arXiv subjects

Koshy George

Publications and source records attributed to Koshy George.

At least 19 recordsLinked to original sources

arXiv2026

Bar-driven gas redistribution suppresses star formation in spiral galaxies: Evidence from dust lanes in NGC 3351

We present observational evidence, based on high-resolution imaging from HST, ALMA, and AstroSat/UVIT, that the redistribution of gas driven by the bar in the face-on spiral galaxy NGC 3351 results in suppressed star formation in its central regions. Dust and molecular gas coexist in galaxies, allowing dust lanes observed in galaxies to be used to probe the distribution of gas. In the central regions of NGC 3351, covered by the stellar bar, dust lanes are visible in the HST F438W-F814W color map, but surprisingly, these areas lack molecular gas and recent star formation. The inward orientation of the dust lane morphology towards the galaxy's center suggests that molecular gas may have once been present in this region, but was redistributed to the center due to the stellar bar's action. The direction of dust lanes, therefore, indicates the past inflows of gas toward the galaxy center, with their morphology consistently oriented inward along the bar. These findings support a scenario where the stellar bar has efficiently channeled molecular gas into the nucleus, building the central reservoir while suppressing star formation along the bar.

astro-ph.GA↗

arXiv2026

With arms wide open: a VLT/MUSE view of the mechanisms driving unwinding spiral arms in cluster galaxies

The environmental mechanisms driving unwinding spiral arms in cluster galaxies remain debated. While earlier studies attributed it mainly to gravitational interactions, recent works suggest that RPS alone can induce unwinding. We present a VLT/MUSE spatially resolved analysis to investigate the mechanisms responsible for spiral-arm unwinding in two galaxies, UG101 and UG103, drawn from a larger sample. They are selected as tidal and RPS-driven candidates, respectively, based on the proximity of close neighbors. We estimate the galactocentric radius at which tidal forces, from a companion or the cluster potential, become relevant ($R_{\mathrm{tid}}$). We examine gas and stellar kinematics, exploiting their different responses to gravitational and hydrodynamical perturbations. SINOPSIS is used to map stellar populations in age bins and constrain unwinding timescales. For UG101, we find $R_{\mathrm{tid}} \sim 1.5 R_e$, while the unwound features extend beyond this radius. UG101 shows irregular stellar and gas kinematics; its rotation curve indicates similar motions, although the gas is truncated on one side and extended on the other. For UG103, neither the closest companion nor the cluster appear capable of triggering unwinding. UG103 displays regular stellar but disturbed gas kinematics, with truncation on the disk side likely facing the ICM wind and gas extended in the opposite direction. Stellar population maps show the emergence and unwinding of the spiral arms in UG103 on timescales consistent with its cluster infall time ($\sim 1.6$ Gyr). We conclude that unwinding in UG101 and UG103 is primarily driven by tidal interactions and RPS, respectively, although a combined effect cannot be excluded for UG101. Our methodology provides a framework to identify the mechanisms driving unwinding in cluster galaxies from spatially resolved properties.

astro-ph.GA↗

arXiv2026

Tracing evolutionary pathways of bar-driven quenching in local Universe disc galaxies

Bars play an integral role in regulating star formation (SF) in spiral galaxies, from triggering central starbursts to driving quenching. The diverse SF morphologies observed in local barred galaxies reflect different evolutionary stages of the bar, motivating studies across these stages. Here we study 12 nearby barred galaxies (z=0.01-0.06) identified as centrally quenched galaxies (having extended star-forming discs but quenched inner regions) by leveraging the differences in SFRs between the MPA-JHU and GSWLC catalogues. However, they exhibit residual central emission in the SDSS 3" fibre spectral region. Emission line analysis shows that this emission originates from either ongoing SF or LINER-like activity, suggesting diverse central ionization mechanisms. Using spatially resolved UV-optical colour maps from SDSS (r-band) and GALEX (FUV and NUV band) imaging data, we find that discs are star-forming and bluer in colour (NUV-r < 4 mag) while the bulge and bar regions are systematically redder (NUV-r > 4 mag) and dominated by older stellar populations. The NUV-r radial colour profiles show a clear transition from red to blue colours at the bar end with a corresponding median stellar age of ~ 1 Gyr. Compared to fully centrally quenched barred galaxies from our earlier work which lack SDSS fibre emission, these galaxies remain systematically bluer at similar radii, despite showing NUV-r > 4 mag inside the bar suggesting an intermediate stage in bar-driven quenching. We also estimate black hole masses associated with kinetic-mode AGN feedback and find them below the threshold (logM_BH < 8.0). Adding this with the presence of pseudo bulges, our results support bars as the primary drivers of quenching, with these galaxies representing an evolutionary phase just before their inner regions are completely quenched.

astro-ph.GA↗

arXiv2026

The supersonic nature of jellyfish galaxies

All gas-rich galaxies in cluster environments are expected to experience ram-pressure stripping from the intra-cluster medium. However, only a fraction of these develop ongoing star-formation in their stripped tail, becoming the so-called ``jellyfish'' galaxies. In this work we provide observational evidence that magnetic fields can signal differences in the extraplanar star formation and explore what are the physical conditions that lead to the formation of a jellyfish galaxy. We first focus on JO147, a jellyfish galaxy that features weak star formation activity in its tail. Using MeerKAT radio continuum observations, we discover polarized emission only in a small fraction of its tail, with an average fraction of $~10\%$, and a low Mach number $\mathcal{M}=1.3-1.6$, which suggests a possible association between magnetic field draping, shock-compression of the gas, and extraplanar star formation activity. Then, we test this scenario in a sample of 17 jellyfish galaxies from the GASP project. We combine dynamical models for their orbits within the host clusters with realistic cluster temperature profiles to infer their Mach number, and we find a positive correlation between it and the star formation activity in their tail. We conclude that supersonic motion is a necessary condition for triggering star formation in the stripped tails of jellyfish galaxies. Our findings provide empirical evidence that the critical factor preventing the stripped gas evaporation is the shock compression induced by the supersonic motion through the cluster. This process likely enhances the magnetic field surrounding the galaxy and the properties of the stripped material.

astro-ph.GA↗

arXiv2026

Star formation outside galaxies undergoing gravitational and hydrodynamic interactions: Dust attenuation and the star formation rate

Galaxies undergo perturbations, either gravitational or hydrodynamic in origin, which can generate extragalactic structures such as rings and tails, where in situ star formation may take place. We selected a sample consisting of JO201 and JW100, undergoing ram-pressure stripping, and NGC 5291 and NGC 7252, formed through gravitational interactions, to investigate how different perturbation mechanisms influence dust content and star formation in extragalactic features. In both cases, star formation can be observed outside the main disks of the galaxies. We present new results of dust attenuation for JO201 and JW100, while for NGC 5291 and NGC 7252 we use results from our previous study, based on high-resolution observations obtained with the Ultraviolet Imaging Telescope onboard AstroSat. Dust attenuation is determined from the ultraviolet continuum slope ($\beta$) calculated using the FUV-NUV colour, and the star formation rates of the star-forming knots are corrected accordingly. It is seen that dust attenuation and dust-corrected SFR densities of the knots in the ram-pressure stripped tails of JO201 and JW100 are comparable to those in the collisional ring of the NGC 5291 system and the tidal tails of the NGC 7252 system. We conclude that, though the formation scenarios of the tails of JO201 and JW100, the NGC 5291 ring, and the NGC 7252 tails are different, their dust content and star formation activity are notably similar.

astro-ph.GA↗

arXiv2025

Distinguishing ram pressure from gravitational interactions: Applying the Size-Shape Difference method to real galaxies

In dense environments, mechanisms like ram pressure stripping (RPS) and gravitational interactions can induce similar morphological features in galaxies, distinguishable only through detailed study of their stellar properties. While RPS affects recently formed stars by displacing the gas disk from which they form, gravitational interactions perturb stars of all ages rather similarly. We present the first observational test of the Size-Shape Difference (SSD) measure, a novel approach validated for simulated galaxies, that quantifies morphological differences between young and intermediate-age stellar populations to distinguish RPS from gravitationally interacting galaxies. We analyze 67 galaxies from the GASP survey using spatially-resolved star formation histories derived using SINOPSIS. In our fiducial model, we compare stellar populations in two age bins (t < 20 Myr and 20 Myr <= t < 570 Myr) to calculate SSD values. The sample includes confirmed RPS cases with different stripping intensities, as well as undisturbed and gravitationally interacting galaxies. We find that extreme cases of RPS show SSD values ~3.5x higher than undisturbed and gravitationally interacting galaxies (56(+24/-15) as compared to 16(+6/-2) and 16(+6/-3), respectively), confirming simulation predictions. This enhancement reflects RPS-induced asymmetries: youngest stars are compressed along the leading edge and/or displaced into the extended tails of cold gas, while older populations remain undisturbed. In contrast, gravitational interactions perturb all stars uniformly, producing lower SSD values. SSD robustly distinguishes strong RPS cases, even adopting different age bins. This holds even without correcting for disk inclination, or when single-band imaging are used to trace stellar distributions. This makes SSD a promising tool to select RPS candidates for spectroscopic follow-up in upcoming surveys.

astro-ph.GA↗

arXiv2025

Star formation at different stages of ram-pressure stripping as observed through far-ultraviolet imaging of 13 GASP galaxies

Galaxies undergoing ram-pressure stripping develop gaseous tails that can extend several kiloparsecs outside the galaxy disc. We used far-ultraviolet and H$α$ imaging from the GASP survey to investigate how different stages of stripping affect star formation properties in the tail and disc of 13 galaxies undergoing stripping. These galaxies have different stripping strengths, as identified from the MUSE integral field spectroscopy. The star-forming knots in the disc and tails show a good correspondence between the measured FUV and H$α$ flux. This is especially true for strong and extreme cases of stripping, which have developed extended ionized gaseous tails featuring clumpy structures. The mechanism behind the H$α$ emission on the tails of these regions, which correlates well with FUV emission, is photoionization caused by young massive stars. The optical emission line ratio maps enable us to understand the emission mechanism, which can be attributed to star formation, LINER activity, or a combination of both phenomena and AGN. The star-forming regions in the emission line maps correspond well to the areas with significant FUV flux in these galaxies. Six galaxies exhibit minimal star formation in their tails, with two cases star formation is limited to the central regions and their discs are truncated. In galaxies with truncated discs, star formation is confined to a smaller region on the disc, as indicated by the FUV flux, compared to H$α$. Galaxies with strong stripping are undergoing recent star formation and are likely recent infalls. Galaxies with truncated discs confine star formation to the center, likely because they have completed a cluster crossing that depleted most of their outer gaseous disc. Galaxies with minimal FUV flux along their tails exhibit unresolved H$α$ emission which may be attributed to processes other than star formation.

astro-ph.GA↗

arXiv2025

Euclid: Early Release Observations of ram-pressure stripping in the Perseus cluster. Detection of parsec scale star formation with in the low surface brightness stripped tails of UGC 2665 and MCG +07-07-070

Euclid is delivering optical and near-infrared imaging data over 14,000 deg$^2$ on the sky at spatial resolution and surface brightness levels that can be used to understand the morphological transformation of galaxies within groups and clusters. Using the Early Release Observations (ERO) of the Perseus cluster, we demonstrate the capability offered by Euclid in studying the nature of perturbations for galaxies in clusters. Filamentary structures are observed along the discs of two spiral galaxies with no extended diffuse emission expected from tidal interactions at surface brightness levels of $\sim$ $30\,{\rm mag}\,{\rm arcsec}^{-2}$. The detected features exhibit a good correspondence in morphology between optical and near-infrared wavelengths, with a surface brightness of $\sim$ $25\,{\rm mag}\,{\rm arcsec}^{-2}$, and the knots within the features have sizes of $\sim$ 100 pc, as observed through $I_E$ imaging. Using the Euclid, CFHT, UVIT, and LOFAR $144\,{\rm MHz}$ radio continuum observations, we conduct a detailed analysis to understand the origin of the detected features. We constructed the \textit{Euclid} $I_E-Y_E$, $Y_E-H_E$, and CFHT $u - r$, $g - i$ colour-colour plane and showed that these features contain recent star formation events, which are also indicated by their H$α$ and NUV emissions. Euclid colours alone are insufficient for studying stellar population ages in unresolved star-forming regions, which require multi-wavelength optical imaging data. The morphological shape, orientation, and mean age of the stellar population, combined with the presence of extended radio continuum cometary tails can be consistently explained if these features have been formed during a recent ram-pressure stripping event. This result further confirms the exceptional qualities of Euclid in the study of galaxy evolution in dense environments.

astro-ph.GA↗

arXiv2025

The MUSE view of ram pressure stripped galaxies in clusters: the GASP sample

We present the full sample of 76 galaxies in 39 galaxy cluster fields at z=0.04-0.07 observed with VLT/MUSE by the GASP survey. Most of them (64) were observed as possible ram pressure stripped galaxies (stripping candidates) based on optical B-band images, while the remaining 12 were a control sample of both star-forming and passive galaxies. Based on spatially resolved ionized gas and stellar kinematics, we assess the physical origin of the gas asymmetries and find that 89% of the stripping candidates are confirmed by the VLT/MUSE data. In addition, also 3 of the 4 star-forming galaxies in the control sample show signs of ram pressure. These control galaxies display a ring of unusual emission line ratios, which we see also in field galaxies, possibly originating from the interaction with a hotter surrounding medium. The stripped galaxies are classified into various classes corresponding to different degrees of stripping, from weakest stripping to strong and extreme (jellyfish galaxies) stripping, as well as truncated gas disks with gas left only in the galaxy center. Our results show that selecting cluster stripping candidates based on optical imaging yields a sample that is indeed largely dominated by galaxies affected by ram pressure at different stages and stripping strength, though some contamination is present, mostly by tidal processes. Strong ram pressure cases are found in galaxies over the whole range of stellar masses studied (10^9-10^11.5 Msun) both in low-mass and high-mass clusters (cluster velocity dispersions sigma = 500-1100 km/s). We examine the possible connection between the progressive stages of stripping, up to the phase of a truncated gas disk, and the subsequent complete stripping of gas. We discuss the incompleteness intrinsic to this and other methods of selection to obtain a complete census of ram pressure stripping in clusters.

astro-ph.GA↗

arXiv2025

Star formation in interacting galaxy systems: UVIT imaging of NGC 7252 and NGC 5291

Interactions play a significant role in the formation and evolution of galaxies in the Universe. The galaxy systems, NGC 7252 and NGC 5291 are two nearby interacting systems that are hosting Tidal Dwarf Galaxies (TDGs) and star-forming knots. The present work aims (a) To determine the attenuation-corrected star formation rate (SFR) of the interacting system NGC 7252 (b) To compare the star formation in the NGC 7252 system with that of the NGC 5291 system (c) To explore the relation between surface densities of gas and SFR in these two systems. The study utilises high-resolution FUV and NUV imaging data from the Ultraviolet Imaging Telescope (UVIT) on board AstroSat. Six star-forming regions, including the merger remnant, were identified in the NGC 7252 system. The SFR corrected for attenuation of the knots in the NGC 7252 system is determined using the continuum slope (β) calculated from the FUV-NUV colour. It has been observed that the attenuation-corrected SFR values of the knots in this system fall within the range of SFR values determined for the NGC 5291 knots. The TDGs in both systems adhere to the same Kennicutt-Schmidt (KS) relation as regular spiral galaxies.

astro-ph.GA↗

arXiv2025

Investigating the role of bars in quenching star formation using spatially resolved UV-optical colour maps

Bars are ubiquitously found in disc galaxies and are known to drive galaxy evolution through secular processes. However, the specific contribution of bars in suppression of star formation (SF) is still under inspection which we investigate in this paper using spatially resolved UV-optical colour maps & radial color profiles of a sample of 17 centrally quenched (CQ) barred galaxies in redshift range 0.02-0.06. The sample is selected based on their location in M$_{\ast}$ - SFR plane. They are classified as passive based on parameters from MPA-JHU VAC, but non-passive based on the GSWLC catalog, indicating a passive inner region & recent star formation in their extended disc. We use archival SDSS r-band and GALEX FUV & NUV imaging data and created spatially resolved (FUV-NUV vs NUV-r) colour maps to understand the nature of UV emission from different regions of these galaxies. We then analyse their NUV-r radial profiles using NUV-r colour as a proxy for stellar population (SP) age. A control sample of 8 CQ unbarred galaxies is also analysed to disentangle the effect of bulge and bar in quenching SF. The CQ barred galaxies display redder colors (NUV-r $>$ 4-4.5 mag) in inner regions, up to the length of the bar, indicating the age of stellar population in these regions to be older than $>$ 1 Gyr. Most barred galaxies in our sample host pseudo bulges and do not host AGN, emphasizing most probable reason for the internal quenching is action of stellar bar. In comparison to the unbarred, the barred galaxies show redder colors to a larger spatial extent. We conclude that bars in their later stages of evolution turn the inner regions of galaxies redder, leading to quenching, with the effect being most prominent up to the ends of the bar and creating a region dominated by older SP. This may occur when bars have already funneled gas to the galactic centre leaving behind no fuel for further SF.

astro-ph.GA↗

arXiv2024

Candidate ram-pressure stripped galaxies in six low-redshift clusters revealed from ultraviolet imaging

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.

astro-ph.GA↗

arXiv2024

Spatially resolved comparison of SFRs from UV and H$α$ in GASP gas-stripped galaxies

Star-formation rates (SFR) in galaxies offer a view of various physical processes across them and are measured using various tracers, such as H$α$ and UV. Different physical mechanisms can affect H$α$ and UV emission, resulting in a discrepancy in the corresponding SFR estimates ($ΔSFR$). We investigate the effects of ram pressure on the SFR measurements and $ΔSFR$ across 5 galaxies from the GASP survey caught in the late stages of gas stripping due to ram pressure. We probe spatially resolved $ΔSFR$ 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$α$ 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 $ΔSFR$, with no difference between the disks and the tails. We discuss the potential causes of variations in $ΔSFR$ 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 $ΔSFR$. The areal coverage of the tracers shows areas with H$α$ and no UV emission; these areas have LINER-like emission (excess in $[OIλ\,6300]/Hα$ line ratio), indicating that they are ionized by processes other than star-formation.

astro-ph.GA↗

arXiv2024

Multiple Estimation Models for Discrete-time Adaptive Iterative Learning Control

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.

eess.SY↗

arXiv2023

What drives the wheels of evolution in NGC 1512? A UVIT study

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.

astro-ph.GA↗

arXiv2023

Deep optical imaging of star-forming blue early-type galaxies: Color map structures and faint features indicative of recent mergers

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.

astro-ph.GA↗

arXiv2023

UVIT view of NGC 5291: Ongoing star formation in tidal dwarf galaxies at ~ 0.35 kpc resolution

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 $β$, 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.

astro-ph.GA↗

arXiv2023

Decoding NGC 7252 as a blue elliptical galaxy

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.

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