Research

📌 Galaxy Deep Field

FUV and NUV color composite image of the AstroSat UV Deep Field north (AUDFn)

Observation taken using F154W, N242W, N245M filters of the UVIT

Identifying high-redshift galaxies in UV Deep fields

I am working on different aspects of the AUDF-north field. We have published the FUV and NUV photometric source catalogs of the AUDF-north field utilizing deep UVIT observations. The key aim is to explore rest-frame FUV properties of galaxies between redshift ~ 0 – 1.0. The superior quality of the AUDF data has helped us to efficiently detect fainter distant galaxies. Our FUV and NUV flux measurements of the identified sources complement existing multiband data in the GOODS-N field, and enable us to probe rest-frame FUV properties of galaxies at redshift z < 1 and search for candidate Lyman continuum leakers at redshift z > 0.97

More can be found in Mondal et al. 2023a

I am utilizing the multi-band photometric and spectroscopic data (eg., UVIT, HST, JWST, Keck) of Deep fields to investigate the following two key science aspects of distant galaxies:

(a) Properties of interstellar dust in star-forming galaxies after the cosmic noon (z ~ 0.4 - 0.8)

(b) Detection and characterization of Lyman continuum leakers around the cosmic noon (z ~ 1 - 3)

Interstellar Dust of star-forming galaxies

Evolution of dust extinction with Cosmic time. Our measurements using UVIT observation fills ~6 Gyr age gap between the cosmic noon and present day, which further supports the gradual reddening of galaxy's UV continuum with time

Is the dust content in galaxies changing with cosmic time?

Combining the measurements from our AUDFn catalog and the HST archival data, we have studied the rest-frame UV continuum slope (which indirectly probes the dust extinction) of galaxies between redshift 0.4 and 0.75. The aim is to understand the nature of dust extinction in galaxies at this epoch which is not explored well. Our measurements add new data points to the global β–z relation in the least-explored redshift regime, further reinforcing the gradual reddening of galaxy UV continuum with cosmic time.

More can be found in Mondal et al. 2023b

Our SED model-based IRX–β relation shows a preference for steeper SMC-like attenuation specially for massive galaxies at redshift ~ 0.6

Do galaxies follow the same attenuation law at different redshifts?

We used measurements from 18 different photometric bands (UVIT, HST, Spitzer, Herschel) to probe the nature of attenuation laws in 83 UV-selected galaxies at z~0.6. Our analysis reports a large diversity in attenuation laws at this epoch. We also find that massive galaxies prefer an SMC-type dust law and they have produced more dust than the less-massive galaxies.

More can be found in Mondal & Saha 2024

Lyman continuum leakers at redshift ~ 1 - 3

RGB image of the galaxy GNHeII J1236+6215 composed of HST F336W, F606W and JWST F090W bands.

GNHeII J1236+6215: A He II1640 emitting potential LyC leaker at z = 2.9803

We identified a low-mass star-forming galaxy emitting HeII1640 emission at redshift 2.9803 in the GOODS-north field. The ISM of the galaxy is highly ionized (including the signature of density-bounded ionization), metal-poor, and has low dust content. In addition, the galaxy has a compact morphology with high SFR surface density, which together makes it a potential LyC leaker. The nature of the HeII1640 line infers that the He+ ionization in the galaxy could be driven by small pockets of young Population III stars or low-metallicity Very Massive Stars (VMSs) rather than AGN or metal-rich Wolf-Rayet stars. This discovery highlights a potential connection between He+ ionization and favorable ISM conditions for the leakage of ionizing photons from galaxies.

More can be found in Mondal et al. 2025

📌 Galaxy kinematics - nearby collisional ring galaxies

Secular vs collision-driven evolution of central stellar structure - discovery of a distinct kinematic component in AM0644-741 collisional ring galaxy

The inner part of the collisional ring galaxy AM0644-741 hosts a distinct stellar structure of effective radius ~ 1kpc. It shows the signature of a disk-like rotating stellar structure with significantly high velocity dispersion. The central structure has a completely different orientation than the larger outer disk, which strengthens the presence of a distinct inner component in the post-collisional system. While the observed trend of stellar rotation implies a minor impact of the recent collision on stellar orbits within the inner part, the elevated value of the velocity dispersion and the Mg b line index could also indicate some level of dynamical heating in the central region. Our study conveys that the dynamical evolution of post-collisional systems can be unique and needs more attention to explore with detailed simulations.

More can be found in Mondal & Barway 2025

Can a stellar bar survive a drop-through collision between two galaxies?

Intensity map for the inner region of the collisional ring galaxy Cartwheel. The extent of the stellar bar and bulge are shown by ellipses.

Intensity, LOS velocity, and velocity dispersion profiles of the Cartwheel’s bar.

Our findings signify the preservation of the pre-collisional structures in the inner region of the Cartwheel, an important point to note when attempting to determine the evolution of collisional galaxy systems, particularly when investigating pre-collisional central regions in theoretical studies.

More can be found here Mondal & Barway 2024

📌 Star-forming galaxies in the Local volume

Can expanding H I shells control galaxies' star formation?

FUV bright star-forming clumps (brown contours) and the identified H I shells (black ellipses) in a specific part of the galaxy IC 2574 - demonstrate that expanding H I shells can trigger secondary star formation in galaxies.

Stellar feedback affecting the galaxy ISM

Using deep FUV imaging of the nearby irregular galaxy IC 2574, we found that 28.6% of the FUV bright star-forming clumps are located in H I shells, 12.6% inside holes, and 60.1% away from the holes. In other words, we found 30 of the 48 H I holes show triggered star formation in their shells. Cross-matching with Hα emission, we found that 23 holes have both FUV and Hα emission in their shells, signifying a very recent trigger. In conclusion, we found that star formation in IC 2574 has been partly triggered due to the expanding HI holes, whereas in the majority of the sites, it is driven by other mechanisms. Our findings indicate that internal stellar feedback can be crucial to induce star formation at a later phase of galaxy's lifetime.

More can be found here Mondal, Subramaniam & George 2019

How can an isolated dwarf galaxy (aka 'cosmic fossil') efficiently form new stars?

A star-forming dwarf irregular galaxy - WLM.

Observation taken with FUV and NUV bands of UVIT

Decoding the demographics of star-forming regions using UV color map

Combining FUV and NUV multi-band images, we identified several OB associations in the galaxy WLM, which is believed to be evolving on its own without any major interaction. Such an active star-forming state of this dwarf galaxy highlights that the secular evolution of low-mass systems can sustain star formation for a longer time. With the help of UVIT's superior angular resolution (FWHM ~ 1.2"), we could also resolve young star-forming knots and study their structural characteristics up to smaller length scales ~ 10 pc.

More can be found here Mondal, Subramaniam & George 2018 and 2021

Can flocculent spiral arms trigger star formation in galaxies?

UVIT FUV image of the galaxy NGC 7793. The red dots mark the location of the youngest (Age < 10 Myr) star-forming clumps in the galaxy. The distribution of these clumps delineates the flocculent arms of the galaxy.

Nature of star-forming clumps in a nearby flocculent spiral galaxy NGC 7793

We ﬁnd that the disk scale length estimated in Far-UV (2.64 ± 0.16 kpc) is larger than that in Near-UV (2.21 ± 0.21 Kpc) and optical (1.08 Kpc), which supports the inside-out growth scenario of the galaxy disk. Around 61% of the UV-bright star-forming clumps identiﬁed in our study are younger than 20 Myr, which points to a recent enhancement in star formation across the galaxy NGC 7793. We also find that the youngest star-forming regions, with an age of <10 Myr, distinctly trace the ﬂocculent arms of the galaxy. Our study indicates that density wave in flocculent spiral galaxies is efficient to induce recent star formation.

More can be found here Mondal et al. 2021

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