarantees in particular that X(Γ) has genus g 1. To X≡(Γ) we attach two kinds of zeta functions: ≥ – let Y (Γ) := X(Γ) cusps be the open modular curve. Then Y (Γ) is \{ } a hyperbolic Riemann surface of finite type. We denote by Z(Y (Γ),s) the Selberg zeta function of Y (Γ) (see Section 2). It is a meromorphic function defined over C, with a simple zero at s = 1;

– let Prim (Γ) be a basis of normalized Hecke eigenforms for Γ. To f 2 ∈ Prim2(Γ) we can attach a Chow motive (f) over Q, with coeﬃcients M 2 in a suitable ﬁnite extension F of Q(µp), independent of f. If χ is a Dirichlet character with values in F ×, we denote by Q(χ) its Artin motive. For instance we may take χ = χf , for the Dirichlet character χf 2 associated to f Prim2(Γ). If Sym denotes the square symmetrization projector and (2)∈ the Tate twist by 2, we put

:= Sym2 (f) Q(χ )(2) Ob(M (Q) ). MΓ M ⊗ f ∈ rat F f∈PrimM2(Γ)

The motivic L-function of Γ, L(s, Γ), can be deﬁned –with the appropriate deﬁnition of theM local factorM at p– so that we have the

1In particular, with the formalism of [9, Sec. 4.3], the assumption of regularity of can be weakened to π : generically smooth. X 2The construction ofX →(f S) amounts to the decomposition of the jacobian Jac(X(Γ)) under the action of the HeckeM algebra. More generally, Deligne [12, Sec. 7] and Scholl [49, Th. 1.2.4] associate a Grothendieck –i.e. homological– motive to any normalized new Hecke eigenform of weight k 2, level n and character χ. ≥

4 relation

(1.1) L(s, )= L(s +2, Sym2f, χ ). MΓ f f∈PrimY2(Γ) The reader is referred to [10], [12, Sec. 7], [28, Sec. 5], [49] and [52] for details.3

Denote by Γ2 the Barnes double Gamma function [4] (see also [47] and [60]).

Theorem B. Let p 11 be a prime number and Γ=Γ0(p) or Γ1(p). Assume p 11 mod 12 whenever≥ Γ=Γ (p). Then there exist rational numbers a, b, c ≡ 0 such that ′ a b c Z (Y (Γ), 1) × e π Γ (1/2) L(0, ), ∼Q 2 MΓ × 4 where α × β means α = qβ for some q Q . ∼Q ∈ The proof relies on Theorem A and the computation of Bost [6] and K¨uhn

[37] for the arithmetic self-intersection number of ωX1(p)/Q(µp)(cusps)hyp. Un- dertaking the proof of Bost and K¨uhn –under the form of Rohrlich’s modular 1 version of Jensen’s formula [45]– and applying Theorem A to (PZ;0, 1, ), one can also show the equality ∞

′ 5/3 −8/3 (1.2) Z (Γ(2), 1)=4π Γ2(1/2) , where Z(Γ(2),s) is the Selberg zeta function of the congruence group Γ(2). The details are given in our thesis [20, Ch. 8]. However, our method fails to ′ provide the exact value of Z (PSL2(Z), 1). To the knowledge of the author, the special values Z′(Y (Γ), 1) remained unknown. Even though it was expected that they encode interesting arithmetic information (see [26] and [46]), it is quite remarkable that they can be expressed in terms of the special values L(0, ). The introduction of in MΓ MΓ the formulation of the theorem was suggested by Beilinson’s conjectures (see [53] for an account) and two questions of Fried [21, Sec. 4, p. 537 and App., Par. 4]. Fried asks about the number theoretic content of the special values of Ruelle’s zeta function and an interpretation in terms of regulators.5 Also Theorem B may be seen as an analogue of the product formula for number ﬁelds x = 1. This analogy alone deserves further study. ν | |ν 3 2 TheQ factors L(s +2, Sym f, χf ) where already studied by Hida [28], Shimura [52] and Sturm [54]. 4The exponents a,b,c can actually be computed in terms of p. 5The Ruelle zeta function R(s) of a hyperbolic Riemann surface is related to the Selberg zeta function Z(s) by R(s)= Z(s)/Z(s + 1). For instance, R′(1) = Z′(1)/Z(2).

5 So far there have been other attempts of proof of Theorem A. This is the case of [61, Part II]. The method of loc. cit. seems to lead to an analogous statement up to an unknown universal constant. The advantage of our approach is that explicit computations –such as Theorem B and (1.2)– are allowed. Moreover, in contrast with [61, Fund. rel. IV’, p. 280], our result is available for pointed stable curves of any genus.6 Remark 6.7 below strengthens the importance of the case g = 0 and n = 3. In his forthcoming thesis [25], T. Hahn obtains different results related to Theorem A. His approach is much in the spirit of Jorgenson–Lundelius [31]–[33]. In contrast with our geometric considerations, Hahn works with a degenerating family of metrics on a fixed compact Riemann surface and studies the behavior of the corresponding family of heat kernels. Consequences are derived for the family of heat trace regularizations and spectral zeta functions. It is likely that the two approaches can be combined to produce more general statements. Let us briefly describe the structure of this paper. Section 2 fixes the normalizations to be followed throughout the paper. Specially we define the Wolpert and Quillen metrics occurring in the statement of Theorem A. In Section 3 we review the definition and properties of the clutching morphisms of Knudsen. We also recall the Mumford isomorphism on . With these tools at hand, we show how to derive the Mg,0 Mumford isomorphsim on g,n (Theorem 3.10). Sections 4 and 5 are de- voted to the analytic partM of the proof of the main theorem. In Section 4 we introduce the Liouville metric on the first tautological line bundle and establish its continuity and behavior under pull-back by the clutching morphisms. In Section 5 we recall the results of Wolpert on the degeneration of the Selberg zeta function for degenerating families of compact hyperbolic Riemann surfaces [62]. We also review the theorem of Burger on the small eigenvalues of such families of curves [7]. We derive consequences for the Sel- berg zeta function as well as for the Quillen metric. In Section 6 we prove a metrized version of the Mumford isomorphism on (Theorem 6.1). The Mg,n strategy relies on sections 3–5 and the arithmetic Riemann-Roch theorem of Gillet and Soulé. Theorem A is then deduced as an immediate application of Theorem 3.10 and Theorem 6.1. The article finishes with the proof of Theorem B in Section 7. 6The proof of loc. cit. presents a gap in genus g 2 (last two lines in page 279). The case g = 2 requires a justification whereas there are≤ counterexamples to the principle of the proof in genus 0 and 1: there exist non-constant harmonic functions on g,n;C, for g = 0, n 4 and g = 1, n 1. Also the case g = 0 and n = 3 is beyond the reachM in [61]. ≥ ≥

6 2 Conventions and notations

We fix some conventions and notations that will hold throughout this paper. Let g, n 0 be integers with 2g 2+ n> 0. We define the real constants ≥ − ζ′( 1) 1 C(g, n) = exp (2g 2+ n) − + , − ζ( 1) 2 − E(g, n) =2(g+2−n)/3π−n/2 1 1 exp (2g 2+ n) 2ζ′( 1) + log(2π) , · − − − 4 2 where ζ denotes the Riemann zeta function. Notice the relations (2.1) C(g + n, 0) = C(g, n)C(1, 1)n, (2.2) E(g + n, 0) = πnE(g, n)E(1, 1)n. Let X be a compact and connected Riemann surface of genus g and p1,..., pn distinct points in X. The open subset U = X p1,...,pn admits a complete hyperbolic riemannian metric, of constant curvature\{ -1.} Denote 2 it by dshyp,U . Via a fuchsian uniformization U Γ H, Γ PSL2(R) torsion 2 ≃ \ ⊂ free, the metric dshyp,U is obtained by descent from the Γ invariant Riemann tensor on H dx2 + dy2 ds2 = , z = x + iy H. hyp,H y2 ∈ 2 Associated to dshyp,U there is a hermitian metric on the complex line TU , that we write hU . It is obtained by descent from the metric hH on TH defined by the rule ∂ ∂ 1 hH , = . ∂z ∂z 2y2 The hermitian metric h extends to a pre-log-log hermitian metric on U k·khyp ωX (p1 +...+pn) [19, Sec. 7.3.2]. The first Chern form of ωX (p1 +...+pn)hyp, which is defined on U, coincides with the normalized Kähler form ω of hU (curvature 1 condition). The form ω is locally given by − i ∂ ∂ ω = h , dz dz. 2π U ∂z ∂z ∧ The volume of X with respect to ω is 2g 2+ n. − For every puncture pj there is a conformal coordinate z with z(pj) = 0, by means of which a small punctured disc D∗(0,ε) C with the Poincaré metric ⊂ dz 2 ds2 = | | P z log z | | | | 7 2 isometrically embeds into (U,dshyp,U ). Such a coordinate is unique up to rotation and is called a rs coordinate at the cusp pj. Definition 2.1 (Wolpert metric [64], Def. 1). Let z be a rs coordinate at the cusp pj. The Wolpert metric on the complex line ωX,pj is defined by dz =1. k kW,pj The tensor product ω is equipped with the tensor product of Wolpert ⊗j X,pj metrics, and we write for the resulting metric. k·kW The complex vector space ∞(X,ω )( H0(X,ω )) is equipped with the C X ⊃ X non-degenerate hermitian form i α, β = α β. h i0 2π ∧ ZX 1 ∨ 0 The space H (X,ωX ) is canonically isomorphic to H (X, X ) = C via the 2 O ∞ analytic Serre duality. Since ω is integrable, the L metric on (X, X )( 0 C O ⊃ H (X, X )) with respect to hU is well defined. If 1 is the function with constantO value 1, then

1, 1 = ω =2g 2+ n. h i1 − ZX The complex line λ(ω ) = det H0(X,ω ) det H1(X,ω )−1 is endowed with X X ⊗ X the determinant metric build up from , 0 and , 1. We refer to it by L2 . We next recall the definition of theh· Selberg·i h· zeta·i function of U (seek·k [27]). For every real l> 0 the function ∞ Z (s)= (1 e−(s+k)l)2 l − kY=1 is holomorphic in Re s> 0. In a first step, the Selberg zeta function of U is defined by the absolutely convergent product

Z(U, s)= Zl(γ)(s), Re s> 1, γ Y running over the simple closed non-oriented geodesics of the hyperbolic sur- 2 face (U,dshyp,U ). Then one shows that Z(U, s) extends to a meromorphic function on C, with a simple zero at s = 1.

Deﬁnition 2.2 (Quillen metric). We deﬁne the Quillen metric on λ(ωX ), attached to the hyperbolic metric on U, to be

′ −1/2 =(E(g, n)Z (U, 1)) 2 . k·kQ k·kL

8 We denote by (C(g, n)) the trivial line bundle equipped with the norm C(g, n) , where O stands for the usual absolute value. |·| |·| Let ( , Σ, F ) be an arithmetic ring of Krull dimension at most 1. Put O ∞ = Spec and denote its generic point by η. Let (π : ; σ1,...,σn) beS a n-pointedO stable curve of genus g, in the sense of KnudsenX → and S Mumford [36, Def. 1.1]. Assume η regular. We write = jσj( ). By definition is geometrically connected.X For every complexU X embedding \ ∪ S τ Σ, the pre- Xη ∈ ceding constructions apply to τ (C) τ (C). Varying τ, we obtain arakelo- U ⊂ X ∗ vian hermitian line bundles ωX /S (σ1 + ... + σn)hyp, ψW = jσj (ωX /S )W , λ(ω ) and (C(g, n)). Similar notations will be employed⊗ for the anal- X /S Q O ogous constructions over more general bases. In this way, we shall consider the “universal situation” (π : g,n g,n; σ1,...,σn), where g,n is the Deligne-Mumford stack of n-pointedC → stableM curves of genus g, M the uni- Cg,n versal family and σ1,...,σn the universal sections. We then have “universal hermitian line bundles” λ(ω ) , σ∗(ω ) , etc. Cg,n/Mg,n |Mg,n;Q j Cg,n/Mg,n |Mg,n;W ( g,n is the open substack of smooth curves). When the context is clear M ∗ enough, we freely write λg,n;Q, σj (ωCg,n/Mg,n )W , ψg,n;W , etc. If F is an algebraic stack of finite type over Spec Z, then we denote by F an the analytic stack associated to FC. For instance, applied to g,n and g,n, an M M we obtain the analytic stack g,n of n-punctured Riemann surfaces of genus M an g and its Deligne-Mumford stable compactification g,n. If 1 2 is a M anE → Ean morphism of sheaves over F , there is an associated morphism 1 2 over F an. Finally, to a morphism F G between algebraic stacksE of→E finite type over Spec Z, corresponds a morphism→ between analytic stacks F an Gan. → Our standard references for the theory of algebraic stacks are [14] and [38]. For pointed stable curves we refer to the original article of Knudsen [36]. Concerning the arithmetic intersection theory, we follow the extension of the theory of Gillet-Soulé[22] developed by Burgos, Kramer and Kühn [8].

3 Knudsen’s clutching and the Mumford isomorphism on Mg,n 3.1 Preliminaries Let g, n 0 be integers with 2g 2+ n> 0. We denote by Spec Z ≥ − Mg,n → the Deligne-Mumford stack of n-pointed stable curves of genus g, and by π : the universal curve [36]. The morphism π has n universal Cg,n → Mg,n sections, σ1,...,σn. The ﬁrst theorem compiles some geometric features of . Mg,n

9 Theorem 3.1. i. g,n is a proper and smooth algebraic stack over Spec Z of relative dimensionM 3g 3+ n. The substack ∂ classifying singular − Mg,n curves is a divisor with normal crossings, relative to Spec Z. ii. The fibers of over Spec Z are geometrically irreducible. Mg,n Proof. The first assertion is [36, Th. 2.7]. The second assertion is due to Deligne-Mumford [14] in the case n = 0 and g 2, to Deligne-Rapoport ≥ [15] in the case n = g = 1 and Keel [35] in the case g = 0 and n 3. For the remaining cases we proceed by induction. Assume the claim≥ for some couple (g, n), with 2g 2+ n > 0. We derive the claim for (g, n + − 1). Recall that g,n gets identified with g,n+1 via Knudsen’s contraction morphsim c : C [36, Sec. 2].M It then suffices to show that Mg,n+1 → Cg,n Cg,n has geometrically irreducible fibers over Spec Z. If g,n is the dense open substack of classifying non-singular curves, thenMπ−1( ) is Mg,n Mg,n →Mg,n proper, smooth with geometrically connected fibers (by definition of pointed −1 stable curve [36, Def. 1.1]). It follows that π ( g,n) has geometrically irreducible fibers over Spec Z. To conclude, we noticeM that π−1(∂ ) Mg,n ∪ σ1( g,n) ... σn( g,n) is a divisor with normal crossings relative to Spec Z. M ∪ −1∪ M Therefore π ( g,n) is fiberwise dense in g,n, over Spec Z. The proof is complete. M C Corollary 3.2. Let N 1 be an integer. For an algebraic stack of the form ≥ := ... we have M Mg1,n1 × × Mgr,nr H0( Spec Z[1/N], G )= Z[1/N]×. M × m Proof. By Theorem 3.1, Spec Z[1/N] is a proper and smooth algebraic M × stack over Spec Z[1/N], with geometrically irreducible fibers. The corollary follows. We now define the tautological line bundles on . Mg,n Definition 3.3 (Tautological line bundles [36], [43]). The tautological line bundles on are Mg,n

λg,n = λ(ωCg,n/Mg,n ) = det(Rπ∗ωCg,n/Mg,n ), δ = (∂ ), g,n O Mg,n (j) ∗ ψg,n = σj ωCg,n/Mg,n , j =1, . . . , n, ψ = ψ(j) , g,n ⊗j g,n κ = ω (σ + ... + σ ),ω (σ + ... + σ ) , g,n h Cg,n/Mg,n 1 n Cg,n/Mg,n 1 n i where , denotes the Deligne pairing [3, XVIII], [18]. h· ·i

10 3.2 Clutching morphisms We proceed to recall Knudsen’s clutching morphism [36, Part II, Sec. 3]. The basic construction is resumed in the following theorem. Theorem 3.4 (Knudsen). Let π : X S be a pre-stable curve (i.e. ﬂat, proper, whose ﬁbers are geometrically connected→ with at worst ordinary double points). Let σ , σ : S X be given disjoint sections of π. Suppose that π 1 2 → is smooth along σ1, σ2. Then there is a diagram

p X / X′ I U ′ σi π π σ S S such that: i. σ = pσ1 = pσ2 and p is universal with this property; ii. p is finite; iii. π′ : X′ S is a prestable curve, fiberwise obtained by identification → of σ1 and σ2 in an ordinary double point; iv. for every open U X′ we have ⊆ 0 0 −1 ∗ ∗ H (U, ′ )= f H (p (U), ) σ (f)= σ (f) ; OX { ∈ OX | 1 2 } (j) ∗ v. let ψ = σj ωX/S, j =1, 2. There is an exact sequence

(1) (2) α (3.1) 0 σ (ψ ψ ) Ω ′ p Ω 0. → ∗ ⊗ → X /S → ∗ X/S →

The arrow α of (3.1) is defined as follows. Let j (resp. ) be the ideal sheaf ′ I I of the image of σj (resp. σ) in X (resp. X ), j =1, 2. Consider the natural morphisms p : / 2 p ( / 2)= σ ψ(j). There is a natural isomorphism j I I → ∗ Ij Ij ∗ 2 / 2 σ (ψ(1) ψ(2)) (3.2) I I −→ ∗ ⊗ ^ u v p u p v p v p u. ∧ 7−→ 1 ⊗ 2 − 1 ⊗ 2 Via the isomorphism (3.2), α gets identified with u v udv. ∧ 7→ The clutching morphism is defined by a three step construction. Construction 3.5 (Clutching morphism). Step 1. Let π1 : X1 S be a n1 + 1-pointed stable curve of genus g1, with (1) (1)→ sections σ1 ,...,σn1+1. In addition consider the 3-pointed stable curve of genus 0, (π : P1 S;0, 1, ). By Theorem 3.4 we can attach P1 to X by S → ∞ S 1 11 (1) identification of the sections 1 and σn1+1. We obtain a new pointed stable ′ (1) (1) curve X of genus g , with sections σ ,...,σn and 0 , . We proceed 1 1 1 1 1 ∞1 analogously with a n2 + 1 pointed stable curve π2 : X2 S, with sections (2) (2) → σ1 ,...,σn2+1. Step 2. We apply Theorem 3.4 to glue X′ and X′ along the sections and 1 2 ∞1 2. We obtain a new pointed stable curve X of genus g1 + g2 with sections ∞(1) (1) (2) (2) σ1 ,...,σn1 , σ1 ,...,σn2 , 01, 02. Step 3. We contract the sections 01 and 02 [36, Prop. 2.1]. We obtain a n1 + n2 pointed stable curve of genus g1 + g2. Theorem 3.6 (Knudsen). Construction 3.5 defines a morphism of algebraic stacks β : , Mg1,n1+1 × Mg2,n2+1 −→ Mg1+g2,n1+n2 which is representable, finite and unramified. If moreover g1 = g2 or n1+n2 = 0, then β is a closed immersion. 6 6

Proof. This is [36, Th. 3.7 and Cor. 3.9]. The following statement describes the behavior of the tautological line bundles under pull-back by the clutching morphism.

Proposition 3.7. Let g = g1 + g2, n = n1 + n2. There are isomorphisms of line bundles, uniquely determined up to a sign,

(3.3) β∗λ ∼ λ ⊠ λ , g,n → g1,n1+1 g2,n2+1 (3.4) β∗δ ∼ (δ ψ(n1+1) −1) ⊠ (δ ψ(n2+1) −1), g,n → g1,n1+1 ⊗ g1,n1+1 g2,n2+1 ⊗ g2,n2+1 (3.5) β∗ψ ∼ (ψ ψ(n1+1) −1) ⊠ (ψ ψ(n2+1) −1), g,n → g1,n1+1 ⊗ g1,n1+1 g2,n2+1 ⊗ g2,n2+1 (3.6) β∗κ ∼ κ ⊠ κ . g,n → g1,n1+1 g2,n2+1 Proof. Once the existence of (3.3)–(3.6) is proven, the uniqueness assertion already follows from Corollary 3.2. For the isomorphisms (3.3)–(3.5) we refer to [36, Th. 4.3] (they are easily constructed by means of Theorem 3.4 above). We now focus on (3.6). The formation of the relative dualizing sheaf is compatible with base change [36, Sec. 1], as well as for the Deligne pairing [18, Par. I.3, p. 202]. By the deﬁnition of the clutching morphism (Construction 3.5), we ﬁrst reduce to the following situation. Let S be a noetherian integral scheme and (πi : Xi (i) (i) → S; σ1 ,...,σni+1), i =1, 2, two pointed stable curves of genus gi, respectively. We apply Theorem 3.4 to the pre-stable curve X = X X S with sections 1⊔ 2 →

12 (1) (2) σn1+1, σn2+1. With the notations of the theorem, we have to construct a natural isomorphism

n1 n2 n1 n2 (1) (2) (1) (2) ∼ (3.7) ω ′ ( σ + σ ),ω ′ ( σ + σ ) X /S j j X /S j j −→ * j=1 j=1 j=1 j=1 + X X X X n1+1 n1+1 (1) (1) (3.8) ωX1/S( σj ),ωX1/S( σj ) * j=1 j=1 + X X n2+1 n2+1 (3.9) ω ( σ(2)),ω ( σ(2)) . ⊗ X2/S j X2/S j * j=1 j=1 + X X

We denote by L the line bundle of (3.7) and L1, L2 the line bundles of (3.8), (3.9), respectively. After localizing for the étale topology on S, we can find n1 (1) n2 (2) ′ rational sections s, t of ωX /S( j=1 σj + j=1 σj ) whose divisors are finite and flat over S, with mutually disjoint components, disjoint with the image (1)P (2)P ′ of the section σ (image of σn1+1 and σn2+1 in X ). Denote by s X1 , t X1 (resp. s , t ) the pull-backs of s and t to X (resp. X ), respectively.| | |X2 |X2 1 2 By the properties of the relative dualizing sheaf, s X1 , t X1 (resp. s X2 , n1+1 (1) | | n2+1 (2)| t X2 ) are rational sections of ωX1/S( j=1 σj ) (resp. ωX2/S( j=1 σj )). They| have finite and flat divisors over S, with mutually disjoint components and disjoint from σ(1) (resp. σ(2) ).P The symbols s, t , s P, t and n1+1 n2+1 h i h |X1 |X1 i s X2 , t X2 are non-zero sections of L, L1 and L2, respectively. We define hthe| assignment| i

Φ: s, t s , t s , t . h i 7−→ h |X1 |X1 i⊗h |X2 |X2 i The symbols of the form s, t generate L. Clearly Φ defines a morphism h i L L1 L2, compatible with base change by noetherian integral schemes. Observe→ ⊗ that Φ is injective. Indeed, with the assumptions and notations above, s , t s , t is non-zero by integrality of S. We h |X1 |X1 i⊗h |X2 |X2 i now prove that Φ is an isomorphism. By Nakayama’s lemma, we reduce to S = Spec k, where k is an algebraically closed field. In this case, by definition of the relative dualizing sheaf, we have

n1 n2 ni+1 (1) (2) (i) ω ′ ( σ + σ ) = ω ( σ ), i =1, 2. X /S j j |Xi Xi/S j j=1 j=1 j=1 X X X

ni+1 (i) For i =1, 2, let si, ti be rational sections of ωXi/S( j=1 σj ), whose divisors (i) have mutually disjoint components, disjoint from σni+1. Consider si, ti as P × sections of ωXi/S. We can deﬁne a(si) = Resσ(i) si k , b(ti) = Resσ(i) ti ni+1 ∈ ni+1 ∈

13 n1 (1) n2 (2) × ′ k , i = 1, 2. We introduce the sections s, t of ωX /S( j=1 σj + j=1 σj ) characterized by P P a(s )s on X , s = 2 1 1 a(s )s on X , (− 1 2 2 and b(t )t on X , t = 2 1 1 b(t )t on X . (− 1 2 2 The divisors of s and t have mutually disjoint components, disjoint from σ. We compute

Φ s, t = a(s )a(s )b(t )b(t ) s , t s , t . h i 1 2 1 2 h 1 1i⊗h 2 2i Since s , t s , t is a frame of L L and a(s )a(s )b(t )b(t ) k×, h 1 1i⊗h 2 2i 1 ⊗ 2 1 2 1 2 ∈ we conclude that Φ is surjective. Notice that the construction of Φ naturally extends to a base S equal to an arbitrary disjoint union of noetherian integral schemes, in particular to objects of the ´etale site of g1,n1+1 g2,n2+1. Applying the functoriality of the Deligne pairing andM the relative× M dualizing sheaf, it is easily checked that Φ is compatible with base change on this site. Therefore it descends to the required isomorphism. The proof of the proposition is complete.

×n Corollary 3.8. Let γ : g,n 1,1 g+n,0 be obtained by reiterated applications of clutching morphisms.M × M Then→ M we have isomorphisms, uniquely determined up to a sign,

(3.10) γ∗λ ∼ λ ⊠ λ⊠n, g+n,0 → g,n 1,1 (3.11) γ∗δ ∼ (δ ψ−1 ) ⊠ (δ ψ−1)⊠n, g+n,0 → g,n ⊗ g,n 1,1 ⊗ 1,1 (3.12) γ∗κ ∼ κ ⊠ κ⊠n. g+n,0 → g,n 1,1 Proof. This is a straightforward application of Proposition 3.7. Remark 3.9. The isomorphisms (3.10)–(3.12) are described, locally for the ´etale topology, by means of Theorem 3.4 and the proof of Proposition 3.7 (see also Knudsen [36, Part III, Sec. 4]).

3.3 The Mumford isomorphism on Mg,n The next theorem generalizes to the so called Mumford isomorphism on Mg,n g,0. The statement is known for g,n over a ﬁeld [2, Eq. 3.15, p. 109]. Our Mapproach is well suited for the analyticM part of the proof of the main theorem.

14 The idea is based on two points: a) pull-back the Mumford isomorphism on by the clutching morphism of Corollary 3.8; b) deduce the Mumford Mg+n,0 isomorphism on from point a) and the Mumford isomorphism on . Mg,n M1,1 Theorem 3.10. There is an isomorphism of line bundles on /Z, unique- Mg,n ly determined up to a sign,

: λ⊗12 δ−1 ψ ∼ κ . Dg,n g,n ⊗ g,n ⊗ g,n −→ g,n Proof. For the cases g 2, n = 0 and g = n = 1, we refer to Moret-Bailly ≥ [41, Th. 2.1] (which is based on Mumford [42] and Deligne-Rapoport [15]). For the latter, a comment is in order: [41] provides an isomorphism

(3.13) λ⊗12 δ−1 ∼ ω ,ω . 1,1 ⊗ 1,1 −→ h C1,1/M1,1 C1,1/M1,1 i

Since π : 1,1 1,1 is smooth along the universal section σ1, we have the adjunctionC isomorphism→ M

(σ ), (σ ) ∼ ω , (σ ) −1, hO 1 O 1 i −→ h C1,1/M1,1 O 1 i and hence an isomorphism

(3.14) ω ,ω ∼ κ ψ−1. h C1,1/M1,1 C1,1/M1,1 i −→ 1,1 ⊗ 1,1

The isomorphism 1,1 is then constructed with (3.13)–(3.14). For the general case, we ﬁrst claimD that there is an isomorphism,

(3.15) ′ : pr∗(λ⊗12 δ−1 ψ ) ∼ pr∗κ Dg,n 1 g,n ⊗ g,n ⊗ g,n −→ 1 g,n

(pr1 is the projection onto the ﬁrst factor). Indeed, consider the clutching ×n morphism γ : . From Corollary 3.8 we deduce Mg,n × M1,1 → Mg+n,0 ∗ ⊗12 −1 ⊗12 −1 ⊠n ∼ γ g+n,0 :(λ δ ψg,n) ⊠ (λ δ ψ1,1) (3.16) D g,n ⊗ g,n ⊗ 1,1 ⊗ 1,1 ⊗ −→ ⊠ ⊠n κg,n κ1,1 .

The claim follows tensoring (3.16) by pr∗( ⊠n)⊗−1. 2 D1,1 Let p1 = p2 be prime numbers and X1 Spec Z[1/p1], X2 Spec Z[1/p2] 6 → 7 → two smooth 1-pointed stable curves of genus 1. Associated to the curves Xj, j =1, 2, there are morphisms

×n ϕ : Spec Z[1/p ] Spec Z[1/p ] ∆ Spec Z[1/p ], j j −→ M1,1 × j −→ M1,1 × j 7 For instance, for p = 11, 17 the modular curve X0(p) can be given the structure of an elliptic curve over Q with good reduction over Spec Z[1/p] [15].

15 where ∆ is the n-diagonal map. Pulling ′ back by pr∗ϕ , we obtain an Dg,n 2 j isomorphism of line bundles on Spec Z[1/p ] Mg,n × j (j) : λ⊗12 δ−1 ψ Z[1/p ] ∼ κ Z[1/p ]. Dg,n g,n ⊗ g,n ⊗ g,n ⊗ j −→ g,n ⊗ j ⊗12 −1 −1 (j) Deﬁne the line bundle L =(λ δ ψ ) κ on . Then g,n g,n ⊗ g,n ⊗ g,n ⊗ g,n Mg,n D induces a trivialization τj of L Z[1/pj ]. Over the open subset Spec Z[1/p1p2] ⊗ × Z Z of Spec Z, the trivializations τ1, τ2 diﬀer by a unit in Z[1/p1p2] = p1 p2 m n ± (Corollary 3.2). Write τ1 = ǫp1 p2 τ2, over Spec Z[1/p1p2], with ǫ 1, 1 . Then τ ′ := ǫp−mτ , τ ′ := pnτ are new trivializations of L Spec Z[1∈/p {−], L} 1 1 1 2 2 2 ⊗ 1 ⊗ Spec Z[1/p ], respectively. By construction, they coincide over Spec Z[1/p ] 2 1 ∩ Spec Z[1/p2]. Therefore they glue into a trivialization of L over Spec Z. This establishes the existence of an isomorphism as in the statement. The uniqueness follows from Corollary 3.2.

×n Corollary 3.11. For the clutching morphism γ : g,n 1,1 g+n,0, ⊠ M × M → M the isomorphisms γ∗ and ⊠ n coincide up to a sign. Dg+n,0 Dg,n D1,1 Proof. The corollary is a straightforward application of Theorem 3.10, Corol- lary 3.8 and Corollary 3.2.

4 The Liouville metric on κg,n 4.1 Local description of an Mg,n Let (X; p1,...,pn) be a pointed stable curve over C. Teichm¨uller’s theory provides a small stable deformation (f : X Ω; p ,..., p )of(Xan; p ,...,p ), → 1 n 1 n where Ω C3g−3+n is some open analytic neighborhood of 0. Let F : Ω an ⊂ → g,n be the induced morphism of analytic stacks. After possibly shrinking M an Ω, the image F(Ω) is an open substack of . It is the stack theoretic quo- Mg,n tient of Ω by a ﬁnite group. Varying (X; p1,...,pn) in g,n(C), the open an subsets Ω as above cover . This subsection is basedM on [39, Sec. 2] and Mg,n [63, Sec. 2], and reviews the construction of the small stable deformation (f : X Ω; p ,..., p ). → 1 n

Construction 4.1. i. Fix (X; p1,...,pn) a n-pointed stable curve of genus an g over C. We shall identify X with X by Chow’s theorem. Let q1,...,qm ◦ be the singular points of X. Define X := X p1,...,pn, q1,...,qm and ◦ \{ } X a smooth completion of X . Then X has a pair of punctures aj, bj at ◦ the place of each cusp qj. The surface X has a unique complete riemannian metric of constant curvature 1. Let (W ,w ) be a rs analytic chart at p − i i i 16 and (Uj,uj), (Vj, vj) rs analytic charts at aj, bj respectively (see Section 2). We suppose that all the Wi, Uj, Vk are mutually disjoint. Finally, let U0 be a relatively compact open subset of X ( i,jWi Uj Vj). ii. The deformation space D of X\◦ ∪is the∪ product∪ of the Teichmüller spaces of the connected components of X◦. We can choose smooth Beltrami differentials ν1,...,νr compactly supported in U0, spanning the tangent space at X◦ of D. Let s Cr and define ν(s) = s ν . Then ν(s) is a smooth ∈ j j j Beltrami differential compactly supported in U0. For s small, ν(s) ∞ < 1. The solution of the Beltrami equation on XP◦, for the| | Beltramik differentialk ◦ ◦ ν(s), produces a new Riemann surface Xs diffeomorphic to X . For this, let (U , z ) be an analytic atlas of X◦. For every α we take w a homeo- { α α }α α morphism solution of wzα = ν(s)wzα . We can normalize wα to depend holo- morphically on s. Then (Uα,wα zα) is an atlas for the Riemann surface ◦ { ◦ } Xs. Notice that wα is in general not holomorphic in zα, but quasi-conformal. However w z is holomorphic on U ( W U V ), since ν(s) is α ◦ α α ∩ ∪i,j i ∪ j ∪ j supported in U0. This means that (Uα ( i,jWi Uj Vj), zα) is an analytic ◦ ∩ ∪ ∪ ∪ chart of Xs. In particular, (Wi pi ,wi), (Uj aj ,uj) and (Vj bj , vj) ◦ \{ } \{ } \{ } serve as analytic charts on Xs. iii. Let 0

p t /c < u

Proposition 4.2. i. The tuple (f : X Ω; p1,..., pn) is a n-pointed stable → an curve of genus g, whose ﬁber at 0 equals (X; p1,...,pn). Let F : Ω g,n be the induced morphism of analytic stacks. → M

17 ii. After possibly shrinking Ω, F is a local manifold cover: the image an F(Ω) is an open substack of and is the stack theoretic quotient of Ω by Mg,n a ﬁnite group acting on Ω. Proof. We refer to [63, Sec. 2].

4.2 Definition of the Liouville metric Construction 4.3. Let p (C) be a point corresponding to a pointed ∈ Mg,n stable curve (X; p1,...,pn). Let X1,...,Xm be the decomposition of Xreg ◦ into connected components. Every Riemann surface Xj = Xj p1,...,pn \{ 2 } admits a complete hyperbolic metric of constant curvature 1, dshyp,j. If ◦ ◦ ◦ − 2 Xj is a compactification of Xj and ∂Xj = Xj Xj , then dshyp,j induces \ ◦ a pre-log-log hermitian metric hyp,j on ωXj (∂Xj ). For its first Chern ◦ k·k ◦ form we write c1(ωXj (∂Xj )hyp) (well defined and smooth on Xj ). If σ, τ are rational sections of ωX (p1 + ... + pn), whose divisors have mutually disjoint components, disjoint from Xsing, then the integral

◦ Ij(σ, τ) := log σ hyp,jδdiv t + log τ hyp,j c1(ωXj (∂Xj )hyp) X◦ k k k k Z j h i is convergent [8, Sec. 7]. The norm σ, τ of σ, τ is characterized by kh ik h i m log σ, τ = I (σ, τ). kh ik j j=1 X This construction deﬁnes a hermitian metric (at the archimedian places) on the tautological line bundle κg,n.

Definition 4.4 (Liouville metric). The hermitian metric on κg,n defined by Construction 4.3 is called the Liouville metric. We write κg,n to refer to the line bundle κg,n together with the Liouville metric. Lemma 4.5. Let be the hermitian metric on ω (σ + ... + k·khyp Cg,n/Mg,n 1 σn), induced by the hyperbolic metric on (the regular locus) of the fibers of σ ( ) . Then we have Cg,n \ ∪j j Cg,n → Mg,n (4.1) κ = ω (σ + ... + σ ) ,ω (σ + ... + σ ) , g,n h Cg,n/Mg,n 1 n hyp Cg,n/Mg,n 1 n hypi where the right hand side of (4.1) is endowed with the Deligne metric [13, Sec. 6]. Proof. This is a reformulation of the definition of the Liouville metric.

18 Lemma 4.6. Let g = g1 + g2, n = n1 + n2 and β : g1,n1+1 g2,n2+1 be Knudsen’s clutching morphism. Then the isomorphismM × (3.6M) induces→ Mg,n an isometry β∗κ ∼ κ ⊠ κ . g,n −→ g1,n1+1 g2,n2+1 ×n In particular, for the clutching morphism γ : of Mg,n × M1,1 → Mg+n,0 Corollary 3.8, we have the isometry

⊠ γ∗κ ∼ κ ⊠ κ n. g+n,0 −→ g,n 1,1 Proof. One easily checks that the isomorphism (3.6), constructed in the proof of Proposition 3.7, is compatible with the Liouville metric. The main result of this section is the following theorem. an Theorem 4.7. The Liouville metric is continuous on . Mg,0 We postpone the proof of Theorem 4.7 until 4.3. For the moment we § derive a consequence of the theorem. an Corollary 4.8. The Liouville metric is continuous on . Mg,n ⊠ ⊠n Proof. We ﬁrst observe that the metric on κg,n κ1,1 is continuous. Indeed, ×n consider the clutching morphism γ : g,n 1,1 g+n,0. By Lemma 4.6, we have an isometry M × M → M

γ∗κ ∼ κ ⊠ κ⊠n. g+n,0 −→ g,n 1,1 The claim already follows from Theorem 4.7. Notice that, applied to the particular case g = n = 1, this implies that the metric on κ1,1 ⊠ κ1,1 is continuous. Let ∆ : be the diagonal morphism. Then we have M1,1 → M1,1 × M1,1 an isometry ∆∗(κ ⊠ κ ) ∼ κ⊗2. 1,1 1,1 −→ 1,1 We deduce that the metric on κ1,1 is continuous. Together with the continuity ⊠ ⊠n ∗ of the metric on κg,n κ1,1 , this shows that pr1κg,n is a continuous hermitian line bundle. Hence so does κg,n. The proof is complete. Remark 4.9. i. By means of Teichm¨uller theory it can be shown that the an Liouville metric is actually smooth on g,n. ii. In [20, Ch. 6] we show that theM Liouville metric is pre-log-log along an ∂ g,n. Miii. The name of Liouville metric is inspired by the Liouville action of Takhtajan-Zograf on an [55]. M0,n

19 4.3 Proof of Theorem 4.7 The proof of Theorem 4.7 is based on the next statement and the pinching expansion for the family hyperbolic metric established by Wolpert [63, Exp. 4.2, p. 445].

Proposition 4.10 (Masur [39], Sec. 6, Eq. 6.6). Let f : X Ω be a small stable deformation in compact curves of genus g 2, as in Construction→ 4.1. 2 ≥ For the hyperbolic metric dshyp;s,t on Xs,t and for all j =1,...,m we write

du 2 ds2 = ρ (u ) | j| , on A (t)= t 1/2 u 0, independent of s, t, such that for all j =1,...,m we have 1 ρ (u ) C on A (t) C ≤ s,t j ≤ j and 1 ρ (v ) C on B (t). C ≤ s,t j ≤ j Proof of Theorem 4.7. Let f : X Ω be a small stable deformation of a → stable curve X of genus g 2. After possibly restricting Ω, we can ﬁnd ≥ meromorphic sections σ, τ of ωX/Ω whose divisors are relative over Ω, with mutually disjoint irreducible components, disjoint from the singular points of the ﬁbers of f. We can further assume that div τ does not meet (A (t) ∪j j ∪ B (t)), (s, t) Ω. We have to prove that the function j ∈ (s, t) log σ , τ 7−→ kh |Xs,t |Xs,t ik is continuous at 0. Introduce the functions

F (s, t)= log σ δ , k khyp div τ ZXs,t G(s, t)= log τ c (ω ), k khyp 1 X/Ω hyp ZXs,t so that log τ Xs,t , σ Xs,t = F (s, t)+ G(s, t). It suﬃces to show that F and G are continuouskh | at| 0.ik The continuity of F at 0 is a consequence of the

20 ﬂatness of div σ over Ω and [63, Exp. 4.2, p. 445]. For the continuity of G we proceed in two steps, according to the decomposition G = G1 + G2, with

G (s, t)= log τ c (ω ), 1 k khyp 1 X/Ω hyp ZXs,t\∪j (Aj (t)∪Bj (t)) G (s, t)= log τ c (ω ). 2 k khyp 1 X/Ω hyp Z∪j (Aj (t)∪Bj (t)) Step 1. That G1(s, t) is continuous readily follows from [63, Exp. 4.2, p. 445], the curvature 1 constraint for the hyperbolic metric on X and Lebe- − s,t segue’s dominate convergence theorem. Step 2. We treat G2(s, t). Observe that over the annulus Aj(t), any diﬀeren- tial form can be expressed in terms of the holomorphic coordinate uj (even for t = 0, provided we exclude uj = 0). In the coordinate uj we have the pointwise convergence log( τ ) c (ω ) k khyp |Aj (t) 1 X/Ω hyp |Aj (t)→ log( τ ) c (ω ) as (s, t) 0. k khyp |Aj (0) 1 X/Ω hyp |Aj (0) → Indeed, this is a consequence of [63, Exp. 4.2, p. 445] and the curvature 1 condition for the hyperbolic metric on X . The corresponding fact is − s,t true for the annuli Bj(t), as well. Now, by assumption, div τ Aj(t) = , (s, t) Ω. From Proposition 4.10 and the curvature 1 constraint,| | ∩ we derive∅ ∈ − a uniform bound

−1 duj duj log( τ hyp Aj (t)) c1(ωX/Ω hyp) Aj (t) log log uj | 2 ∧ | 2 . k k | | ≪ | | uj (log uj ) | | | | Notice that we used that div τ is away from the singular points of the ﬁbers of f. An analogous bound holds on Bj(t). By Lebesgue’s dominate convergence theorem we arrive to G (s, t)= log τ c (ω ) 2 k khyp 1 X/Ω hyp Z∪j (Aj (t)∪Bj (t)) log τ c (ω ). → k khyp 1 X/Ω hyp Z∪j (Aj (0)∪Bj (0)) This completes the proof of the theorem.

5 On the degeneracy of the Quillen metric

5.1 Statement of the theorem

Let (X; a1,...,an) be a smooth n-pointed stable curve of genus g and (T1; b1), ..., (Tn; bn) n smooth 1-pointed stable curves of genus 1, all over C. They

21 deﬁne complex valued points P g,n(C), Q1,...,Qn 1,1(C), respec- ∈ M ∈ ×Mn tively. We apply the clutching morphism γ : to Mg,n × M1,1 → Mg+n,0 (P, Q1,...,Qn). We obtain a complex valued point R g+n,0(C). The curve represented by R is constructed as the quotient analytic∈ M space

Y =(X T ... T )/(a b ,...,a b ). ⊔ 1 ⊔ ⊔ n 1 ∼ 1 n ∼ n Since Y is compact, Chow’s theorem ensures the algebraicity of Y . Construct a small stable deformation f : Y Ω of Y as described by Construction 4.1. We build the family g : Z D→by restriction of f to the → −1 locus s1 = ... = sr = 0 and t1 = ... = tn = t D. The ﬁber Zt = g (t) ∈ ◦ is non-singular for t = 0, of genus g + n 2. Let X := X a1,...,an , ◦ 6 ≥ \{ } Tj = Tj bj , j = 1,...,n. Following the conventions of Section 2, we \{ } ◦ ◦ denote by Z(Zt,s), Z(X ,s) and Z(Tj ,s) the Selberg zeta functions of Zt, X◦ and T ◦, t = 0, j =1,...,n, respectively. j 6 Theorem 5.1. For t D 0 , we have the convergence ∈ \{ } Z′(Z , 1) t −n/6 t | | → (5.1) 1 n +1 Z′(X◦, 1) Z′(T ◦, 1) as t 0. πn 2g 2+ n j → j − Y The proof of the theorem is detailed throughout the next subsections.

5.2 Degeneracy of the Selberg zeta function We undertake the notations in Theorem 5.1. For every t D, t = 0, denote ∗ ∈ 6 by ∆t = d d the scalar hyperbolic laplacian on Zt. We notice that ∆t is obtained, via a fuchsian uniformization, by descent of y2(∂2/∂x2 + ∂2/∂y2) − on H. It is well-known that ∆t admits a unique non-negative and self-adjoint 2 extension to the Hilbert space L (Zt, C) [29]. For the eigenvalues of ∆t, counted with multiplicity, we write λ (t)=0 <λ (t) λ (t) .... 0 1 ≤ 2 ≤ Theorem 5.2 (Wolpert et al.). i. As t 0, the eigenvalues that converge → to 0 are exactly λ1(t),...,λn(t). ii. Let γ1(t),...,γn(t) Zt be the simple closed geodesics that are pinched to a node as t 0. Then⊂ the holomorphic function → Z(Z ,s) 1 t , Re s> j Zl(γj (t))(s) 2 uniformly converges to Z(QX◦,s) Z(T ◦,s) as t 0. j j → Q 22 iii. Let K1 D(1, 1/2) and K2 D be relatively compact open subsets. Then there is a⊂ uniform bound ⊂

Z(Zt,s) n 2 β, s K1, t K2 0 . j Zl(γj (t))(s) j=0(s s + λj(t)) ≤ ∈ ∈ \{ } −

Proof. QThe ﬁrst assertionQ is established in [48]. The second and third items are proven in [50, Th. 35 and Th. 38] and [62, proof of Conj. 1 and Conj. 2]. Theorem 5.3. For t D 0 , we have the convergence ∈ \{ } 1 n l(γ (t)) π2 Z′(Z , 1) j exp (2π)n t λ (t) 3l(γ (t)) → j=1 j j (5.2) Y n Z′(X◦, 1) Z′(T ◦, 1) as t 0. j → j=1 Y Proof. For every t D, deﬁne the meromorphic function ∈ Z(Z ,s) Q (s)= t , s D(1, 1/2). t Z (s) n (s2 s + λ (t)) ∈ j l(γj (t)) j=0 − j By Theorem 5.2 Qiii, Qt(s) extendsQ to a holomorphic function on D(1, 1/2). Furthermore, for every sequence tn n 0, tn = 0, there exists a subse- quence t such that Q converges{ } → to a holomorphic6 function H, uni- nk tnk k formly over{ } compact subsets{ } of D(1, 1/2) (Montel’s theorem). In particular Qt (1) H(1) as k + . By Theorem 5.2 i–ii, we see that nk → → ∞ Z(X◦,s) n Z(T ◦,s) H(s)= j , s D(1, 1/2). s2 s s2 s ∈ j=1 − Y − ◦ ◦ The Selberg zeta functions Z(Zt,s), Z(X ,s) and Z(Tj ,s) all have a simple zero at s = 1. The local factors Zl(γj (t))(s) are holomorphic and non-vanishing at s = 1. Thus we compute ′ Z (Zt, 1) Qt(1) = n , j=1 Zl(γj (t))(1)λj(t) n Q′ ◦ ′ ◦ H(1) = Z (X , 1) Z (Tj , 1). j=1 Y Furthermore the following asymptotic estimate evaluated at s = 1 holds [50, Lemma 39]: for Re s> 0 we have 2 2 2s−1 lim Γ(s) Zl(s) exp(π /3l)l =2π. l→0 The proof follows from these considerations.

23 5.3 Asymptotics of small eigenvalues and proof of The- orem 5.1 The notations of Section 5.2 are still in force.

Theorem 5.4. The product of the eigenvalues λ1(t),...,λn(t) satisﬁes

n λ (t) 1 n (5.3) lim j = +1 . t→0 l(γ (t)) (2π2)n 2g 2+ n j=1 j Y − This result actually appears as a particular instance of [7, Th. 1.1] (see Theorem 5.5 below), that the author learned from [30, Sec. 4]. We shall detail how to apply loc. cit. to derive Theorem 5.4. The ﬁrst step is to attach a weighted graph to every ﬁber Z , t = 0. Let t 6 G be the graph consisting of:

– n + 1 vertices, V = v ,...v ; { 0 n} – n edges, E = e ,...e . The edge e links the vertices v and v . { 1 n} i 0 i Therefore the shape of G is a n-edged star. For every t D, t = 0, we attach ∈ 6 to Zt two functions mt and lt:

m : V R t −→ v 2g 2+ n, 0 7−→ − v 1, j =1,...,n j 7−→ and l : E R t 7−→ e l(γ (t)). j 7−→ j n ∗ Deﬁne R[V ] := j=0 Rvj and R[V ] the vector space of linear functionals on R[V ]. For every F R[V ]∗ we put L ∈ n Q F = (F (v ) F (v ))2l (e ). t j − 0 t j j=1 X We also introduce the scalar product on R[V ]∗

n F , F = m (v )F (v )F (v ). h 1 2it t j 1 j 2 j j=0 X

24 ∗ There exists a unique symmetric operator M EndR(R[V ] ) such that t ∈ M F, F = Q F, F R[V ]∗. h t it t ∈ ∗ ∗ The matrix of Mt in the base v0,...,vn dual to v0,...,vn is (l (t)+ ... + l (t))/α l (t)/α l (t)/α 1 n − 1 ··· ··· − n l1(t) l1(t) 0 0  − . . ···  A(t) := . 0 .. 0 ,  . . . .   . . .. .     l (t) 00 l (t)   n n   − ···  where l (t) = l(γ (t)), α = 2g 2+ n. It is easily seen that 0 is a simple j j − eigenvalue of A(t). Let µ (t) ... µ (t) be the strictly positive eigenvalues 1 ≤ ≤ n of Mt, counted with multiplicity. Theorem 5.5 (Burger). For every j =1,...,n we have

λj(t) 1 lim = 2 . t→0 µj(t) 2π Proof. See [7, Th. 1.1 and App. 2]. Therefore, to prove Theorem 5.4, it suﬃces to establish the next proposition.

Proposition 5.6. For the product of the eigenvalues µ1(t),...,µn(t) we have

n µ (t) n lim j = +1. t→0 l(γ (t)) 2g 2+ n j=1 j Y − Proof. Introduce the function l(t)=2π2/ log t −1, t D, t = 0. By [63, Ex. 4.3, p. 446] there is an estimate | | ∈ 6 1 (5.4) l(γ (t)) = l(t)+ O . j (log t )4 | | Deﬁne the matrix nl(t)/α l(t)/α l(t)/α l(t) − l(t)··· 0 ··· − 0  − . . ···  B(t) := . 0 .. 0 .  . . . .   . . .. .     l(t) 0 0 l(t)     − ···  25 Let ν0(t)=0 < ν1(t) ... νn(t) be the eigenvalues of B(t). They are easily computed to be ≤ ≤ n ν (t)= ... = ν (t)= l(t) < ν (t)= +1 l(t). 1 n−1 n 2g 2+ n − By comparison of the characteristic polynomials of A(t), B(t) and the estimate (5.4) we arrive to n µ (t) n ν (t) n lim j = lim j = +1. t→0 l(γ (t)) t→0 l(t) 2g 2+ n j=1 j j=1 Y Y − This completes the proof of the proposition. Proof of Theorem 5.4. The theorem is a straightforward consequence of The- orem 5.5 and Proposition 5.6. We are now in position to prove Theorem 5.1. Proof of Theorem 5.1. The limit property (5.1) is a conjunction of (5.2) (The- orem 5.3), (5.3) (Theorem 5.4) and Wolpert’s estimate (5.4) for the length l(γj(t)).

5.4 Consequences: degeneracy of the Quillen metric

We apply Theorem 5.1 to study the behavior of the Quillen metric Q on an k·k λg+n,0, near ∂ g,0. We maintainM the notations of Section 5.1. For the clutching morphism ×n γ : , we have the isomorphism (3.10) Mg,n × M1,1 → Mg+n,0 Φ: γ∗λ ∼ λ ⊠ λ⊠n. g+n,0 −→ g,n 1,1 At the point N =(P, Q1,...,Qn), Φ induces a natural isomorphism n Φ : λ(ω ) ∼ λ(ω ) λ(ω ). N Z0 −→ X ⊗ Tj j=1 O Observe that λ(ω )= λ(ω ) is the stalk at 0 D of λ(ω ). Z0 Y ∈ Z/D 2 Proposition 5.7. i. The L metric 2 on λ(ω ) extends contin- k·kL Z/D |D\{0} uously to λ(ωZ/D). ii. Let µ = n/(2g 2+ n)+1. Then Φ induces an isometry − N ∼ ΦN :(λ(ωZ/D), L2 ) 0 k·k | −→ n 1/2 (λ(ω ), 2 ) (λ(ω ), 2 ) (γ ). X k·kL ⊗ Tj k·kL ⊗ O j=1 O

26 Proof (see also [5], Prop. 13.5, case (II), pp. 96–98). We shall prove the two assertions simultaneously. Write D∗ = D 0 . First of all we have isometries \{ } ∼ 0 1/2 (λ(ω ) ∗ , 2 ) (det R g ω ∗ ,V ), Z/D |D k·kL → ∗ Z/D |D g+n,0 ·k·k0 ∼ 0 1/2 (λ(ω ), 2 ) (det H (X,ω ),V ), X k·kL → X g,n ·k·k0 ∼ 0 1/2 (λ(ω ), 2 ) (det H (T ,ω ),V ), Tj k·kL → j Tj 1,1 ·k·k0 2 0 where 0 is the L metric on the line det R g∗ωZ/D D∗ (respectively 0 k·k 0 | det H (X,ωX ), det H (Tj,ωTj )) and Vg+n,0 =2g +2n 2, Vg,n =2g 2+ n, − 2 − V1,1 = 1. Since µ = Vg+n,0/Vg,n, it suﬃces to prove that the L metric on 0 det R g ω ∗ extends continuously at 0 and Φ induces an isometry ∗ Z/D |D N 0 ∼ (det R g∗ωZ/D, 0) 0 k·k | −→ n (det H0(X,ω ), ) (det H0(T ,ω ), ). X k·k0 ⊗ j Tj k·k0 j=1 O 0 0 Let α1,...,αg be a basis of H (X,ωX ) and βj a basis of H (Tj,ωTj ), j =

1,...,n. The diﬀerential forms αi, βj satisfy the residue conditions Respk αi =

0, Resqj βj = 0. Therefore there exist global sections αi and βj of ωY extend- ing αi, βj by 0, respectively. The sections αi, i = 1,...,g, βj, j = 1,...,n 0 0 form a basis of H (Y,ωY ). Besides, R g∗ωZ/D is locallye freee of rank g + n. After possibly shrinking D, we can ﬁnd ae frame αi(t), i =e 1,...,g, βj(t), 0 j =1,...,n of R g∗ωZ/D over D, such that αi(0) = αi and βj(0) = βj. Write θ (t) g+n for the whole ordered set α (t) g eβ (t) n . An easye local { i }i=1 { i }i=1 ∪{ j }j=1 computation shows that the function e e e e e e i t det θ θ , t =0 7→ 2π j ∧ k 6 ZZt 1≤j≤k≤g+n extends continuously to 0, with value

i n i det α α β β . 2π j ∧ k 2π j ∧ j X 1≤j,k≤g j=1 Tj Z Y Z Notice that

Φ :(θ ... θ )(0) (α ... α ) β ... β . N 1 ∧ ∧ g+n 7→ ± 1 ∧ ∧ g ⊗ 1 ⊗ ⊗ n The proposition follows from these observations.

27 Attached to the one-parameter deformation g : Z D, there is a classifying map → an (g): D . C −→ Mg+n,0 The line bundle λ(ω ) = (g)∗λan is endowed with the Quillen metric Z/D C g+n,0 . k·kQ 0 Corollary 5.8. Let α1,..., αg, β1,..., βn be a frame of the sheaf R g∗ωZ/D over D, as in the proof of Proposition 5.7. Then we have

e e e e n/12 lim α1 ... αg β1 ... βn Q,t t t→0 k ∧ ∧ ∧ ∧ ∧ k | | = α1 ... αg Q ... β1 Q ... βn Q. e e e k ∧ e ∧ k · ·k k · ·k k Proof. This is easily derived from the very deﬁnition of Q (see Section 2), Theorem 5.1, Proposition 5.7 and the relation (2.2). k·k

6 Metrized Mumford isomorphism on Mg,n In this section we establish the following metrized version of the Mumford isomorphism (Theorem 3.10). ◦ Theorem 6.1. Let g,n be the restriction to g,n of the Mumford isomorphism . Then D◦ induces an isometry M Dg,n Dg,n ◦ : λ⊗12 ψ ∼ κ (C(g, n)) on . Dg,n g,n;Q ⊗ g,n;W −→ g,n ⊗ O Mg,n Remark 6.2. For the sake of simplicity, in the theorem we wrote λg,n, ψg,n, etc. instead of λ , ψ etc. respectively. g,n |Mg,n g,n |Mg,n To lighten the forthcoming arguments, it is worth introducing some notations. Notation 6.3. We define the line bundles L := λ⊗12 ψ δ−1 . g,n g,n ⊗ g,n ⊗ g,n L := (λ⊗12 ψ δ−1) ⊠ (λ⊗12 ψ δ−1)⊠n g,n g,n ⊗ g,n ⊗ g,n 1,1 ⊗ 1,1 ⊗ 1,1 and the isomorphism ⊠ ⊠ D := ⊠ n : L ∼ κ ⊠ κ n. g,n Dg,n D1,1 g,n −→ g,n 1,1 We let g,n be the continuous hermitian metric on Lg,n such that the Mumfordk·k isomorphism becomes an isometry Dg,n : L := (L , ) ∼ κ (C(g, n)). Dg,n g,n g,n k·kg,n −→ g,n ⊗ O ⊠n ⊠n Finally, we write L = L ⊠ L and D = ⊠ . g,n g,n 1,1 g,n Dg,n D1,1 28 The first observations towards the proof of Theorem 6.1 is summarized in the next proposition. Proposition 6.4. Let g 2 be an integer. Then: ≥ i. Theorem 6.1 holds true for (g, n)=(g, 0); −1 ii. endow δg,0 = ( ∂ g,0) with the trivial singular metric coming from O − M−1 the absolute value; write δg,0 for the resulting hermitian line bundle. Then ⊗12 ⊗12 −1 λg,0;Q extends to a continuous hermitian line bundle λg,0;Q δg,0 on g,0. ◦ ⊗ M Moreover, extends to an isometry Dg,0 −1 λ⊗12 δ ∼ κ (C(g, 0)). g,0;Q ⊗ g,0 −→ g,0 ⊗ O Proof. In the present form, the first item is a theorem of Deligne [13, Th. 11.4] and Gillet-Soulé[22]. Indeed, it is enough to point out the following facts:

◦ – Deligne’s functorial isomorphism coincides with g,0, up to a sign. This is justiﬁed by H0( , G )= 1 [41, LemmaD 2.2.3]; Mg,0 m {± }

– the normalization of the Quillen metric on λ(ωX ) of Deligne and Gillet- Souléin loc. cit. coincides with ours (see Section 2). Let X be a compact Riemann surface of genus g, with hyperbolic metric of curvature 2 2 1, dshyp. Let h be the hermitian metric induced by dshyp on TX . De- − ∗ note by ∆∂ = ∂ ∂ the associated ∂-laplacian acting on functions, and ∗ ∆d = d d the hyperbolic scalar laplacian. Recall the Kähler identity [59, Ch. 5] 1 (6.1) ∆ = ∆ . ∂ 2 d Deligne and Gillet-Souléwork with the Quillen metric

′ −1/2 8 = (det ∆ ) 2 . k·k ∂ k·kL We now check the relation

′ ′ (6.2) det ∆∂ = E(g, 0)Z (X, 1). First of all, since X has genus g and by (6.1), we compute

′ (g+2)/3 ′ (6.3) det ∆∂ =2 det ∆d 8This deﬁnition agrees with the one by Deligne and Gillet-Soul´edue to the remark preceding [44, Prop. 2.7, p. 159].

29 (see [5, Eq. 13.2, p. 88]). By a theorem of d’Hoker-Phong [16] and Sarnak [47, Cor. 1], we have the expression

(6.4) det′ ∆ = Z′(X, 1) exp((2g 2)(2ζ′( 1) 1/4+1/2 log 2π)). d − − − Equations (6.3)–(6.4) together already imply (6.2), and hence the equality = . k·k k·kQ The second assertion ii is derived by combination of the Mumford isomorphism on (Theorem 3.10), the continuity of the Liouville metric (The- Mg,0 orem 4.7) and the ﬁrst point i. Corollary 6.5. i. For every integer g 2, the continuous hermitian line ≥ bundle Lg,0 = Lg,0 satisﬁes the following factorization formula:

−1 L = λ⊗12 δ . g,0 g,0;Q ⊗ g,0 ii. There is a diagram of isometries of continuos hermitian line bundles on Mg,n

∗D γ g+n,0 ∗ ∼ ∗ (6.5) γ L / γ (κg+n,0 (C(g + n, 0))) g+n,0 ⊗ O ≀ ≀ ⊠ ⊠n Lg,n ∼ / (κg,n (C(g, n))) (κ1,1 O(C(1, 1))) , Dg,n ⊗ O ⊗ commutative up to a sign. The isomorphisms underlying the vertical arrows are induced by (3.10)–(3.12) (Corollary 3.8). Proof. The first claim is a reformulation of Proposition 6.4 ii. For the second assertion, we first observe that if we forget the hermitian structures, then (6.5) is a consequence of Corollary 3.2, Corollary 3.8 and Corollary 3.11. The existence of the whole diagram (6.5) is a conjunction of the very definition of Lg,n and Dg,n (Notation 6.3), relation (2.1), Lemma 4.6 and Proposition 6.4 ii. Theorem 6.1 is actually equivalent to the next apparently weaker statement. Proposition 6.6. There is a factorization of hermitian line bundles on ×n Mg,n ×M1,1 ⊠ × ⊗12 ⊠ ⊗12 n (6.6) Lg,n M ×M n =(λg,n;Q ψg,n;W ) (λ1,1;Q ψ1,1;Q) . | g,n 1,1 ⊗ ⊗

30 Proof. We first observe that proving (6.6) is tantamount to proving the factorization formula an ⊠ × an ⊗12 an ⊠ an ⊗12 an n (6.7) Lg,n Man ×Man n =(λg,n;Q ψg,n;W ) (λ1,1;Q ψ1,1;Q) . | g,n 1,1 ⊗ ⊗ Since we aim to show the factorization of the underlying hermitian structures, we shall establish (6.7) pointwise. By Corollary 6.5 there is an isometry of continuous hermitian line bundles ×n on Mg,n × M1,1 Ψ: γ∗L ∼ L . g+n,0 −→ g,n The isomorphism of line bundles underlying Ψ is build up with (3.10)–(3.11). Fix a complex valued point N = (P, Q ,...,Q ) (C) (C)×n; 1 n ∈ Mg,n ×M1,1 let R be its image in g+n,0(C) by the clutching morphism γ. At the point N, Ψ induces an isometryM of complex hermitian lines (6.8) Ψ : R∗L = N ∗γ∗L ∼ N ∗L . N g+n,0 g+n,0 −→ g,n ∗ We focus on R Lg+n,0. Let(X; a1,...,an), (T1; b1),..., (Tn, bn) be the pointed stable curves corresponding to P , Q1,...,Qn, respectively. Let Y be the curve represented by R. By means of Construction 4.1 we obtain a small stable deformation f : Y Ω of Y . Attached to f there is a classifying morphism → an (f) : Ω . C −→ Mg+n,0 an an an We agree in denoting the pull-backs of λg+n,0, δg+n,0, Lg+n,0, etc. to Ω by λ(f), δ(f), L(f), etc. respectively. We also set R(f) := 0 Ω. We have to study the hermitian complex line R(f)∗L(f). With the notations∈ of Construction 4.1, the equation of the divisor of the singular fibers of f is t ... t = 0. 1 · · n After possibly shrinking Ω, the holomorphic section e := t1 ... tn is a frame of δ(f)−1. We introduce two auxiliary metrics: · · – a modified Quillen metric ′ on λ(f): k·kQ ′ = ′ t ... t 1/12 at the point (s, t) Ω; k·kQ;(s,t) k·kQ| 1 · · n| ∈ – the smooth metric ′ on δ(f)−1 defined by the rule k·k e ′ =1. k k With these choices, there is an obvious equality

−1 ′−1 (6.9) L(f)= λ(f)⊗12 δ(f) = λ(f)′⊗12 δ(f) . Q ⊗ Q ⊗ We point out two features concerning ′ and ′: k·kQ k·k 31 – by the continuity of the metric of L(f), the smoothness of ′ and equality (6.9), we see that the metric ′ is actually continuous;k·k k·kQ – the pull-back of the isomorphism (3.11) by N yields

n Θ : R∗δ−1 = R(f)∗δ(f)−1 ∼ P ∗ψ Q∗ψ N g,n −→ g,n ⊗ j 1,1 j=1 O R(f)∗e ( n du ) ( n dv ), 7−→ ± ⊗j=1 j ⊗ ⊗j=1 j

where uj, vj are the rs coordinates at aj, bj, respectively, used to construct f : Y Ω. This is so because the degeneration of the family → f in a neighborhood of the node aj bj is modeled by ujvj = tj (see [17, Sec. 4] for a detailed proof). Therefore,∼ if we endow δ(f)−1 with the metric ′, then Θ becomes an isometry k·k N n ′−1 (6.10) Θ : R(f)∗δ(f) ∼ P ∗ψ Q∗ψ . N −→ g,n;W ⊗ j 1,1;W j=1 O Indeed, it suﬃces to recall that the Wolpert metric assigns the value1 to duj and dvj (see Deﬁnition 2.1). We claim that the isomorphism (3.10) induces an isometry

n (6.11) Φ : R(f)∗λ(f)′ ∼ P ∗λ Q∗λ . N Q −→ g,n;Q ⊗ j 1,1;Q j=1 O Let g : Z D be the restriction of f to the locus s = ... = s = 0, → 1 r t1 = ... = tn = t D, for some small disc D C centered at R(g) := 0. Let (g) be the associated∈ classifying map. By the⊂ very deﬁnition of g there is a Ccommutative diagram

(6.12) R(g)=0 / D _ F FF FFC(g) FF FF " an ι . Mg+n,0 xx< xx xx xx C(f) x R(f)=0 / Ω

From (6.12) we derive the equality of line bundles

λ(g)= (g)∗λan = ι∗λ(f). C g+n,0 32 ′ Therefore, the pull-back of Q by ι is a continuous hermitian structure on ′ k·k λ(g). Write λ(g)Q for the resulting hermitian line bundle. The claim (6.11) is equivalent to asserting an isometry induced by (3.10)

n (6.13) Φ : R(g)∗λ(g)′ ∼ P ∗λ Q∗λ . N Q −→ g,n;Q ⊗ j 1,1;Q j=1 O The validity of (6.13) has been established in 5.4, Corollary 5.8. We come up to the conclusion by (6.8)–(6.11). §

Proof of Theorem 6.1. By the very deﬁnition of Lg,n (Notation 6.3) it suﬃces to show a decomposition of hermitian line bundles

(DEC ) L = λ⊗12 ψ . g,n g,n |Mg,n g,n;Q ⊗ g,n;W As a ﬁrst step, we treat the case g = n = 1. Proposition 6.6 yields

⊠ × ⊗12 2 (6.14) L1,1 M 2 =(λ1,1;Q ψ1,1;W ) . | 1,1 ⊗ ⊠2 × Recall that L1,1 M 2 = L1,1. Therefore the pull-back of (6.14) by the diagonal | 1,1 morphism ∆ : leads to M1,1 →M1,1 ×M1,1 ⊗2 ⊗12 ⊗2 (6.15) L1,1 =(λ ψ1,1;W ) . |M1,1 1,1;Q ⊗ Because L = λ⊗12 ψ , the formula (DEC ) follows from (6.15). To 1,1 |M1,1 1,1 ⊗ 1,1 1,1 establish (DECg,n) for general g, n, we tensor equation (6.6) in Proposition ∗ ⊠n ⊗−1 ×n 6.6 by the identity (pr2(DEC1,1) ) (pr2 is the projection g,n 1,1 ×n). We obtain M ×M → M1,1 (6.16) pr∗L = pr∗(λ⊗12 ψ ). 1 g,n |Mg,n 1 g,n;Q ⊗ g,n;W By deﬁnition of L , L = λ⊗12 ψ . Consequently, (6.16) already g,n g,n |Mg,n g,n ⊗ g,n implies the coincidence of the metric on L and the metric on λ⊗12 g,n |Mg,n g,n;Q ⊗ ψg,n;W , hence (DECg,n). This completes the proof of the theorem. Remark 6.7. The Wolpert metric on ψ naturally extends to . For g,n Mg,n the clutching morphism β : g1,n1+1 g2,n2+1 g1+g2,n1+n2 we have an isometry of hermitian line bundlesM × M → M

(6.17) β∗ψ ∼ ψ ⊠ ψ . g1+g2,n1+n2;W −→ g1,n1+1;W g2,n2+1;W In view of the results in [20, Chap. 4] –specially Section 4.3 of loc. cit.–, we expect that ψ is a continuous hermitian line bundle on . In this g,n;W Mg,n 33 ◦ case, the isometry g,n of Theorem 6.1 extends to an isometry of continuous hermitian line bundlesD

−1 : λ⊗12 δ ψ ∼ κ (C(g, n)) on , Dg,n g,n;Q ⊗ g,n ⊗ g,n;W −→ g,n ⊗ O Mg,n −1 where δg,n is equipped with the trivial singular metric coming from the absolute value. A parallel argument as for the proof of Theorem 6.1 and Propo- sition 6.6 –combining Lemma 4.6, equation (6.17), [50, Th. 35 and Th. 38] and [7, Th. 1.1]– will then lead to a factorization

(6.18) β∗ = ⊠ . Dg1+g2,n1+n2 Dg1,n1+1 Dg2,n2+1 An analogous compatibility formula is expected for the clutching morphism α : of [36, Def. 3.8 and Th. 4.2]: Mg−1,n+2 → Mg,n (6.19) α∗ = . Dg,n Dg−1,n+2 Conversely, assume that for a suitable choice of constant E(g, n) –and the subsequent choice of Quillen metric determined by Deﬁnition 2.2– the relations (6.18)–(6.19) hold. An algebraic manipulation then shows that E(0, 3) determines E(g, n). Furthermore, by Theorem A applied to (P1 ;0, 1, ), Z ∞ E(0, 3) coincides with the constant (2.2) (for g = 0 and n = 3), and so does E(g, n). This explains the signiﬁcance of Theorem A in the case g = 0, n = 3. We plan to deepen in these questions in the future.

Proof of Theorem A. Attached to ( ; σ1,...,σn) there is a classifying morphism X → S : . C S → Mg,n For every embedding τ Σ, τ = τ C is smooth. Hence there is a commutative diagram ∈ X X ×

Cτ ___ (6.20) Spec C / g,n II M _ II II II I$ τ g,n. tt:M tt tt tt C tt S ∗ Let g,n be the pull-back of the Mumford isomorphism g,n by . From (6.20)C weD infer the equality τ ∗ ∗ = ∗ ◦ (recall that ◦D := C ). C Dg,n Cτ Dg,n Dg,n Dg,n |Mg,n

34 Therefore Theorem 3.10 and Theorem 6.1 altogether yield an isometry of hermitian line bundles on S ∗ :λ(ω )⊗12 (∆ )−1 ψ ∼ C Dg,n X /S Q ⊗ O X /S ⊗ W −→ (6.21) ω (σ + ... + σ ) ,ω (σ + ... + σ ) h X /S 1 n hyp X /S 1.2 n hypi (C(g, n)). ⊗ O

The theorem is now obtained from (6.21) applying c1. We close this section with a further application of Theorem 6.1: a significant case of the Takhtajan-Zograf local index theoremb [56]–[57].

Theorem 6.8 (Takhtajan-Zograf). Let ωWP , ωTZ be the Weil-Petersson and Takhtajan-Zograf Kähler forms on an , respectively. The following equality Mg,n of differential forms on an holds: Mg,n 1 1 (6.22) c (λ )= ω ω . 1 g,n;Q 12π2 WP − 9 TZ Proof. First of all we have the equality of differential forms on an Mg,n 1 (6.23) c (κ )= ω . 1 g,n π2 WP For a reference see [63] (case n = 0) and [20, Ch. 5] (general case). Secondly, by [64, Th. 5], there is another identity of differential forms on an Mg,n 4 (6.24) c (ψ )= ω . 1 g,n;W 3 TZ The relation (6.22) is deduced by conjunction of Theorem 6.1 and (6.23)– (6.24). Remark 6.9. In contrast with [61, Fund. th., p. 278], our proof of (6.22) is new and does not require the work of Takhtajan-Zograf.

7 The special values Z′(Y (Γ), 1) and L(0, ) MΓ The aim of this section is to proof Theorem B. The argument relies on The- orem A and a formula of Bost [6] and K¨uhn [37] for the arithmetic self- intersection number of ωX1(p)/Q(µp)(cusps)hyp. Fix K = Q(µp) C the p-th cyclotomic ﬁeld. Denote by ι the inclusion K C and by ι its⊂ complex conjugate. Then = (Spec K, Σ := ι, ι , F ) ⊂ A { } ∞

35 1 is an arithmetic ring. The arithmetic Chow group CH (Spec K) –associated to – comes equipped with an arithmetic degree map: A 1 c deg : CH (Spec K) R/ log K× −→ | | (7.1) 1 [(0, λ ,λ )] [ (λ + λ )] = [λ ] (since λ = λ ). d c { ι ι} 7−→ 2 ι ι ι ι ι If L = (L, ) is a metrized line over Spec K, then the first arithmetic k·k 1 −2 Chern class c1(L) CH (Spec K) is the class of (0, log sσ σ σ∈Σ), for any non-vanishing section∈ s of L. Therefore, taking (7.1){ intok account,k } c (7.2) b deg c (L) = [log s −2], 1 k ιkι which is independent of the choice of s. d × Let Q be the algebraic closureb of Q in C. We will write Q for the group × | | of norms of elements of Q . The modular curve X(Γ),forΓ = Γ0(p)orΓ1(p), and its cusps are defined over the number field K [51, Ch. 6.7]. We still write X(Γ) for a projective 9 model over K. The notations X0(p) and X1(p) will also be employed. Proposition 7.1. Let p be a prime number for which X(Γ) has genus g 1. Then the equality ≥ −2g deg c (λ(ω ), 2 )= log(π L(0, )) 1 X(Γ)/K k·kL − MΓ × holds in R/ log Q . d| b | 0 (see also [58]). First recall that via the q-expansion, H (X(Γ),ωX(Γ)/K) gets identified with the space of weight 2 cusp forms in S2(Γ, C) whose Fourier series expansion at have coefficients in K [11, Th. 1.33]. Let us write ∞ S2(Γ,K) for this space. Notice that the set Prim2(Γ) S2(Γ,K) K Q is a Q-basis. For every f Prim (Γ), write ω H0(X(Γ)⊂ ,ω )⊗ C for ∈ 2 f ∈ X(Γ)/K ⊗K the corresponding differential form. Viewing ωf as a holomorphic form, we find the relation i 4π f, f = ω ω , h i 2π f ∧ f ZX(Γ) where f, f is the Petersson square norm of f. Indeed, ωf pulls back to the h i 2 Γ invariant tensor 2πif(z)dz on H. By the definition of the L metric L2 × k·k (see Section 2) and (7.2), we infer the equality in R/ log Q | | g (7.3) deg c (λ(ω ), 2 )= log(π f, f ). 1 X(Γ)/K k·kL − h i f∈PrimY2(Γ) 9 d Contrary to theb usual conventions, here X0(p) is assumed to be defined over K, and not over Q.

36 By [28, Th. 5.1] we have

−3g 2 (7.4) f, f × π L(2, Sym f, χf ). h i ∼Q+ f∈PrimY2(Γ) f∈PrimY2(Γ) The equations (1.1) (see the Introduction), (7.3) and (7.4) altogether lead to the conclusion. Lemma 7.2. Suppose that p is a prime number congruent to 11 modulo 12. The following assertions hold: i. the natural morphism ϕ : X (p) X (p) is unramiﬁed as a morphism 1 → 0 of K-schemes; ii. ϕ induces an isometry of pre-log-log hermitian line bundles

ϕ∗(ω ((0) + ( )) ) ∼ ω (cusps) ; X0(p)/K ∞ hyp −→ X1(p)/K hyp

iii. if σ X1(p)(K) is a cusp lying over X0(p)(K) (resp. 0), then ϕ induces an∈ isometry of hermitian line bundles∞ ∈

e ∼ σ∗ (ω ) σ∗(ω ) , ∞ X0(p)/K W −→ X1(p)/K W where σ is the section (resp. σ ). ∞ ∞ 0 e Proof. The first assertion is derived from [40, Ch. 2, Sec. 2, Table I]. Then properties ii–iii are easily checked as a consequence of i. Proposition 7.3. Let p 11 be a prime number. Assume that p 11 ≥ ≡ mod 12 whenever Γ=Γ0(p). Let ψW be the tensor product of the cotangent bundles at the cusps of X(Γ)K, endowed with the Wolpert metric. Then the equality deg c1(ψW )=0 holds in R/ log K× . | | db Proof. We begin with the more delicate case Γ = Γ0(p). Let σ0 and σ∞ be the sections of X0(p) induced by the cusps 0, , respectively. We first show ∗ ∞ that deg σ∞(ωX0(p)/K ) = 0. Let f S2(Γ0(p),K) be a cusp form whose q- expansion has leading coefficient a ∈= 0. It exists because Prim (Γ) is a base 1 6 2 of S2(Γd0(p),K) K Q. Let θ be the global section of ωX0(p)/K corresponding to f via the q-expansion.⊗ We claim that

(7.5) σ∗ θ = a K× . k ∞ kW,∞,ι | 1|∈| | It shall be emphasized that (7.5) is not a formal consequence of the deﬁnition of the Wolpert metric: since Γ0(p) has elliptic ﬁxed points, the hyperbolic

37 10 metric on H does not descend to the hyperbolic metric of X (p)C 0, . 0 \{ ∞} However we may pull-back θ to X1(p), where the hyperbolic metric on X (p)C cusps is obtained by descend from H. By the lemma we have 1 \{ } σ∗ θ = σ∗ ϕ∗θ , k ∞ kW,∞,ι k ∞ kW,∞,ι where σ∞ is the section of X1(p) induced by the cusp X1(p). The ∗e ∞ ∈ restriction of the differential form (ϕ θ)C to X (p)C cusps lifts to the 1 \ { } upper halfe plane as dq f(z)= a qn , q = e2πiz. n q n≥1 X By the very definition of the Wolpert metric we find

σ∗ ϕ∗θ = a , k ∞ kW,∞,ι | 1| thus proving the claim. Notice that by (7.2), equation (7.5) entails e (7.6) deg σ∗ (ω ) = [log a −2] = 0 in R/ log K× . ∞ X0(p)/K W | 1| | | ∗ Now we turnd our attention to deg σ0 (ωX0(p)/K )W . Consider the Atkin-Lehner involution wp : X0(p) X0(p) (see [15, IV, (3.16)] and [40, Ch. 2, Sec. 6, → ∗ Par. 1]). The pull-back wpθ isd a global section of ωX0(p)/K . From [15, VII, (3.18)] and [40, Ch. 2, Sec. 6, Par. 1], the ﬁrst coeﬃcient of the q-expansion ∗ of wpθ at the cusp 0 is a1/p. The same argument as above shows

σ∗w∗θ = a /p K× k 0 p kW,0,ι | 1| ∈| | and hence

(7.7) deg σ∗(ω ) = 0 in R/ log K× . 0 X0(p)/K W | | Equations (7.6) andd (7.7) lead to the conclusion. The argument for Γ = Γ1(p) is analogous. A comment is in order: to produce a global section of ωX1(p)/K not vanishing at a prescribed cusp, besides the Atkin-Lehner involution we need the diamond operators d : X1(p) X (p), for d (Z/pZ)×/ 1 .11 This completes the proof. h i → 1 ∈ {± } 10 The hyperbolic metric on X0(p)C 0, is smooth at the image of the elliptic ﬁxed points, while the “descended” metric\{ is not∞} [37, Par. 4.2]. 11 The diamond operators constitute the Galois group of X1(p) X0(p). This morphism is unramiﬁed at the cusps [40, Sec. 2]. Hence the group generated→ by the Atkin-Lehner involution and the diamond operators acts transitively on the cusps of X1(p).

38 Proposition 7.4. Let p 11 be a prime number and suppose that p 11 ≥ ≡ mod 12 whenever Γ=Γ0(p). The equality deg π (c (ω (cusps) )2) = 4[Γ(1) : Γ](2ζ′( 1) + ζ( 1)) ∗ 1 X(Γ)/K hyp − − × holds in R/ log Q . d b| | Proof. We begin with Γ = Γ1(p). By [34, Tab. 10.13.9.1, Th. 10.13.11] there is a Kodaira-Spencer isomorphism KS : ω⊗2 ∼ ω (cusps), mod −→ X1(p)/K ⊗2 ⊗2 where ωmod is the sheaf of weight 2 modular forms on X1(p)/K. Equip ωmod ∗ with the hermitian structure mod = KS hyp. A theorem of Bost and K¨uhn [37, Th. 6.1, Cor. 6.2 andk·k Rem. 6.3 a)]k·k provides

⊗2 2 deg π∗(c1(ωmod, mod) )= (7.8) k·k ′ ♯ ι, ι 2[Γ(1) : Γ1(p)](2ζ ( 1) + ζ( 1)) d { }· − − × b in R/ log Q .12 We now| focus| on Γ (p), p 11 mod 12. Let d denote the degree of 0 ≡ ϕ : X1(p) X0(p). By Lemma 7.2 and the functoriality of the arithmetic self-intersection→ numbers, we have degπ (c (ω ((0) + ( )) )2)= ∗ 1 X0(p)/K ∞ hyp (7.9) 1 deg π (c (ω (cusps) )2). dd b ∗ 1 X1(p)/K hyp Because d = [Γ (p):Γ (p)], the claim follows from (7.8) and (7.9). 0 1 d b Proof of Theorem B. The result follows from Theorem A, the relation (7.2), Proposition 7.1–7.4 and the identity exp(ζ′( 1)) = 2−1/36π1/6Γ (1/2)−2/3 − 2 (see [60, App.]).

Remark 7.5. We expect that × can be refined to an equality. A possible ∼Q approach would be to work with the regular models of Deligne-Rapoport [15] and, at least in the case Γ = Γ0(p), apply a theorem of Ullmo on the Faltings height of the Jacobian J0(p) [58]. However, the necessary considerations have not been effected here: the singular fiber of (p)/Z, together with the X0 sections 0, , is not a pointed stable curve [40, Ch. II, Sec. 1 and App.]. ∞ 12 The discussion of [37, Par. 4.14] shows that mod is 1/√2 times the Petersson metric (or L2 metric) used by Bost and Kühn. Thisk·k disagreement contributes to 0 in the × arithmetic self-intersection number (7.8), whose value is in R/ log Q . | | 39 Acknowledgements The starting point of this work was the suggestion of my thesis advisors J.-B. Bost and J. I. Burgos of studying the Arakelov geometry of the moduli spaces g,n. I am deeply indebted to them for the many hours of mathe- maticalM discussions on the topic of this work, as well as for their constant insistence and encouragement. I also thank J.-M. Bismut, G. Chenevier, T. Hahn, M. Harris, S. Keel, J. Kramer, U. Kühn, X. Ma, V. Maillot, C. Soulé, L. Takhtajan and E. Ullmo for discussions on this and related topics. Special thanks go to Scott Wolpert, who kindly shared his ideas with me. A substantial part of this work was done while the author was visiting the Universitàdegli Studi di Padova, with the support of the European Com- mission’s “Marie Curie early stage researcher” programme (6th Framework Programme RTN Arithmetic Algebraic Geometry). I thank these institutions for their hospitality and financial support.

References

[1] L.V. Ahlfors, Complex analysis: an introduction to the theory of analytic functions of one complex variable, International series in pure and applied mathematics, McGraw-Hill, 1979. [2] E. Arbarello and M. Cornalba, Calculating cohomology groups of moduli spaces of curves via algebraic geometry, Pub. Math. I.H.E.S. 88, 1998, 97–127. [3] M. Artin, A. Grothendieck and J.-L. Verdier, Séminaire de Géométrie Algébrique du Bois Marie 1963/64, tome 3 (Lect. Notes Math., Vol 305), Springer. [4] E. W. Barnes, The theory of the G-function, Q. J. Math. 31, 1900, 264–314. [5] J.-M. Bismut and J.-B. Bost, Fibrés déterminants, métriques de Quillen et dégénérescence des courbes, Acta Math. 165, 1990, 1–103.

2 [6] J.-B. Bost, Intersection theory on arithmetic surfaces and L1 metrics, Letter dated March, 1998. [7] M. Burger, Small eigenvalues of Riemann surfaces and graphs, Math. Z. 205, 1990, 395–420. [8] J. I. Burgos, J. Kramer and U. K¨uhn, Cohomological arithmetic Chow rings, J. Inst. Math. Jussieu 6, 2007, 1–172.

40 [9] J. I. Burgos, J. Kramer and U. K¨uhn, Arithmetic characteristic classes of automorphic vector bundles, Documenta Math. 10, 2005, 619–716.

[10] H. Carayol, Sur les repr´esentations ℓ-adiques associ´ees aux formes modulaires de Hilbert, Ann. Sci. Ec. Norm. Sup 19, 1986, 409–468.

[11] H. Darmon, F. Diamond and R. Taylor, Fermat’s Last Theorem in Ellip- tic curves, modular forms & Fermat’s last theorem (Hong Kong 1993), Int. Press, Cambridge, MA, 1997.

[12] P. Deligne, Formes modulaires et repr´esentations ℓ-adiques, S´em. Bour- baki, exp. 355, LNM 179, 139–172, Berlin-Heidelberg-New York, Springer 1969.

[13] P. Deligne, Le d´eterminant de la cohomologie, Contemporary Math. 67, 1987, 93–177.

[14] P. Deligne and D. Mumford, The irreductibility of the moduli space of curves of given genus, Pub. Math. I.H.E.S. 36, 1969, 75–110.

[15] P. Deligne and M. Rapoport, Les sch´emas de modules de courbes ellip- tiques, in Deligne, P., Kuijk, W. (eds) Modular functions of one variable II. Proceedings Antwerp 1972, LNM 349, Berlin-Heidelberg-New York, Springer 1973.

[16] E. d’Hoker and D.H. Phong, On determinants of Laplacians on Riemann surfaces, Commun. Math. Phys. 104, 1986, 537–545.

[17] B. Edixhoven, On N´eron models, divisors, and modular curves, Ramanu- jan J. 13, 1998, 157–194.

[18] R. Elkik, Fibr´es d’intersection et int´egrales de classes de Chern, Ann. scient. Ec.´ Norm. Sup., 22, 1989, 195–226.

[19] G. Freixas i Montplet, Heights and metrics with logarithmic singularities, to appear in J. Reine Angew. Math.

[20] G. Freixas i Montplet, Généralisations de la théorie de l’intersection arithmétique, Thèse de doctorat, Orsay, Décembre 2007.

[21] D. Fried, Analytic torsion and closed geodesics on hyperbolic manifolds, Invent. Math. 84, 1986, 523–540.

[22] H. Gillet and C. Soul´e, Arithmetic intersection theory, Pub. Math. IHES 72, 1990, 94–174.

41 [23] H. Gillet and C. Soul´e, Characteristic classes for algebraic vector bundles with Hermitian metrics I, II, Annals of Math. 131, 1990, 163–203 and 205–238.

[24] H. Gillet and C. Soul´e, An arithmetic Riemann-Roch theorem, Invent. Math. 110, 1992, 473–543.

[25] T. Hahn, Doctoral thesis, in preparation, Humboldt-Universit¨at zu Berlin.

[26] Y. Hashimoto, Arithmetic expressions of Selberg’s zeta functions for congruence subgroups, J. Number Theory 122, 2007, 324–335.

[27] D. A. Hejhal, The Selberg Trace formula for PSL2(R) Vol I, II, Lect. Notes Math. 548, 1001, Springer.

[28] H. Hida, Congruences of cusp forms and special values of their zeta functions, Invent. Math. 63, 1981, 225–261.

[29] H. Iwaniec, Spectral Methods of Automorphic Forms, Graduate Texts in Math., vol 53, Amer. Math. Soc. 2002.

[30] L. Ji, The asymptotic behavior of Green’s functions for degenerating hyperbolic surfaces, Math. Z. 212, 1993, 375–394.

[31] J. Jorgenson, R. Lundelius, Continuity of relative hyperbolic spectral theory through metric degeneration, Duke Math. J. 84, 1996, 47–81.

[32] J. Jorgenson, R. Lundelius, Convergence of the normalized spectral counting function on degenerating hyperbolic Riemann surfaces of ﬁnite volume, J. Funct. Anal. 149, 1997, 25–57.

[33] J. Jorgenson, R. Lundelius, A regularized heat trace for hyperbolic Rie- mann surfaces of ﬁnite volume, Comment. Math. Helv. 72, 1997, 636– 659.

[34] N. Katz and B. Mazur, Arithmetic moduli of elliptic curves, Annals of Mathematics Studies, Princeton University Press, 1985.

[35] S. Keel, Intersection theory of moduli space of stable N-pointed curves of genus zero, Trans. Amer. Math. Soc. 330, 1992, 545–574.

[36] F. F. Knudsen, The projectivity of the moduli space of stable curves II,III, Math. Scand. 52, 1983, 161–212.

42 [37] U. K¨uhn, Generalized arithmetic intersection numbers, J. Reine Angew. Math. 534, 2001, 209–236.

[38] G. Laumon and L. Moret-Bailly, Champs alg´ebriques, Ergeb. der Math. und ihrer Grenzgebiete, 3. Folge, vol 39, Springer-Verlag, 2000.

[39] H. Masur, Extension of the Weil-Petersson metric to the boundary of Teichm¨uller space, Duke. Math. J. 43, 1976, 623–635.

[40] B. Mazur, Modular curves and the Eisenstein ideal, Pub. Math. IHES 47, 1977, 33–186.

[41] L. Moret-Bailly, La formule de Noether pour les surfaces arithm´etiques, Invent. Math 98, 1989, 491–498.

[42] D. Mumford, Stability of projective varieties, L’Ens. Math. 23, 1977, 39–110.

[43] D. Mumford, Towards an enumerative geometry of the moduli space of curves, Arithmetic and geometry, Vol. II, 271–328, Progr. Math. 26, Birkh¨auser, 1983.

[44] D. B. Ray and I. M. Singer, Analytic torsion for complex manifolds, Annals of Math. 98, 1973, 154–177.

[45] D. Rohrlich, A modular version of Jensen’s formula, Math. Proc. Camb. Philos. Soc. 95, 1984, 15–20.

[46] P. Sarnak, Class numbers of indeﬁnite binary quadratic forms, J. Num- ber Theory 15, 1982, 229–247.

[47] P. Sarnak, Determinants of Laplacians, Commun. Math. Phys. 110, 1987, 113–120.

[48] R. Schoen, S. Wolpert and S.T. Yau, Geometric bounds on the low eigenvalues of a compact surface, Proc. Symp. Pure Math. 36, 1980, 279–285.

[49] A. J. Scholl, Motives and modular forms, Invent. Math. 100, 1990, 419– 430.

[50] M. Schulze, On the resolvent of the Laplacian on functions for degenerating surfaces of ﬁnite geometry, J. Funct. Anal. 236, 2006, 120–160.

[51] G. Shimura, Introduction to the Arithmetic Theory of Automorphic Functions, Princeton Uni. Press, Princeton, 1971.

43 [52] G. Shimura, On the holomorphy of certain Dirichlet series, Proc. London Math. Soc. 31, 1975, 79–98.

[53] C. Soulé, Régulateurs, in Sém. Bourbaki, Astérisque 133–134, 1986, exp. 644, 237–253.

[54] J. Sturm, Special values of zeta functions, and Eisenstein series of half integral weight, Amer. J. Math. 102, 1980, 219–240.

[55] L. A. Takhtadzhyan and P. G. Zograf, On Liouville’s equation, accessory parameters, and the geometry of Teichm¨uller space for Riemann surfaces of genus 0, Math. UUSR Sbornik 45, 1988, 143–164.

[56] L. Takhtajan and P. Zograf, The Selberg zeta function a new a K¨ahler metric on the moduli space of punctured Riemann surfaces, J. Geom. Phys. 5 (4), 1988, 551–570.

[57] L. A. Takhtajan and P. G. Zograf, A local index theorem for families of ∂-operators on punctured Riemann surfaces and a new K¨ahler metric on their moduli spaces, Commun. Math. Phys. 137, 1991, 399–426.

[58] E. Ullmo, Hauteurs de Faltings de quotients de J0(N), discriminants d’alg`ebres de Hecke et congruences entre formes modulaires, Amer. J. Math. 122, 2000, 83–115.

[59] C. Voisin, Théorie de Hodge et Géométrie Algébrique complexe, Cours Spécialisés, vol. 10., Soc. Math. France, 2002.

[60] A. Voros, Spectral functions, special functions and the Selberg zeta function, Commun. Math. Phys. 110, 1987, 439–465.

[61] L. Weng, Ω-amdissible theory II. Deligne pairings over moduli spaces of punctured Riemann surfaces, Math. Ann. 320, 2001, 239–283.

[62] S. A. Wolpert, Asymptotics of the Spectrum and the Selberg Zeta Func- tion on the Space of Riemann Surfaces, Commun. Math. Phys. 112, 1987, 283–315.

[63] S. A. Wolpert, The hyperbolic metric and the geometry of the universal curve, J. Diﬀerential Geom. 31, 1990, 417–472.

[64] S. A. Wolpert, Cusps and the family hyperbolic metric, Duke Math. J. 138, 2007, 423–443.