Environmental dependence

Several authors have addressed the dependence on environment of the prop-erties of the stellar populations in early-type galaxies. Studies of the colour-magnitude relation and the relations between absorption indices and velocity dispersion on relatively small samples of early-type galaxies in different en-vironments have shown that galaxies in low-density enen-vironments tend to be younger and more metal-rich than those in high-density environments (e.g.

Kuntschner et al. 2002; Thomas et al. 2005; Denicol´o et al. 2005). Bernardi et al. (1998), later confirmed by Bernardi et al. (2006) on a larger sample of SDSS early-type galaxies, found differences in the Mg₂−σ_V relation of galax-ies in different environments, implying that galaxgalax-ies in dense environments are at most 1 Gyr older than galaxies in low-density environments and that they have the same metallicity.

Kauffmann et al. (2004) provide estimates of environmental density for a sample of SDSS DR2 galaxies in the redshift range 0.03 < z <0.1 and with apparent r-band magnitude in the range 14.5 < r < 17.77, complete down to a stellar mass of 2×10⁹M. The density is expressed in terms of the

3.3 Physical origin of observed relations for early-type galaxies

Table 3.1: Parameters of the relations fitted by minimising the absolute vertical deviations. Sample A refers to our primary sample, while sample B is obtained by selecting galaxies according to Bernardi et al. (2005) criteria.

Sample Slope Intercept Scatter

Colour-magnitude relation

sample A^a −0.015±0.001 0.40 0.051 sample B^a −0.014±0.002 0.41 0.043 sample A^b −0.024±0.002 0.22 0.050 sample B^b −0.028±0.001 0.14 0.043 sample A^c −0.025±0.001 0.14 0.051 sample B^c −0.015±0.002 0.37 0.043 sample A^d −0.035±0.002 −0.05 0.050 sample B^d −0.029±0.002 0.09 0.044

Mg₂–σ_V relation

sample A^e 0.23±0.01 −0.29 0.024 sample B^e 0.17±0.003 −0.15 0.023 sample A^f 0.25±0.01 −0.31 0.024 sample B^f 0.19±0.00 −0.18 0.023

ak-corrected colour and magnitude. ^bk+e-corrected colour and magnitude (the evolution correction is the one provided by Bernardi et al. 2005). ^ck-corrected colour and magnitude dereddened applying the average dust corrections of Fig. 3.5. ^dFully corrected colour and mag-nitude. ^eMg2 index strength corrected for velocity dis-persion. ^fMg2 index strength corrected for velocity dis-persion and evolution.

number of spectroscopically-observed neighbouring galaxies (down to a fixed absolute magnitude) within 2 Mpc of projected radius and ±500 km/s in ve-locity difference from the target galaxy, corrected for galaxies missed due to fibre collisions (N_neigh). We take advantage of these density estimates to ad-dress any environmental dependence of the physical properties of the stellar populations for the galaxy sample studied here. This can be achieved for only 1765 galaxies in our sample, for which an estimate of N_neigh is avail-able. We consider three bins in environmental density, defined by N_neigh<4, 4≤Nneigh <7 and Nneigh≥7, which contain, respectively, 693, 388 and 684 galaxies. As mentioned in Section 4.2.1, we can classify the galaxies in our sample on the basis of their emission-line properties. As expected, the sample is dominated by ‘unclassifiable’ galaxies (without emission lines), but there is also contamination by galaxies with a low level of star formation (SF and C galaxies). Fig. 3.12 illustrates the fraction of unclassifiable, SF, low-S/N SF, C

3 Physical origin of the colour-magnitude and the Mg₂–σ_V relations for early-type galaxies

and AGN galaxies as a function of environment. This plot quantifies the state-ment of Section 3.3.1.1 that the fraction of galaxies showing emission lines in our sample increases in lower-density environments. This class of galaxies also contributes to increase the scatter blueward of the colour-magnitude relation.

In Fig. 3.13, we explore how the CMR (left-hand panels) and the Mg2−σ_V relation (right-hand panels) depend on environment. The relations found in the highest-density bin are compared to those defined in the lowest-density bin.

The results of the linear fits (also for the intermediate bin of N_neigh) are given in Table 3.2. Fig. 3.13 and Table 3.2 show that there is no systematic variation in the slope of the CMR as a function of environment, while the Mg2−σ_V relation appears to steepen at low densities because of a larger fraction of galaxies with low Mg₂ index strength at low velocity dispersions (columns 3 and 6).

Between the two extreme density bins there are differences of 0.006±0.003 and 0.007±0.003 in the zero-points of the CMR and the Mg2−σ_V relation, respectively (columns 4 and 7). This is in agreement with the small shift of 0.007±0.002 mag measured by Bernardi et al. (1998) in the Mg₂−σ_V relation of a sample of 931 early-type galaxies in different environments. We also identify a systematic increase of the scatter about both relations from high-to low-density environments, in agreement with earlier findings, as mentioned above (e.g. Hogg et al. 2004).

It is of interest to understand how the change in the scatter along the two scaling relations reflects differences in the physical parameters of galaxies in different environments. We note that the distribution in stellar mass does not vary significantly¹¹ with environment, but that the median stellar mass of galaxies in the lowest-density bin is lower by about 0.05 dex than the me-dian stellar mass in the highest-density bin (it increases from 8 ×10¹⁰ to 9×10¹⁰M). Since stellar metallicity, age and element abundance ratio all in-crease with stellar mass, any effect induced by changes in stellar mass must be removed when quantifying variations in these parameters with environment.

To do this, we calculate the median stellar metallicity, light-weighted age and element abundance ratio as a function of stellar mass for the sample as a whole (see Fig. 3.17). For the 1765 galaxies with an estimate of environmental density, we then consider the offsets in log(Z/Z), log(t_r/yr), ∆(Mgb/hFei) from the median values of these parameters at fixed stellar mass in the whole sample. The distributions in ∆[logZ] and ∆[logt_r] are skewed toward nega-tive values, independently of environment, and this effect is stronger at small masses. If galaxies at low densities are distributed preferentially to smaller

11When comparing the stellar mass distribution of galaxies in the low-density bin with that of galaxies in the high-density bin, the probability obtained from a Kolmogorov-Smirnov test is not low enough to reject the hypothesis that the two distributions are drawn from the same parent distribution.

3.3 Physical origin of observed relations for early-type galaxies

masses than galaxies in dense environments, the distributions in ∆[logZ] and

∆[logtr] will show a stronger tail toward negative values in the low-density bin. To separate this effect from an intrinsic dependence of metallicity and age on environment, we further distinguish between galaxies with stellar masses above and below 10¹¹M.

The result of this analysis is shown in Fig. 3.14 for the same three environ-mental bins as considered above. The distributions of the offsets in log(Z/Z), log(t_r/yr) and ∆(Mgb/hFei) for the two low-density bins (N_neigh < 4 and 4 ≤ Nneigh < 7, solid lines) are compared to the corresponding ones in the high-density bin (N_neigh ≥ 7, dotted line in each panel), for massive and low-mass galaxies separately (red and blue histograms, respectively). The comparison is also summarized in Table 3.3, where we give the difference of the average parameter offset between the highest- and the lowest-density bins, for galaxies with stellar mass above and below 10¹¹M separately. There, we also indicate the probability for the two distributions to be drawn from the same parent distribution, according to a Kolmogorov-Smirnov test. Element abundance ratio, as expressed by ∆(Mgb/hFei), does not show any significant variation with environment, either in the average value or in the scatter. In contrast, light-weighted age (independently of mass) and metallicity (for mas-sive galaxies) show a small dependence on environment in the sense that there is a higher fraction of young, metal-poor galaxies in low-density environments.

Massive early-type galaxies in dense environments tend to be∼0.03 dex more metal-rich than their field counterparts. Similarly, the light-weighted ages of galaxies in dense environments are ∼ 0.02 dex older than in the field. It is interesting to mention that we also find a systematic increase in the scatter of metallicity and age from dense to low-density environments. From the highest-to the lowest-density bins the scatter in both metallicity and age increases by about 0.02 dex for massive galaxies (from 0.118 to 0.135 for log(Z/Z), from 0.083 to 0.103 for log(t_r/yr)) and by about 0.01 dex for M∗ <10¹¹M (from 0.16 to 0.17 for log(Z/Z) and from 0.12 to 0.136 for log(t_r/yr)). Although very small, these trends hint at a possibly very relevant environmental de-pendence of metallicity and age. Future analysis of larger samples of galaxies (provided, e.g., by the complete SDSS) with well characterized environmental properties will allow to draw firmer conclusions.