arXiv subjects

Smriti Mahajan

Publications and source records attributed to Smriti Mahajan.

At least 19 recordsLinked to original sources

arXiv2023

An AstroSat/UVIT study of galaxies in the cluster Abell 2199

(abridged) We present the newly acquired data for an AstroSat/UVIT field centered on a face-on spiral starburst galaxy UGC 10420, located in the cluster Abell 2199. We have analysed the FUV data for this field along with the archival data from the Galex mission, optical photometric data from the SDSS, and low-frequency radio data from the LoTSS survey, respectively. The stars were separated from the galaxies using the SDSS pipeline classification, while the spectroscopic redshifts available for 35% of the detected UVIT sources were used to identify member galaxies of the cluster Abell 2199. We find that (a) the non-cluster galaxies are on average fainter than the cluster galaxies at fixed magnitude, (b) stars and galaxies are indistinguishable in the r vs NUV-r plane, and (c) bright stars are ~1.5 mag bluer than the galaxies in the FUV-r vs NUV-r colour-colour plane. Besides UGC 10420 which is the only known cluster galaxy with an extended-UV disk, we identify five more galaxies with asymmetric FUV morphology and extended radio emission in this field. All the asymmetric member galaxies of Abell 2199, lie within the virial boundaries of the cluster. This observation, together with the fact that these asymmetric cluster galaxies have low-frequency radio tails or FUV emission pointing away from the cluster centre leads us to hypothesise that these galaxies are likely undergoing ram-pressure stripping (RPS) under the influence of cluster-environment related mechanisms. A comparison of optical and FUV star formation rate of UVIT detected galaxies shows enhanced star formation in half of the RPS candidates, suggesting that environment-related mechanisms may lead to a burst of star formation in RPS galaxies. Our analysis indicates the presence of at least two more groups or clusters at z~0.077 and 0.260, coincident with Abell 2199 along the line of sight of the field of view studied here.

astro-ph.GA↗

arXiv2023

Supermassive black holes in a mass-limited galaxy sample

The observed scaling relations between supermassive black hole masses and their host galaxy properties indicate that supermassive black holes influence the evolution of galaxies. However, the scaling relations may be affected by selection biases. We propose to measure black hole masses in a mass-limited galaxy sample including all non-detections to inprove constraints on galaxy mass - black hole mass scaling relations and test for selection bias. We use high spatial resolution spectroscopy from the Keck and Gemini telescopes, and the Jeans Anisotropic Modelling method to measure black hole masses in early type galaxies from the Virgo Cluster. We present four new black hole masses and one upper limit in our mass-selected sample of galaxies of galaxy mass (1.0-3.2) x $10^{10} M_\odot$. This brings the total measured to 11 galaxies out of a full sample of 18 galaxies, allowing us to constrain scaling relations. We calculate a lower limit for the average black hole mass in our sample of 3.7 x $10^{7} M_\odot$. This is at an average galaxy stellar mass of (1.81 +/- 0.14) x $10^{10} M_\odot $ and an average bulge mass of (1.31 +/- 0.15) x $10^{10} M_\odot$. This lower limit shows that black hole masses in early type galaxies are not strongly affected by selection biases.

astro-ph.GA↗

arXiv2023

Dashing through the cluster: An X-ray to radio view of UGC 10420 undergoing ram-pressure stripping

We present multi-wavelength data and analysis, including new FUV AstroSat/UVIT observations of the spiral galaxy UGC 10420 (z=0.032), a member of the cluster Abell 2199. UGC 10420 is present on the edge of the X-ray emitting region of the cluster at a distance of ~ 680 kpc from the centre. The FUV data shows intense knots of star formation on the leading edge of the galaxy, accompanied by a tail of the same on the diametrically opposite side. Our analysis shows that the images of the galaxy disk in the optical and mid-infrared are much smaller in size than that in the FUV. While the broadband optical colours of UGC 10420 are typical of a post-starburst galaxy, the SFR derived from a UV-to-IR spectral energy distribution is at least a factor of nine higher than that expected for a star-forming field galaxy of similar mass at its redshift. A careful removal of the contribution of the diffuse intracluster gas shows that the significant diffuse X-ray emission associated with the inter-stellar medium of UGC 10420 has a temperature, T_X = 0.24^{+0.09}_{-0.06} keV (0.4-2.0 keV) and luminosity, L_X = 1.8+/-0.9 x 10^{40} erg/s, which are typical of the X-ray emission from late-type spiral galaxies. Our analysis favours a scenario where the interaction of a galaxy with the hot intra-cluster medium of the cluster, perturbs the gas in the galaxy causing starburst in the leading edge of the disk. On the other hand, the turbulence thus developed may also push some of the gas out of the disk. Interactions between the gas ejected from the galaxy and the intracluster medium can then locally trigger star formation in the wake of the galaxy experiencing ram-pressure stripping. Our data however does not rule out the possibility of a flyby encounter with a neighbouring galaxy, although no relevant candidates are observed in the vicinity of UGC 10420. (abridged)

astro-ph.GA↗

arXiv2022

Deepest far ultraviolet view of a central field in the Coma cluster by AstroSat UVIT

We present analysis of the far ultraviolet (FUV) emission of sources in the central region of the Coma cluster (z=0.023) using the data taken by the UVIT aboard the multi-wavelength satellite mission AstroSat. We find a good correlation between the UVIT FUV flux and the fluxes in both wavebands of the Galex mission, for the common sources. We detect stars and galaxies, amongst which the brightest (r <= 17 mag) galaxies in the field of view are mostly members of the Coma cluster. We also detect three quasars (z = 0.38, 0.51, 2.31), one of which is likely the farthest object observed by the UVIT so far. In almost all the optical and UV colour-colour and colour-magnitude planes explored in this work, the Coma galaxies, other galaxies and bright stars could be separately identified, but the fainter stars and quasars often coincide with the faint galaxies. We have also investigated galaxies with unusual FUV morphology which are likely to be galaxies experiencing ram-pressure stripping in the cluster. Amongst others, two confirmed cluster members which were not investigated in the literature earlier, have been found to show unusual FUV emission. All the distorted sources are likely to have fallen into the cluster recently, and hence have not virialised yet. A subset of our data have optical spectroscopic information available from the archives. For these sources (~ 10% of the sample), we find that 17 galaxies identify as star-forming, 18 as composite and 13 as host galaxies for active galactic nuclei, respectively on the emission-line diagnostic diagram.

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arXiv2022

Galaxy And Mass Assembly (GAMA): Data Release 4 and the z < 0.1 total and z < 0.08 morphological galaxy stellar mass functions

In Galaxy And Mass Assembly Data Release 4 (GAMA DR4), we make available our full spectroscopic redshift sample. This includes 248682 galaxy spectra, and, in combination with earlier surveys, results in 330542 redshifts across five sky regions covering ~250deg^2. The redshift density, is the highest available over such a sustained area, has exceptionally high completeness (95 per cent to r_KIDS=19.65mag), and is well suited for the study of galaxy mergers, galaxy groups, and the low redshift (z<0.25) galaxy population. DR4 includes 32 value-added tables or Data Management Units (DMUs) that provide a number of measured and derived data products including GALEX, ESO KiDS, ESO VIKING, WISE and Herschel Space Observatory imaging. Within this release, we provide visual morphologies for 15330 galaxies to z<0.08, photometric redshift estimates for all 18million objects to r_KIDS~25mag, and stellar velocity dispersions for 111830 galaxies. We conclude by deriving the total galaxy stellar mass function (GSMF) and its sub-division by morphological class (elliptical, compact-bulge and disc, diffuse-bulge and disc, and disc only). This extends our previous measurement of the total GSMF down to 10^6.75 M_sol h^-2_70 and we find a total stellar mass density of rho_*=(2.97+/-0.04)x10^8 M_sol h_70 Mpc^-3 or Omega_*=(2.17+/-0.03)x10^-3 h^-1_70. We conclude that at z<0.1, the Universe has converted 4.9+/-0.1 per cent of the baryonic mass implied by Big Bang Nucleosynthesis into stars that are gravitationally bound within the galaxy population.

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arXiv2020

Study of galaxies on large-scale filaments in simulations

We use data from the Evolution and Assembly of GaLaxies in their Environment (EAGLE) cosmological simulation to study properties of galaxies in the cosmic web. Galaxies become more redder and form stars at a lower rate relative to their counterparts further away from the cylindrical axis of the large-scale filaments. These trends are particularly strong for galaxies with $M_*/M_{\odot}\lesssim10^{10}$. We also find that at distances $<0.5$ Mpc from the spine of the filaments, the median gas and stellar mass fraction in filament galaxies rises sharply with decreasing distance from the spine of the filament. These results, together with matching trends in the SFR/$M_*$ and the $g-r$ colour of filament galaxies suggest that (i) the intrafilamentary gas condenses into the filament galaxies thus fuelling star formation in them, and (ii) increased number density of galaxies closer to the central axis of the filament enhances the rate of gravitational interactions among filament galaxies closer to the spine.

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arXiv2020

Age and Metallicity of galaxies in different environments of the Coma supercluster

We analyse luminosity-weighted ages and metallicity (Z) of galaxies in a continuous range of environments, i.e. clusters, filaments and voids prevalent in the Coma supercluster ($\sim 100 h^{-1}$ Mpc). Specifically, we employ two absorption line indices, H$β$ and $\langle\rm{Fe}\rangle$ as tracers of age and metallicity of galaxies. We find that the stellar-phase metallicity of galaxies declines with increasing age as a function of stellar mass ($M^*$) as well as environment. On the filaments, metallicity of galaxies varies as a function of their distance from the spine of the filament, such that galaxies closer to the centre of the filaments have lower metallicity relative to their counterparts 1 Mpc away from it. The mean age of intermediate mass galaxies ($10^{10} < M^*/M_{\odot} < 10^{10.5}$) galaxies is statistically significantly different in different environments such that, the galaxies in clusters are older than the filament galaxies by 1-1.5 Gyr, while their counterparts in the voids are younger than filament galaxies by $\sim 1$ Gyr. The massive galaxies ($M^*/M_{\odot} > 10^{10.5}$), on the other hand show no such difference for the galaxies in clusters and filaments, but their counterparts in voids are found to be younger by $\sim 0.5$ Gyr. At fixed age however, Z of galaxies is independent of their $M^*$ in all environments, except the most massive ($M^*/M_{\odot} \gtrsim 10^{10.7}$), oldest galaxies ($\gtrsim 9$ Gyr) which show a sharp decline in their Z with $M^*$. Our results support a scenario where galaxies in the nearby Universe have grown by accreting smaller galaxies or primordial gas from the large-scale cosmic web.

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arXiv2019

Galaxy And Mass Assembly (GAMA): Properties and evolution of red spiral galaxies

We use multi-wavelength data from the Galaxy and Mass Assembly (GAMA) survey to explore the cause of red optical colours in nearby (0.002 = 47% of all spirals in our sample. Furthermore, for a small subsample of our spiral galaxies which are detected in HI, we find that the SFR of gas-rich red spiral galaxies is lower by ~1 dex than their blue counterparts.

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arXiv2018

The Star Formation Reference Survey III: A Multi-wavelength View of Star Formation in Nearby Galaxies

We present multi-wavelength global star formation rate (SFR) estimates for 326 galaxies from the Star Formation Reference Survey (SFRS) in order to determine the mutual scatter and range of validity of different indicators. The widely used empirical SFR recipes based on 1.4 GHz continuum, 8.0 $μ$m polycyclic aromatic hydrocarbons (PAH), and a combination of far-infrared (FIR) plus ultraviolet (UV) emission are mutually consistent with scatter of $\raise{-0.8ex}\stackrel{\textstyle <}{\sim }$0.3 dex. The scatter is even smaller, $\raise{-0.8ex}\stackrel{\textstyle <}{\sim }$0.24 dex, in the intermediate luminosity range 9.3 }5\times10^9$ L$_{\odot}$ or with implied far-UV extinction <1 mag, the UV spectral slope gives extinction corrections with 0.22~dex uncertainty.

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arXiv2018

Ultraviolet and optical view of galaxies in the Coma Supercluster

The Coma supercluster (100 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 square degrees 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 Halpha 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 around 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 multi-wavelength data.

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arXiv2018

Galaxy And Mass Assembly (GAMA): The effect of galaxy group environment on active galactic nuclei

In galaxy clusters, efficiently accreting active galactic nuclei (AGN) are preferentially located in the infall regions of the cluster projected phase-space, and are rarely found in the cluster core. This has been attributed to both an increase in triggering opportunities for infalling galaxies, and a reduction of those mechanisms in the hot, virialised, cluster core. Exploiting the depth and completeness ($98\,$per cent at $r<19.8\,$mag) of the Galaxy And Mass Assembly survey (GAMA), we probe down the group halo mass function to assess whether AGN are found in the same regions in groups as they are in clusters. We select 451 optical AGN from 7498 galaxies with $\log_{10}(M_*/\text{M}_\odot) > 9.9$ in 695 groups with $11.53\leq \log_{10}(M_{200}/\text{M}_\odot) \leq 14.56$ at $z<0.15$. By analysing the projected phase-space positions of these galaxies we demonstrate that when split both radially, and into physically derived infalling and core populations, AGN position within group projected phase-space is dependent on halo mass. For groups with $\log_{10}(M_{200}/\text{M}_\odot)>13.5$, AGN are preferentially found in the infalling galaxy population with $3.6σ$ confidence. At lower halo masses we observe no difference in AGN fraction between core and infalling galaxies. These observations support a model where a reduced number of low-speed interactions, ram pressure stripping and intra-group/cluster medium temperature, the dominance of which increase with halo mass, work to inhibit AGN in the cores of groups and clusters with $\log_{10}(M_{200}/\text{M}_\odot)>13.5$, but do not significantly affect nuclear activity in cores of less massive structures.

astro-ph.GA↗

arXiv2017

Galaxy And Mass Assembly (GAMA): Blue spheroids within 87 Mpc

In this paper we test if nearby blue spheroid (BSphs) galaxies may become the progenitors of star-forming spiral galaxies or passively-evolving elliptical galaxies. Our sample comprises 428 galaxies of various morphologies in the redshift range 0.002<z<0.02 (8-87 Mpc) with panchromatic data from the Galaxy and Mass Assembly survey. We find that BSph galaxies are structurally (mean effective surface brightness, effective radius) very similar to their passively-evolving red counterparts. However, their star-formation and other properties such as colour, age and metallicity are more like star-forming spirals than spheroids (ellipticals and lenticulars). We show that BSph galaxies are statistically distinguishable from other spheroids as well as spirals in the multi-dimensional space mapped by luminosity-weighted age, metallicity, dust mass and specific star formation rate. We use HI data to reveal that some of the BSphs are (further) developing their disks, hence their blue colours. They may eventually become spiral galaxies --- if sufficient gas accretion occurs --- or more likely fade into low-mass red galaxies.

astro-ph.GA↗

arXiv2017

Galaxy and Mass Assembly (GAMA): Formation and Growth of Elliptical Galaxies in the Group Environment

There are many proposed mechanisms driving the morphological transformation of disk 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 10$^{11}$ - 10$^{15}$ M$_{\odot}$. 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$\pm$0.012 per dex of group mass for groups more massive than 10$^{12.5}$ M$_{\odot}$. 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 10$^{11}$M$_{\odot}$. We tested if this observed relation is consistent with 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$\pm$0.010 per dex of group mass. These results demonstrate a strong correlation between the frequency of merger events and disk-to-elliptical galaxy transformation in galaxy group environments.

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arXiv2016

The SAMI galaxy survey: Galaxy Interactions and Kinematic Anomalies in Abell 119

Galaxy mergers are important events that can determine the fate of a galaxy by changing its morphology, star-formation activity and mass growth. Merger systems have commonly been identified from their disturbed morphologies, and we now can employ Integral Field Spectroscopy to detect and analyze the impact of mergers on stellar kinematics as well. We visually classified galaxy morphology using deep images ($μ_{\rm r} = 28\,\rm mag\,\, arcsec^{-2}$) taken by the Blanco 4-m telescope at the Cerro Tololo Inter-American Observatory. In this paper we investigate 63 bright ($M_{\rm r}<-19.3$) spectroscopically-selected galaxies in Abell 119; of which 53 are early type and 20 galaxies show a disturbed morphology by visual inspection. A misalignment between the major axes in the photometric image and the kinematic map is conspicuous in morphologically-disturbed galaxies. Our sample is dominated by early-type galaxies, yet it shows a surprisingly tight Tully-Fisher relation except for the morphologically-disturbed galaxies which show large deviations. Three out of the eight slow rotators in our sample are morphology disturbed. The visually-selected morphologically-disturbed galaxies are generally more asymmetric, visually as well as kinematically. Our findings suggest that galaxy interactions, including mergers and perhaps fly-bys, play an important role in determining the orientation and magnitude of galaxy's angular momentum.

astro-ph.GA↗

arXiv2015

Galaxy And Mass Assembly (GAMA): Trends in galaxy colours, morphology, and stellar populations with large scale structure, group, and pair environments

We explore trends in galaxy properties with Mpc-scale structures using catalogues of environment and large scale structure from the Galaxy And Mass Assembly (GAMA) survey. Existing GAMA catalogues of large scale structure, group and pair membership allow us to construct galaxy stellar mass functions for different environmental types. To avoid simply extracting the known underlying correlations between galaxy properties and stellar mass, we create a mass matched sample of galaxies with stellar masses between $9.5 \leq \log{M_*/h^{-2} M_{\odot}} \leq 11$ for each environmental population. Using these samples, we show that mass normalised galaxies in different large scale environments have similar energy outputs, $u-r$ colours, luminosities, and morphologies. Extending our analysis to group and pair environments, we show galaxies that are not in groups or pairs exhibit similar characteristics to each other regardless of broader environment. For our mass controlled sample, we fail to see a strong dependence of Sérsic index or galaxy luminosity on halo mass, but do find that it correlates very strongly with colour. Repeating our analysis for galaxies that have not been mass controlled introduces and amplifies trends in the properties of galaxies in pairs, groups, and large scale structure, indicating that stellar mass is the most important predictor of the galaxy properties we examine, as opposed to environmental classifications.

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arXiv2014

Galaxy And Mass Assembly (GAMA): The unimodal nature of the dwarf galaxy population

In this paper we aim to (i) test the number of statistically distinct classes required to classify the local galaxy population, and, (ii) identify the differences in the physical and star formation properties of visually-distinct galaxies. To accomplish this, we analyse the structural parameters (effective radius r_e, effective surface brightness within r_e (mu_e), central surface brightness (mu_0), and S'ersic index (n)), obtained by fitting the light profile of 432 galaxies (0.002<z<=0.02 Viking Z-band), and their spectral energy distribution using multi-band photometry in 18 broadbands to obtain the stellar mass (M*), the star formation rate (SFR), the specific SFR (sSFR) and the dust mass (M_{dust}), respectively. We show that visually distinct, star-forming dwarf galaxies (irregulars, blue spheroids and low surface brightness galaxies) form a unimodal population in a parameter space mapped by mu_e, mu_0, n, r_e, SFR, sSFR, M*, M_{dust} and (g-i). The SFR and sSFR distribution of passively evolving (dwarf) ellipticals on the other hand, statistically distinguish them from other galaxies with similar luminosity, while the giant galaxies clearly segregate into star-forming spirals and passive lenticulars. We therefore suggest that the morphology classification scheme(s) used in literature for dwarf galaxies only reflect the observational differences based on luminosity and surface brightness among the apparent distinct classes, rather than any physical differences between them.

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arXiv2014

Galaxy and Mass Assembly (GAMA): Fine filaments of galaxies detected within voids

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 6 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 SDSS-DR7 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, that may be defined as `void galaxies,' will have local galaxy number densities that depend on such observational limits and are likely higher than can be directly measured.

astro-ph.CO↗

arXiv2013

Evidence for galaxies being pre-processed before accreted into clusters

I use the spectroscopic data for galaxies in and around nearby (0.02 3 ang in absorption) may be correlated with the accretion of small galaxy groups in clusters. At r<2r200, 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|/sigma_v, where v_los is the difference between the line-of-sight velocity of the galaxy and the cluster's mean, and sigma_v is the cluster's velocity dispersion. This distribution is statistically different from that of other cluster galaxies within 2r200, and in the same absolute velocity range. Moreover, 87% of clusters in the sample studied here show statistically significant substructure in their velocity distribution, and 91.4% of all 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.

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