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Gradients of Absorption Line Strengths in Elliptical Galaxies

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Gradients of Absorption Line Strengths in Elliptical Galaxies View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server To appear in the Astrophysical Journal Preprint typeset using LATEX style emulateapj GRADIENTS OF ABSORPTION LINE STRENGTHS IN ELLIPTICAL GALAXIES Chiaki KOBAYASHI Department of Astronomy, School of Science, University of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; e-mail: [email protected] Nobuo ARIMOTO Institute of Astronomy, School of Science, University of Tokyo 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan; e-mail: [email protected] ABSTRACT We have re-studied line-strength gradients of 80 elliptical galaxies. Typical metallicity gradients of elliptical galaxies are ∆[Fe/H]/∆logr 0.3 which is flatter than the gradients predicted by monolithic collapse simulations. The metallicity gradients'− do not correlate with any physical properties of galaxies, including central and mean metallicities, central velocity dispersions σ0, absolute B-magnitudes MB, absolute effective radii Re, and dynamical masses of galaxies. By using the metallicity gradients, we have calculated mean stellar metallicities for individual ellipticals. Typical mean stellar metallicities are [Fe/H] 0.3, and range from [Fe/H] 0.8to+0.3, which is contrary to what Gonzalez & Gorgas (1996)h claimed;i'− the mean metallicitiesh ofi'− ellipticals are not universal. The mean metallicities correlate well with σ0 and dynamical masses, though relations for MB and Re include significant scatters. We find fundamental planes defined by surface brightnesses SBe, [Fe/H] ,andRe (or M ), and the scatters of h i B which are much smaller than those of the [Fe/H] - Re (or M ) relations. The [Fe/H] -logσ0 relation h i B h i is nearly in parallel to the [Fe/H]0 -logσ0 relation but systematically lower by 0.3 dex; thus the mean metallicities are about a half of the central values. The metallicity-mass relation, or equivalently, the color-magnitude relation of ellipticals holds not only for the central part but also for the whole part of galaxies. Assuming that Mg2 and Fe1 give [Mg/H] and [Fe/H], respectively, we find [Mg/Fe] +0.2in h i' most of elliptical galaxies. [Mg/Fe] shows no correlation with galaxy mass tracers such as σ0, in contrast to what was claimed for theh centrali [Mg/Fe]. This can be most naturally explained if the star formation had stopped in elliptical galaxies before the bulk of Type Ia supernovae began to explode. Elliptical galaxies can have significantly different metallicity gradients and [Fe/H] even if they have the same galaxy mass. This may result from galaxy mergers, but no evidenceh is foundi from presently available data to support the same origin for metallicity gradients, the scatters around metallicity-mass relation, and dynamical disturbances. This may suggest that the scatters have their origin at the formation epoch of galaxies. Subject headings: galaxies: abundances — galaxies: elliptical — galaxies: evolution — galaxies: formation 1. INTRODUCTION elliptical galaxy could form if many small galaxies accreted onto a massive disk galaxy (e.g., Cole et al. 1994; Baugh How elliptical galaxies formed is one of key questions et al. 1998). Observationally, the dynamical disturbances of modern astronomy. Two competing scenarios have so far been proposed: Elliptical galaxies should form mono- such as shells/ripples, transient dust lanes, and multiple and/or counter-rotating cores (e.g., Kormendy & Djorgov- lithically by gravitational collapse of gas cloud with con- ski 1989) all seem to support the scenario that elliptical siderable energy dissipation (e.g., Larson 1974b; Arimoto & Yoshii 1987), or alternatively ellipticals should form galaxies formed via hierarchical clustering of smaller galax- ies (Schweizer & Seitzer 1992), which is a picture predicted via mergers of relatively small galaxies (e.g., Toomre & by cold dark matter (CDM) cosmology. The merger hy- Toomre 1972; Kauffmann & White 1993; Cole et al. 1994). Elliptical galaxies show apparently little evidence for on- pothesis may easily explain morphology-density relation of galaxies in clusters (Dressler 1980; Dressler et al. 1997), a going star formation, the bulk of their stars are old (e.g., significant number of interacting galaxies at high redshifts Kodama & Arimoto 1997; Stanford, Eisenhardt, & Dick- inson 1997; Kodama et al. 1998a), and yet some of ellip- (Driver, Windhorst, & Griffiths 1995), Butcher-Oemler ef- fects (Butcher & Oemler 1978; 1984), and E+A galaxies ticals show strong signs of recent dynamical disturbances (Schweizer et al. 1990; Schweizer & Seitzer 1992). Obser- (Dressler & Gunn 1983; 1992). However, collisions and vational evidences are confusing and controversial. mergers of galaxies should be less frequent at the clus- ter center because of large velocity dispersions of galaxies The merger hypothesis assumes that gaseous disk-like galaxies formed first by assembling subgalactic clumps, there (Ostriker 1980). A recent HST observation reveals then two grown-up disk galaxies of similar mass collided conspicuous spiral arms dominated by A-type stars in the E+A galaxies (Franx et al. 1996), which suggests that into a single giant elliptical galaxy (e.g., Kauffmann & White 1993; Kauffman & Charlot 1998). Alternatively, an the Butcher-Oemler galaxies are not ellipticals, but spiral galaxies falling onto a cluster potential for the first time 1 2 Line Strength Gradients in Ellipticals and are witnessed at an instance of being transformed into the two hydrogen lines are sensitive to stellar age (Burstein S0 galaxies. Elliptical galaxies are surrounded by a huge et al. 1984; Faber et al. 1985; Gonzalez 1993; Worthey number of globular clusters (GCs). The number of glob- 1994). Recently, Vazdekis & Arimoto (1999) have bro- ular clusters per galaxy luminosity (specific frequency) is ken up the age-metallicity degeneracy of Hγ by including almost twice of that in spiral galaxies (Harris 1991). If neighboring metallic lines in such a way that a resulting Hγ ellipticals formed via mergers of spiral galaxies, at least depends only on age. The approach is thus very promising, similar amount of GCs should be born during the merger but no attempt has yet been conducted for estimating ages events (van den Bergh 1982; 1990). True, young GCs are of giant ellipticals. We therefore should keep it in our mind discovered in merging galaxies (Whitmore & Schweizer that some of giant elliptical galaxies in the field might be 1995), but these GCs are far less numerous than those significantly younger than cluster ellipticals (Franceschini required for explaining high specific frequencies of ellipti- et al. 1998; Kodama, Bower, & Bell 1998b) and that the cals. On the other hand, GCs in elliptical galaxies have radial gradients of metallic lines may be caused by not bimodal [Fe/H] distributions (Forbes, Brodie, & Grillmair only a radial variation of stellar metallicity but also an 1997), which has been considered to support the merging equivalent variation of mean stellar age. This paper dis- hypothesis. cusses the gradients of the four metallic lines and Hβ, but The dissipative collapse hypothesis assumes that a bulk mainly Mg2 and Mgb gradients, because Fe1 and Fe2 are of stars in ellipticals formed during an initial burst of easily influenced by a possible gradient of stellar velocity star formation, which was induced by the collisions of dispersion whose spatial distribution in a galaxy is still fragmented clouds in proto-galaxies, and terminated by a difficult to measure precisely. supernovae-driven galactic wind that expelled the left-over Previous studies showed that (Mg2)0 correlate with the interstellar gas from galaxies (e.g., Larson 1974b; Arimoto velocity dispersion σ0 at the galaxy center (Davies et al. & Yoshii 1987). The galactic wind is supposed to play 1987; Burstein et al. 1988; Bender, Burstein, & Faber an essential role in enriching heavy elements in hot intr- 1993). A similar relation holds for (Mg2)0 and total abso- acluster gas (Ciotti et al. 1991). Elliptical galaxies after lute magnitudes MB (Faber 1973; Burstein 1979; Terlevich the wind should evolve passively (Kodama et al. 1998a). et al. 1981; Dressler 1984). If, admittedly a big if,onecan The shells and ripples would appear when ellipticals cap- assume that elliptical galaxies are old, one can convert tured nearby dwarf galaxies. These dynamical distur- Mg2 into the metallicity Z either empirically (Burstein bances would be detectable for a couple of Gyrs, but the et al. 1984; Faber et al. 1985) or theoretically with a capture itself would not introduce any significant change in help of population synthesis models (Mould 1978; Peletier the stellar constituents of galaxies, since the mass involved 1989; Buzzoni, Gariboldi, & Mantegazza 1992; Barbuy in the secondary formation of stars is at most 10% as the 1994; Worthey 1994; Casuso et al. 1996; Bressan, Chiosi, study of Hβ absorption and broad band colors of elliptical & Tantalo 1996; Kodama & Arimoto 1997). For exam- galaxies shows (Kodama & Arimoto 1998). The galactic ple, if Worthey’s (1994) calibration is adopted, one finds wind predicts tight correlations among global properties that a typical metallicity gradient of elliptical galaxies is of galaxies including a color-magnitude relation (Bower, ∆logZ/∆logr 0.3. The radial gradient of metallic Lucey, & Ellis 1992) and a fundamental plane (Djorgovski line strength can'− be naturally explained by the dissipa- & Davies 1987; Dressler et al. 1987). Recent observa- tive collapse picture. However, the measured gradients are tions of clusters at high redshifts confirm that these re- less steep than those predicted by numerical simulations lationships are holding even at z 1 (Dickinson 1996; of collapse model: For example, Larson’s hydrodynamical Schade, Barrientos & Lopez-Cruz 1997;∼ Kelson et al. 1997; simulations gave ∆ log Z/∆logr 0.35 (Larson 1974a), Stanford et al. 1998). A progressive change of the color- and 1.0 (Larson 1975); Carlberg’s∼− N-body simulations magnitude relation as a function of look-back time clearly gave− ∆ log Z/∆logr 0.5 (Carlberg 1984).
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