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S. Kaviraj

Publications and source records attributed to S. Kaviraj.

70 records · Page 4Linked to original sources
arXiv2009

Galaxy Zoo: The properties of merging galaxies in the nearby Universe - local environments, colours, masses, star-formation rates and AGN activity

Following the study of Darg et al. (2009; hereafter D09a) we explore the environments, optical colours, stellar masses, star formation and AGN activity in a sample of 3003 pairs of merging galaxies drawn from the SDSS using visual classifications from the Galaxy Zoo project. While D09a found that the spiral-to-elliptical ratio in (major) mergers appeared higher than that of the global galaxy population, no significant differences are found between the environmental distributions of mergers and a randomly selected control sample. This makes the high occurrence of spirals in mergers unlikely to be an environmental effect and must, therefore, arise from differing time-scales of detectability for spirals and ellipticals. We find that merging galaxies have a wider spread in colour than the global galaxy population, with a significant blue tail resulting from intense star formation in spiral mergers. Galaxies classed as star-forming using their emission-line properties have average star-formation rates approximately doubled by the merger process though star formation is negligibly enhanced in merging elliptical galaxies. We conclude that the internal properties of galaxies significantly affect the time-scales over which merging systems can be detected (as suggested by recent theoretical studies) which leads to spirals being `over-observed' in mergers. We also suggest that the transition mass $3\times10^{10}{M}_{\astrosun}$, noted by \citet{kauffmann1}, below which ellipticals are rare could be linked to disc survival/destruction in mergers.

astro-ph.GA↗
arXiv2009

Galaxy Zoo: the fraction of merging galaxies in the SDSS and their morphologies

We present the largest, most homogeneous catalogue of merging galaxies in the nearby universe obtained through the Galaxy Zoo project - an interface on the world-wide web enabling large-scale morphological classification of galaxies through visual inspection of images from the Sloan Digital Sky Survey (SDSS). The method converts a set of visually-inspected classifications for each galaxy into a single parameter (the `weighted-merger-vote fraction,' $f_m$) which describes our confidence that the system is part of an ongoing merger. We describe how $f_m$ is used to create a catalogue of 3003 visually-selected pairs of merging galaxies from the SDSS in the redshift range $0.005 < z <0.1$. We use our merger sample and values of $f_m$ applied to the SDSS Main Galaxy Spectral sample (MGS) to estimate that the fraction of volume-limited ($M_r < -20.55$) major mergers ($1/3 < {M}^*_1/{M}^*_2 < 3$) in the nearby universe is $1 - 3 \times C%$ where $C \sim 1.5$ is a correction factor for spectroscopic incompleteness. Having visually classified the morphologies of the constituent galaxies in our mergers, we find that the spiral-to-elliptical ratio of galaxies in mergers is higher by a factor $\sim 2$ relative to the global population. In a companion paper, we examine the internal properties of these merging galaxies and conclude that this high spiral-to-elliptical ratio in mergers is due to a longer time-scale over which mergers with spirals are detectable compared to mergers with ellipticals.

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arXiv2009

AGN Jet-induced Feedback in Galaxies. II. Galaxy colours from a multicloud simulation

We study the feedback from an AGN on stellar formation within its host galaxy, mainly using one high resolution numerical simulation of the jet propagation within the interstellar medium of an early-type galaxy. In particular, we show that in a realistic simulation where the jet propagates into a two-phase ISM, star formation can initially be slightly enhanced and then, on timescales of few million years, rapidly quenched, as a consequence both of the high temperatures attained and of the reduction of cloud mass (mainly due to Kelvin-Helmholtz instabilities). We then introduce a model of (prevalently) {\em negative} AGN feedback, where an exponentially declining star formation is quenched, on a very short time scale, at a time t_AGN, due to AGN feedback. Using the Bruzual & Charlot (2003) population synthesis model and our star formation history, we predict galaxy colours from this model and match them to a sample of nearby early-type galaxies showing signs of recent episodes of star formation (Kaviraj et al. 2007). We find that the quantity t_gal - t_AGN, where t_gal is the galaxy age, is an excellent indicator of the presence of feedback processes, and peaks significantly around t_gal - t_AGN \approx 0.85 Gyr for our sample, consistent with feedback from recent energy injection by AGNs in relatively bright (M_{B} \lsim -19) and massive nearby early-type galaxies. Galaxies that have experienced this recent feedback show an enhancement of 3 magnitudes in NUV(GALEX)-g, with respect to the unperturbed, no-feedback evolution. Hence they can be easily identified in large combined near UV-optical surveys.

astro-ph.GA↗
arXiv2009

Evidence for recent star formation in BCGs: a correspondence between blue cores and UV excess

We present a joint analysis of near-ultraviolet (NUV) data from the GALEX mission and (optical) colour profiles for a sample of 7 Brightest Cluster Galaxies (BCGs) in the Canadian Cluster Comparison Project. We find that every BCG which has a blue rest-frame UV colour also shows a blue-core in its optical colour profile. Conversely, BCGs that lack blue cores and show monotonic colour gradients typical of old elliptical galaxies, are red in the UV. We interpret this as evidence that the NUV enhancement in the blue BCGs is driven by recent star formation and not from old evolved stellar populations such as horizontal branch stars. The recent star formation in the blue BCGs typically has an age less than 200 Myrs and contributes mass fractions of less than a percent. Although the sample studied here is small, we demonstrate, for the first time, a one-to-one correspondence between blue cores in elliptical galaxies (in particular BCGs) and a NUV-enhancement observed using GALEX. The combination of this one-to-one correspondence and the consistently young age of recent star formation, coupled with additional correlations with the host cluster's X-ray properties, strongly suggests that the star formation is fueled by gas cooling out of the intracluster medium. In turn, this implies that any AGN heating of the intracluster medium in massive clusters only acts to reduce the magnitude of the cooling flow and that once this flow starts, it is nearly always active. (abridged)

astro-ph↗
arXiv2008

GalICS II: the [alpha/Fe]-mass relation in elliptical galaxies

We aim at reproducing the mass- and sigma-[alpha/Fe] relations in the stellar populations of early-type galaxies by means of a cosmologically motivated assembly history for the spheroids. We implement a detailed treatment for the chemical evolution of H, He, O and Fe in GalICS, a semi-analytical model for galaxy formation which successfully reproduces basic low- and high-redshift galaxy properties. The contribution of supernovae (both type Ia and II) as well as low- and intermediate-mass stars to chemical feedback are taken into account. We find that this chemically improved GalICS does not produce the observed mass- and sigma-[alpha/Fe] relations. The slope is too shallow and scatter too large, in particular in the low and intermediate mass range. The model shows significant improvement at the highest masses and velocity dispersions, where the predicted [alpha/Fe] ratios are now marginally consistent with observed values. We show that this result comes from the implementation of AGN (plus halo) quenching of the star formation in massive haloes. A thorough exploration of the parameter space shows that the failure of reproducing the mass- and sigma-[alpha/Fe] relations can partly be attributed to the way in which star formation and feedback are currently modelled. The merger process is responsible for a part of the scatter. We suggest that the next generation of semi-analytical model should feature feedback (either stellar of from AGN) mechanisms linked to single galaxies and not only to the halo, especially in the low and intermediate mass range. The integral star formation history of a single galaxy determines its final stellar [alpha/Fe] as it might be expected from the results of closed box chemical evolution models. (abridged)

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arXiv2007

The UV properties of E+A galaxies: constraints on feedback-driven quenching of star formation

We present the first large-scale study of E+A (post-starburst) galaxies that incorporates photometry in the ultraviolet (UV) wavelengths. We find that the starburst that creates the E+A galaxy typically takes place within the last Gyr and creates a high fraction (20-60 percent) of the stellar mass in the remnant over a short timescale (< 0.1 Gyrs). We find a tight correlation between the luminosity of our E+A galaxies and the implied star formation rate (SFR) during the starburst. While low-luminosity E+As (M(z) > -20) exhibit implied SFRs of less than 50 solar masses per year, their luminous counterparts (M(z) < -22) shows SFRs greater than 300 and as high as 2000 solar masses per year, suggesting that luminous and ultra-luminous infrared galaxies in the low-redshift Universe could be the progenitors of massive nearby E+A galaxies. We perform a comprehensive study of the characteristics of the quenching that truncates the starburst in E+A systems.We find that, for galaxies less massive than 10^10 MSun, the quenching efficiency decreases as the galaxy mass increases. However, for galaxies more massive than 10^10 MSun, this trend is reversed and the quenching efficiency increases with galaxy mass. Noting that the mass threshold at which this reversal occurs is in excellent agreement with the mass above which AGN become significantly more abundant in nearby galaxies, we use simple energetic arguments to show that the bimodal behaviour of the quenching efficiency is consistent with AGN and supernovae (SN) being the principal sources of negative feedback above and below M ~ 10^10 MSun respectively. (abridged)

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arXiv2007

Better age estimations using UV-optical colours: breaking the age-metallicity degeneracy

We demonstrate that the combination of GALEX UV photometry in the FUV (~1530 angstroms) and NUV (~2310 angstroms) passbands with optical photometry in the standard U,B,V,R,I filters can efficiently break the age-metallicity degeneracy. We estimate well-constrained ages, metallicities and their associated errors for 42 GCs in M31, and show that the full set of FUV,NUV,U,B,V,R,I photometry produces age estimates that are ~90 percent more constrained and metallicity estimates that are ~60 percent more constrained than those produced by using optical filters alone. The quality of the age constraints is comparable or marginally better than those achieved using a large number of spectrscopic indices.

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arXiv2007

UV-optical colours as probes of early-type galaxy evolution

We have studied ~2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range 0 < z < 0.11. Combining GALEX UV photometry with corollary optical data from the SDSS, we find that, at a 95 percent confidence level, at least ~30 percent of galaxies in this sample have UV to optical colours consistent with some recent star formation within the last Gyr. In particular, galaxies with a NUV - r colour less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semi-analytical LCDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have ~1 to 3 percent of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ~300 to 500 Myrs. We also find that monolithically evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colours (NUV - r < 5.5) seen in a significant fraction (~30 percent) of our observed sample.

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arXiv2007

The star formation histories of early-type galaxies: insights from the rest-frame ultra-violet

Our current understanding of the star formation histories of early-type galaxies is reviewed, in the context of recent observational studies of their ultra-violet (UV) properties. Combination of UV and optical spectro-photometric data indicates that the bulk of the stellar mass in the early-type population forms at high redshift (z > 2), typically over short timescales (< 1 Gyr). Nevertheless, early-types of all luminosities form stars over the lifetime of the Universe, with most luminous (-23 < M(V) < -21) systems forming 10-15% of their stellar mass after z = 1 (with a scatter to higher value), while their less luminous (M(V) > -21) counterparts form 30-60% of their mass in the same redshift range. The large scatter in the (rest-frame) UV colours in the redshift range 0 < z < 0.7 indicates widespread low-level star formation in the early-type population over the last 8 billion years. The mass fraction of young (< 1 Gyr old) stars in luminous early-type galaxies varies between 1% and 6% at z~0 and is in the range 5-13% at z~0.7. The intensity of recent star formation and the bulk of the UV colour distribution is consistent with what might be expected from minor mergers (mass ratios < 1:6) in an LCDM cosmology.

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arXiv2007

The UV colours of high-redshift early-type galaxies: evidence for recent star formation and stellar mass assembly over the last 8 billion years

We combine deep UBVRIzJK photometry from the Multiwavelength Survey by Yale-Chile (MUSYC) with redshifts from the COMBO-17 survey to perform a large-scale study of the rest-frame ultraviolet (UV) properties of 674 high-redshift (0.5 -20.5) counterparts potentially forming 30-60 percent of their mass in the same redshift range. (abridged)

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arXiv2007

UV Bright Globular Clusters in M87: More Evidence for Super-helium-rich Stellar Populations?

We study the UV and optical properties of 38 massive GCs in M87, imaged using the HST/STIS and WFPC2 instruments. The majority of these GCs appear extremely bright in the FUV - roughly a magnitude brighter than their Galactic counterparts with similar metallicities. The observed FUV flux is several times larger than predictions of canonical old stellar population models. These canonical models, which assume a fiducial helium enrichment parameter, dY/dZ=2, are able to reproduce the observed FUV fluxes only if ages are 3--5 Gyr larger than the WMAP age of the Universe, although the same models fit the UV photometry of Galactic and M31 GCs for ages less than the WMAP age. A similar discrepancy is found between the mass-weighted and UV-luminosity weighted ages of the massive Galactic GC omega Cen, whose CMD (including peculiar features on its well-populated horizontal branch) can be accurately reproduced by invoking a small super-He-rich (dY/dZ ~ 90) stellar component. By comparison to omega Cen, we propose that the majority of M87 GCs in our sample contain strong signatures of similarly minor super-He-rich sub-components. Although we cannot prove or disprove the extreme helium scenario at the moment, we show that the same phenomenon that causes the extended horizontal branch of omega Cen explains the UV brightness of our sample. If this is indeed due to the extreme helium, this study would be the first to find its signatures in extragalactic objects.

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arXiv2006

The progenitor set of present-day early-type galaxies

We present a comprehensive theoretical study, within a fully realistic semi-analytical framework, of the photometric properties of early-type progenitors in the redshift range 0 0.7) spirals have ~75-95 percent chance of being a progenitor, while the corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is ~50-75 percent. Finally, we explore the correspondence between the true progenitor set of present-day early-types and the commonly used `red-sequence', defined as the set of galaxies within the part of the colour-magnitude space which is dominated by early-type objects. While large members (M_V<-22) of the `red sequence' trace the progenitor set accurately in terms of numbers and mass, the relationship breaks down severely at fainter luminosities (M_V>-21). Hence the red sequence is generally not a good proxy for the progenitor set of early-type galaxies.

astro-ph↗
arXiv2006

The effect of environment on the UV colour-magnitude relation of early-type galaxies

We use \textit{GALEX} (Galaxy Evolution Explorer) near-UV (NUV) photometry of a sample of early-type galaxies selected in \textit{SDSS} (Sloan Digital Sky Survey) to study the UV color-magnitude relation (CMR). $NUV-r$ color is an excellent tracer of even small amounts ($\sim 1$% mass fraction) of recent ($\la 1$ Gyr) star formation and so the $NUV-r$ CMR allows us to study the effect of environment on the recent star formation history. We analyze a volume-limited sample of 839 visually-inspected early-type galaxies in the redshift range $0.05 < z < 0.10$ brighter than $M_{r}$ of -21.5 with any possible emission-line or radio-selected AGN removed to avoid contamination. We find that contamination by AGN candidates and late-type interlopers highly bias any study of recent star formation in early-type galaxies and that, after removing those, our lower limit to the fraction of massive early-type galaxies showing signs of recent star formation is roughly $30 \pm 3%$ This suggests that residual star formation is common even amongst the present day early-type galaxy population. We find that the fraction of UV-bright early-type galaxies is 25% higher in low-density environments. However, the density effect is clear only in the lowest density bin. The blue galaxy fraction for the subsample of the brightest early-type galaxies however shows a very strong density dependence, in the sense that the blue galaxy fraction is lower in a higher density region.

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arXiv2006

On the globular cluster formation history of NGC 5128

We deduce the globular cluster formation history of the nearby elliptical NGC 5128 by using a phenomenological chemical enrichment model to reproduce its observed metallicity distribution function (MDF). We derive the observed MDF using recently obtained U and B photometry of the NGC 5128 GC system, with (U-B) used as the metallicity indicator. We test whether the observed MDF can be reproduced using a single starburst at high redshift followed by passive evolution (monolithic scenario). Our results strongly indicate that this scenario is incapable of reproducing the observed MDF of the NGC 5128 GC system. At least one more major starburst is required. A persistent feature of our best-fit models is a strong GC formation episode in the first 1-2 Gyrs, followed by a second formation episode typically after 10-12 Gyrs. Roughly one-third of the stellar mass in the GC system is created in this late starburst, producing clusters that are potentially less than 1 Gyr old. Although we do not have adequate resolution in the model or in the observed MDF to make a reliable claim about the possibility of additional GC formation episodes, our results suggest that the GCs in this galaxy did not form solely in a monolithic collapse and seem to support observational evidence which indicates that NGC 5128 is a merger remnant.

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arXiv2004

Globular Clusters as Probes of Galaxy Evolution: NGC 5128

We present the results of our analysis of the photometric data of globular clusters in the elliptical galaxy NGC 5128 (Cen A). We show that the integrated colour U-B can be an effective metallicity indicator for simple stellar populations. This is because it is sensitive to metallicity via the opacity effect but relatively insensitive to the effective main sequence turn-off temperature of the population (and thus to age) when Teff ~ 7000-12000K, that is, when the Balmer Jump is strong. This flat U-B vs Teff relation is a result of the fact that the blueing effect of the optical continuum with increasing temperature is temporarily stopped by the Balmer Jump which becomes greater with temperature in this range. In this study we use U-B and B-V as metallicity and age indicators, respectively. We first show that the use of the U-B vs B-V two-colour diagram roughly yields the metallicities and ages of the Milky Way globular clusters independently determined, and then apply the technique to the clusters in NGC 5128. There is a large range in U-B, which corresponds to [Fe/H] of -2.0 through over +0.3. The large uncertainties from the data and the models currently prevent us from pinning down their ages and metallicities. Although a constant age for all these clusters cannot be ruled out, there is a hint of the metal-rich clusters being younger. Significance of these results and caveats of the analysis are discussed.

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