Galaxy formation and evolution

Abstract:

We report on near-infrared medium-resolution spectroscopy of a sample of luminous and ultraluminous infrared galaxies (LIRGs-ULIRGs), carried out with SOFI at the ESO 3.5 m New Technology Telescope. Because of wavelength dependence of the attenuation, the detection of the Paα or Brγ line in the Ks band should provide relevant constraints on SFR and the contribution of an AGN. We find, however, that the intensities of the Paα and Brγ lines, even corrected for slit losses, are on average only 10% and 40%, respectively, of that expected from a normal starburst of similar bolometric luminosity. The corresponding star formation rates, after correcting for the attenuation derived from the NIR-optical emission line ratios, are 14% and 60% of that expected if the far infrared luminosity were entirely powered by the starburst. This confirms the existence of a recombination photon deficit, particularly in the case of the Paα line, already found in the Brγ line in other infrared galaxies of similar luminosity. In discussing the possible causes of the discrepancy, we find unlikely that it is due to the presence of an AGN, though two objects show evidence of broadening of the Paα line and of the presence of coronal line emission. In fact, from our own observations and data collected from the literature we argue that the studied galaxies appear to be predominantly powered by a nuclear starburst. Two scenarios compatible with the present data are that either there exists a highly attenuated nuclear star forming region, and/or that a significant fraction (≃80%) of the ionizing photons are absorbed by dust within the HII regions. We suggest that observations in the Brα spectral region could constitute a powerful tool to disentangle these two possibilities. © ESO 2005.

Abstract:

We investigate a spectroscopic sample of 48 early-type galaxies in the rich cluster Abell 2390 at z = 0.23 and 48 early-type galaxies from a previously published survey of Abell 2218 at z = 0.18. The spectroscopic data of A 2390 are based on multi-object spectroscopy using the multi-object spectrograph for Calar Alto at the 3.5-m telescope on Calar Alto Observatory and are complemented by ground-based imaging using the 5.1-m Hale telescope and Hubble Space Telescope (HST) observations in the F555W and F814W filters. Our investigation spans a broad range in luminosity (-20.5 ≥ Mr ≥ -23.0) and a rather wide field of view of 1.53 h70-1 × 1.53 h70-1 Mpc2. As the A 2218 and A2390 samples are very similar, we can combine them and analyse a total number of 96 early-type (E+S0) galaxies at z ∼ 0.2. Using the ground-based data only, we construct the Faber-Jackson relation (FJR) for all 96 E+S0 galaxies and detect a modest luminosity evolution with respect to the local reference. The average offset from the local FJR in the Gunn r band is Δ ̄Mr = 0.32 ± 0.22 mag. Similar results are derived for each cluster separately. Less massive galaxies show a trend for a larger evolution than more massive galaxies. HST/WFPC2 surface brightness profile fits were used to derive the structural parameters for a subsample of 34 E+S0 galaxies. We explore the evolution of the Fundamental Plane (FP) in Gunn r, its projections on to the Kormendy relation and the M/L ratios as a function of velocity dispersion. The FP for the cluster galaxies is offset from the local Coma cluster FP. At a fixed effective radius and velocity dispersion our galaxies are brighter than their local counterparts. For the total sample of 34 E+S0 cluster galaxies which enter the FP we deduce only a mild evolution with a zero-point offset of 0.10 ± 0.06, corresponding to a brightening of 0.31 ± 0.18 mag. Elliptical and lenticular galaxies are uniformly distributed along the FP with a similar scatter of 0.1 dex. Within our sample we find little evidence for differences between the populations of elliptical and S0 galaxies. There is a slight trend that lenticulars induce on average a larger evolution of 0.44 ± 0.18 mag than ellipticals with 0.02 ± 0.21 mag. The M/L ratios of our distant cluster galaxies at z = 0.2 are offset by Δlog (M/L r) = -0.12 ± 0.06 dex compared with those of Coma. Our results can be reconciled with a passive evolution of the stellar populations and a high formation redshift for the bulk of the stars in early-type galaxies. However, our findings are also consistent with the hierarchical formation picture for rich clusters, if ellipticals in clusters had their last major merger at high redshift. © 2005 RAS.

Abstract:

We present a study of the stellar populations of galaxies in the cluster RX J0 152.7-1357 at a redshift of 0.83. The study is based on new high signal-to-noise ratio spectroscopy of 29 cluster members covering the wavelength range 5000-10000 Å, as well as r′i′z′ photometry of the cluster. We use scaling relations between the central velocity dispersions of the galaxies and their luminosities, Balmer line strengths, and various metal line strengths to parameterize the differences between the members of RX J0152.7-1357 and our low-redshift comparison sample. The luminosities of the RX J0152.7-1357 galaxies and the strengths of the higher order Balmer lines H γ and Rδ (for non-emission-line galaxies) appear to be in agreement with pure passive evolution of the stellar populations with a formation redshift Z form ≈ 4. However, the strengths of the D4000 indices and the metal indices do not support this interpretation. Compared with our low-redshift comparison sample, the metal indices (C4668, Fe4383, CN3883, G4300, and CN 2) show that at least half of the non-emission-line galaxies in RX J0152.7-1357 have an Q-element abundance ratio [a/Fe] of 0.2 dex higher, and about half of the galaxies have significantly lower metal content. X-ray data have previously shown that RX J0152.7-1357 is in the process of merging from two sub-clumps. We find that differences in stellar populations of the galaxies are associated with the location of the galaxies relative to the X-ray emission. The galaxies with weak C4668 and G4300, as well as galaxies for which weak [0 II] emission indicates a very recent star formation episode involving about 1% of the mass, are located in areas of low X-ray luminosity, on the outskirts of the two subclumps. It is possible that these galaxies are experiencing the effect of the cluster merger as (short) episodes of star formation, while the galaxies in the cores of the subclumps are unaffected by the merger. The spectroscopy of the RX JO 152.7-1357 galaxies shows for the first time galaxies in a rich cluster at intermediate redshift that cannot evolve passively into the present-day galaxy population in rich clusters. Additional physical processes may be at work, and we speculate that merging with infalling (disk) galaxies in which stars have formed over an extended period might produce the required reduction in [a/Fe]. However, the merging could not be accompanied by star formation involving a substantial mass fraction. We note that our conclusions, in part, rely on stellar population models for which the predictions of the indices in the rest-frame blue have not yet been tested extensively. © 2005. The American Astronomical Society. All rights reserved.