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.
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.
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.
I use the spectroscopic data for galaxies in and around 28 nearby (0.02 ≤ z ≤ 0.06) X-ray bright galaxy clusters, to show that the incidence of k+A (or post-starburst) galaxies (EW(Hα) < 2 Å in ...emission and EW(Hδ) > 3 Å in absorption) may be correlated with the accretion of small galaxy groups in clusters. At r < 2r
200, the k+A galaxies are found in regions of higher galaxy density relative to other cluster galaxies. The k+A galaxies have a positively skewed distribution of absolute velocity, |v
los|/σ
v
, where v
los is the difference between the line-of-sight velocity of the galaxy and the cluster's mean, and σ
v
is the cluster's velocity dispersion. This distribution is statistically different from that of other cluster galaxies within 2r
200, and in the same absolute velocity range. Moreover, 87 per cent of clusters in the sample studied here show statistically significant substructure in their velocity distribution, and 91.4 per cent of all the k+A galaxies are found to be a part of one of these substructures with 4-10 members. These results suggest that star formation in these k+A galaxies is likely to have been quenched due to 'pre-processing' in a poor group-like environment before they are accreted into clusters. I also find a mild, but statistically significant trend in the fraction of k+A galaxies increasing with the temperature of the X-ray emitting gas in clusters.
Abstract
There are many proposed mechanisms driving the morphological transformation of disc galaxies to elliptical galaxies. In this paper, we determine if the observed transformation in low-mass ...groups can be explained by the merger histories of galaxies. We measured the group mass–morphology relation for groups from the Galaxy and Mass Assembly group catalogue with masses from 1011 to 1015 M⊙. Contrary to previous studies, the fraction of elliptical galaxies in our more complete group sample increases significantly with group mass across the full range of group mass. The elliptical fraction increases at a rate of 0.163 ± 0.012 per dex of group mass for groups more massive than 1012.5 M⊙. If we allow for uncertainties in the observed group masses, our results are consistent with a continuous increase in elliptical fraction from group masses as low as 1011 M⊙. We tested if this observed relation is consistent with the merger activity using a gadget-2 dark matter simulation of the galaxy groups. We specified that a simulated galaxy would be transformed to an elliptical morphology either if it experienced a major merger or if its cumulative mass gained from minor mergers exceeded 30 per cent of its final mass. We then calculated a group mass–morphology relation for the simulations. The position and slope of the simulated relation were consistent with the observational relation, with a gradient of 0.184 ± 0.010 per dex of group mass. These results demonstrate a strong correlation between the frequency of merger events and disc-to-elliptical galaxy transformation in galaxy group environments.
Based on data from the Galaxy and Mass Assembly (GAMA) survey, we report on the discovery of structures that we refer to as 'tendrils' of galaxies: coherent, thin chains of galaxies that are rooted ...in filaments and terminate in neighbouring filaments or voids. On average, tendrils contain six galaxies and span 10 h
−1 Mpc. We use the so-called line correlation function to prove that tendrils represent real structures rather than accidental alignments. We show that voids found in the Sloan Digital Sky Survey, 7th data release survey that overlap with GAMA regions contain a large number of galaxies, primarily belonging to tendrils. This implies that void sizes are strongly dependent on the number density and sensitivity limits of a survey. We caution that galaxies in low-density regions, which may be defined as 'void galaxies,' will have local galaxy number densities that depend on such observational limits and are likely higher than those can be directly measured.
Mental disorders in medical students
Indian journal of social psychiatry : official publication of Indian Association for Social Psychiatry,
04/2021
Journal Article
ABSTRACT
The Coma supercluster (100 h−1 Mpc) offers an unprecedented contiguous range of environments in the nearby Universe. In this paper, we present a catalogue of spectroscopically confirmed ...galaxies in the Coma supercluster detected in the ultraviolet (UV) wavebands. We use the arsenal of UV and optical data for galaxies in the Coma supercluster covering ∼500 deg2 on the sky to study their photometric and spectroscopic properties as a function of environment at various scales. We identify the different components of the cosmic-web: large-scale filaments and voids using Discrete Persistent Structures Extractor, and groups and clusters using Hierarchical Density-based spatial clustering of applications with noise, respectively. We find that in the Coma supercluster the median emission in H α inclines, while the g − r and FUV − NUV colours of galaxies become bluer moving further away from the spine of the filaments out to a radius of ∼1 Mpc. On the other hand, an opposite trend is observed as the distance between the galaxy and centre of the nearest cluster or group decreases. Our analysis supports the hypothesis that properties of galaxies are not just defined by its stellar mass and large-scale density, but also by the environmental processes resulting due to the intrafilament medium whose role in accelerating galaxy transformations needs to be investigated thoroughly using multiwavelength data.
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