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Register a new userDwarf Galaxies in the Coma Cluster: I. Velocity Dispersion Measurements
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E. Kourkchi
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We present the study of a large sample of early-type dwarf galaxies in the Coma cluster observed with DEIMOS on the Keck II to determine their internal velocity dispersion. We focus on a subsample of 41 member dwarf elliptical galaxies for which the velocity dispersion can be reliably measured, 26 of which were studied for the first time. The magnitude range of our sample is $-21<M_R<-15$ mag. This paper (paper I) focuses on the measurement of the velocity dispersion and their error estimates. The measurements were performed using {it pPXF (penalised PiXel Fitting)} and using the Calcium triplet absorption lines. We use Monte Carlo bootstrapping to study various sources of uncertainty in our measurements, namely statistical uncertainty, template mismatch and other systematics. We find that the main source of uncertainty is the template mismatch effect which is reduced by using templates with a range of spectral types. Combining our measurements with those from the literature, we study the Faber-Jackson relation ($Lproptosigma^alpha$) and find that the slope of the relation is $alpha=1.99pm0.14$ for galaxies brighter than $M_Rsimeq-16$ mag. A comprehensive analysis of the results combined with the photometric properties of these galaxies is reported in paper II.
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We present velocity dispersion measurements for 69 faint early-type galaxies in the core of the Coma cluster, spanning -22.0<M_R<-17.5 mag. We examine the L-sigma relation for our sample and compare it to that of bright ellipticals from the literature. The distribution of the the faint early-type galaxies in the L-sigma plane follows the relation L ~ sigma^{2.01pm0.36}, which is significantly shallower from L ~ sigma^4 as defined for the bright ellipticals. While increased rotational support for fainter early-type galaxies could account for some of the difference in slope, we show that it cannot explain it. We also investigate the Colour-sigma relation for our Coma galaxies. Using the scatter in this relation, we constrain the range of galaxy ages as a function of their formation epoch for different formation scenarios. Assuming a strong coordination in the formation epoch of faint early-type systems in Coma, we find that most had to be formed at least 6 Gyrs ago and over a short 1 Gyr period.
We use ACS data from the HST Treasury survey of the Coma cluster (z~0.02) to study the properties of barred galaxies in the Coma core, the densest environment in the nearby Universe. This study provides a complementary data point for studies of barred galaxies as a function of redshift and environment. From ~470 cluster members brighter than M_I = -11 mag, we select a sample of 46 disk galaxies (S0--Im) based on visual classification. The sample is dominated by S0s for which we find an optical bar fraction of 47+/-11% through ellipse fitting and visual inspection. Among the bars in the core of the Coma cluster, we do not find any very large (a_bar > 2 kpc) bars. Comparison to other studies reveals that while the optical bar fraction for S0s shows only a modest variation across low-to-intermediate density environments (field to intermediate-density clusters), it can be higher by up to a factor of ~2 in the very high-density environment of the rich Coma cluster core.
Deep B- and R-band CCD images of the central ~700 arcmin^2 of the Coma cluster core have been used to measure the dwarf-galaxy population in Coma. In this paper, we describe a newly developed code for automated detection, photometry and classification of faint objects of arbitrary shape and size on digital images. Intensity-weighted moments are used to compute the positions, radial structures, ellipticities, and integrated magnitudes of detected objects. We demonstrate that Kron-type 2r_1 aperture aperture magnitudes and surface brightnesses are well suited to faint-galaxy photometry of the type described here. Discrimination between starlike and extended (galaxy) objects is performed interactively through parameter-space culling in several possible parameters, including the radial moments, surface brightness, and integrated color versus magnitude. Our code is tested and characterized with artificial CCD images of star and galaxy fields; it is demonstrated to be accurate, robust and versatile. Using these analysis techniques, we detect a large population of dE galaxies in the Coma cluster core. These dEs stand out as a tight sequence in the R, (B-R) color-magnitude diagram.
We use Keck/DEIMOS spectroscopy to confirm the cluster membership of 16 ultra-diffuse galaxies (UDGs) in the Coma cluster, bringing the total number of spectroscopically con- firmed UDGs to 24. We also identify a new cluster background UDG. In this pilot study of Coma UDGs in velocity phase-space, we find evidence that most present-day Coma UDGs have a recent infall epoch while a few may be ancient infalls. These recent infall UDGs have higher absolute relative line-of-sight velocities, bluer optical colors, and are smaller in size, unlike the ancient infalls. The kinematics of the spectroscopically confirmed Coma UDG sample is similar to that of the cluster late-type galaxy population. Our velocity phase-space analysis suggests that present-day cluster UDGs have a predominantly accretion origin from the field, acquire velocities corresponding to the mass of the cluster at accretion as they are accelerated towards the cluster center, and become redder and bigger as they experience the various physical processes at work within the cluster.
(ABRIDGED) We use high resolution (~0.1) F814W ACS images from the HST ACS Treasury survey of the Coma cluster at z~0.02 to study bars in massive disk galaxies (S0s), and in dwarf galaxies in the Coma core. Our study helps constrain the evolution of bars and disks in dense environments and provides a comparison point for studies in lower density environments and at higher redshifts. (1) We characterize the fraction and properties of bars in a sample of 32 bright (M_V <= -18, M_* > 10^9.5 M_sun) S0 galaxies, which dominate the population of massive disk galaxies in the Coma core. Measuring the S0 bar fraction must be handled carefully, as the results depend on the method used: the bar fraction for bright S0s in the Coma core is 50%+/-11%, 65%+/-11%, and 60%+/-11% for three methods of bar detection: strict ellipse fit criteria, relaxed ellipse fit criteria, and visual classification. (2) We compare the S0 bar fraction across different environments (Coma core, A901/902, Virgo). We find that the bar fraction among bright S0 galaxies does not show a statistically significant variation across environments spanning two orders of magnitude in galaxy number density (n~300-10,000 gal/Mpc^3). We speculate that the S0 bar fraction is not significantly enhanced in rich clusters because S0s in rich clusters are less prone to bar instabilities as they are dynamically hot and gas poor due to ram pressure stripping and accelerated star formation. In addition, high-speed encounters in rich clusters may be less effective than slow, strong encounters in inducing bars. (3) We analyze a sample of 333 faint (M_V > -18) dwarf galaxies in the Coma core. Using unsharp-masking, we find only 13 galaxies with bar and/or spiral structure. The paucity of disk structures in Coma dwarfs suggests that either disks are not common in these galaxies, or that any disks present are too hot to develop instabilities.
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