On the Origin of the Colour-Magnitude Relation in The

arXiv:astro-ph/0104084v1 4 Apr 2001 rpittpstuigL using typeset Preprint 2018 29, October version Draft con o the for account nedGn´lz(93,Jresn(99 n rgre al. variations. et Trager to metallicity and and expected (1999) age González Jørgensen be of Indeed (1993), might combination CMR formation a the from star events, arise recent merger some late have to galaxies due Char- where & Kauffmann 1998) by 1994; lot implied al. In as et bluer such (Cole younger. with merging histories, hierarchical are galaxies formation luminosity star the their complex that for more and expected old, galax- than are cluster colors CMR Coma the the that sup- on showing provided by ies view (1999) galax- this al. for luminous port et achieve more Terlevich therefore scenario can metallicities. and this higher energies the In binding of greater have formation stars. ies early the an of with bulk ellipticals of for formation picture collapse the dissipational a suggested 1999). (1974) Silk & Larson age & Kodama of Charlot Ferreras, combination 1987; (González a 1993; variations Yoshii by or & 1997), metallicity (Arimoto stellar Arimoto sequence mean the the of variation along a by either caused stellar constituent its to luminosity, related populations. its the through with together galaxy, relation, tight redder et This Mg Bower be 1977; 1992a). to Sandage & al. observed (Visvanathan are ones fainter clusters than in galaxies early-type oudrtn o aaisfr n vle ene to need we evolve, and form galaxies how understand To h rgno h M shtydbtd tcudbe could it debated: hotly is CMR the of origin The 2 - σ eain(ols ta.19)lnstems fa of mass the links 1999) al. et (Colless relation reo h ffcso ealct.W n httelmnst weighted luminosity the that find n We a metallicity. use than we of greater particular selected effects are In galaxies the dispersions. model elliptical of velocity synthesis six free their population for by stellar given ratio evolutionary resolution new S/N a high using very analysed of spectra using h M eursteaayi fsetao eyhg ult,sc a such quality, clusters. high other very headings: and of Subject Virgo spectra in abu of galaxies metallicity, driven analysis of total be number rat the the may requires abundance by rôle played CMR CMR solar the the the from of departure understanding that full systematic speculate A a not we by However, and and metallicity correlation. dispersion. increase luminosity-metallicity total velocity a the and by with color primarily luminosity, driven with is metallicity of correlation eepoeteoii fteclrmgiuerlto CR feryt early of (CMR) relation color-magnitude the of origin the explore We NTEOII FTECLRMGIUERLTO NTEVROCL VIRGO THE IN RELATION COLOR-MAGNITUDE THE OF ORIGIN THE ON oo-antd relation color-magnitude A 1. T 3 4 E 1 tl mltajv 04/03/99 v. emulateapj style X nttt fAtooy nvriyo oy,Oaa2-21-1, Osawa Tokyo, of University Astronomy, of Institute colo hsc n srnm,Uiest fNottingham, of University Astronomy, and Physics of School INTRODUCTION eateto hsc,Uiest fDra,Dra H 3LE DH1 Durham Durham, of University Physics, of Department oapa nTeAtohsclJunlLtes 01Arl2 April 2001 Letters, Journal Astrophysical The in appear To lxnr Vazdekis Alexandre Vro aais litcladlniua,c aais vlto ga — evolution galaxies: — cD lenticular, and content elliptical stellar galaxies: — (Virgo) ∼ aais bnacs—glxe:cutr:gnrl—glxe:clus galaxies: — general clusters: galaxies: — abundances galaxies: 2 y,adso ocertedwt aaylmnst.W lofidapos a find also We luminosity. galaxy with trend clear no show and Gyr, 8 nttt eAto´sc eCnra,E320L aua T Laguna, La E-38200 Canarias, Astrof´ısica de de Instituto [email protected] Roger.Davies@durham. [email protected], [email protected],[email protected]. [email protected], CR:luminous (CMR): and .Arimoto N. [email protected] metallicity 1 , 2 aadKuntschner Harald , .F Peletier F. R. ABSTRACT 3 .Nakamura O. , 1 hssmdlo adks(99,wiheposteex- the employs which (1999), Vazdekis of model thesis g niaost eieaeetmtsta r virtually are that estimates metallicity. the age population sensitive of derive more stellar independent to new, new indicators with a combination age use in We model have spectra. synthesis we S/N which for high CMR very the along selected ellipticals Virgo appropri- of yet lack analysis. the not of of is tools because CMR ate part, the in of Peletier understood, origin & fully Sadler The (Davies, González emission 1993). by 1993; affected 1994) be (Worthey can metallicity and on dependence non-negligible sdaeidctri h ikISsse,H system, widely Lick/IDS the the Even in ellip- 1994). indicator of (Worthey age metallicities CMR used and the age ages along the in ticals of changes analysis both ambiguous to sensitive and are populations stellar rise give CMR. might tight relation a this galaxies that to rich conjecture metal and dispersion, young velocity They are fixed a old. relation at generally dispersion that age-metallcity-velocity such are tight but clusters a al. sample in find full et also galaxies their Trager the in range that age galaxies. note large elliptical a reported some intermediate(2000b) in significant a population for age evidence strong find (2000a) eew s h vltoayselrpplto syn- population stellar evolutionary the use we Here nti etrw td h tla ouain fsix of populations stellar the study we Letter this In ra adclr n iesrnt nie fintegrated of indices strength line and colors band Broad ealct n hsdgnrc a rvne nun- an prevented has degeneracy this and metallicity 4 iaa oy 8-05 Japan; 181-0015, Tokyo Mitaka, 1 oe.L Davies L. Roger. , 2. o fsm lmnsaogteCMR. the along elements some of ios otnhm G R,U.K; 2RD, NG7 Nottingham, 3 dnerto n g ngenerating in age and ratios ndance TLA OUAINMODELS POPULATION STELLAR hs eotdhr,fralarger a for here, reported those s .;[email protected], U.K; , ognrt aaysetaa the at spectra galaxy generate to waeidctrta svirtually is that indicator age ew 51 ubr2) number (551, 0 p aaisi h ig cluster Virgo the in galaxies ype nrf,Spain enerife, ac.jp ln h M.Tedt are data The CMR. the along enae fVroellipticals Virgo of ages mean ac.uk l lmnsices equally increase elements all ecnld htteCMR the that conclude We ybt oa metallicity total a both by 1 es individual ters: USTER laxies: itive β hw a shows , 2 tensive empirical stellar spectral library of Jones (1999). ISIS double-beam spectrograph with the EEV12 CCD and The model predicts spectral energy distributions (SEDs) in R600B grating in the blue channel, which provided a dis- the optical wavelength range for simple stellar populations persion of 0.44 A/pix˚ and a spectral range of λλ ∼4000- (SSPs) at a resolution of 1.8 A˚ (FWHM). Previous mod- 5500 A.˚ The slit width was set to 1′′. 6 giving a spectral els (e.g., Worthey 1994; Vazdekis et al. 1996) used mostly resolution of 2.4 A˚ (FWHM). The seeing was 2-3′′ in the the Lick polynomial fitting functions (Worthey et al. 1994; first night and ∼1′′. 5 in the second. The CCD was binned Worthey & Ottaviani 1997) to relate the strengths of se- ×3 in the spatial direction giving 0′′. 57 per bin. Multiple lected absorption features to stellar atmospheric param- exposures of 35 min length were obtained for each galaxy. eters. The fitting functions are based on the Lick/IDS Data reduction was done with standard IRAF packages. stellar library (FWHM∼9 A,˚ Worthey et al. 1994), thus We made use of tungsten flatfields obtained for each galaxy limiting the application of the models to strong features. pointing. The continuum shape was corrected to a relative Furthermore the Lick stellar library has not been flux cal- flux scale with spectrophotometric standards. ibrated, so offsets had to be applied to data obtained with We summed up the spectra within re/10, where effective different spectrographs (Worthey & Ottaviani 1997). radii were taken from Burstein et al. (1987). Before sum- The new model provides flux calibrated, high resolution mation the central galaxy spectra were carefully aligned spectra, therefore allowing an analysis of galaxy spectra with the rows of the array, and each row in spatial direc- at the natural resolution given by their internal velocity tion was corrected for the velocity shift due to rotation. −1 broadening and instrumental resolution. Furthermore, as Variations of σ within re/10 were small (< 25 km s ) for the model outputs are SEDs rather than predicted index all galaxies and therefore no correction was applied. Ta- strengths, it is easy to define new indices and even to con- ble 1 lists the S/N ratio per A˚ achieved in the summed front the model predictions with the detailed structure of spectra around the Hγ feature. observed absorption features. Vazdekis & Arimoto (1999) 4. defined a new age indicator [Hγ+ 1 (FeI+MgI)] (hereafter THE AGES AND METALLICITIES OF VIRGO 2 σ ELLIPTICALS Hγσ), which is virtually free from metallicity dependence. To cover a large range in σ, Vazdekis & Arimoto defined In this section (i) we compare the sensitivity and preci- three slightly different indices which give stable and sen- sion of different age indicators, (ii) demonstrate that not sitive age predictions within the σ ranges quoted in the all metals are enhanced at the same rate in more luminous sub-indices: Hγ100<σ<175, Hγ150<σ<225 and Hγ225<σ<300. (higher dispersion) galaxies and (iii) show that for the six We emphasise that the model spectra we are fitting have Virgo ellipticals studied here metallicity increases with lu- a single metallicity and age. We are measuring luminosity- minosity, velocity dispersion and color but no trend with weighted properties, which means that the presence of a age is apparent. young (i.e., bright) stellar component will disproportion- Our primary goal is to measure the ages of the central ally affect the mean age and therefore will be detected regions of the galaxy sample along the CMR. We use dif- more easily. ferent age indicators: Hγσ, HδF and Hβ (as defined by Vazdekis & Arimoto (1999), Worthey & Ottaviani 1997, 3. DATA and Worthey et al. 1994, respectively). Instead of ap- We obtained major axis spectra of very high S/N ratios plying σ corrections to the line strength measurements we for six Virgo ellipticals NGC 4365, NGC 4387, NGC 4464, modified the model predictions. We divided our sample NGC 4473, NGC 4478, and NGC 4621, selected along into three groups containing galaxies with similar velocity the CMR of Bower, Lucey & Ellis (1992a,b) (see Fig. 1, dispersions and smoothed the SEDs of Vazdekis (1999) ac- and also Table 1). The observations were performed at cordingly. We chose (a) σtot = 145, (b) σtot = 190 and (c) −1 2 2 2 the WHT (4.2m) at the Observatorio del Roque de Los σtot = 260 kms (where σtot = σgalaxy +σinstr.). To min- Muchachos, La Palma, in April 21-22, 1999. We used the imize the effect of small velocity dispersion differences the spectrum of NGC 4387 was convolved with a Gaussian of σ = 85 kms−1 and that of NGC 4621 with σ = 105 kms−1. Fig. 2 shows age/metallicity diagnostic diagrams for the age indicators as a function of the mean metallicity indicator [MgFe] (González 1993). Velocity dispersion increases in each column from top to bottom. Note that for a given age and metallicity the strength of the [MgFe] index is smaller for larger σ. The left panels of Fig. 2 show how well Hγσ is able to break the age-metallicity degeneracy, much better than, e.g., HδF . The lines of constant age are essentially horizontal, which means that a given mea- surement of Hγσ corresponds to a unique age. Hβ is also a good age indicator, the new models predict that Hβ is Fig. 1.— The CMR in the Virgo cluster. Data and a best-fit more sensitive to age, and less sensitive to metallicity than regression (ellipticals only) from Bower, Lucey & Ellis (1992a,b). For NGC 4464 we have taken VT from RC3. For NGC 4478 we models using the Lick fitting functions. used Michard (1982) U −V color (60′′aperture) applying an offset of Overall we find good agreement between the ages in- − − 0.047, obtained by a comparison of his non dereddened U V colors ferred from Hγσ and Hβ. All our galaxies, except for with those of Bower, Lucey & Ellis (1992b) which are dereddened by ∼ − NGC 4478, are older than 11 Gyr. NGC 4478 is 8-9 Gyr 0.02. Solid symbols represent our sample of galaxies, the shapes − are used to identify the six galaxies in the following figures. old and has a U V color 0.06 mag bluer than other Virgo ellipticals of equivalent luminosities (see Fig. 1). 3

Fig. 2.— Plots of Hγσ,HδF and Hβ (from left to right) versus the Fig. 3.— Plots of Hγσ vs Fe3, Mg b, Ca4227 and CN2. Symbols metallicity index [MgFe] (defined in González 1993). The central ve- and line styles are the same as in Fig. 2. locity dispersion increases in three groups from top to bottom, rep- resenting the CMR. Overplotted are the models by Vazdekis (1999). Lines of constant [Fe/H] = −0.7, −0.4, 0.0 and +0.2 are shown by thick dot-dashed, thick dotted, thick solid and thick dashed lines, “metallicity”, we emphasize that by using Hγσ these un- respectively. Thin dotted lines represent models of constant ages certainies do not influence our derived ages. which are quoted in Gyr. Fig. 4 shows our estimates for luminosity weighted mean age and metallicity (derived from Hγσ and Hβ vs. [MgFe] in Fig. 2) versus galaxy luminosity, color, and velocity dis- The middle panels of Fig. 2 show that the age estimates persion. Except for NGC 4478, all ellipticals are old with based on HδF remain strongly coupled with metallicity ages ranging from ∼11 Gyr to ∼20 Gyr. There is no trend estimates. (NB. the HγF index is similar and is not plot- in age along the CMR but there is a clear, positive corre- ted.) The age estimates based on HδF are younger than 0 − lation of metallicity with VT , U V and σ. If we correct those derived from Hγσ or Hβ but we are unable to dis- the metallicity estimates to the value they would have if tinguish between genuinely younger ages and the possible all the galaxies were 14 Gyr old the scatter in Fig. 4 is systematic effects of non-solar abundance ratios. For ex- reduced. Furthermore our [MgFe] index yields very simi- ample, using HδF or HγF as the age indicator one derives lar metallicities to those derived by correcting the Fe in- younger ages when Mg b and/or Mg2 are used as the metal dices for non-solar abundance ratios, following Trager et indicator than when the Fe lines alone are used (Worthey al. (2000a). We conclude that for these six galaxies in 1998; Kuntschner 2000). Virgo the metal content increases along the CMR. Fig. 3 plots the Hγσ indices against the strength of four metal absorption lines and illustrates the effects of non- 5. CONCLUSIONS solar abundance ratios in determining metallicity. The We have performed a detailed spectral analysis of six left-hand columns in Fig. 3 show only a very modest in- 1 elliptical galaxies in the Virgo cluster. We have combined crease in the metallicity derived from Fe3 or Ca4227 as very high S/N spectra with a new evolutionary stellar pop- velocity dispersion increases from top to bottom. We note ulation synthesis model. This model enables us to study that the G-band also follows Fe3. In contrast the right- galaxy spectra at the resolution given by their velocity dis- hand columns in Fig. 3 show that the metallicity derived persions and determine ages that are virtually free of any from Mg b or CN2 increases significantly towards high ve- dependence on metallicity. We find that elliptical galaxies locity dispersion galaxies, extending beyond the model in Virgo are generally old. grids in some cases. Although Ca, like Mg, is an α-element, We find that the metallicity as measured by [MgFe] cor- we find that Ca4227 does not track Mg b confirming the relates with luminosity, U − V and velocity dispersion. results reported by Vazdekis et al. (1997), Worthey (1998) We conclude that the CMR in Virgo is driven primarily and Trager et al. (2000a). We conclude that there are at by an increase in metallicity with luminosity. We find that least two families of metal lines which give rise to very not all elements increase equally with the total metallic- different metallicity estimates and exhibit different trends ity estimates. For example the increase in metallicity as along the CMR. Despite these complexities in estimating measured from the Fe or Ca indices is very modest along 1 1 Fe3 = 3 (Fe4383 + Fe5270 + Fe5335), Kuntschner (2000). the CMR, whereas the Mg or CN indices indicate a more rapid increase with galaxy luminosity. This is consistent 4

Fellowship.

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The authors thank J. Blakeslee, R. Bower, J. Gorgas, T. Kodama, J. Lucey and the referee, J.J. González, for valuable discussions and suggestions. The authors thank the referee for very useful suggestions. A.V. and H.K. ac- knowledge the support of the PPARC rolling grant ’Extra- galactic Astronomy and Cosmology in Durham 1998-2002’. This work was financially supported in part by a Grant-in- Aid for Scientific Research (No.1164032) by the Japanese Ministry of Education, Culture, Sports and Science. RLD acknowledges the support of a Leverhulme Trust Research 5

Table 1 Virgo galaxy sample.

˚ −1 0 a − 0 b NGC Exposure (minutes) re/10 S/N per A σ(kms ) VT (U V )10 4464 105 0′′. 6 ∼160 ∼135 12.46c 1.46a 4387 105 2′′. 6 ∼130 ∼105 11.99 1.50 4478 105 1′′. 7 ∼135 ∼135 11.23 1.50 4473 70 2′′. 7 ∼280 ∼180 10.06 1.61 4621 140 6′′. 0 ∼490 ∼230 9.81 1.64 4365 70 6′′. 7 ∼250 ∼255 9.62 1.67

aBower, Lucey & Ellis (1992b). b − 0 − Michard (1982). (U V )10 corrected to re/10 from (U V )e using the mean gradient of Trager et al. (2000b). Galactic extinction corrected using Schlegel et al. (1998). cRC3.