Analía V. Smith Castelli

Galaxy populations in the Antlia cluster. I. Photometric properties of early-type galaxies ⋆

We present the first colour‐magnitude relation (CMR) of early-type galaxies in the central region of the Antlia cluster, obtained from CCD wide-field photometry in the Washington photometric system. Integrated (C − T1) colours, T1 magnitudes, and effective radii have been measured for 93 galaxies (i.e. the largest galaxies sample in the Washington system till now) from the FS90 Antlia Group catalogue. Membership of 37 objects can be confirmed through new radial velocities and data collected from the literature. The resulting colour‐ magnitude diagram shows that early-type FS90 galaxies that are spectroscopically confirmed Antlia members or that were considered as definite members by FS90, follow a well-defined CMR [σ(C−T1) ∼ 0.07 mag] that spans 9 mag in brightness with no apparent change of slope. This relation is very tight for the whole magnitude range but S0 galaxies show a larger dispersion, apparently due to a separation of ellipticals and S0s. Antlia displays a slope of −13.6 in a T1 versus (C − T1) diagram, in agreement with results for clusters like Fornax, Virgo, Coma and Perseus, which are dynamically different to Antlia. This fact might indicate that the build-up of the CMR in cluster of galaxies is more related to galaxies internal processes than to the influence of the environment. Interpreting the CMR as a luminosity‐metallicity relation of old stellar systems, the metallicities of the Antlia galaxies define a global relation down to MV ≈− 13. We also find, for early-type dwarfs, no clear relation between luminosity and effective radius, indicating a nearly constant mean effective radius of ∼1 kpc. This value is also found in several samples of dwarf galaxies in Virgo and Coma.

The bright end of the colour-magnitude relation of cluster galaxies

We investigate the origin of the colour-magnitude relation (CMR) followed by early-type cluster galaxies by using a combination of cosmological N-body simulations of cluster of galaxies and a semi-analytic model of galaxy formation (Lagos, Cora & Padilla 2008). Results show good agreement between the general trend of the simulated and observed CMR. However, in many clusters, the most luminous galaxies depart from the linear fit to observed data displaying almost constant colours. With the aim of understanding this behaviour, we analyze the dependence with redshift of the stellar mass contributed to each galaxy by different processes, i.e., quiescent star formation, and starburst during major/minor and wet/dry mergers, and disk instability events. The evolution of the metallicity of the stellar component, contributed by each of these processes, is also investigated. We find that the major contribution of stellar mass at low redshift is due to minor dry merger events, being the metallicity of the stellar mass accreted during this process quite low. Thus, minor dry merger events seem to increase the mass of the more luminous galaxies without changing their colours.

Galaxy populations in the Antlia cluster – II. Compact elliptical galaxy candidates★

Continuing our study of galaxy populations in the Antlia cluster, we present a photometric analysis of four galaxies classified as compact elliptical (cE) galaxies in the 1990 Antlia Group catalogue of Ferguson and Sandage. Until now, there have been only six known members of this rare type of galaxy. Using data from various photometric systems (Washington C, KronCousins R, Bessel V and I, Hubble Space Telesecope F814W and F435W), we measured the brightness and colour profiles, as well as the structural parameters. By comparing these with those of other galaxies in the Antlia cluster, as well as with confirmed cE galaxies from the literature, we found that two of the cE candidates, although spectroscopically confirmed Antlia members, are not cE galaxies. However, one of these objects presents strong ellipticity and position angle variations that resemble those already reported for M32, leading us to speculate about this type of object being a progenitor of a cE galaxy. The other two cE candidates, for which radial velocities are not available, match some features typical of cE galaxies, such as being close in projection to a larger galaxy, displaying flat colour profiles, and having a high degree of compactness. Only one of the remaining cE candidates shows a high central surface brightness, two components in its brightness profile and distinct changes in ellipticity and position angle where the outer component begins to dominate. It seems to follow the same trend as other confirmed cE galaxies in a luminosity versus mean effective surface brightness diagram. Moreover, it shows a distorted inner structure with similar characteristics to those found by simulations of interacting galaxies. Also, an extremely faint structure, which seems to link this object with one of the Antlia dominant galaxies, has been detected in images from the Cerro Tololo Inter-American Observatory MOSAIC, the Very Large Telescope FORS 1 and the Hubble Space Telescope Advanced Camera for Surveys. The cE nature of this galaxy and the possible interaction with its bright companion still have to be confirmed using spectroscopy.

Ultracompact dwarfs around NGC 3258 in the Antlia cluster

Fil: Caso, Juan Pablo. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Astrofisica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronomicas y Geofisicas. Instituto de Astrofisica La Plata; Argentina

Galaxy populations in the Antlia cluster – III. Properties of faint early-type galaxies

We present a new analysis of the early-type galaxy population in the central region of the Antlia cluster, focusing on the faint systems like dwarf ell ipticals (dE) and dwarf spheroidals (dSph). The colour‐magnitude relation (CMR) and the relation between luminosity and mean effective surface brightness for galaxies in the central re gion of Antlia have been previously studied in Paper I of the present series. Now we confirm 22 earl y-type galaxies as Antlia members, using GEMINI‐GMOS and MAGELLAN‐MIKE spectra. Amo ng them, 15 are dEs from the FS90 Antlia Group catalogue, 2 belong to the rare type of compact ellipticals (cE), and 5 are new faint dwarfs that had never been catalogued before. In addition, we present 16 newly identified low surface brightness galaxy candidate s, almost half of them displaying morphologies consistent with being Antlia’s counterparts of Local Group dSphs, that extend the faint luminosity limit of our study down to MB = 10.1 (BT = 22.6) mag. With these new data, we built an improved CMR in the Washington photometric system, i.e. integrated T1 magnitudes versus (C T1) colours, which extends � 4 mag faintwards the limit of spectroscopically confirmed Antlia members. When only confirmed early-type members are considered, this relation extends over 10 mag in luminosity with no apparent change in slope or increase in colour dispersion towards its faint end. The int rinsic colour scatter of the relation is compared with those reported for other clusters of galaxies; we argue that it is likely that the large scatter of the CMR, usually reported at faint magnitudes, is mostly due to photometric errors along with an improper membership/morphological classification. The distinct behaviour of the luminosity versus mean effective surface brightness relation at the bright and faint ends is analyzed, while it is confirmed that dE galaxies on the same relation present a very similar effective radius, regardless of their colour. The projected spatial distribution of the member sample confirms the existence of two groups in Antl ia, each one dominated by a giant elliptical galaxy and with one cE located close to each giant. Size and position, with respect to massive galaxies, of the dSph candidates are estimated and compared to Local Group couterparts.

Globular clusters: DNA of early-type galaxies?

This paper explores if the mean properties of Early-Type Galaxies (ETG) can be reconstructed from “genetic” information stored in their GCs (i.e., in their chemical abundances, spatial distributions and ages). This approach implies that the formation of each globular occurs in very massive stellar environments, as suggested by some models that aim at explaining the presence of multi-populations in these systems. The assumption that the relative number of globular clusters to diffuse stellar mass depends exponentially on chemical abundance, [Z/H], and the presence of two dominant GC sub-populations (blue and red), allows the mapping of low metallicity halos and of higher metallicity (and more heterogeneous) bulges. In particular, the masses of the low-metallicity halos seem to scale up with dark matter mass through a constant. We also find a dependence of the globular cluster formation efficiencywith the mean projected stellar mass density of the galaxies within their effective radii. The analysis is based on a selected sub-sample of galaxies observed within the ACS Virgo Cluster Survey of the Hubble Space Telescope. These systems were grouped, according to their absolute magnitudes, in order to define composite fiducial galaxies and look for a quantitative connection with their (also composite) globular clusters systems. The results strengthen the idea that globular clusters are good quantitative tracers of both baryonic and dark matter in ETGs.

Multicolour–metallicity relations from globular clusters in NGC 4486 (M87)

We present Gemini griz photometry for 521 globular cluster (GC) candidates in a 5.5 x 5.5 arcmin field centered 3.8 arcmin to the south and 0.9 arcmin to the west of the center of the giant elliptical galaxy NGC 4486. All these objects have previously published (C-T1) photometry. We also present new (C-T1) photometry for 338 globulars, within 1.7 arcmin in galactocentric radius, which have (g-z) colors in the photometric system adopted by the Virgo Cluster Survey of the Advanced Camera for Surveys of the Hubble Space Telescope. These photometric data are used to define a self-consistent multicolor grid (avoiding polynomial fits) and preliminary calibrated in terms of two chemical abundance scales. The resulting multicolor color-chemical abundance relations are used to test GC chemical abundance distributions. This is accomplished by modelling the ten GC color histograms that can be defined in terms of the Cgriz bands. Our results suggest that the best fit to the GC observed color histograms is consistent with a genuinely bimodal chemical abundance distribution NGC(Z). On the other side, each (blue and red) GC subpopulation follows a distinct color-color relation.

Deep MOS Spectroscopy of NGC 1316 Globular Clusters

The giant elliptical galaxy NGC 1316 is the brightest galaxy in the Fornax cluster, and displays a number of morphological features that might be interpreted as an intermediate age merger remanent (∼3 Gyr). Based on the idea that globular clusters systems (GCS) constitute genuine tracers of the formation and evolution of their host galaxies, we conducted a spectroscopic study of approximately 40 globular clusters (GCs) candidates associated with this interesting galaxy. We determined ages, metallicities, and α -element abundances for each GC present in the sample, through the measurement of different Lick indices and their subsequent comparison with simple stellar populations models (SSPs).

Development of the Red Sequence in Galaxy Clusters

Fil: Jimenez, Noelia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico La Plata. Instituto de Astrofisica de La Plata; Argentina