Chapter 1

Inventory of the Local Universe

1 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.1 The major types of : Hubble-Sandage system

Hubble-Sandage Tuning Fork:

Kormendy & Bender, ApJ 464, L119 (1996), revised for ellipticals. Other classification schemes: e.g. de Vaucouleurs (1959), van den Bergh (1960/66), Yerkes (Morgan, 1957 ff)

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Primary classification criteria of commonly used Hubble-Sandage system:

Bulge-to-disk ratio (S0/Sa: 5 to 0.3, Sb: 1 to 0.1, Sc/Irr: 0.2 to 0) Opening angle of spiral arms (Sa: 0 to 10, Sb: 5 to 20, Sc: 10 to 30 degrees) Bars

Physical parameters varying along the Hubble-Sandage system:

Stellar mass M increases from irregulars (108M ) to ellipticals (1012M )

Specific Angular Momentum J/M of baryons increases from ellipticals to spirals Mean age increases from irregulars through spirals to ellipticals (B-V increases from 0.3 to 1.0, mass-to-light M/LB ratio increases from about 2 to 10) Mean stellar density of spheroids increases with decreasing spheroid luminosity Mean surface brightness of disks increases with luminosity cold gas content increases along Hubble sequence (fraction of baryonic mass: 0 in E/S0, 0.1 to 0.3 in Sa to Sc, up to 0.9 in Irr) hot gas content only significant in massive E (few percent of baryonic mass)

3 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Examples for Normal Galaxies: Elliptical (E) Galaxies:

The ellipticals M 84 (right) and M 86 (middle) in the Virgo cluster (NOAO).

4 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Lenticular (S0) Galaxies:

NGC 3115: S0- NGC 4371: SB0-galaxy

5 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Spiral (Sa) Galaxies:

M 104 (Sombrero), Sa-galaxy NGC 3223: Sa-galaxy (P.Barthel, VLT)

6 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Spiral (Sb) Galaxies:

M 31 (Andromeda): Sb-galaxy M 81: Sb-galaxy

7 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Spiral (Sc) Galaxies:

M 51: Sc-galaxy M 101: Sc-galaxy courtesy: C. Gossl,¨ Wendelstein Observatory, USM courtesy: C. Gossl,¨ Wendelstein Observatory, USM

8 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Barred-Spiral (SBa) Galaxies:

M 83 (Southern Pinwheel): SBa-galaxy

9 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Barred-Spiral (SBb) Galaxies:

NGC 2523: SBb-galaxy M 95: SBb-galaxy

10 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Barred-Spiral (SBc) Galaxies:

NGC 613: SBc-galaxy NGC 1365: SBc-galaxy

11 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Irregular (Irr) Galaxies:

LMC: Irr-galaxy SMC: Irr-galaxy

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Milkyway, Sbc galaxy (all-sky projection in optical)

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Milkyway, Sbc-galaxy (all sky projection in near IR, COBE satellite)

14 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Luminosities of Bulges and Disks

bulge luminosity γ = total luminosity

Yoshizawa, Wakamatsu A&A 44, 363 (1975) the Hubble sequence is primarily a bulge sequence →

15 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.2 Important other galaxy types not contained in the Hubble- Sandage system:

dwarf galaxies: dE: dwarf ellipticals or dwarf spheroidals, similar to E but of low luminosity and low surface brightness BCD: Blue Compact Dwarfs, concentrated starburst, faint old stellar components cD-galaxies: Yerkes classification for “extra (c) large and diffuse (D)” galaxies, found in the centers of clusters and groups low surface brightness (LSB) galaxies: mostly luminous but very low surface brightness disk galaxies active galaxies: radio galaxies; galaxies with unusual nuclear emission lines and/or extreme nuclear luminosity (QuasiStellarObjects-QSOs, Seyfert galaxies) and/or with powerful non-thermal radio emission (radio galaxies, quasars) interacting, merging and starbursting galaxies (IRAS mergers, ULIRGs, i.e. Ultra- Luminous Infra-Red Galaxies)

16 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Dwarf galaxies

Leo 1 (MV = 12), dwarf elliptical (dwarf spheroidal) companion of Milky Way −

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NGC 205 (MV = 16.3), dwarf elliptical companion of M31 (NOAO) −

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Blue Compact Dwarf (BCD) NGC 1705, blue: blue continuum, green: red continuum, red: Hα (G. Meurer)

19 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Active galaxies

NGC 7742, a Seyfert galaxy

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NGC 383 (= 3C31), a radio galaxy, blue: optical, red: radio (A. Bridle)

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The currently most distant object known, a quasar at 5.8 (April 2000, Sloan Digitial Sky Survey)

22 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Interacting, merging and starbursting galaxies

Interacting galaxy pair. Note that spiral disks are not optically thick!

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Cartwheel galaxy

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Antenna galaxies

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M 82, a starburst galaxy, white/brown: stellar light and dust, red: hot expanding gas in Hα (Subaru telescope)

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Various evolutionary steps of spiral-spiral mergers

27 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Luminosity and surface brightness bias

see: Mihalas / Binney: Galactic Astronomy

28 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Galaxies at non-optical wavelengths

Spirals in ultraviolett (dominated by massive ) and visual (average population), Ultraviolet Imaging Telescope, Astro mission. Note: Redshifted spirals observed in the optical will show rest-frame UV morphology!

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Dwarf irregular NGC 2915 yellow: optical blue: HI

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optical M81-group HI

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NGC 2300 group (black&white = optical, blue/pink = X-rays)

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Milkyway (Sbc-galaxy) in different wavebands

33 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.3 Luminosity Functions of Galaxies

Schechter (ApJ 203, p297, 1976) proposed a global fitting function for all galaxies (indi- vidual types do not follow the Schechter-function)

L L α L Φ   = Φ   exp −  L ∗ L L ∗ ∗ ∗

typical values (averaged over large volumes)

10 2 LB, 10 LB, h− ∗ ' or: MB, 19.5 + 5 log h ∗ ' − 3 3 Φ 0.01Mpc− h ∗ ' α 1 1.3 (see Peebles 1993) ' − · · · −

H0 L with: h = and MB = 2.5 log + 5.48 100km/s LB, Mpc −

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Galaxy luminosity function in the Virgo cluster (Binggeli et al. (1985), AJ 90, 1759

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Φ L dL is the number density of galaxies with luminosities in the range (L, L + dL) L  (strong∗ variation of Φ depending on environment) ∗

averaged luminosity per volume:

∞ L L j = Z L Φ   d   = Γ(α + 2)Φ L L L ∗ ∗ 0 ∗ ∗

8 L j 10 ⇒ ' Mpc3

M 9 M Using 10 yields a mass density of ρ 10 3 Ω 0.004 L ' ∗ ' Mpc ⇒ ∗ '

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see: Thomas: ESO Astrophysics Symposia (1999)

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Luminosity Function Φ(M) versus Absolute Blue Magnitude MBT

Binggeli, Sandage, Tammann: ARAA 26 (1988)

38 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.4 Basic Components of Galaxies

Spheroid (bulge): metal poor to super metal rich, generally old stars; surface brighness profile I(r) follows roughly a de Vaucouleurs law:

0.25 3.33((r/re) 1) I = Ie10− − re = half light radius −

Disk: metal rich stars, strong rotation, stars show a wide range of ages, Sa and later types show formation, HI, H2-gas, molecular clouds, dust, hot gas (heating by star formation and supernovae); surface brighness profile I(r) follows exponential law:

r/ro I = Ioe− ro = disk scale length −

Baryonic halo: metal poor stars with little/no rotation and a wide variety of orbits (only detected in Milky Way); globular clusters, X-ray gas (prominent in ellipticals), low density HI/HII-gas Dark halo: dominating mass outside of 10kpc, total mass in dark matter 5 to 10 times larger than baryonic mass, dark halo (maybe) slightly flattened

39 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.5 Kinematics of Spirals, Tully-Fisher-Relation

Rotation curves of spiral galaxies are flat, because of dark matter.

see: van Albada et al. ApJ 295, 305 (1985)

2 2 2 2 ∂Φ Φ = Φhalo + Φdisc v = v + v (v = r ) ⇒ c c,halo c,disc c ∂r

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The circular speeds vcirc of spiral galaxies are closely correlated with their luminosities (Tully-Fisher-relation (1977, A&A 54, 661)): L v3...4 ∼ (The power depends on the wavelength of L, because of changing M/L, star formation etc.) From the virial theorem, v2 M , and the identity L Σr2 one obtains: ∼ r ∼ 2 M − 4 1 L   v Σ− ∼ L 2 M − 1 Evidently, the observed Tully-Fisher relation implies: L Σ− const. Indeed, most  · ' M bright spirals have similar mass-to-light ratios and surface brightnesses: L const and Σ const (the latter is called the “Freeman law”). ∼ ∼ Dwarf spirals and irregulars deviate from the Tully-Fisher relation because their bary- onic densities are lower and dark matter contributes significantly to the mass within the luminous part of the galaxies. The Tully-Fisher relation is an important distance indicator at low (z < 0.1) and can serve as an indicator for evolution at higher redshifts.

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Tully-Fisher relation

top: local calibrators middle: Ursa major cluster bottom: Virgo cluster

i WR = 2vcirc vcirc can be deduced from HI observations; correction for incli- nation from flattening of optical image.

Jacoby et al. (1992) PASP 104, 599

42 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Typical Rotation and Dispersion Curve for an Elliptical Galaxies:

from Bender (1990) A&A, 229, 441. Because elliptical galaxies are pressure supported, the rotation velocities are MUCH lower than the circular velocities. The circular velocity of NGC 4621 is about 350 km/s.

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Faber-Jackson relation (1974) between central velocity dispersion and total magnitude of 4 elliptical galaxies (LB σ ) (’7 Samurai’ data set, Faber et al 1989, ApJS 71, 173). ∝

44 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.6 Fundamental Plane of Elliptical Galaxies

A comprehensive set of global structural parameters for elliptical galaxies is:

The half light (or effective) radius re

The mean surface brightness Σe (or Ie) within re

The central velocity dispersion σ0 or the corresponding circular velocity Vcirc The luminosity L The mass M within the luminous part of the galaxies

The following two relations relate these quantities: L/2 Σe = 2 (Definition of mean surface brightness) πre

M 2 = cσ0 (Virial equilibrium) re with the structure parameter c which contains all unknown details about the galaxies’ struc- ture.

45 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Multiplication yields an expected relation for these parameters: 1 c M − 2 1 re = !   σ Σ− 2π L 0 e

Because neither M/L or c are varying very much, the brackets are nearly constant and imply that ellipticals are distributed in a (thick) plane in the 3-space of their global paramters 2 (re, Σe, σ0). Astonishingly, this plane is much better defined than naively expected, with very low dis- persion perpendicular to the plane (implying a variance in the product of the brackets less than 10%) and a small but significant tilt (implying small but significant changes in the struc- ture of ellipticals as a function of their luminosity or mass), see Djorgovski & Davis 1987, Dressler et al. 1987.

The observed so-called ”fundamental plane” relation reads:

1.4 0.85 re σ Σ− ∝ 0 e This is consistent with the theoretical expectation, if 2π M     M 0.2 L0.25 c L ∝ ∝

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M This implies a limitation of the variation of the dynamical structure (σ0), of the L of the stellar population, of the amount of dark matter within re, of the slope of the stellar initial mass function, and all other possibly varying parameters. Even though the kinematics of ellipticals can appear to be highly complicated in detail, the objects must in fact be rather similar with respect to their global structure and their M stellar L ! The fundamental plane is an important distance indicator for elliptical galaxies, like the Tully-Fisher relation for spirals. At higher redshifts it is a useful indicator for the evolution of elliptical galaxies. The fundamental plane can conveniently be visualized in the κ-parameter space, using the parameters (Bender, Burstein & Faber ApJ 1992 ff):

2 log(σ re) κ1 = log M √2 ∝

2 2 1/3 log(σ Σe/re) M κ2 = log Σe   √6 ∝ L 2 log(σ /Σe/re) M κ3 = log   √3 ∝ L

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E: squares S0: crosses dE: diamonds dSph: small squares compact E: circles

curved lines indicate the surface density of baryons if collapsed by a factor 10 relative to their dark matter halos

arrows indicate major physical processes acting on galaxies Bender, Burstein, Faber 1992, ApJ 399

49 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Spiral galaxies in κ-space

Spiral galaxies also lie near the fundamental plane of ellipticals. The tilt of the plane is such that the Tully-Fisher relation emerges as a simple relation between luminosity and rotation velocity for spirals.

50 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.7 Star Formation History in Spirals and Irregulars

The Hα-line strength provides information about the present star formation rate Ψ in galaxies. The Hα strength is directly proportional to the ionizing flux of massive, hot stars and in turn to the number of these stars which again is a measure of the actual star formation rate Ψ: Ψ LHα M /yr ' 1041erg/sec

(The underlying assumption is the so-called case B approx.: photons of the Lyman- continuum are completely absorbed and reemitted as Lyα, Hα, etc, see Osterbrock: Astrophysics of Gaseous Nebulae). Hα line strengths reach up to 1042.5 ergs/s and correspond to SFR up to 20 M /yr. Ultraluminous starburst galaxies can reach SFRs of several hundred M /yr.

The far-IR-luminosity also provides a measure for the star formation activity in a galaxy. The far-IR luminosity is produced by warm dust grains which are surrounding the star formation regions. These grains absorb a large fraction of the UV photons, are heated up and radiate blackbody radiation at their equilibrium temperature (10K to several 100K): log LFIR/L log LHα/L + 2.2 '

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Major absorption features: Hδ: 4102A˚ Hγ: 4342A˚ Hβ: 4861A˚ Hα: 6563A˚ Ca H+K: 3934A,˚ 3969A˚ G-band: 4308A˚ MgI: 5180A˚ NaI: 5893A˚ 4000A˚ break Balmer break: 3646A˚

Major emission features: Hβ: 4861A˚ Hα: 6563A˚ [OII]: 3727A˚ [OIII]: 4959A,˚ 5007A˚ [OI]: 6300A˚ [SII]: 6716A,˚ 6731A˚

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see: Devereux et al. ApJ 350, 25 (1990)

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Colors and Hα-equivalent widths provide information about the star formation history. A high star formation rate in the past will have produced many low-mass red stars which cause the galaxy to appear redder and to have more continuum light. The Hα-equivalent width (a measure for the strength of the Hα emission relative to the un- derlying continuum) is an indicator of how many stars are currently formed relative to the average SF rate over the history of the galaxy.

Comparing the Hα-flux with colours leads to: < 1 for Sa...Sb presentΨ =  1 for Sc < Ψ >  '> 1 for Sd...Im  (for a schematic star formation history of different Hubble types see Sandage 1996)

The relation between the equivalent width of Hα compared to a color (e.g. B-V) is an indicator for the slope of the initial mass function (IMF). Current data show compatibility with the Salpeter IMF and are probably incompatible with a Miller-Scalo IMF. Typical equivalent widths of Hα for spirals range between a few up to 100A.˚

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The three evolutionary models use different IMF (from top to bottom): 2 Φ m− ∼ 2.35 Φ m− Φ ∼ Miller-Scalo IMF ∼

Hαflux present SFR Wλ ' continuum flux ' luminosity of old stars

see: Kennicutt et al. ApJ 272, 54 (1983)

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see: Kennicutt et al. ApJ 272, 54 (1983)

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from: Sandage et al. A&A 161, 89 (1989) very schematic

57 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.8 The Large–Scale Distribution of Galaxies

Galaxies are not uniformly distributed in space. They rather form large superclusters of galaxies, which surround regions with very low galaxy density (voids), as is obvious from the following wedge diagrams:

Center for Astrophysics (CFA, Harvard) Survey (from Peebles 1993)

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2dF Galaxy Redshift Survey

59 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Typical Scales of Large–Scale Structure

Galaxies 10 kpc Groups & Clusters ∼ (0.3 ... 5) Mpc Superclusters ∼ 50 Mpc ∼ Superclusters of galaxies are the largest known structures in the universe.

The Local Group is a member of the Local Supercluster, a huge flattened distribution of galaxies centred on the Virgo cluster.

Distribution of different galaxy types

Ellipticals and S0-galaxies prefer regions of high galaxy density, spirals and irregulars are found in lower denisty environment. Nevertheless all galaxy types cluster along filaments and in groups and galaxy clusters.

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Giovanelli et al. (1986) ApJ, 300, 77.

61 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.9 The two-point correlation function of galaxies

The two-point correlation function ξ(r) is a quantitative measure of galaxy clustering and is defined via the probability to find pairs of galaxies at a distance r: 2 dNpair = No (1 + ξ(r))dV1dV2 where No is the mean background density and dV1 and dV2 are volume elements around the two positions under consideration.

The two-point correlation funtion is related to the relative overdensity ∆(x) = δρ/ρo because we also have: 2 dNpair = ρ(~x)dV1ρ(~x + ~r)dV2 = ρo(1 + ∆(~x))(1 + ∆(~x + ~r))dV1dV2

Averaging over a large volume removes the linear terms in ∆(~x) and we obtain: 2 < dNpair >= ρo(1+ < ∆(~x)∆(~x + ~r) >)dV1dV2 and therefore: ξ(r) =< ∆(~x)∆(~x + ~r) >

62 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Observationally one obtains averaged over all galaxy types: γ r − ξ(r) =   ro with: γ = 1.8 and ro = 5/hMpc (h = Ho/100km/s/Mpc) which is valid for scales from 100kpc to 10Mpc. Beyond 10Mpc the correlation function falls more rapidly.

angular correlation function of galaxies from APM survey Maddox et al. 1990, MN 242, 43

63 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE 1.10 Galaxy Clusters

Virgo Cluster:

Nearest large with more than 2000 galaxies • 7.8 brighter than MB 14 (LB 10 L ) ' − ∼ Distance 17Mpc (dependent on H0) • ∼ Extend 10◦ ˆ=3Mpc 3Mpc • Irregular∼ cluster, densest× regions dominated by ellipticals • Velocity dispersion of galaxies about 600km/s •

Coma Cluster:

One of the most luminous clusters known • Distance 100Mpc (dependent on H0) • Regular cluster∼ with probably subcluster merging from SW • Dominated by ellipticals and S0s, two central cDs and one in subcluster • Velocity dispersion of galaxies about 1000km/s • Strong X-ray source •

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central part of Coma cluster with two cDs.

65 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE Morphology-Radius Relation:

Ferguson, Sandage (1989) ApJ, 346, L53. Ellipticals and S0’s are more concentrated than spirals and irregulars.

66 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE X-Ray Gas in Galaxy Clusters

Coma cluster (left: optical image, right: X-ray image)

67 CHAPTER 1. INVENTORY OF THE LOCAL UNIVERSE The X-ray spectra show the characteristics of Bremsstrahlung of a 108K hot gas. dP 27 1/2 2 erg ∼ The volume emissivity is: dV = 2.410− T − Ne cm3 s  11  The cooling time of the plasma is: t = 3NekT 10 T 1/2 [s] cool dP Ne dV ' For the center of the Coma cluster we have: L 1044erg/s ' 3 3 ne 10− cm− ' 10 τcool 10 yrs ' 13 Mgas 10 M ' The following correlations exist between the different components of galaxy clusters: The central galaxy density is higher for higher LX. • The fraction of spirals is lower for higher LX. • 8 The temperature T is proportional to LX and typically 10 K. • The gas metallicity is lower for higher T and typically 1/3 of solar. • The ratio of gas-mass to galaxy-mass increases with T up to 5 or more. • The dominant component in all clusters is dark matter. This follows consistently from the dynamics• of galaxies, the hydrostatic equilibrium of the X-ray gas and from gravitational lensing. The typical mass ratios are:

galaxies : X-ray-gas : dark-matter 1 : 5 : 25 '

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Galaxy Cluster Cl0024+1645, strong lensing reconstruction (left, courtesy S. Seitz) of HST image (right, Colless et al.); light blue = caustic structure, bold green = critical lines of ’infinite’ amplification, squares = observed positions of multiple imaged source (A,B,C,D,E in color image), yellow crosses = predicted position of the lensmodel, yellow circle = position of source in source plane, red crosses = mass centers used for the lens model. The caustics are obtained by mapping the critical lines into the source plane.

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