arXiv subjects

Boris S. Kalita

Publications and source records attributed to Boris S. Kalita.

25 records · Page 2Linked to original sources

arXiv2023

A rest-frame near-IR study of clumps in galaxies at 1 < z < 2 using JWST/NIRCam: connection to galaxy bulges

A key question in galaxy evolution has been the importance of the apparent `clumpiness' of high redshift galaxies. Until now, this property has been primarily investigated in rest-frame UV, limiting our understanding of their relevance. Are they short-lived or are associated with more long-lived massive structures that are part of the underlying stellar disks? We use JWST/NIRCam imaging from CEERS to explore the connection between the presence of these `clumps' in a galaxy and its overall stellar morphology, in a mass-complete ($log\,M_{*}/M_{\odot} > 10.0$) sample of galaxies at $1.0 < z < 2.0$. Exploiting the uninterrupted access to rest-frame optical and near-IR light, we simultaneously map the clumps in galactic disks across our wavelength coverage, along with measuring the distribution of stars among their bulges and disks. Firstly, we find that the clumps are not limited to rest-frame UV and optical, but are also apparent in near-IR with $\sim 60\,\%$ spatial overlap. This rest-frame near-IR detection indicates that clumps would also feature in the stellar-mass distribution of the galaxy. A secondary consequence is that these will hence be expected to increase the dynamical friction within galactic disks leading to gas inflow. We find a strong negative correlation between how clumpy a galaxy is and strength of the bulge. This firmly suggests an evolutionary connection, either through clumps driving bulge growth, or the bulge stabilizing the galaxy against clump formation, or a combination of the two. Finally, we find evidence of this correlation differing from rest-frame optical to near-IR, which could suggest a combination of varying formation modes for the clumps.

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arXiv2023

IllustrisTNG in the HSC-SSP: image data release and the major role of mini mergers as drivers of asymmetry and star formation

At fixed galaxy stellar mass, there is a clear observational connection between structural asymmetry and offset from the star forming main sequence, $Δ$SFMS. Herein, we use the TNG50 simulation to investigate the relative roles of major mergers (stellar mass ratios $μ\geq0.25$), minor ($0.1 \leq μ< 0.25$), and mini mergers ($0.01 \leq μ< 0.1$) in driving this connection amongst star forming galaxies (SFGs). We use dust radiative transfer post-processing with SKIRT to make a large, public collection of synthetic Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) images of simulated TNG galaxies over $0.1\leq z \leq 0.7$ with $\log M_{\star} / \mathrm{M}_{\odot}\geq9$ ($\sim750$k images). Using their instantaneous SFRs, known merger histories/forecasts, and HSC-SSP asymmetries, we show (1) that TNG50 SFGs qualitatively reproduce the observed trend between $Δ$SFMS and asymmetry and (2) a strikingly similar trend emerges between $Δ$SFMS and the time-to-coalescence for mini mergers. Controlling for redshift, stellar mass, environment, and gas fraction, we show that individual mini merger events yield small enhancements in SFRs and asymmetries that are sustained on long timescales (at least $\sim3$ Gyr after coalescence, on average) -- in contrast to major/minor merger remnants which peak at much greater amplitudes but are consistent with controls only $\sim1$ Gyr after coalescence. Integrating the boosts in SFRs and asymmetries driven by $μ\geq0.01$ mergers since $z=0.7$ in TNG50 SFGs, we show that mini mergers are responsible for (i) $55$ per cent of all merger-driven star formation and (ii) $70$ per cent of merger-driven asymmetric structure. Due to their relative frequency and prolonged boost timescales, mini mergers dominate over their minor and major counterparts in driving star formation and asymmetry in SFGs.

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arXiv2022

Bulge formation inside quiescent lopsided stellar disks: connecting accretion, star formation and morphological transformation in a z ~ 3 galaxy group

We present well-resolved near-IR and sub-mm analysis of the three highly star-forming massive ($>10^{11}\,\rm M_{\odot}$) galaxies within the core of the RO-1001 galaxy group at $\rm z=2.91$. Each of them displays kpc-scale compact star-bursting cores with properties consistent with forming galaxy bulges, embedded at the center of extended, massive stellar disks. Surprisingly, the stellar disks are unambiguously both quiescent, and severely lopsided. Therefore, `outside-in' quenching is ongoing in the three group galaxies. We propose an overall scenario in which the strong mass lopsidedness in the disks (ranging from factors of 1.6 to $>$3), likely generated under the effects of accreted gas and clumps, is responsible for their star-formation suppression, while funnelling gas into the nuclei and thus creating the central starbursts. The lopsided side of the disks marks the location of accretion streams impact, with additional matter components (dust and stars) detected in their close proximity directly tracing the inflow direction. The interaction with the accreted clumps, which can be regarded as minor-mergers, leads the major axes of the three galaxies to be closely aligned with the outer Lyman-$α$-emitting feeding filaments. These results provide the first observational evidence of the impact of cold accretion streams on the formation and evolution of the galaxies they feed. In the current phase, this is taking the form of the rapid buildup of bulges under the effects of accretion, while still preserving massive quiescent and lopsided stellar disks at least until encountering a violent major-merger.

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arXiv2021

An ancient massive quiescent galaxy found in a gas-rich z ~ 3 group

Deep ALMA and HST observations reveal the presence of a quenched massive galaxy within the $z=2.91$ galaxy group RO-1001. With a mass-weighted stellar age of $1.6 \pm 0.4 \,$Gyr this galaxy is one of the oldest known at $z\sim3$, implying that most of its $10^{11}\rm \, M_{\odot}$ of stars were rapidly formed at $z>6$--8. This is a unique example of the predominantly passive evolution of a galaxy over at least $3<z<6$ following its high-redshift quenching and a smoking-gun event pointing to the early imprint of an age-environment relation. At the same time, being in a dense group environment with extensive cold-gas reservoirs as betrayed by a giant Ly$α$ halo, the existence of this galaxy demonstrates that gas accretion shutdown is not necessary for quenching and its maintenance.

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arXiv2021

Feedback Factory: Multiple faint radio-jets detected in a cluster at z=2

We report the detection of multiple faint radio sources, that we identify as AGN-jets, within CLJ1449+0856 at z=2 using 3 GHz VLA observations. We study the effects of radio-jet based kinetic feedback at high redshifts, which has been found to be crucial in low redshift clusters to explain the observed thermodynamic properties of their ICM. We investigate this interaction at an epoch featuring high levels of AGN activity and a transitional phase of ICM in regards to the likelihood of residual cold-gas accretion. We measure a total flux of $\rm 30.6 \pm 3.3~μJy$ from the 6 detected jets. Their power contribution is estimated to be $1.2 ~(\pm 0.6)~ \times 10^{44} ~\rm ergs~ s^{-1}$, although this value could be up to $4.7 ~ \times 10^{44} ~\rm ergs~ s^{-1}$. This is a factor $\sim 0.25 - 1.0$ of the previously estimated instantaneous energy injection into the ICM of CLJ1449+0856 from AGN outflows and star formation, that have already been found to be sufficient in globally offsetting the cooling flows in the cluster core. In line with the already detected abundance of star formation, this mode of feedback being distributed over multiple sites, contrary to a single central source observed at low redshifts, points to accretion of gas into the cluster centre. This also suggests a 'steady state' of the cluster featuring non cool-core like behaviour. Finally, we also examine the TIR-radio luminosity ratio for the known sample of galaxies within the cluster core and find that dense environments do not have any serious consequence on the compliance of galaxies to the IR-radio correlation.

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arXiv2019

Jellyfish: Resolving the kinematics of extreme ram-pressure stripping at $z\sim0.3$

We present and discuss results from the first spatially resolved kinematic study of ram-pressure stripping of a massive late-type galaxy at intermediate redshifts. Our target, the spectacular "jellyfish" galaxy A1758N\_JFG1, was previously identified as a fast-moving member of the equal-mass merger A1758N ($z=0.28$) with a star-formation rate of 48 M$_\odot$ yr$^{-1}$, far above the galaxy main sequence. IFU data obtained by us unambiguously confirm ram-pressure stripping as the physical mechanism driving the optical morphology and high star-formation rate of this system by revealing extended [\ion{O}{2}]$λ$3727Å emission up to 40 kpc (in projection) downstream, as well as an ordered radial-velocity field generated by (a) conservation of angular momentum of the interstellar gas stripped from the edge of the galactic disk and (b) drag forces exerted by the intra-cluster medium on the "tentacles" of stripped material. We find no evidence of significant nuclear activity in A1758N\_JFG1, although an AGN might, at this early stage of the stripping process, be obscured by high column densities of gas and dust near the galactic core. Finally, our exploration of possible trajectories of A1758N\_JFG1 found solutions consistent with the notions (a) that the A1758N merger proceeds along an axis that is substantially inclined with respect to the plane of the sky and (b) that A1758N\_JFG1 participated in the merger, rather than having been accreted independently from the field.

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arXiv2019

Jellyfish: Ram-pressure stripping as a diagnostic tool in studies of cluster collisions

Prompted by the discovery of A1758N_JFG1, a spectacular case of ram-pressure stripping (RPS) in the galaxy cluster A1758N, we investigate the properties of other galaxies suspected to undergo RPS in this equal-mass, post-collision merger. Exploiting constraints derived from Hubble Space Telescope images and Keck longslit spectroscopy, our finding of apparent debris trails and dramatically enhanced star formation rates in an additional seven RPS candidates support the hypothesis that RPS, and hence rapid galaxy evolution in high-density environments, is intricately linked to cluster collisions. Unexpectedly, we find the vast majority of RPS candidates in A1758N to be moving toward us, and in a shared direction as projected on the plane of the sky. We hypothesize that this directional bias is the result of two successive events: (1) the quenching, during and after the first core passage, of star formation in galaxies with an approximately isotropic velocity distribution within the central region of the merger, and (2) RPS events triggered in late-type galaxies falling into the merging system along a filament, possibly enhanced by a shock front expanding into the outskirts of the south-eastern subcluster. Since this explanation implies that the merger axis of A1758N must be significantly inclined with respect to the plane of the sky, our findings open the possibility of RPS events becoming important diagnostic tools to constrain the geometry of cluster collisions that, due to the orientation of the merger axis, lack the classic observational signatures of face-on mergers.

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