arXiv subjects

Tomotsugu Goto

Publications and source records attributed to Tomotsugu Goto.

At least 199 records · Page 11Linked to original sources

arXiv2005

Velocity Dispersion of 335 Galaxy Clusters Selected from the Sloan Digital Sky Survey: Statistical Evidence for Dynamical Interaction, and Against Ram-Pressure Stripping

There has been plenty of observational evidence of cluster galaxy evolution. However, it has been difficult to identify the origin of the cluster galaxy evolution. Here we show that gravitational interaction/friction between galaxies is the statistically dominant physical mechanism responsible for the cluster galaxy evolution, and that the well-favored ram-pressure stripping by the cluster gas is not statistically driving the cluster galaxy evolution. We have constructed the largest composite cluster with 14548 member galaxies out of 335 clusters with ~>300 km/s carefully selected from the SDSS. By measuring velocity dispersions of various subsamples of galaxies in this composite cluster, we found that bright cluster galaxies (Mz<-23) have significantly smaller velocity dispersion than faint galaxies (Mz>-23). We interpret this as direct evidence of the dynamical interaction/friction between cluster galaxies, where massive galaxies lose their velocity through the energy equipartition during the dynamical interaction/friction with less massive galaxies. We also found that star-forming late-type galaxies have a larger velocity dispersion than passive late-type galaxies. This is inconsistent with the ram-pressure stripping model; since the ram-pressure is proportional to sigma v^2 (i.e., stronger for galaxies with high velocity), the ram-pressure stripping cannot explain the observed trends of passive (evolved) galaxies having low velocity rather than high velocity. On the other hand, the result is again consistent with the dynamical galaxy-galaxy interaction/friction, where more evolved (passive) galaxies lose their velocity through dynamical interaction/friction.

astro-ph↗

arXiv2004

The Luminosity Functions of the Galaxy Cluster MS1054-0321 at z=0.83 based on ACS Photometry

We present new measurements of the galaxy luminosity function (LF) and its dependence on local galaxy density, color, morphology, and clustocentric radius for the massive z=0.83 cluster MS1054-0321. Our analyses are based on imaging performed with the ACS onboard the HST in the F606W, F775W and F850LP passbands and extensive spectroscopic data obtained with the Keck LRIS. Our main results are based on a spectroscopically selected sample of 143 cluster members with morphological classifications derived from the ACS observations. Our three primary findings are (1) the faint-end slope of the LF is steepest in the bluest filter, (2) the LF in the inner part of the cluster (or highest density regions) has a flatter faint-end slope, and (3) the fraction of early-type galaxies is higher at the bright end of the LF, and gradually decreases toward fainter magnitudes. These characteristics are consistent with those in local galaxy clusters, indicating that, at least in massive clusters, the common characteristics of cluster LFs are established at z=0.83. We also find a 2sigma deficit of intrinsically faint, red galaxies (i-z>0.5, Mi>-19) in this cluster. This trend may suggest that faint, red galaxies (which are common in z<0.1 rich clusters) have not yet been created in this cluster at z=0.83. The giant-to-dwarf ratio in MS1054-0321 starts to increase inwards of the virial radius or when Sigma>30 Mpc^-2, coinciding with the environment where the galaxy star formation rate and the morphology-density relation start to appear. (abridged)

astro-ph↗

arXiv2004

Are E+A Galaxies Dusty-Starbursts?: VLA 20cm Radio Continuum Observation

E+A galaxies are characterized as a galaxy with strong Balmer absorption lines but without any [OII] or Halpha emission lines. Strong Balmer absorption lines indicate that E+A galaxies have experienced a starburst within the last <1-1.5 Gyr. However, the lack of emission lines indicates that E+A galaxies do not have any on-going star formation. Therefore, E+A galaxies are interpreted as a post-starburst galaxy. However, it has been a mystery why E+A galaxies underwent a starburst then stopped abruptly. One possible explanation to E+A galaxies is the dusty starburst scenario, where E+A galaxies have on-going star formation, but optical emission lines are invisible due to the heavy obscuration by dust. To test this dusty starburst scenario, we have observed 36 E+A galaxies carefully selected from the SDSS DR1 in 20cm radio continuum using the VLA. Since the radio emission is not affected by the dust extinction, the SFR in dusty galaxies can be revealed by the 20 cm continuum. All of our 36 target E+A galaxies are selected to have HδEW greater than 6A and no detection of [OII] or Halpha emission lines within 1 sigma. These selection criteria are much stronger than previous E+A selections in our attempt to select galaxies in the pure post-starburst phase without any remaining star formation. Except for the two galaxies with a nearby radio source, none of our 34 E+A galaxies are detected in 20 cm continuum to the limits reported in Table 1. The obtained upper limits on the radio estimated SFR suggest that E+A galaxies do not possess strong starburst (> 100 Msun yr^-1) hidden by dust extinction for the whole sample while 15 (z<0.08) E+As have lower SFR upper limits of ~15 Msun yr^-1. (Abridged)

astro-ph↗

arXiv2004

Do star formation rates of galaxy clusters depend on mass?: blue/late-type fractions, total star formation rates of 115 galaxy clusters as a function of cluster virial mass

There has been plenty of observational evidence of cluster galaxy evolution such as the Butcher-Oemler effect and the decrease in S0 fraction with increasing redshift. As a modern version, the redshift evolution of total star formation rate (SFR) in clusters has been actively debated recently. However, these studies of redshift trend have been always hampered by the possible mass dependence; due to the observational selection effects, clusters found at higher redshift inevitably tend to be more massive and luminous than local counterparts. Consequently, one has to correct for the mass trend before interpreting the redshift trend. As an attempt to break this degeneracy, we investigate the mass dependence of blue/late-type fractions and total SFR of 115 clusters at $z\leq 0.09$ selected from the Sloan Digital Sky Survey. We find that none of blue/late-type fractions, total SFR and total SFR normalized by cluster mass shows significant dependence on cluster virial mass. The scatter is much larger at each cluster mass than a possible trend. Our results indicates that physical mechanisms that depend on cluster mass (such as the ram-pressure stripping) are not likely to be solely responsible for cluster galaxy evolution. Our results also provide an excellent low redshift comparison sample for future high redshift cluster SFR studies.

astro-ph↗

arXiv2004

The Origin of E+A Galaxies

E+A galaxies are galaxies with strong Balmer absorption lines and with no significant emission in [OII] or H$α$ lines. The existence of strong Balmer absorption lines shows that these galaxies have experienced a starburst within the last 1 Gyr. At the same time, these galaxies do not show any sign of on-going star formation as indicated by the lack of [OII] or H$α$ emission lines. Therefore, E+A galaxies are interpreted as a post-starburst galaxy -- a galaxy which truncated starburst suddenly. However, for more than 20 years, the origin of this post-starburst phenomena remained a mystery. Using the 100,000 galaxy spectra of the Sloan Digital Sky Survey Data Release 1, we tested three popular explanations to the E+A phenomenon.

astro-ph↗

arXiv2004

The Environmental Dependence of Galaxy Properties in the Local Universe: Dependence on Luminosity, Local Density, and System Richness

We investigate the environmental dependence of star formation and the morphology of galaxies in the local universe based on a volume-limited sample (0.030<z<0.065 and M_r<M_r^*+2) constructed from the data of the Sloan Digital Sky Survey. First, we focus on how galaxy properties change with local density. The star formation and morphology show a 'break' at the critical local density of log Sigma_crit~0.4 galaxies h_75^2 Mpc^-2, which is in agreement with previous studies. However, the break can be seen only for faint galaxies (M^*_r+1 < M_r < M^*_r+2), and bright galaxies (M_r < M^*_r+1) show no break. Thus, galaxies of different luminosities are found to show different environmental dependencies. Next, we examine dependencies on richness of galaxy systems. Star formation activities of galaxies are different from those of field galaxies even in systems as poor as sigma~100 km/s. This result suggests that environmental mechanisms that are effective only in rich systems, such as ram-pressure stripping of cold gas and harassment, have not played a major role in transforming galaxies into red early-type galaxies. Strangulation and interactions between galaxies, however, remain candidates of the driver of the environmental dependence. In the dense environments in the local universe, the slow transformation of faint galaxies occurs to some extent, but the transformation of bright galaxies is not clearly visible. We suggest that the evolution of bright galaxies is not strongly related to galaxy system, such as groups and clusters, while the evolution of faint galaxies is likely to be closely connected to galaxy system.

astro-ph↗

arXiv2004

Are Passive Spiral Galaxies Truly "Passive" and "Spiral"? : Near-Infrared Perspective

Passive spiral galaxies -- unusual galaxies with spiral morphology without any sign of on-going star formation -- have recently been discovered to exist preferentially in cluster infalling regions (at about the virial radius, or at a local galaxy density of $\sim 1$ Mpc$^{-2}$). The discovery directly connects the passive spiral galaxies to the cluster galaxy evolution studies such as the Butcher-Oemler effect or the morphology-density relation, i.e., passive spiral galaxies are likely to be transition objects between high-z blue, spiral galaxies and low-z red, cluster early-type galaxies. Thus, detailed study of passive spiral galaxies potentially could bring a new insight on the underlying physical mechanisms governing cluster galaxy evolution. However, in previous work, passive spiral galaxies are selected from the low resolution optical images with $\sim 1.5$ arcsec of seeing. Therefore, passive spirals could be a mis-identification of S0 galaxies; or dusty-starburst galaxies which are not passive at all. To answer these questions, we performed a deep, high-resolution, near-infrared imaging of 32 passive spiral galaxies. Our high resolution $K$ band images show clear spiral arm structures. Thus, passive spirals are not S0s. Optical-infrared colour does not show any signs of dusty-starburst at all. Therefore, it is likely that they are truly ``passive'' and ``spiral'' galaxies in the midst of cluster galaxy evolution.

astro-ph↗

arXiv2004

The Evolution of Galaxies in and around Clusters at High-Redshift

In this paper, we focus on ram-pressure stripping and evaporation of disk galaxies in and around a cluster. We show that the evolution of the disk surface density affects the efficiency of ram-pressure stripping of galaxies at z>~1. We also consider the saturation of thermal conduction in detail and show that it cannot be ignored at larger radii of a cluster, which makes the time-scale of the evaporation larger. Both the ram-pressure stripping and evaporation could affect the evolution of galaxies even around a cluster. In particular, the observed gradual decline of the star-formation rates of galaxies in and around clusters could be explained by evaporation without resorting to speculative strangulation (stripping of warm gas in galactic halos).

astro-ph↗

arXiv2003

The Environmental Effects on Galaxy Evolution Based on the SDSS Data

By constructing a large, uniform galaxy cluster catalog from the SDSS data, we have found that cluster galaxies evolve both spectrally and morphologically. By studying the morphology--cluster-centric-radius relation, we have found two characteristic environments where galaxy morphologies change dramatically. We found passive spiral galaxies in the infalling region of clusters. They are likely to be a galaxy population in transition due to the cluster environment. We also studied E+A galaxies, which have also been thought to be cluster originated. We found that E+As live in all environments including the field region.

astro-ph↗

arXiv2003

The Morphology-Density Relation in the Sloan Digital Sky Survey

We have studied the morphology-density relation and morphology--cluster-centric-radius relation using a volume limited sample (0.05$<z<$0.1, $Mr^*<-$20.5) of the Sloan Digital Sky Survey (SDSS) data. We found there are two characteristic changes in both the morphology-density and the morphology-radius relations, suggesting two different mechanisms are responsible for the relations. In the sparsest regions (below 1 Mpc$^{-2}$ or outside of 1 virial radius), both relations become less noticeable, suggesting the responsible physical mechanisms for galaxy morphological change require denser environment. In the intermediate density regions (density between 1 and 6 Mpc$^{-2}$ or virial radius between 0.3 and 1), intermediate-type fractions increase toward denser regions, whereas late-disc fractions decrease. In the densest regions (above 6 Mpc$^{-2}$ or inside of 0.3 virial radius), intermediate-type fractions decrease radically and early-type fractions increase in turn. This is a contrasting result to that in intermediate regions and it suggests that yet another mechanism is more responsible for the morphological change in these regions. We also compared the morphology-density relation from the SDSS (0.01$<z<$0.054) with that of the MORPHS data ($z\sim$0.5). Two relations lie on top of each other, suggesting that the morphology-density relation was already established at $z\sim$0.5 as is in the present universe.

astro-ph↗

arXiv2003

Evolution of the Colour--Radius Relation and the Morphology--Radius Relation in the SDSS Galaxy Clusters

We have investigated the redshift evolution of the colour--cluster-centric-radius relation and the morphology--cluster-centric-radius relation in three redshift bins, $0.02\leq z\leq 0.14$, $0.14< z\leq 0.20$, and $0.20< z\leq 0.30$ using a homogeneous sample of 736 galaxy clusters selected from the Sloan Digital Sky Survey. Both the relations are well-defined in all the redshift bins; the fraction of blue/late-type galaxies increases toward outside of clusters.Blue/late-type galaxy fractions are found to decrease with decreasing redshift at any cluster-centric radius. The trend is consistent with the Butcher-Oemler effect and the morphological Butcher-Oemlereffect. In addition, we find that colour (spectral) evolution is almost completed by $z\sim0.2$, whereas morphological evolution continues to the present day. The colour-radius relation is smooth inall the redshift bins, while the morphology--radius relation has a break only in the highest redshift bin. It is also found thatfractions of blue-late type galaxies decreases mostly between the highest and the intermediate redshift bins, while fractions of red-late type galaxies continuously decreases with decreasing redshift through all the redshift bins. These results are consistent with the interpretation that the timescale of the colour (spectral) evolution is shorter than that of the morphological evolution. It is suggested that red-late type galaxies in the middle of the transformation are observed as passive spiral galaxies.

astro-ph↗

arXiv2003

Environmental Effects on Galaxy Evolution

It is a remarkable feature that various properties of galaxies vary according to their environments. However, only little has been known on the origin of this variety in galaxies. Using the Sloan Digital Sky Survey (SDSS) data, we investigate the morphology-density relation to find two characteristic environment where the morphology-density relation abruptly changes (0.3 and 2 Rvir).The result has significant implication for the underlying physical mechanism since it suggests that two different mechanisms might be responsible for the morphology-density relation. In addition, we study the environment of unusual spiral galaxies with no [OII] or Halpha emission lines (passive spirals), and find that passive spiral galaxies preferentially live in cluster infalling regions (1-10 Rvir}). Thus, the origin of passive spiral galaxies is likely to be cluster related. Based on all of our observational results, we propose possible fate of cluster infalling galaxies.

astro-ph↗

arXiv2003

The Environment of Passive Spiral Galaxies in the SDSS

In previous work on galaxy clusters, several authors reported a discovery of an unusual population of galaxies, which have spiral morphologies, but do not show any star formation activity. These galaxies are called ``passive spirals'', and have been interesting since it has been difficult to understand the existence of such galaxies. Using a volume limited sample (0.05<z<0.1 and Mr<-20.5; 25813 galaxies) of the Sloan Digital Sky Survey data, we have found 73 (0.28$\pm$0.03%) passive spiral galaxies and studied their environments. It is found that passive spiral galaxies live in local galaxy density 1-2 Mpc$^{-2}$ and 1-10 cluster-centric virial radius. Thus the origins of passive spiral galaxies are likely to be cluster related. These characteristic environments coincide with the previously reported environment where galaxy star formation rate suddenly declines and the so-called morphology-density relation turns. It is likely that the same physical mechanism is responsible for all of these observational results. The existence of passive spiral galaxies suggests that a physical mechanism that works calmly is preferred to dynamical origins such as major merger/interaction since such a mechanism can destroy spiral arm structures. Compared with observed cluster galaxy evolution such as the Butcher-Oemler effect and the morphological Butcher-Oemler effect, passive spiral galaxies are likely to be a galaxy population in transition between red, elliptical/S0 galaxies in low redshift clusters and blue, spiral galaxies more numerous in higher redshift clusters.

astro-ph↗

arXiv2003

Selection and photometric properties of K+A galaxies

Two different simple measurements of galaxy star formation rate with different timescales are compared empirically on $156,395$ fiber spectra of galaxies with $r<17.77$ mag taken from the Sloan Digital Sky Survey in the redshift range $0.05<z<0.20$: a ratio $\Aamp / \Kamp$ found by fitting a linear sum of an average old stellar poplulation spectrum (\Kamp) and average A-star spectrum (\Aamp) to the galaxy spectrum, and the equivalent width (EW) of the $\Halpha$ emission line. The two measures are strongly correlated, but there is a small clearly separated population of outliers from the median correlation that display excess $\Aamp /\Kamp$ relative to \Halpha EW. These ``K+A'' (or ``E+A'') galaxies must have dramatically decreased their star-formation rates over the last $\sim 1$ Gyr. The K+A luminosity distribution is very similar to that of the total galaxy population. The K+A population appears to be bulge-dominated, but bluer and higher surface-brightness than normal bulge-dominated galaxies; it appears that K+A galaxies will fade with time into normal bulge-dominated galaxies. The inferred rate density for K+A galaxy formation is $\sim 10^{-4} h^3 Mpc^{-3} Gyr^{-1}$ at redshift $z\sim 0.1$. These events are taking place in the field; K+A galaxies don't primarily lie in the high-density environments or clusters typical of bulge-dominated populations.

astro-ph↗

arXiv2003

Composite Luminosity Functions Based on the Sloan Digital Sky Survey "Cut & Enhance" Galaxy Cluster Catalog

We present here results on the composite luminosity functions of galaxies in the clusters of galaxies selected from the SDSS Cut and Enhance cluster catalog (CE; Goto et al. 2001). We construct the composite luminosity function in the five SDSS bands, u,g,r,i and z, using 204 CE clusters ranging from z=0.02 to z=0.25. We use photometric redshifts to construct composite luminosity functions. The robustness of the weighting scheme was tested using Monte Carlo simulation. We find the slope of composite LFs become flatter toward redder color band. Comparing with field LFs of SDSS (Blanton et al. 2001), cluster LFs have brighter characteristic magnitude and flatter slopes in g, r, i and z bands. These results are consistent with the hypothesis that the cluster LF has two distinct underlying populations i.e. the bright end of the LF is dominated by bright early types that follow a gaussian--like luminosity distribution, while the faint--end of the cluster LF is a steep power--law like function dominated by star--forming (bluer) galaxies. We also study the composite luminosity functions for early--type and late--type galaxies using profile fits, a concentration parameter and u-r color to classify galaxy morphology. The strong dependence of LF on galaxy morphology is found.The faint end slope of the LF is always flatter for early--type galaxies than late--type regardless of passband and methodology. These results are consistent with the hypothesis that the cluster region are dominated by bright elliptical galaxies.

astro-ph↗

arXiv2003

The Morphological Butcher-Oemler effect in the SDSS Cut&Enhance Galaxy Cluster Catalog

We investigate the evolution of the fractions of late type cluster galaxies as a function of redshift, using one of the largest, most uniform cluster samples available. The sample consists of 514 clusters of galaxies in the range 0.02<z<0.3 from the Sloan Digital Sky Survey Cut & Enhance galaxy cluster catalog. This catalog was created using a single automated cluster finding algorithm on uniform data from a single telescope, with accurate CCD photometry, thus, minimizing selection biases. We use four independent methods to analyze the evolution of the late type galaxy fraction. Specifically, we select late type galaxies based on: restframe g-r color, u-r color, galaxy profile fitting and concentration index. The first criterion corresponds to the one used in the classical Butcher-Oemler analyses. The last three criteria are more sensitive to the morphological type of the galaxies. In all four cases, we find an increase in the fraction of late type galaxies with increasing redshift, significant at the 99.9% level. The results confirm that cluster galaxies do change colors with redshift (the Butcher-Oemler effect) and, in addition, they change their morphology to later-type toward higher redshift -- indicating a morphological equivalent of the Butcher-Oemler effect. We also find a tendency of richer clusters to have lower fractions of late type galaxies. The trend is consistent with a ram pressure stripping model, where richer clusters have more effective ram pressure due to their higher temperature.

astro-ph↗

arXiv2003

A Merged Catalog of Clusters of Galaxies from Early SDSS Data

We present a catalog of 799 clusters of galaxies in the redshift range z_est = 0.05 - 0.3 selected from ~400 deg^2 of early SDSS commissioning data along the celestial equator. The catalog is based on merging two independent selection methods -- a color-magnitude red-sequence maxBCG technique (B), and a Hybrid Matched-Filter method (H). The BH catalog includes clusters with richness Λ>= 40 (Matched-Filter) and N_gal >= 13 (maxBCG), corresponding to typical velocity dispersion of σ_v >~ 400 km s^{-1} and mass (within 0.6 h^{-1) Mpc radius) >~ 5*10^{13} h^{-1} M_sun. This threshold is below Abell richness class 0 clusters. The average space density of these clusters is 2*10^{-5} h^3 Mpc^{-3}. All NORAS X-ray clusters and 53 of the 58 Abell clusters in the survey region are detected in the catalog; the 5 additional Abell clusters are detected below the BH catalog cuts. The cluster richness function is determined and found to exhibit a steeply decreasing cluster abundance with increasing richness. We derive observational scaling relations between cluster richness and observed cluster luminosity and cluster velocity dispersion; these scaling relations provide important physical calibrations for the clusters. The catalog can be used for studies of individual clusters, for comparisons with other sources such as X-ray clusters and AGNs, and, with proper correction for the relevant selection functions, also for statistical analyses of clusters.

astro-ph↗

arXiv2003

The Interplay of Cluster and Galaxy Evolution

We review here the interplay of cluster and galaxy evolution. As a case study, we consider the Butcher-Oemler effect and propose that it is the result of the changing rate of cluster merger events in a hierarchical universe. This case study highlights the need for new catalogs of clusters and groups that possess quantified morphologies. We present such a sample here, namely the Sloan Digital Sky Survey (SDSS) C4 Catalog, which has been objectively-selected from the SDSS spectroscopic galaxy sample. We outline here the C4 algorithm and present first results based on the SDSS Early Data Release, including an X-ray luminosity-velocity dispersion (L_x-sigma) scaling relationship (as a function of cluster morphology), and the density-SFR relation of galaxies within C4 clusters (Gomez et al. 2003). We also discuss the merger of Coma and the NGC4839 group, and its effect on the galaxy populations in these systems. We finish with a brief discussion of a new sample of Hdelta-selected galaxies (i.e., k+a, post--starburst galaxies) obtained from the SDSS spectroscopic survey.

astro-ph↗