Marijn Franx

Ellipticals with kinematically distinct cores: V-I color images with WFPC2

We have analyzed Hubble Space Telescope WFPC2 F555W and F814W (i.e., V and I) images for 15 elliptical galaxies with kinematically distinct cores. For each of them? we have derived surface brightness and isophotal parameter profiles in the two bands, color maps, and radial profiles in V-I. Most galaxies show patchy dust absorption close to their nuclei, However, there are generally no indications of homogeneous, diffuse dust components close to the nuclei. The nuclear colors in the unobscured regions are most likely representative of the central stellar populations. We have detected photometric evidence for faint stellar disks, on scales of a few tens to a few arcseconds, in seven galaxies, namely NGC 1427, 1439, 1700, 4365, 4406, 4494, and 5322, In NGC 1700, the isophotes are slightly boxy at the scale of the counterrotating component and disky at larger radii. We find no difference in V-I color greater than 0.02 mag between these disks and the surrounding galactic regions. Hence, the stellar populations in the kinematically distinct cores are not strongly deviant from the population of the main body. Specifically, there is no evidence for a dominating population of blue, very metal weak stars as predicted by some of the formation scenarios. This argues against models in which small galaxies fall in and survive in the nuclei, unless supermassive black holes are present. These would likely disrupt the accreted small systems. For one galaxy, NGC 4365, the innermost region is bluer than the surrounding regions. This area extends to similar to 15 pc and contains a luminosity of similar to 2.5 x 10(6) L.. If interpreted as a stellar population effect, an age difference of similar to 3-4 Gyr, or an [Fe/H] variation of about 0.2 dex? is derived. The nuclear intensity profiles show a large variety: some galaxies have steep cusp profiles, while others have shallow cusps and a break radius. The nuclear cusps of galaxies with kinematically distinct cores follow the same trends as the nuclei of normal galaxies. We have not been able to identify a unique, qualifying feature in the WFPC2 images that distinguishes the galaxies with kinematically distinct cores from the kinematically normal cares. It is possible that statistical differences exist: possibly, the kinematically distinct cores have a higher fraction of nuclear disks. The similarity of both types of cores puts strong constraints on the formation scenarios. Simulations of galaxy mergers, with the inclusion of star formation and nuclear black holes, are needed to resolve the question of how these structures may have formed. Spectra with high spatial resolution are needed to study the nuclear structure of the distinct component in detail.

Morphology and kinematics in clusters of galaxies

The existence of subclustering, infall, and the morphology-local density relation suggests that there should be differences in the kinematics of different morphological populations in clusters. From published data, we compile a sample of galaxy redshifts and types in six rich clusters of galaxies: A548, A1060, A1644, A1656, A2151, and DC 2048-52. In three clusters, the velocity distribution of spiral galaxies is different from those of other morphological populations: (1) spirals and ellipticals differ in A1656 and A2151, and (2) spirals and SOs differ in DC 2048-52. The differences in the distributions result primarily from differences in the mean velocities

Measuring the Evolution of the Mass-to-Light Ratio from z = 0 to z = 0.6 from the Fundamental Plane

Galaxy evolution is probably a complex process. Mergers, infall, and star bursts may change galaxy properties systematically with time. As a result, the interpretation of the luminosity function is ambiguous, and information on the mass evolution of galaxies is needed. Such information can be retrieved from the evolution of the Tully-Fisher relation, Faber-Jackson relation, or the Fundamental Plane with redshift.

Stellar populations and morphology on the red sequence at z [approximate] 1

We present results from a detailed study of cluster red sequence at z≈1 from the ACS Intermediate Redshift Cluster Survey (Mei et al. 2009). Our analysis shows that the red sequence is well defined at z≈1 and elliptical and lenticular galaxies lie on similar color‐magnitude relations. We analyze the parameters of the early‐type color‐magnitude relations ‐scatter, slope and zero‐point—as a function of redshift, galaxy properties and cluster mass. Our results suggest that bright elliptical galaxies in cluster cores are on average older than S0 galaxies and peripheral elliptical galaxies (by about 0.5 Gyr, using a simple, single burst solar metallicity Bruzual & Charlot (2003) stellar population model). The red sequence does not show significant evolution out to redshift z≈1.3 nor significant dependence on cluster mass. The fraction of morphological early‐type galaxies on the red sequence is 80% to 90% of the total red sequence population for most of our clusters. In the highest redshift, low mass cluster of...

A z = 2.72 Galaxy Gravitationally Lensed by the Cluster MS 1512+36: Reconstruction and Near-infrared Spectroscopy

Using archival HST/WFPC2 optical data and near-infrared VLT/ISAAC spectroscopy, the properties of the Lyman-break galaxy cB58 are analyzed. The z = 0.37 galaxy cluster MS1512+36 lenses it into strongly magnified arcs with ample substructure. The data enables an accurate reconstruction of the source, yielding milliarcseconds resolution; an unparalleled view of the central kpc of the z = 2.72 starburst galaxy.

Line-Strength Profiles in Early-Type Galaxies

Line-strengths and their gradients in Mg, Fe, and Hγ have been determined for a sample of 9 brightest cluster (BCG), 7 elliptical, and 15 S0 galaxies in order to study their stellar populations and investigate their relationship to one another. We find that BCGs follow the same relationship between central Mgb line-strength and central velocity dispersion found for ellipticals while the S0 galaxies show significant scatter with respect to this relation. Brightest cluster galaxies are in agreement with the known trend towards more massive ellipticals having larger [Mg/Fe] ratios while the internal gradients within our BCG and E galaxies are consistent with a roughly constant [Mg/Fe] ratio. We find that a correlation exists between the central [Mg/Fe] ratio and average Hγ line-strength in the sense that BCG and E galaxies with larger [Mg/Fe] ratios have lower Hγ strengths. For our BCG and E galaxies, Hγ is the best predictor of [Mg/Fe] ratio. The Mgb metallicity gradients for BCGs and ellipticals are similar and consistent with a reduction in the mean metallicity of the stellar population by about a factor of 2 over a factor often in radius. No strong correlations are found between the metallicity gradient sizes and either kinematic or line-strength parameters of the E and BCG galaxies. The S0 disks display roughly constant Mg, Fe, and Hγ line-strengths with radius indicating that they have uniform age and metallicity throughout. S0 galaxy minor axes ‘bulge’ metal line-strength gradients are similar to elliptical gradients and fall to values lower than those found in the disks.