ABSTRACT
We take a close look at the galaxies in the Coma Supercluster and assess the role of the environment (in the form of cluster, group, and supercluster filament) in their evolution, in ...particular, examining the role of groups. We characterize the groups according to intrinsic properties such as richness and halo mass, as well as their position in the supercluster and proximity to the two rich clusters, Abell 1656 (Coma) and Abell 1367. We devise a new way of characterizing the local environment using a kernel density estimator. We find that apart from the dominant effects of the galaxy mass, the effect of the environment on galaxies is a complex combination of the overdensities on various scales, which is characterized in terms of membership of groups, and also of the position of the galaxy on filaments and their proximity to the infall regions of clusters. Whether the gas can be turned into stars depends upon the level of pre-processing, which plays a role in how star formation is enhanced in a given environment. Our results are consistent with gas accreted in the cold mode from the filaments, being made available to enhance star formation. Finally, we show that the Abell 1367 end of the supercluster is in the process of assembly at present, leading to heightened star formation activity, in contrast with the Coma-end of the filament system.
Ongoing or recent star formation in galaxies is known to increase with increasing projected distance from the centre of a cluster out to several times its virial radius (R
v). Using a complete sample ...(M
r≤−20.5, 0.02 ≤z≤ 0.15) of galaxies in and around 268 clusters from the Sloan Digital Sky Survey's Fourth Data Release, we investigate how, at a given projected radius from the cluster centre, the stellar mass and star formation properties of a galaxy depend on its absolute line-of-sight velocity in the cluster rest frame, |v
LOS|. We find that for projected radii R < 0.5 R
v, the fraction of high-mass non-brightest cluster galaxies increases towards the centre for low |v
LOS|, which may be the consequence of the faster orbital decay of massive galaxies by dynamical friction. At a given projected radius, the fraction of Galaxies with Ongoing or Recent (<1-3 Gyr) Efficient Star Formation GORES; with EW(Hδ) > 2 Å & Dn
4000 > 1.5 is slightly but significantly lower for low |v
LOS| galaxies than for their high-velocity counterparts. We study these observational trends with the help of a dark matter (DM) cosmological simulation. We classify DM particles as virial, infall and backsplash according to their present positions in (r, v
r) radial phase space and measure the frequencies of each class in cells of (R, |v
LOS|) projected phase space. As expected, the virial class dominates at projected radii R < R
v, while the infall particles dominate outside, especially at high |v
LOS|. However, the backsplash particles account for at least one-third (half) of all particles at projected radii slightly greater than the virial radius and |v
LOS| < σv (|v
LOS| ≪σv). The deprojection of the GORES fraction leads to a saturated linear increase with radius. We fit simple models of the fraction of GORES as a function of class only or class and distance to the cluster centre (as in our deprojected fraction). While GORES account for 18 ± 1 per cent of all galaxies within the virial cylinder, in our best-fitting model, they account for 13 ± 1 per cent of galaxies within the virial sphere, 11 ± 1 per cent of the virial population, 34 ± 1 per cent of the distant (for projected radii R < 2 R
v) infall population and 19 ± 4 per cent of the backsplash galaxies. Also, 44 ± 2 per cent of the GORES within the virial cylinder are outside the virial sphere. These fractions are very robust to the precise good-fitting model and to our scheme for assigning simulation particle classes according to their positions in radial phase space (except for two of our models, where the fraction of GORES reaches 27 ± 4 per cent). Given the 1-3 Gyr lookback time of our GORES indicators, these results suggest that star formation in a galaxy is almost completely quenched in a single passage through the cluster.
Abstract
Giant radio galaxies (GRGs) are one of the largest astrophysical sources in the Universe with an overall projected linear size of ∼0.7 Mpc or more. The last six decades of radio astronomy ...research has led to the detection of thousands of radio galaxies. However, only ∼300 of them can be classified as GRGs. The reasons behind their large size and rarity are unknown. We carried out a systematic search for these radio giants and found a large sample of GRGs. In this paper, we report the discovery of 25 GRGs from the National Radio Astronomy Observatory Very Large Array Sky Survey, in the red-shift range z ∼ 0.07 to 0.67. Their physical sizes range from ∼0.8 Mpc to ∼4 Mpc. Eight of these GRGs have sizes ≥2 Mpc, which is a rarity. Here, for the first time, we investigate the mid-infrared (IR) properties of the optical hosts of the GRGs and classify them securely into various active galactic nuclei types using the WISE mid-IR colours. Using radio and IR data, four of the hosts of the GRGs were observed to be radio-loud quasars that extend up to 2 Mpc in radio size. These GRGs missed detection in earlier searches possibly because of their highly diffuse nature, low surface brightness and lack of optical data. The new GRGs are a significant addition to the existing sample. They will contribute to a better understanding of the physical properties of radio giants.
Integrated star formation rate (SFR) and specific star formation rate (SFR/M
*), derived from the spectroscopic data obtained by the Sloan Digital Sky Survey (SDSS) data release 4 (DR4), are used to ...show that the star formation activity in galaxies (M
r
≤ −20.5) found on the outskirts (1-2r
200) of some nearby clusters (0.02 ≤ z ≤ 0.15) is enhanced. By comparing the mean SFR of galaxies in a sample of clusters with at least one starburst galaxy (log SFR/M
* ≥ −10 yr−1
and SFR ≥ 10 M⊙yr−1) to a sample of clusters without such galaxies ('comparison' clusters), we find that despite the expected decline in the mean SFR of galaxies towards the cluster core, the SFR profile of the two samples is different. Compared to the clusters with at least one starburst galaxy on their outskirts, the galaxies in the 'comparison' clusters show a lower mean SFR at all radius (≤3r
200) from the cluster centre. Such an increase in the SFR of galaxies is more likely to be seen in dynamically unrelaxed (σv ≳ 500 km s−1) clusters. It is also evident that these unrelaxed clusters are currently being assembled via galaxies falling in through straight filaments, resulting in high velocity dispersions. On the other hand, 'comparison' clusters are more likely to be fed by relatively low density filaments. We find that the starburst galaxies on the periphery of clusters are in an environment of higher local density than other cluster galaxies at similar radial distances from the cluster centre. We conclude that a relatively high galaxy density in the infalling regions of clusters promotes interactions amongst galaxies, leading to momentary bursts of star formation. Such interactions play a crucial role in exhausting the fuel for star formation in a galaxy, before it is expelled due to the environmental processes that are operational in the dense interiors of the cluster.
Abstract
Superclusters are the largest massive structures in the cosmic web, on tens to hundreds of megaparsec scales. They are the largest assembly of galaxy clusters in the Universe. Apart from a ...few detailed studies of such structures, their evolutionary mechanism is still an open question. In order to address and answer the relevant questions, a statistically significant, large catalog of superclusters covering a wide range of redshifts and sky areas is essential. Here, we present a large catalog of 662 superclusters identified using a modified friends-of-friends algorithm applied on the WHL (Wen–Han–Liu) cluster catalog within a redshift range of 0.05 ≤
z
≤ 0.42. We name the most massive supercluster at
z
∼ 0.25 as the
Einasto Supercluster
. We find that the median mass of superclusters is ∼5.8 × 10
15
M
⊙
and the median size ∼65 Mpc. We find that the supercluster environment slightly affects the growth of clusters. We compare the properties of the observed superclusters with the mock superclusters extracted from the Horizon Run 4 cosmological simulation. The properties of the superclusters in the mocks and observations are in broad agreement. We find that the density contrast of a supercluster is correlated with its maximum extent with a power-law index,
α
∼ −2. The phase-space distribution of mock superclusters shows that, on average, ∼90% of part of a supercluster has a gravitational influence on its constituents. We also show the mock halos’ average number density and peculiar velocity profiles in and around the superclusters.
Understanding the interaction between galaxies and their surroundings is central to building a coherent picture of galaxy evolution. Here we use Galaxy Evolution Explorer imaging of a statistically ...representative sample of 23 galaxy groups at z approximately 0.06 to explore how local and global group environments affect the UV properties and dust-corrected star formation rates (SFRs) of their member galaxies. The data provide SFRs out to beyond 2R sub(200) in all groups, down to a completeness limit and limiting galaxy stellar mass of 0.06 M sub(middot in circle) yr super(-1) and 1 x 10 super(8) M sub(middot in circle), respectively. At fixed galaxy stellar mass, we find that the fraction of star-forming group members is suppressed relative to the field out to an average radius of R approximately 1.5 Mpc approximately 2R sub(200), mirroring results for massive clusters. For the first time, we also report a similar suppression of the specific SFR within such galaxies, on average by 40% relative to the field, thus directly revealing the impact of the group environment in quenching star formation within infalling galaxies. At fixed galaxy density and stellar mass, this suppression is stronger in more massive groups, implying that both local and global group environments play a role in quenching. The results favor an average quenching timescale of gap2 Gyr and strongly suggest that a combination of tidal interactions and starvation is responsible. Despite their past and ongoing quenching, galaxy groups with more than four members still account for at least ~25% of the total UV output in the nearby universe.
We explore the relation between colour (measured from photometry) and specific star formation rate (derived from optical spectra obtained by the Sloan Digital Sky Survey Data Release 4) of over 6000 ...galaxies (Mr≤−20.5) in and around (<3 r200) low-redshift (z < 0.12) Abell clusters. Even though, as expected, most red sequence galaxies have little or no ongoing star formation, and most blue galaxies are currently forming stars, there are significant populations of red star-forming and blue passive galaxies. This paper examines various properties of galaxies belonging to the latter two categories, to understand why they deviate from the norm. These properties include morphological parameters, internal extinction, spectral features such as EW(Hδ) and the 4000 Å break, and metallicity. Our analysis shows that the blue passive galaxies have properties very similar to their star-forming counterparts, except that their large range in Hδ equivalent width indicates recent truncation of star formation. The red star-forming galaxies fall into two broad categories, one of them being massive galaxies in cluster cores dominated by an old stellar population, but with evidence of current star formation in the core (possibly linked with active galactic nuclei). For the remaining red star-forming galaxies, it is evident from spectral indices, stellar and gas-phase metallicities and mean stellar ages that their colours result from the predominance of a metal-rich stellar population. Only half of the red star-forming galaxies have extinction values consistent with a significant presence of dust. The implication of the properties of these star-forming galaxies on environmental studies, like that of the Butcher–Oemler effect, is discussed.
We analyse Spitzer Multiband Imaging Photometer (MIPS) 24-μm observations, and Sloan Digital Sky Survey Data Release 7 optical broad-band photometry and spectra, to investigate the star formation ...(SF) properties of galaxies residing in the Coma supercluster region. We find that SF in dwarf galaxies is quenched only in the high-density environment at the centre of clusters and groups, but that passively evolving massive galaxies are found in all environments, indicating that massive galaxies can become passive via internal processes. The SF–density relation observed for the massive galaxies is weaker relative to the dwarfs, but both show a trend for the fraction of star-forming galaxies (fSF) declining to ∼0 in the cluster cores. We find that active galactic nucleus activity is also suppressed among massive galaxies residing in the cluster cores. We present evidence for a strong dependence of the mechanism(s) responsible for quenching SF in dwarf galaxies on the cluster potential, resulting in two distinct evolutionary pathways. First, we find a significant increase (at the 3σ level) in the mean equivalent width of Hα emission among star-forming dwarf galaxies in the infall regions of the Coma cluster and the core of Abell 1367 with respect to the overall supercluster population, indicative of the infalling dwarf galaxies undergoing a starburst phase. We identify these starburst galaxies as the precursors of the post-starburst k + A galaxies. Extending the survey of k + A galaxies over the whole supercluster region, we find 11.4 per cent of all dwarf (z mag >15) galaxies in the Coma cluster and 4.8 per cent in the Abell 1367 have post-starburst like spectra, while this fraction is just 2.1 per cent when averaged over the entire supercluster region (excluding the clusters). This points to a cluster-specific evolutionary process in which infalling dwarf galaxies undergo a starburst and subsequent rapid quenching due to their passage through the dense intracluster medium. In galaxy groups, the SF in infalling dwarf galaxies is instead slowly quenched due to the reduced efficiency of ram-pressure stripping. We show that in the central ∼2 h−170 Mpc of the Coma cluster, the (24 −z) near-infrared/mid-infrared (MIR) colour of galaxies is correlated with their optical (g−r) colour and Hα emission, separating all MIR-detected galaxies into two distinct classes of ‘red’ and ‘blue’. By analysing the spatial and velocity distribution of galaxies detected at 24 μm in Coma, we find that the (optically) red 24-μm detected galaxies follow the general distribution of ‘all’ the spectroscopic members, but their (optically) blue counterparts (i) are almost completely absent in the central ∼0.5 h−170 Mpc of Coma and (ii) have a remarkable peak in their velocity distribution, corresponding to the mean radial velocity of the galaxy group NGC 4839, suggesting that a significant fraction of the ‘blue’ MIR galaxies are currently on their first infall towards the cluster. The implications of adopting different SF-rate tracers for quantifying evolutionary trends like the Butcher–Oemler effect are also discussed.
ABSTRACT
This article presents the results of a spectroscopic analysis of the X-CLASS-redMaPPer (XC1-RM) galaxy cluster sample. X-CLASS is a serendipitous search for clusters in X-ray wavebands based ...on the XMM–Newton archive, whereas redMaPPer is an optical cluster catalogue derived from the Sloan Digital Sky Survey (SDSS). The present sample comprises 92 X-ray extended sources identified in optical images within 1 arcmin separation. The area covered by the cluster sample is ∼ 27 deg2. The clusters span a wide redshift range (0.05 < z < 0.6) and 88 clusters benefit from spectrosopically confirmed redshifts using data from SDSS Data Release 14. We present an automated pipeline to derive the X-ray properties of the clusters in three distinct apertures: R500 (at fixed mass overdensity), Rfit (at fixed signal-to-noise ratio) and ${R}_{300\, {\rm kpc}}$ (fixed physical radius). The sample extends over wide temperature and luminosity ranges: from 1–10 keV and from 6 × 1042 to 11 × 1044 erg s−1, respectively. We investigate the luminosity–temperature (L–T) relation of the XC1-RM sample and find a slope equal to 3.03 ± 0.26. It is steeper than predicted by self-similar assumptions, in agreement with independent studies. A simplified approach is developed to estimate the amount and impact of selection biases that might be affecting our recoveredL–Tparameters. The result of this simulation process suggests that the measuredL–Trelation is biased to a steeper slope and higher normalization.
Abstract
IRAS 09104+4109 is a rare example of a dust enshrouded type 2 quasi-stellar object (QSO) in the centre of a cool-core galaxy cluster. Previous observations of this z = 0.44 system showed ...that, as well as powering the hyperluminous infrared emission of the cluster-central galaxy, the QSO is associated with a double-lobed radio source. However, the steep radio spectral index and misalignment between the jets and ionized optical emission suggested that the orientation of the QSO had recently changed. We use a combination of new, multiband Giant Metrewave Radio Telescope observations and archival radio data to confirm that the jets are no longer powered by the QSO, and estimate their age to be 120-160 Myr. This is in agreement with the ∼70-200 Myr age previously estimated for star formation in the galaxy. Previously unpublished Very Long Baseline Array data reveal a 200 pc scale double radio source in the galaxy core which is more closely aligned with the current QSO axis and may represent a more recent period of jet activity. These results suggest that the realignment of the QSO, the cessation of jet activity and the onset of rapid star formation may have been caused by a gas-rich galaxy merger. X-ray observations reveal a spiral structure in the intracluster medium (ICM) which suggests that the cluster is in the process of relaxation after a tidal encounter or merger with another system; such a merger could provide a mechanism for transporting a gas-rich galaxy into the cluster core without stripping its cold gas. A Chandra X-ray observation confirms the presence of cavities associated with the radio jets, and we estimate the energy required to inflate them to be ∼7.7 ×1060 erg. The mechanical power of the jets is sufficient to balance radiative cooling in the cluster, provided that they are efficiently coupled to the ICM. We find no evidence of direct radiative heating and conclude that the QSO either lacks the radiative luminosity to heat the ICM, or that it requires longer than 100-200 Myr to significantly impact its environment.
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