Search

The assembly of galaxy clusters is understood to be a hierarchical process with a continuous accretion of galaxies over time, which increases the cluster size and mass. Late-type galaxies that fall into clusters can undergo ram-pressure stripping, forming extended gas tails within which star formation can happen. The number, location, and tail orientations of such galaxies provide clues about the galaxy infall process, the assembly of the cluster over time, and the consequences of infall for galaxy evolution. Here, we utilise the $\sim$ 0.5 degree diameter circular field of the Ultraviolet Imaging Telescope to image six galaxy clusters at z < 0.06 that are known to contain jellyfish galaxies. We searched for stripping candidates in the ultraviolet images of these clusters, which revealed 54 candidates showing signs of unilateral extra-planar emission, due to ram-pressure stripping. Seven candidates had already been identified as likely stripping based on optical B-band imaging. We identified 47 new candidates through UV imaging. Spectroscopic redshift information is available for 39 of these candidate galaxies, of which 19 are associated with six clusters. The galaxies with spectroscopic redshifts that are not part of the clusters appear to be within structures at different redshifts identified as additional peaks in the redshift distribution of galaxies, indicating that they might be ram-pressure stripped or disturbed galaxies in other structures along the line of sight. We examine the orbital history of these galaxies based on their location in the position-velocity phase-space diagram and explore a possible connection to the orientation of the tail direction among cluster member candidates. The tails of confirmed cluster member galaxies are found to be oriented away from the cluster centre., Comment: Accepted for publication in A&A

Star-formation rates (SFR) in galaxies offer a view of various physical processes across them and are measured using various tracers, such as H$\alpha$ and UV. Different physical mechanisms can affect H$\alpha$ and UV emission, resulting in a discrepancy in the corresponding SFR estimates ($\Delta SFR$). We investigate the effects of ram pressure on the SFR measurements and $\Delta SFR$ across 5 galaxies from the GASP survey caught in the late stages of gas stripping due to ram pressure. We probe spatially resolved $\Delta SFR$ at pixel scales of 0.5 kpc, and compare disks to tails, and regions dominated by the dense gas to diffuse ionized gas (DIG) regions. The regions dominated by dense gas show similar SFR values for UV and H$\alpha$ tracers, while the regions dominated by the DIG show up to 0.5 dex higher SFR(UV). There is a large galaxy-by-galaxy variation in $\Delta SFR$, with no difference between the disks and the tails. We discuss the potential causes of variations in $\Delta SFR$ between the dense gas and DIG areas. We conclude that the dominant cause of discrepancy is recent variations in star formation histories, where star formation recently dropped in the DIG-dominated regions leading to changes in $\Delta SFR$. The areal coverage of the tracers shows areas with H$\alpha$ and no UV emission; these areas have LINER-like emission (excess in $[OI\lambda\,6300]/H\alpha$ line ratio), indicating that they are ionized by processes other than star-formation., Comment: 18 pages, 9 figures, 3 tables

Environmental and secular processes play a pivotal role in the evolution of galaxies. These can be due to external processes such as interactions or internal processes due to the action of bar, bulge and spiral structures. Ongoing star formation in spiral galaxies can be affected by these processes. Studying the star formation in the galaxy can give insights into the evolution of the galaxy. The ongoing interaction between barred-spiral galaxy NGC 1512 and its satellite NGC 1510 offers an opportunity to investigate how galactic interactions and the presence of a galactic bar influence the evolution of NGC 1512. We aim to understand the recent star formation activity in the galaxy pair and thus gain insight into the evolution of NGC 1512. The UltraViolet Imaging Telescope (UVIT) onboard AstroSat enables us to study the star-forming regions in the galaxy with a spatial resolution of ~85 pc in the galaxy rest frame. We identified and studied 175 star-forming regions in UVIT FUV image of NGC 1512 and correlated with the neutral hydrogen (HI) distribution. We detected localized regions of star formation enhancement and distortions in the galactic disk. This is consistent with HI distribution in the galaxy. This is evidence of past and ongoing interactions affecting the star formation properties of the galaxy. We studied the properties of the inner ring. We find that the regions of the inner ring show maximum star formation rate density (log(SFRDmean[Msolaryr-1kpc-2]) ~ -1.7) near the major axis of the bar, hinting at a possible crowding effect in these regions. The region of the bar in the galaxy is also depleted of UV emission. This absence suggests that the galactic bar played an active role in the redistribution of gas and quenching of star formation inside identified bar region. Hence, we suggest that both the secular and environmental factors might influence the evolution of NGC 1512., Comment: 8 pages, 4 figures, accepted for publication in Astronomy and Astrophyics journal

Blue early-type galaxies with galaxy-scale ongoing star formation are interesting targets in order to understand the stellar mass buildup in elliptical and S0 galaxies in the local Universe. We study the star-forming population of blue early-type galaxies to understand the origin of star formation in these otherwise red and dead stellar systems. The legacy survey imaging data taken with the dark energy camera in the $g$, $r$, and $z$ bands for 55 star-forming blue early-type galaxies were examined, and $g-r$ color maps were created. We identified low surface brightness features near 37 galaxies, faint-level interaction signatures near 15 galaxies, and structures indicative of recent merger activity in the optical color maps of all 55 galaxies. These features are not visible in the shallow Sloan Digital Sky Survey imaging data in which these galaxies were originally identified. Low surface brightness features found around galaxies could be remnants of recent merger events. The star-forming population of blue early-type galaxies could be post-merger systems that are expected to be the pathway for the formation of elliptical galaxies. We hypothesize that the star-forming population of blue early-type galaxies is a stage in the evolution of early-type galaxies. The merger features will eventually disappear, fuel for star formation will cease, and the galaxy will move to the passive population of normal early-type galaxies., Comment: Accepted in A&A

NGC 5291, an early-type galaxy surrounded by a giant HI ring, is believed to be formed from collision with another galaxy. Several star forming complexes and tidal dwarf galaxies are distributed along the collisional ring which are sites of star formation in environments where extreme dynamical effects are involved. Dynamical effects can affect the star formation properties and the spatial distribution of star forming complexes along the tidal features. To study and quantify the star formation activity in the main body and in the ring structure of the NGC 5291 system, we use high spatial resolution FUV and NUV imaging observations from the Ultraviolet Imaging Telescope onboard AstroSat. A total of 57 star-forming knots are identified to be part of this interacting system out of which 12 are new detections (star forming complexes that lie inside the HI contour) compared to the previous measurements from lower resolution UV imaging. We estimate the attenuation in UV for each of the resolved star-forming knots using the UV spectral slope $\beta$, derived from the FUV-NUV colour. Using the extinction corrected UV fluxes, we derive the star formation rate of the resolved star forming complexes. The extinction corrected total star formation rate of this system is estimated as 1.75 $\pm$ 0.04 $M_{\odot}/yr$. The comparison with dwarf galaxy populations (BCD, Sm and dIm galaxies) in the nearby Universe shows that many of the knots in the NGC 5291 system have SFR values comparable to the SFR of BCD galaxies., Comment: Accepted for publication in MNRAS. Appendix table can be found in published version

Elliptical galaxies with blue optical colours and significant star formation are hypothesised to be major merger remnants of gas-rich spiral galaxies or normal elliptical galaxies with a sudden burst of star formation. We present here a scenario in which blue elliptical galaxies identified in shallow imaging surveys may fail to recover faint features that are indicative of past merger activity using a nearby major merger remnant. Based on deep optical imaging data of the post-merger galaxy, NGC 7252, we demonstrate that the galaxy can appear as an elliptical galaxy if it is observed at higher redshifts. The main body and the low surface brightness merger features found at the outskirts of the galaxy are blue in the optical g - r colour map. We argue that the higher-redshift blue elliptical galaxies discovered in surveys as shallow as the SDSS or DECaLS may be advanced mergers whose defining tidal features fall below the detection limits of the surveys. This should be taken into consideration during the morphological classification of these systems in future and ongoing surveys., Comment: Accepted for publication in A&A

Stellar bars have been found to substantially influence the stellar populations properties in galaxies, affecting their ability of forming stars. While this can be easily seen when studying galaxies in relatively isolated environments, such kind of analysis takes a higher degree of complexity when cluster galaxies are considered, due to the variety of interactions which can potentially occur in these denser environments. We use IFU MUSE data from the GASP survey to study the combined effect of the presence of a stellar bar and of ram pressure, on spatially resolved properties of stellar populations. We have analyzed spatially resolved indicators of both recent SFR and average stellar population ages to check for signatures of anomalous central SF activity, also taking into account for the possible presence of nuclear activity. We found an increase of central SFR in ram pressure affected galaxies when compared with unperturbed ones. The most extreme cases of increase SFR and central rejuvenation occur in barred galaxies that are at advanced stages of ram pressure stripping. For low-mass barred galaxies affected by ram pressure, the combined effect is a systematic enhancement of the star formation activity as opposed to the case of high-mass galaxies that present both enhancement and suppression. Barred galaxies that present a suppression of their star formation activity also present signatures of nuclear activity. Our results indicate that the combined effect of the presence of a bar and a strong perturbation by ram pressure is able to trigger the central SF activity and probably ignite nuclear activity., Comment: 22 pages, 9 figures, 2 tables. Accepted for publication in Apj

Spiral galaxies undergo strong ram-pressure effects when they fall into the galaxy cluster potential. As a consequence, their gas is stripped to form extended tails within which star formation can happen, giving them the typical jellyfish appearance. The ultraviolet imaging observations of jellyfish galaxies provide an opportunity to understand ongoing star formation in the stripped tails. We report the ultraviolet observations of the jellyfish galaxies JW39, JO60, JO194 and compare with observations in optical continuum and $\mathrm{H}{\alpha}$. We detect knots of star formation in the disk and tails of the galaxies and find that their UV and H$\alpha$ flux are well correlated. The optical emission line ratio maps of these galaxies are used to identify for every region the emission mechanism, due to either star formation, LINER or a mix of the two phenomena. The star-forming regions in the emission line maps match very well with the regions having significant UV flux. The central regions of two galaxies (JW39, JO194) show a reduction in UV flux which coincides with composite or LINER regions in the emission line maps. The galaxies studied here demonstrate significant star formation in the stripped tails, suppressed star formation in the central regions and present a possible case of accelerated quenching happening in jellyfish galaxies., Comment: Accepted for publication in MNRAS

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., Comment: 20 Pages; Accepted for publication in the journal Publications of the Astronomical Society of Australia (PASA)

The energetic feedback from supermassive black holes can influence star formation at the centres of galaxies. Observational evidence for active galactic nucleus (AGN) impact on star formation can be searched for in galaxies by combining ultraviolet imaging and optical integral field unit data. The ultraviolet flux directly traces recent star formation, and the integral field unit data can reveal dust attenuation, gas ionisation mechanisms, and gas kinematics from the central regions of the galaxy disk. A pilot study on NGC 3982 shows star formation suppression in the central regions of the galaxy, likely due to negative AGN feedback, and enhanced star formation in the outer regions. The case of NGC 3982 could be observational evidence of AGN feedback operating in a Seyfert galaxy., Comment: Published in A&A, 7 pages, 8 figures, interactive online figure at https://prajwel.github.io/NGC3982/

The spatial distribution and physical sizes of star forming clumps at the smallest scales provide valuable information on hierarchical star formation (SF). In this context, we report the sites of ongoing SF at ~120 pc along the interacting galaxies in Stephan's Quintet (SQ) compact group using AstroSat-UVIT and JWST data. Since ultraviolet radiation is a direct tracer of recent SF, we identified star forming clumps in this compact group from the FUV imaging which we used to guide us to detect star forming regions on JWST IR images. The FUV imaging reveals star forming regions within which we detect smaller clumps from the higher spatial resolution images of JWST, likely produced by PAH molecules and dust ionised by FUV emission from young massive stars. This analysis reveals the importance of FUV imaging data in identifying star forming regions in the highest spatial resolution IR imaging available., Comment: Accepted in RNAAS, 3 pages, 1 figure, online figures at https://prajwel.github.io/stephans_quintet/

$\require{mediawiki-texvc}$ Supermassive black holes at the centre of active galactic nuclei (AGN) produce relativistic jets that can affect the star formation characteristics of the AGN hosts. Observations in the ultraviolet (UV) band can provide an excellent view of the effect of AGN jets on star formation. Here, we present a census of star formation properties in the Northern Star-forming Region (NSR) that spans about 20 kpc of the large radio source Centaurus A hosted by the giant elliptical galaxy NGC 5128. In this region, we identified 352 UV sources associated with Cen A using new observations at an angular resolution of $<$1.5 arcseconds observed with the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat. These observations were carried out in one far-ultraviolet (FUV; $\lambda_{\text{mean}}$ = 1481 $\AA$) and three near-ultraviolet (NUV; with $\lambda_{\text{mean}}$ of 2196 $\AA$, 2447 $\AA$, and 2792 $\AA$, respectively) bands. The star-forming sources identified in UV tend to lie in the direction of the jet of Cen A, thereby suggesting jet triggering of star formation. Separating the NSR into Outer and Inner regions, we found the stars in the Inner region to have a relatively younger age than the Outer region, suggesting that the two regions may have different star formation histories. We also provide the UVIT source catalogue in the NSR., Comment: Accepted for publication by MNRAS. Data and Jupyter notebooks are available at https://github.com/prajwel/Centaurus-A_NSR . 15 pages, 19 figures, 6 tables

Secular and environmental effects play a significant role in regulating the star formation rate and hence the evolution of the galaxies. Since UV flux is a direct tracer of the star formation in galaxies, the UltraViolet Imaging Telescope (UVIT) onboard ASTROSAT enables us to characterize the star forming regions in a galaxy with its remarkable spatial resolution. In this study, we focus on the secular evolution of NGC 628, a spiral galaxy in the local universe. We exploit the resolution of UVIT to resolve up to $\sim$ 63 pc in NGC 628 for identification and characterization of the star forming regions. We identify 300 star forming regions in the UVIT FUV image of NGC 628 using ProFound and the identified regions are characterized using Starburst99 models. The age and mass distribution of the star forming regions across the galaxy supports the inside-out growth of the disk. We find that there is no significant difference in the star formation properties between the two arms of NGC 628. We also quantify the azimuthal offset of the star forming regions of different ages. Since we do not find an age gradient, we suggest that the spiral density waves might not be the possible formation scenario of the spiral arms of NGC 628. The headlight cloud present in the disk of the galaxy is found to be having the highest star formation rate density ($0.23 M_{\odot} yr^{-1} kpc^{-2}$) compared to other star forming regions on spiral arms and the rest of the galaxy., Comment: 11 pages, 11 Figures, Accepted in MNRAS

We present a new approach to observationally constrain where the tails of Jellyfish (JF) galaxies in groups and clusters first appear and how long they remain visible with respect to the moment of their orbital pericenter. This is accomplished by measuring the distribution of their tail directions with respect to their host's center, and their distribution in a projected velocity-radius phase-diagram. We then model these observed distributions using a fast and flexible approach where JF tails are painted onto dark matter halos according to a simple parameterised prescription, and perform a Bayesian analysis to estimate the parameters. We demonstrate the effectiveness of our approach using observational mocks, and then apply it to a known observational sample of 106 JF galaxies with radio continuum tails located inside 68 hosts such as groups and clusters. We find that, typically, the radio continuum tails become visible on first infall when the galaxy reaches roughly three quarters of r$_{200}$, and the tails remain visible for a few hundred Myr after pericenter passage. Lower mass galaxies in more massive hosts tend to form visible tails further out and their tails disappear more quickly after pericenter. We argue that this indicates they are more sensitive to ram pressure stripping. With upcoming large area surveys of JF galaxies in progress, this is a promising new method to constrain the environmental conditions in which visible JF tails exist., Comment: Accepted to ApJ June 2022, 24 pages, 13 figures, 2 tables

This article focuses on making discrete-time Adaptive Iterative Learning Control (ILC) more effective using multiple estimation models. Existing strategies use the tracking error to adjust the parametric estimates. Our strategy uses the last component of the identification error to tune these estimates of the model parameters. We prove that this strategy results in bounded estimates of the parameters, and bounded and convergent identification and tracking errors. We emphasize that the proof does not use the key technical lemma. Rather, it uses the properties of square-summable sequences. We extend this strategy to include multiple estimation models and show that all the signals are bounded, and the errors converge. It is also shown that this works whether we switch between the models at every instant and every iteration or at the end of every iteration. Simulation results demonstrate the efficacy of the proposed method with a faster convergence using multiple estimation models., Comment: 26 pages, 5 figures, 2 tables

We present an ultra-violet study of two nearby dwarf irregular galaxies WLM and IC~2574, using the Far-UV and Near-UV data from the Ultra-Violet Imaging Telescope (UVIT). We used the F148W band Far-UV images and identified 180 and 782 young star-forming clumps in WLM and IC~2574, respectively. The identified clumps have sizes between 7 - 30 pc in WLM and 26 - 150 pc in IC~2574. We noticed more prominent hierarchical splitting in the structure of star-forming regions at different flux levels in IC~2574 than WLM. We found that the majority of the clumps have elongated shapes in the sky plane with ellipticity ($\epsilon$) greater than 0.6 in both the galaxies. The major axis of the identified clumps is found to show no specific trend of orientation in IC~2574, whereas in WLM the majority are aligned along south-west to north-east direction. We estimated (F148W$-$N242W) colour for the clumps identified in WLM and noticed that the younger ones (with (F148W$-$N242W) $<-0.5$) are smaller in size ($<10$ pc) and are located mostly in the southern half of the galaxy between galactocentric radii 0.4 - 0.8 kpc., Comment: 12 pages, 10 figures, Accepted for publication in Journal of Astrophysics and Astronomy

The central regions of star-forming barred spiral galaxies can be devoid of star formation because of the redistribution of gas along the length of the bar. However, there can be gas outside the length of the bar that can host star formation. We study a sample of barred disc galaxies in the local Universe with an aim to discriminate between centrally quenched and globally quenched galaxies based on their positions on star-formation-rate(SFR)--stellar mass plots and to find a connection between the SFR of quenched galaxies and the length of their bar. We classified barred galaxies as centrally quenched and globally quenched based on their position on SFR--stellar mass plots, with SFRs derived from H$\alpha$ flux and spectral energy distribution fits on combined ultraviolet and optical flux. We selected galaxies as passive based on the distance from the main sequence relation. From a total 2514 barred galaxies studied here, we present 651 with suppressed star formation in their central region but hosting star formation outside. We also find a possible correlation between bar length and SFR for the galaxies that are fully quenched because of the stellar bar., Comment: Accepted for publication in A&A

We present the far ultraviolet (FUV) imaging of the nearest Jellyfish or Fireball galaxy IC3418/VCC 1217, in the Virgo cluster of galaxies, using Ultraviolet Imaging Telescope (UVIT) onboard the ASTROSAT satellite. The young star formation observed here in the 17 kpc long turbulent wake of IC3418, due to ram pressure stripping of cold gas surrounded by hot intra-cluster medium, is a unique laboratory that is unavailable in the Milkyway. We have tried to resolve star forming clumps, seen compact to GALEX UV images, using better resolution available with the UVIT and incorporated UV-optical images from Hubble Space Telescope archive. For the first time, we resolve the compact star forming clumps (fireballs) into sub-clumps and subsequently into a possibly dozen isolated stars. We speculate that many of them could be blue supergiant stars which are cousins of SDSS J122952.66+112227.8, the farthest star (~17 Mpc) we had found earlier surrounding one of these compact clumps. We found evidence of star formation rate (4 - 7.4 x 10^-4 M_sun per yr ) in these fireballs, estimated from UVIT flux densities, to be increasing with the distance from the parent galaxy. We propose a new dynamical model in which the stripped gas may be developing vortex street where the vortices grow to compact star forming clumps due to self-gravity. Gravity winning over turbulent force with time or length along the trail can explain the puzzling trend of higher star formation rate and bluer/younger stars observed in fireballs farther away from the parent galaxy., Comment: 12 pages, 9 figures. Accepted for publication in the Special Issue of the Journal of Astrophysics and Astronomy on ASTROSAT

Star formation in galaxies is a hierarchical process with a wide range of scales from smaller clusters to larger stellar complexes. Here, we present an ultra-violet imaging study of the nearby flocculent spiral galaxy NGC 7793, observed with the Ultra-Violet Imaging Telescope (UVIT). We find that the disk scale-length estimated in Far-UV (2.64$\pm$0.16 kpc) is larger than that in Near-UV (2.21$\pm$0.21 kpc) and optical (1.08 kpc), which supports the inside-out growth scenario of the galaxy disk. The star-forming UV disk is also found to be contained within the extent of H~I gas of column density greater than $10^{21}$cm$^{-2}$. With the spatial resolution of UVIT (1 pixel $\sim$ 6.8 pc), we identified 2046 young star-forming clumps in the galaxy with radii between $\sim$ 12 - 70 pc, which matches well with the size of GMCs detected in the galaxy. Around 61\% of the regions identified in our study have age younger than 20 Myr, which points to a recent enhancement of star formation across the galaxy. We also noticed that the youngest star-forming regions, with age $<$ 10 Myr, distinctly trace the flocculent arms of the galaxy. The estimated mass of the clumps cover a range between $10^3 - 10 ^6 M_{\odot}$. We noticed a gradient in the mass distribution of identified clumps along the spiral arms. We have also studied the nuclear star cluster of the galaxy and found that the stellar populations in the cluster outskirts are younger than the inner part., Comment: 17 pages, 14 figures, Accepted for publication in ApJ

The underlying nature of the process of star formation quenching in the central regions of barred disc galaxies that is due to the action of stellar bar is not fully understood. We present a multi-wavelength study of four barred galaxies using the archival data from optical, ultraviolet, infrared, CO, and HI imaging data on star formation progression and stellar and gas distribution to better understand the process of bar quenching. We found that for three galaxies, the region between the nuclear or central sub-kiloparsec region and the end of the bar (bar region) is devoid of neutral and molecular hydrogen. While the detected neutral hydrogen is very negligible, we note that molecular hydrogen is present abundantly in the nuclear or central sub-kiloparsec regions of all four galaxies. The bar co-rotation radius is also devoid of recent star formation for three out of four galaxies. One galaxy shows significant molecular hydrogen along the bar, which might mean that the gas is still being funnelled to the centre by the action of the stellar bar. Significant star formation is also present along the bar co-rotation radius of this galaxy. The study presented here supports a scenario in which gas redistribution as a result of the action of stellar bar clears the bar region of fuel for further star formation and eventually leads to star formation quenching in the bar region., Comment: Accepted for publication in A&A. Continuation on the letter paper (arXiv:1812.04178) to appear in main journal

Galaxies undergoing ram pressure stripping in clusters are an excellent opportunity to study the effects of environment on both the AGN and the star formation activity. We report here on the most recent results from the GASP survey. We discuss the AGN-ram pressure stripping connection and some evidence for AGN feedback in stripped galaxies. We then focus on the star formation activity, both in the disks and the tails of these galaxies, and conclude drawing a picture of the relation between multi-phase gas and star formation., Comment: Proceedings of the IAU Symposium 359 "Galaxy evolution and feedback across different environments", editors T. Storchi-Bergmann, R. Overzier, W. Forman & R. Riffel - final version

The calibration of ground based optical imaging data to photometric accuracy of 10 mmag over the full survey area and to color uniformity better than 5 mmag on the scale of the VIS focal plane is a key science requirement for the Euclid mission. These accuracies enable stable photometric redshifts of galaxies and modeling of the color dependent VIS PSF for weak lensing studies. We use the Gaia photometry to calibrate the $g/r/i/z$ magnitudes of Dark Energy Survey (DES) data to meet the stringent Euclid requirements. The Gaia G band magnitude along with the BP-RP color information of stars observed in the DES single epoch (SE) exposures are used to derive the transformation from Gaia to DES photometry for individual CCDs and to characterize persistent photometric errors across the DECam focal plane. We use the color dependence of these persistent errors to characterize the $g/r/i/z$ bandpass variations across the DECam focal plane., Comment: To appear in proceedings of ADASS XXIX, ASP Conf. Series

With MUSE, Chandra, VLA, ALMA and UVIT data from the GASP programme we study the multiphase baryonic components in a jellyfish galaxy (JW100) with a stellar mass 3.2 X 10^{11} M_sun hosting an AGN. We present its spectacular extraplanar tails of ionized and molecular gas, UV stellar light, X-ray and radio continuum emission. This galaxy represents an excellent laboratory to study the interplay between different gas phases and star formation, and the influence of gas stripping, gas heating, and AGN. We analyze the physical origin of the emission at different wavelengths in the tail, in particular in-situ star formation (related to Halpha, CO and UV emission), synchrotron emission from relativistic electrons (producing the radio continuum) and heating of the stripped interstellar medium (ISM) (responsible for the X-ray emission). We show the similarities and differences of the spatial distributions of ionized gas, molecular gas and UV light, and argue that the mismatch on small scales (1kpc) is due to different stages of the star formation process. We present the relation Halpha--X-ray surface brightness, which is steeper for star-forming regions than for diffuse ionised gas regions with high [OI]/Halpha ratio. We propose that ISM heating due to interaction with the intracluster medium (either for mixing, thermal conduction or shocks) is responsible for the X-ray tail, the observed [OI]-excess and the lack of star formation in the northern part of the tail. We also report the tentative discovery in the tail of the most distant (and among the brightest) currently known ULX, a point-like ultraluminous X-ray source commonly originating in a binary stellar system powered either by an intermediate-mass black hole or a magnetized neutron star., Comment: accepted for publication in ApJ

The Galaxy And Mass Assembly (GAMA) survey has morphologically identified a class of "Little Blue Spheroid" (LBS) galaxies whose relationship to other classes of galaxies we now examine in detail. Considering a sample of 868 LBSs, we find that such galaxies display similar but not identical colours, specific star formation rates, stellar population ages, mass-to-light ratios, and metallicities to Sd-Irr galaxies. We also find that LBSs typically occupy environments of even lower density than those of Sd-Irr galaxies, where ~65% of LBS galaxies live in isolation. Using deep, high-resolution imaging from VST KiDS and the new Bayesian, two-dimensional galaxy profile modeling code PROFIT, we further examine the detailed structure of LBSs and find that their S\'ersic indices, sizes, and axial ratios are compatible with those of low-mass elliptical galaxies. We then examine SAMI Galaxy survey integral field emission line kinematics for a subset of 62 LBSs and find that the majority (42) of these galaxies display ordered rotation with the remainder displaying disturbed/non-ordered dynamics. Finally, we consider potential evolutionary scenarios for a population with this unusual combination of properties, concluding that LBSs are likely formed by a mixture of merger and accretion processes still recently active in low-redshift dwarf populations. We also infer that if LBS-like galaxies were subjected to quenching in a rich environment, they would plausibly resemble cluster dwarf ellipticals., Comment: 15 pages, 14 figures, MNRAS accepted

The suppression of star formation in the inner kiloparsec regions of barred disk galaxies due to the action of bars is known as bar quenching. We investigate here the significance of bar quenching in the global quenching of star formation in the barred galaxies and their transformation to passive galaxies in the local Universe. We do this by measuring the offset of quenched barred galaxies from star-forming main sequence galaxies in the star formation rate-stellar mass plane and comparing it with the length of the bar, which is considered as a proxy of bar quenching. We constructed the star formation rate-stellar mass plane of 2885 local Universe face-on strong barred disk galaxies ($z<0.06$) identified by Galaxy Zoo. The barred disk galaxies studied here fall on the star formation main sequence relation with a significant scatter for galaxies above stellar mass 10$^{10.2}$ M$\odot$. We found that 34.97 $\%$ galaxies are within the intrinsic scatter (0.3 dex) of the main sequence relation, with a starburst population of 10.78 $\%$ (above the 0.3 dex) and a quenched population of 54.25 $\%$ (below the -0.3 dex) of the total barred disk galaxies in our sample. Significant neutral hydrogen (M$_{HI}$ >10$^{9}$ M$\odot$ with log M$_{HI}$/M$\star$ $\sim$ -1.0 to -0.5) is detected in the quenched barred galaxies with a similar gas content to that of the star-forming barred galaxies. We found that the offset of the quenched barred galaxies from the main sequence relation is not dependent on the length of the stellar bar. This implies that the bar quenching may not contribute significantly to the global quenching of star formation in barred galaxies. However, this observed result could also be due to other factors such as the dissolution of bars over time after star formation quenching, the effect of other quenching processes acting simultaneously, and/or the effects of environment., Comment: Accepted for publication in A&A

We present an ultra-violet (UV) study of the galaxy NGC~300 using GALEX far-UV (FUV) and near-UV (NUV) observations. We studied the nature of UV emission in the galaxy and correlated it with optical, H~I and mid-infrared (3.6 $\mu$m) wavelengths. Our study identified extended features in the outer disk, with the UV disk extending up to radius 12 kpc ($>$2R$_{25})$. We estimated the FUV and NUV disk scale-length as 3.05$\pm$0.27 kpc and 2.66$\pm$0.20 kpc respectively. The scale-length in FUV is 2.3 times larger than that at 3.6 $\mu$m, and we also find the disk to gradually become flatter from longer to shorter wavelengths. We performed a statistical source subtraction to eliminate the background contaminants and identified 261 unresolved UV sources between the radii 5.3 kpc and 10 kpc (1 $\sim$ 2 R$_{25}$). The identified UV sources show an age range between 1 - 300 Myr with a peak at 25 Myr and a mass range between $10^3 M_{\odot}$ to $10^6 M_{\odot}$, estimated using starburst99 models. The north-eastern spiral arm is found to be populated by young low mass sources suggesting that the star formation in this spiral arm is a recent phenomenon. The UV emission beyond the R$_{25}$ radius has contribution from these low mass sources and is extended up to $\sim$ 2R$_{25}$ radius. We conclude that NGC~300 has an extended UV disk, mainly populated by young low mass sources. The star formation rate is measured to be $\sim$0.46 $M_{\odot}/yr$ which is comparable to its near optical twin M33., Comment: Accepted for publication in Journal of Astrophysics and Astronomy

Star forming dwarf galaxies in the local volume are diverse and are ideal test beds to understand details of star formation in a variety of environments. Here, we present a deep FUV imaging study of a nearby dwarf irregular galaxy IC 2574 using the Ultraviolet Imaging Telescope (UVIT). We identified 419 FUV bright regions with radii between 15 - 285 pc in the galaxy and found that 28.6\% of them to be located in H~I shells, 12.6\% inside holes and 60.1\% to be away from the holes. The H~I column density is found to be more than $10^{21} cm^{-2}$ for 82.3\% of the identified regions. 30 out of the 48 H~I holes show triggered star formation in their shells while 16 holes do not show any related FUV emission. Cross-matching with H$\alpha$ emission, we found that 23 holes have both FUV and H$\alpha$ emission in their shells signifying very recent trigger. Therefore, star formation in the galaxy has been partly triggered due to the expanding H~I holes whereas in majority of the sites it is driven by other mechanisms. Irrespective of the location, larger star forming complexes were found to have multiple sub-structures. We report two resolved components for the remnant cluster of the super giant shell and estimated their masses. The star formation rate of IC 2574 is found to be 0.57 $M_{\odot}$/yr, which is slightly higher compared to the average value of other nearby dwarf irregular galaxies., Comment: 19 pages, 9 figures, Accepted for publication in Astronomical Journal

We report evidence for star formation quenching in the central 8.6 kpc region of the jellyfish galaxy JO201 which hosts an active galactic nucleus, while undergoing strong ram pressure stripping. The ultraviolet imaging data of the galaxy disk reveal a region with reduced flux around the center of the galaxy and a horse shoe shaped region with enhanced flux in the outer disk. The characterization of the ionization regions based on emission line diagnostic diagrams shows that the region of reduced flux seen in the ultraviolet is within the AGN-dominated area. The CO J$_{2-1}$ map of the galaxy disk reveals a cavity in the central region. The image of the galaxy disk at redder wavelengths (9050-9250 $\overset{\lower.5em\circ}{\mathrm{A}}$) reveals the presence of a stellar bar. The star formation rate map of the galaxy disk shows that the star formation suppression in the cavity occurred in the last few 10$^8$ yr. We present several lines of evidence supporting the scenario that suppression of star formation in the central region of the disk is most likely due to the feedback from the AGN. The observations reported here make JO201 a unique case of AGN feedback and environmental effects suppressing star formation in a spiral galaxy., Comment: Author's accepted manuscript

The physical processes related to the effect of bar in the quenching of star formation in the region between the nuclear/central sub-kpc region and the ends of the bar (bar-region) of spiral galaxies is not fully understood. It is hypothesized that the bar can either stabilize the gas against collapse, inhibiting star formation or efficiently consume all the available gas, with no fuel for further star formation. We present a multi-wavelength study using the archival data of an early-type barred spiral galaxy, Messier 95, which shows signatures of suppressed star formation in the bar-region. Using the optical, ultraviolet, infrared, CO and HI imaging data we study the pattern of star formation progression, stellar/gas distribution and try to provide insights on the process responsible for the observed pattern. The FUV$-$NUV pixel colour map reveals a cavity devoid of UV flux in the bar-region that interestingly matches with the length of the bar ($\sim$ 4.2kpc). The central nuclear region of the galaxy is showing a blue color clump and along the major-axis of the stellar bar the colour progressively becomes redder. Based on a comparison to single stellar population models, we show that the region of galaxy along the major-axis of the bar (unlike the region outside the bar) is comprised of stellar populations with ages $\geq$ 350 Myr, with a star-forming clump in the center of younger ages ($\sim$ 150Myr). Interestingly the bar-region is also devoid of neutral and molecular hydrogen but with an abundant molecular hydrogen present at the nuclear region of the galaxy. Our results are consistent with a picture in which the stellar bar in Messier 95 is redistributing the gas by funneling gas inflows to nuclear region, thus making the bar-region devoid of fuel for star formation., Comment: Accepted for publication in A&A letter to the editor

Designing optimal controllers continues to be challenging as systems are becoming complex and are inherently nonlinear. The principal advantage of reinforcement learning (RL) is its ability to learn from the interaction with the environment and provide optimal control strategy. In this paper, RL is explored in the context of control of the benchmark cartpole dynamical system with no prior knowledge of the dynamics. RL algorithms such as temporal-difference, policy gradient actor-critic, and value function approximation are compared in this context with the standard LQR solution. Further, we propose a novel approach to integrate RL and swing-up controllers.

We present a study of star forming regions and its demographics in the nearby dwarf irregular galaxy WLM using the Ultra-Violet Imaging Telescope (UVIT) multi band observations in three filters F148W, N245M and N263M. We find that the UV emission is extended at least up to 1.7 kpc, with the NUV emission more extended than the FUV. We create UV color maps ((F148W$-$N245M) and (F148W$-$N263M)) to study the temperature morphology of young stellar complexes with the help of theoretical models. We identify several complexes with temperature T $>$ 17500 K which are likely to be the OB associations present in the galaxy. These complexes show good spatial correlation with the H$\alpha$ emitting regions, H$~$I distribution and HST detected hot stars. The hot star forming regions are found to be clumpy in nature and show a hierarchical structure, with sizes in the range of 4 - 50 pc, with a large number with sizes $<$ 10 pc. The south western part of the galaxy shows many hot star forming regions, high level of H$\alpha$ emission and low column density of H$~$I which altogether denote a vigorous recent star formation. WLM is likely to have a large fraction of low mass compact star forming regions with mass M $< 10^3 M_{\odot}$, in agreement with the size and mass of the CO clouds. We estimate the star formation rate of WLM to be $\sim$ 0.008 $M_{\odot}/yr$, which is similar to the average value measured for nearby dwarf irregular galaxies., Comment: Accepted for publication in AJ

Some post-merger galaxies are known to undergo a starburst phase that quickly depletes the gas reservoir and turns it into a red-sequence galaxy, though the details are still unclear. Here we explore the pattern of recent star formation in the central region of the post-merger galaxy NGC7252 using high resolution UV images from the UVIT on ASTROSAT. The UVIT images with 1.2 and 1.4 arcsec resolution in the FUV and NUV are used to construct a FUV-NUV colour map of the central region. The FUV-NUV pixel colour map for this canonical post-merger galaxy reveals a blue circumnuclear ring of diameter $\sim$ 10 " (3.2 kpc) with bluer patches located over the ring. Based on a comparison to single stellar population models, we show that the ring is comprised of stellar populations with ages $\lesssim$ 300 Myr, with embedded star-forming clumps of younger age ($\lesssim$ 150Myr). The suppressed star formation in the central region, along with the recent finding of a large amount of ionised gas, leads us to speculate that this ring may be connected to past feedback from a central super-massive black hole that has ionised the hydrogen gas in the central $\sim$ 4" $\sim$ 1.3 kpc., Comment: Accepted for publication in Astronomy & Astrophysics Letters

The DESI Legacy Imaging Surveys are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image approximately 14,000 deg^2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12 and 22 micorons) observed by the Wide-field Infrared Survey Explorer (WISE) satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project., Comment: 47 pages, 18 figures; accepted for publication in the Astronomical Journal

Jellyfish are cluster galaxies that experience strong ram-pressure effects that strip their gas. Their H$\alpha$ images reveal ionized gas tails up to 100 kpc, which could be hosting ongoing star formation. Here we report the ultraviolet (UV) imaging observation of the jellyfish galaxy JO201 obtained at a spatial resolution $\sim$ 1.3 kpc. The intense burst of star formation happening in the tentacles is the focus of the present study. JO201 is the "UV-brightest cluster galaxy" in Abell 85 ($z \sim$ 0.056) with knots and streams of star formation in the ultraviolet. We identify star forming knots both in the stripped gas and in the galaxy disk and compare the UV features with the ones traced by H$\alpha$ emission. Overall, the two emissions remarkably correlate, both in the main body and along the tentacles. Similarly, also the star formation rates of individual knots derived from the extinction-corrected FUV emission agree with those derived from the H$\alpha$ emission and range from $\sim$ 0.01 -to- 2.07 $M_{\odot} \, yr^{-1}$. The integrated star formation rate from FUV flux is $\sim$ 15 $M_{\odot} \, yr^{-1}$. The unprecedented deep UV imaging study of the jellyfish galaxy JO201 shows clear signs of extraplanar star-formation activity due to a recent/ongoing gas stripping event., Comment: Accepted for publication in MNRAS

The tidal tails of post-merger galaxies exhibit ongoing star formation far from their disks. The study of such systems can be useful for our understanding of gas condensation in diverse environments. The ongoing star formation in the tidal tails of post-merger galaxies can be directly studied from ultraviolet (UV) imaging observations. The post merger galaxy NGC7252 ("Atoms-for-Peace" galaxy) is observed with the Astrosat UV imaging telescope (UVIT) in broadband NUV and FUV filters to isolate the star forming regions in the tidal tails and study the spatial variation in star formation rates. Based on ultraviolet imaging observations, we discuss star formation regions of ages $<$ 200 Myrs in the tidal tails. We measure star formation rates in these regions and in the main body of the galaxy. The integrated star formation rate of NGC7252 (i.e., that in the galaxy and tidal tails combined) without correcting for extinction is found to be 0.81 $\pm$ 0.01 M$_{\odot}$/yr. We show that the integrated star formation rate can change by an order of magnitude if the extinction correction used in star formation rates derived from other proxies are taken into consideration. The star formation rates in the associated tidal dwarf galaxies (NGC7252E, SFR=0.02 M$_{\odot}$/yr and NGC7252NW, SFR=0.03 M$_{\odot}$/yr) are typical of dwarf galaxies in the local Universe. The spatial resolution of the UV images reveals a gradient in star formation within the tidal dwarf galaxy. The star formation rates show a dependence on the distance from the centre of the galaxy. This can be due to the different initial conditions responsible for the triggering of star formation in the gas reservoir that was expelled during the recent merger in NGC7252., Comment: Accepted for publication in Astronomy & Astrophysics

Star-forming blue early-type galaxies at low redshift can give insight to the stellar mass growth of L$*$ elliptical galaxies in the local Universe. We wish to understand the reason for star formation in these otherwise passively evolving red and dead stellar systems. The fuel for star formation can be acquired through recent accretion events such as mergers or flyby. The signatures of such events should be evident from a structural analysis of the galaxy image. We carried out structural analysis on SDSS $r$-band imaging data of 55 star-forming blue elliptical galaxies, derived the structural parameters, analysed the residuals from best-fit to surface brightness distribution, and constructed the galaxy scaling relations. We found that star-forming blue early-type galaxies are bulge-dominated systems with axial ratio $>$ 0.5 and surface brightness profiles fitted by Sersic profiles with index ($n$) mostly $>$ 2. Twenty-three galaxies are found to have $n$ $<$ 2; these could be hosting a disc component. The residual images of the 32 galaxy surface brightness profile fits show structural features indicative of recent interactions. The star-forming blue elliptical galaxies follow the Kormendy relation and show the characteristics of normal elliptical galaxies as far as structural analysis is concerned. There is a general trend for high-luminosity galaxies to display interaction signatures and high star formation rates. The star-forming population of blue early-type galaxies at low redshifts could be normal ellipticals that might have undergone a recent gas-rich minor merger event. The star formation in these galaxies will shut down once the recently acquired fuel is consumed, following which the galaxy will evolve to a normal early-type galaxy., Comment: Accepted for publication in Astronomy & Astrophysics