The Fundamental Plane for cluster E and S0 galaxies

We have analyzed the Fundamental Plane (FP) for a sample of 226 E and S0 galaxies in ten clusters of galaxies. For photometry in Gunn r the best fitting plane is log r_e=1.24 log sigma - 0.82 log _e + cst. The scatter is 0.084 in log r_e. The slope of the FP is not significantly different from cluster to cluster. The residuals of the FP correlate weakly with the velocity dispersion and the surface brightness. Thus, to avoid biases of derived distances the galaxies need to be selected in a homogeneous way. The FP has significant intrinsic scatter. No other structural parameters like ellipticity or isophotal shape can reduce the scatter significantly. The Mg_2-sigma relation differs slightly from cluster to cluster. Galaxies in clusters with lower velocity dispersions have systematically lower Mg_2. With the current stellar population models, it is in best agreement with our results regarding the FP if the offsets are mainly caused by differences in metallicity. Most of the distances that we derive from the FP imply small peculiar motions, <1000km/s. The zero point of the FP must therefore be quite stable. The residuals from the Mg_2-sigma relation may be used to flag galaxies with deviant populations, and possibly to correct the distance determinations for the deviations.Comment: 20 pages, gzipped PostScript, 14 figures included. Accepted for publication in MNRA

An assessment of the evidence from ATLAS3D for a variable initial mass function

The ATLAS3D Survey has reported evidence for a non-universal stellar initial mass function (IMF) for early type galaxies (ETGs) (Cappellari et al. 2012, 2013b,a). The IMF was constrained by comparing stellar mass measurements from kinematic data with those from spectral energy distribution (SED) fitting. Here we investigate possible effects of scatter in the reported stellar mass measurements and their potential impact on the IMF determination. We find that a trend of the IMF mismatch parameter with the kinematic mass to light ratio, comparable to the trend observed by Cappellari et al. (2012), could arise if the Gaussian errors of the kinematic mass determination are typically 30%. Without additional data, it is hard to separate between the option that the IMF has a true large intrinsic variation or the option that the errors in the determination are larger than anticipated. A correlation of the IMF with other properties would help to make this distinction, but no strong correlation has been found yet. The strongest correlation is with velocity dispersion. However, it has a large scatter and the correlation depends on sample selection and distance measurements. The correlation with velocity dispersion could be partly caused by the colour-dependent calibration of the surface brightness fluctuation distances of Tonry et al. (2001). We find that the K-band luminosity limited ATLAS3D Survey is incomplete for the highest M/L galaxies below 10^10.3 M_sun. There is a significant IMF - velocity dispersion trend for galaxies with SED masses above this limit, but no trend for galaxies with kinematic masses above this limit. We also find an IMF trend with distance, but no correlation between nearest neighbour ETGs, which excludes a large environmental dependence. Our findings do not rule out the reported IMF variations, but they suggest that further study is needed.Comment: Accepted for publication in MNRAS, 15 pages, 11 figures, 4 table

On The Robustness of z=0-1 Galaxy Size Measurements Through Model and Non-Parametric Fits

We present the size-stellar mass relations of nearby (z=0.01-0.02) Sloan Digital Sky Survey galaxies, for samples selected by color, morphology, Sersic index n, and specific star formation rate. Several commonly employed size measurement techniques are used, including single Sersic fits, two-component Sersic models, and a non-parametric method. Through simple simulations, we show that the non-parametric and two-component Sersic methods provide the most robust effective radius measurements, while those based on single Sersic profiles are often overestimates, especially for massive red/early-type galaxies. Using our robust sizes, we show for all sub-samples that the mass-size relations are shallow at low stellar masses and steepen above ~ 3-4 x 10^{10}\msun. The mass-size relations for galaxies classified as late-type, low-n, and star-forming are consistent with each other, while blue galaxies follow a somewhat steeper relation. The mass-size relations of early-type, high-n, red, and quiescent galaxies all agree with each other but are somewhat steeper at the high-mass end than previous results. To test potential systematics at high redshift, we artificially redshifted our sample (including surface brightness dimming and degraded resolution) to z=1 and re-fit the galaxies using single Serisc profiles. The sizes of these galaxies before and after redshifting are consistent and we conclude that systematic effects in sizes and the size-mass relation at z ~ 1 are negligible. Interestingly, since the poorer physical resolution at high redshift washes out bright galaxy substructures, single-Sersic fitting appears to provide more reliable and unbiased effective radius measurements at high z than for nearby, well-resolved galaxies.Comment: 18 pages, 15 figures, Accepted for publication in Ap

The average structural evolution of massive galaxies can be reliably estimated using cumulative galaxy number densities

Galaxy evolution can be studied observationally by linking progenitor and descendant galaxies through an evolving cumulative number density selection. This procedure can reproduce the expected evolution of the median stellar mass from abundance matching. However, models predict an increasing scatter in main progenitor masses at higher redshifts, which makes galaxy selection at the median mass unrepresentative. Consequently, there is no guarantee that the evolution of other galaxy properties deduced from this selection are reliable. Despite this concern, we show that this procedure approximately reproduces the evolution of the average stellar density profile of main progenitors of M = 10^11.5 Msun galaxies, when applied to the EAGLE hydrodynamical simulation. At z > 3.5 the aperture masses disagree by about a factor two, but this discrepancy disappears when we include the expected scatter in cumulative number densities. The evolution of the average density profile in EAGLE broadly agrees with observations from UltraVISTA and CANDELS, suggesting an inside-out growth history for these massive galaxies over 0 < z < 5. However, for z < 2 the inside-out growth trend is stronger in EAGLE. We conclude that cumulative number density matching gives reasonably accurate results when applied to the evolution of the mean density profile of massive galaxies.Comment: 5 pages, 4 figures, Accepted by MNRAS Letter

Revealing velocity dispersion as the best indicator of a galaxy's color, compared to stellar mass, surface mass density or morphology

Using data of nearby galaxies from the Sloan Digital Sky Survey we investigate whether stellar mass, central velocity dispersion, surface mass density, or the Sersic n parameter is best correlated with a galaxy's rest-frame color. Specifically, we determine how the mean color of galaxies varies with one parameter when another is fixed. When the stellar mass is fixed we see that strong trends remain with all other parameters, whereas residual trends are weaker when surface mass density, n, or velocity dispersion are fixed. Overall velocity dispersion is the best indicator of a galaxy's typical color, showing the largest residual color dependence when any of the other three parameters are fixed, and stellar mass is the poorest. Other studies have indicated that both the halo and black hole properties are better correlated with velocity dispersion than with stellar mass, surface mass density or Sersic n. Therefore, our results are consistent with a picture where a galaxy's star formation history and present star formation rate are determined to some significant degree by the current properties and assembly history of its dark matter halo and/or the feedback from its central super massive black hole.Comment: 7 pages, 5 figures, submitted to ApJ Letter

Galactic Bulges

We review current knowledge on the structure, properties and evolution of galactic bulges, considering particularly common preconceptions in the light of recent observational results.Comment: in press, Annual Review Astron. Astrophys. 35 1997. Plain tex, 9 figures included. Also available by anonymous ftp at ftp://ftp.ast.cam.ac.uk/pub/gil