M. Yagi

Luminosity Functions of 10 Nearby Clusters of Galaxies. I. Data

Wide field CCD imaging was carried out in the RC band for 10 nearby clusters of galaxies and four control fields to derive the total and type-specific luminosity functions. Observation and data reduction procedures are described. We extract galaxies down to RC ~ 20 mag and classify them into two broad types, r1/4-like and exponential-like, on the basis of the bulge-to-total luminosity ratio B/T estimated from Petrosian quantities. We describe our classification scheme in detail. We apply a single classification scheme to both giant and dwarf galaxies. The consistency of our classification is verified for giant galaxies using both simulated images and real data in the literature. We set the boundary of our two types at B/T = 0.35. This boundary gives 70% completeness to both the r1/4-like sample (for E/S0 galaxies) and the exponential-like sample (for Sa–Irr galaxies). Our classification for dwarf galaxies is investigated using higher resolution images of some 20 dwarf galaxies in the Virgo Cluster. Galaxy catalogs are constructed, which include position, magnitude, and B/T. The projected sky distribution of each type of galaxies is shown for the clusters and control fields.

A comparison of the Galaxy populations in the coma and distant clusters: The evolution of k + a galaxies and the role of the intracluster medium

The spectroscopic properties of galaxies in the Coma Cluster are compared with those of galaxies in rich clusters at z ~ 0.5, to investigate the evolution of the star formation history in clusters. Luminous galaxies with MV ≤ -20 and poststarburst/post-star-forming (k+a) spectra that constitute a significant fraction of galaxies in distant cluster samples are absent in Coma, where spectacular cases of k+a spectra are found instead at MV > -18.5 and represent a significant proportion of the cluster dwarf galaxy population. A simple inspection of their positions on the sky indicates that this type of galaxy does not show a preferential location within the cluster, but the bluest and strongest lined group of k+a galaxies lie in projection toward the central 1.4 Mpc of Coma and have radial velocities significantly higher than the cluster mean. We find a striking correlation between the positions of these young and strong poststarburst galaxies and substructure in the hot intracluster medium (ICM) identified from XMM-Newton data, with these galaxies lying close to the edges of two infalling substructures. This result strongly suggests that the interaction with the dense ICM could be responsible for the quenching of the star formation (thus creating the k+a spectrum) and, possibly, for any previous starburst. The evolution with redshift of the luminosity distribution of k+a galaxies can be explained by a downsizing effect, with the maximum luminosity/mass of actively star-forming galaxies infalling onto clusters decreasing at lower redshift. We discuss the possible physical origin of this downsizing effect and the implications of our results for current scenarios of environmental effects on the star formation in galaxies.

A Photometric and Spectroscopic Study of Dwarf and Giant Galaxies in the Coma Cluster. III. Spectral Ages and Metallicities

We present a detailed analysis of the spectroscopic catalog of galaxies in the Coma Cluster from Mobasher et al. (Paper II of the series). This catalog comprises ~300 spectra of cluster members with absolute magnitudes in the range MB = -20.5 to -14 in two areas of ~1 × 1.5 Mpc toward the center and the southwest region of the cluster. In order to study the star formation and metallicity properties of the Coma galaxies as a function of their luminosity and environment, spectral indices of the Lick/IDS system and equivalent widths of the emission lines were measured in the range λ = 3600-6600 A. In this paper, the analysis is restricted to the 257 galaxies with no emission lines in their spectra. The strength of the age-sensitive indices (such as Hβ, HγF, and HδF) is found to correlate with galaxy magnitude over the whole magnitude range explored in this study. Similarly, the metallicity-sensitive indices (such as Mg2, Fe, and C24668) anticorrelate with magnitude. By comparing the observed indices with model grids based on the Padova isochrones, we derive luminosity-weighted ages and metallicities. We present the distributions of ages and metallicities for galaxies in various magnitude bins. The mean metallicity decreases with galaxy magnitude and, at a given luminosity, appears to be generally lower for galaxies in the southwest region of Coma as compared to the center of the cluster. A broad range of ages, from younger than 3 Gyr to older than 9 Gyr, is found in galaxies of any magnitude. However, systematic trends of age with luminosity are present among galaxies in the central field, including a slight decrease of the mean age for fainter galaxies. Furthermore, in the central Mpc of Coma, a large fraction of galaxies at any luminosity (50%-60% of the giants, more than 30% of the dwarfs) show no evidence in their central regions of star formation occurred at redshift z < 2, while the proportion of galaxies with significant star formation occurring at intermediate (0.35 < z < 2) and low (z < 0.35) redshifts is found to depend on galaxy luminosity. An additional surprising result is that the faint galaxies with young luminosity-weighted ages appear to have a bimodal metallicity distribution that, if confirmed, would point to a composite formation scenario involving different physical processes. Coadding the spectra of these metal-rich and metal-poor galaxies separately supports the reality of the metallicity bimodality, although higher signal-to-noise ratio spectra of the individual galaxies will be needed to draw definite conclusions. An anticorrelation between age and metallicity is found to be present in galaxies of any given luminosity bin, and it is especially evident among the brightest subset with the highest signal-to-noise ratio spectra. Finally, we present an interpretation of the index-magnitude relations observed. We show that the slopes of the indices/magnitude relations are the consequence of both age and metallicity trends with luminosity: each such trend on its own would be sufficient to produce relations similar to those observed.

Ages of S0 and Elliptical Galaxies in the Coma Cluster

The ages of stellar populations in 52 elliptical and S0 galaxies in the Coma Cluster are investigated, using a new spectroscopic sample. More than 40% of the S0ˇs are found to have undergone star formation in their central regions during the last D5 Gyr, while such activity is absent in the ellipticals. Galaxies in this sample have absolute magnitudes in the range and the fraction of S0 [20.5 \ M B \ [17.5, galaxies with recent star formation is higher at fainter luminosities. The observed luminosity range of S0 galaxies with signs of recent star formation activity is consistent with them being the descendants of typical star-forming spirals at intermediate redshift whose star formation has been halted as a consequence of the dense environment.

Approximately A Thousand Ultra Diffuse Galaxies in the Coma cluster

We report the discovery of 854 ultra diffuse galaxies (UDGs) in the Coma cluster using deep R band images, with partial B, i, and Halpha band coverage, obtained with the Subaru telescope. Many of them (332) are Milky Way-sized with very large effective radii of r_e>1.5kpc. This study was motivated by the recent discovery of 47 UDGs by van-Dokkum et al. (2015); our discovery suggests >1,000 UDGs after accounting for the smaller Subaru field. The new UDGs show a distribution concentrated around the cluster center, strongly suggesting that the great majority are (likely longtime) cluster members. They are a passively evolving population, lying along the red sequence in the CM diagram with no Halpha signature. Star formation was, therefore, quenched in the past. They have exponential light profiles, effective radii re ~ 800 pc- 5 kpc, effective surface brightnesses mu_e(R)=25-28 mag arcsec-2, and stellar masses ~1x10^7 - 5x10^8Msun. There is also a population of nucleated UDGs. Some MW-sized UDGs appear closer to the cluster center than previously reported; their survival in the strong tidal field, despite their large sizes, possibly indicates a large dark matter fraction protecting the diffuse stellar component. The indicated baryon fraction ~<1% is less than the cosmic average, and thus the gas must have been removed from the possibly massive dark halo. The UDG population appears to be elevated in the Coma cluster compared to the field, indicating that the gas removal mechanism is related primarily to the cluster environment.

A Photometric and Spectroscopic Study of Dwarf and Giant Galaxies in the Coma Cluster. IV. The Luminosity Function

A large spectroscopic survey is constructed of galaxies in the Coma Cluster. The survey covers a wide area (1 deg2) to deep magnitudes (R ~ 19.5), covering both the core (high density) and outskirts (intermediate to low density) of the cluster. The spectroscopic sample consists of a total of 1191 galaxies, of which 760 are confirmed members of the Coma Cluster. A statistical technique is developed to correct the spectroscopic sample for incompleteness. The corrected sample is then used to construct an R-band luminosity function (LF) spanning a range of 7 mag (-23 1.35) galaxies, at both the core and outskirts of the cluster. This population of low-luminosity red galaxies has a higher surface density than the blue (B-R < 1.35) star-forming population and dominates the faint end of the Coma Cluster LF. It is found that the relative number of high surface brightness galaxies is larger at the cluster core, implying the destruction of low surface brightness galaxies in the dense core environment.

Narrowband Imaging in [O III] and H-alpha to Search for Intracluster Planetary Nebulae in the Virgo Cluster

We have identified intracluster planetary nebulae (PNs) in a Virgo Cluster core field by imaging with the Subaru Suprime-Cam through two narrowband filters centered at the redshifted wavelengths of the [O III] λ5007 and the Hα 6563 A lines; broadband images in V and R were acquired to check for emission in the adjacent continuum. Emission-line objects in Virgo are then selected from the two-color diagram [O III]-Hα versus [O III]-(V+R), which is calibrated using PNs in M84 (Jacoby et al.). Using both [O III] and Hα allows us to distinguish bona fide planetary nebulae from high-redshift emission-line galaxies at the bright end of the [O III] luminosity function. Spectroscopic observations of a subsample of these objects were made at the Telescopio Nazionale Galileo and at the Very Large Telescope in a region around M84 and in an intracluster field, respectively. The observations confirm the efficiency of the combined [O III]+Hα imaging to identify true PNs. We also obtained the first spectrum of an intracluster PN that shows the [O III] doublet with a signal-to-noise ratio greater than 10 and its Hα emission. From the results based on the spectroscopic follow-up, we derive a lower limit to the fraction of the Virgo Cluster light contributed by the intracluster stars at the surveyed position in the cluster core: it amounts to 10%.

Luminosity Functions of 10 Nearby Clusters of Galaxies. II. Analysis of the Luminosity Function

We investigate the total luminosity function (LF) and the type-specific luminosity functions (TSLFs)of r1/4-like galaxies and exponential-like galaxies in 10 nearby clusters. The composite total LF of the 10clusters is described well by the Schechter function with M = -21.3 and α = -1.31 in the range -23.5 < MR + 5 log h < -16. This value of α is consistent with the previous results for cluster galaxies. An extensive comparison of the values of α is made between clusters and fields. We find that the total LF of cluster galaxies has a steeper faint end slope than that of field galaxies. In contrast to the composite total LF, the individual total LFs of some clusters are not fitted well by the Schechter function. They show a hump around MR ~ -20 to -19 associated with a dip around MR ~ -19 to -18. The composite TSLFs of both r1/4-like galaxies and exponential-like galaxies are fitted well by the Schechter function. The best-fit parameters are (M = -21.2, α = -1.08) for the former and (M = -21.1, α = -1.49) for the latter. When seen on a cluster to cluster basis, however, the TSLFs of the two types of galaxies show different behaviors. The TSLF of r1/4-like galaxies shows a considerable diversity among the clusters, while the TSLF of exponential-like galaxies is similar in all the clusters. We demonstrate that the dip at MR ~ -19 to -18 seen in the total LFs of some clusters is almost entirely due to r1/4-like galaxies, rather than exponential-like galaxies. We find that the dip of the TSLF of r1/4-like galaxies is correlated with the velocity dispersion of the cluster in the sense that clusters with larger velocity dispersion show more distinct dips.

A Photometric and Spectroscopic Study of Dwarf and Giant Galaxies in the Coma Cluster. V. Dependence of the Spectroscopic Properties on Location in the Cluster

We investigate the radial dependence of the spectroscopic properties, in particular the Mg2, Fe, and Hβ spectroscopic indices, in a sample of galaxies spanning a wide range of absolute luminosity in the Coma Cluster. After allowing for the magnitude dependence of these indices, we find a significant gradient in Mg2, in the sense that galaxies in the core of the cluster have stronger Mg2. We find only weak gradients in Fe and Hβ. Using the model grids presented in an earlier paper in this series, we attribute the Mg2 gradient to changes in metal abundance. One possible mechanism to create this abundance gradient is pressure confinement by the intracluster medium of material from supernova-driven winds early in the history of the galaxies.

CATALOG OF ULTRA-DIFFUSE GALAXIES IN THE COMA CLUSTERS FROM SUBARU IMAGING DATA*

We present a catalog of ultra-diffuse galaxies (UDGs) in the Coma cluster. UDGs are a subset of low surface brightness (SB) galaxies with very large effective radii defined by van Dokkum et al. We surveyed the Subaru data archive for deep Suprime-Cam/Subaru R-band images, and used data covering the 1o.7 × 2o.7 region of the Coma cluster. The data are ~1 magnitude deeper than the data of van Dokkum et al (2015a) in limiting SB. This paper explains the details of our sample selection procedure. This UDG catalog includes positions, magnitudes, effective radii, mean and central SBs, and colors (when available). Comparisons with previous galaxy catalogs in the literature are performed, and we show that the current catalog is the largest for UDGs. We also discuss that most of the UDGs are members of the Coma cluster, and the major axis of the UDGs tends to align toward the cluster center (radial alignment).