arXiv subjects

Abhijeet Borkar

Publications and source records attributed to Abhijeet Borkar.

11 recordsLinked to original sources

arXiv2026

Two Peas in a Pod: The First Confirmed Dual Active Galactic Nucleus within a Green Pea Galaxy System

The growth of galaxies in the early Universe is thought to be dominated by compact, intensely star-forming systems, yet the corresponding growth of supermassive black holes (SMBHs) within such environments remains poorly constrained. Green Pea galaxies are nearby analogs of rapidly assembling galaxies in the early Universe owing to their compact morphologies, intense star formation, low metallicities, and extreme ionization conditions. Although galaxy interactions are thought to trigger episodes of rapid SMBH growth, direct observations of simultaneous accretion onto multiple SMBHs in compact, intensely star-forming galaxies are lacking. Here we report the discovery of the first confirmed dual active galactic nucleus (AGN) in a Green Pea system, SDSS J162209.41+352107.5. Chandra imaging resolves two luminous hard X-ray sources separated by 8.4 kpc in projection, demonstrating simultaneous accretion onto two SMBHs. Follow-up Keck spectroscopy confirms that the two optical nuclei share a common redshift and independently exhibit broad Balmer emission and high-ionization AGN emission lines. Unlike most known dual AGN, which are typically found in massive mergers, J162209.41+352107.5 is a compact low-mass system analogous to galaxies thought to dominate early phases of galaxy assembly. These findings demonstrate that efficient growth of multiple SMBHs can occur in such environments and establish Green Pea galaxies as nearby laboratories for investigating the interplay between galaxy interactions, star formation, and black-hole growth under conditions analogous to those prevalent in the young Universe.

astro-ph.GA↗

arXiv2026

Blueberry and Green Pea galaxies live in low density environments

Little is currently known about the large-scale environments of Green Pea (GP) and Blueberry (BB) galaxies, which are low-mass, compact systems with extreme specific star-formation rates (sSFR). Their environments are inherently linked to their formation mechanism, and they may serve as crucial local analogues for high-redshift, reionizing galaxies. This paper aims to investigate the clustering properties of GPs and BBs, leveraging large-scale survey data to quantify their spatial distribution relative to the broader galaxy population. We here investigate a sample of these galaxies, consisting of 339 GPs $\rm (0.1 < z \le 0.33)$ and 56 BBs $\rm (0 < z \le 0.1)$, whose clustering properties we analyse relative to an extensive control sample derived from the SDSS MPA-JHU DR8 catalogue, binned by stellar mass and sSFR. We use the number of neighbours within a 5 Mpc radius as a proxy for environmental density, i.e. clustering, and employ a pair-matching and bootstrapping methodology to ensure statistical robustness. We observe that galaxy clustering depends strongly on star-formation activity, with passive galaxies being more clustered than their high star-formation rate counterparts, with GPs and BBs lying at the extreme end of this relation, exhibiting the lowest neighbour counts among all subsamples. The nearest neighbours of BBs also tend to have lower masses than other classes of dwarf galaxies. GPs and BBs predominantly reside in isolated, low-density environments, suggesting that their intense starbursts are unlikely to be triggered by common environmental processes such as mergers or starburst cycles. Their low metallicities and weak clustering instead support scenarios in which recent starbursts are driven by internal processes or pristine gas accretion, reinforcing their role as nearby analogues of young, low-mass galaxies in the early Universe.

astro-ph.GA↗

arXiv2026

Discovery of ultra-fast outflows with v$_{\rm out}>0.3 \rm c$ in local bright active galactic nuclei

Ultra-fast outflows (UFOs) are mildly relativistic (outflow velocity $v_{out}>0.1c$) nuclear winds detected as blueshifted absorption lines from highly ionized, dense gas in the X-ray spectra of active galactic nuclei. The AGN feedback mechanism is believed to be powered by these outflows, which can inject a large amount of energy and momentum into the surrounding interstellar medium, shaping the coevolution of the AGNs and their host galaxies. We performed a systematic search and rigorous statistical assessment of the presence of UFOs in the 7-12 keV band, in a sample of bright local AGNs. This study also aims to understand whether the presence and characteristics of UFOs depend on the state of the sources, by studying the relations between the incidence of UFOs and the accretion properties of AGNs. We collected X-ray spectroscopic flux-limited XMM data of 33 observations of local (z<0.2) type 1 AGNs. We modeled their spectra in the 2-12 keV band using a combination of direct-continuum and reflection components and searched for absorption features. This represents the first systematic search for UFOs up to 12 keV. We performed Monte Carlo simulations to assess the statistical significance of the detected lines. We report strong detections of UFOs in six sources of the sample at the >95% confidence level via MC simulations, corresponding to a fraction of 18% in our sample. From the observed energies of each absorption line, we evaluated the respective wind velocities, which in some cases exceed 40\% of the speed of light. The velocity distribution found in this work is therefore shifted to higher energies than those found in previous searches for UFOs in local sources, which were limited to 10 keV. Moreover, our analysis shows no correlation between the accretion properties of the SMBHs and the presence of winds. Furthermore, our study highlights the temporal variability of UFOs.

astro-ph.HE↗

arXiv2026

Optical and cm follow-ups of the Changing-Look event in Mkn 590

The Changing-Look active galactic nucleus Mkn 590 is currently in a rejuvenated state, exhibiting a contemporaneous flux rise across X-rays, UV, optical and cm wavelengths. In this study, we present three new optical spectra obtained with the Nordic Optical Telescope, alongside three 1.4 GHz continuum measurements from the Giant Meterwave Radio Telescope, acquired since Nov. 2024. We identified a clear increase in the broad hydrogen Balmer line emission in the most recent observational epochs. Additionally, the core radio flux densities appear to track the overall X-ray variability, suggesting a possible connection between the accretion flow and jet activity. Based on these data, we aim to explore the evolution of the circumnuclear gas in this source and potential links between accretion and ejection activity.

astro-ph.HE↗

arXiv2025

XMM-Newton Conclusively Identifies an Active Galactic Nucleus in a Green Pea Galaxy

Green Pea galaxies are a class of compact, low-mass, low-metallicity star-forming galaxies in the relatively local universe. They are believed to be analogues of high-redshift galaxies that re-ionised the universe and, indeed, the James Webb Space Telescope (JWST) is now uncovering such populations at record redshifts. Intriguingly, JWST finds evidence suggestive of active galactic nuclei (AGN) in many of these distant galaxies, including the elusive Little Red Dots, that broadly lack any detectable X-ray counterparts. Intuitively, one would expect to detect an AGN in their low-redshift analogues with X-rays, yet no study to date has conclusively identified an X-ray AGN within a Green Pea galaxy. Here we present the deepest X-ray campaign of a Green Pea galaxy performed to date, obtained with the goal of discerning the presence of a (potentially low-luminosity) AGN. The target $-$ SDSS J082247.66 +224144.0 (J0822+2241 hereafter) $-$ was previously found to display a comparable X-ray spectral shape to more local AGN ($Γ$ $\sim$ 2) and a high luminosity ($L_{2-10\,{\rm keV}}$ $\sim$ 10$^{42}$ erg s$^{-1}$). We show that over 6.2 years (rest-frame), the 2$-$10 keV luminosity of J0822+2241 is constant, whereas the soft 0.5$-$2 keV flux has decreased significantly by $\sim$60%. We discuss possible scenarios to explain the X-ray properties of J0822+2241, finding transient low-column density obscuration surrounding an AGN to be the only plausible scenario. J0822+2241 thus provides further evidence that low-luminosity AGN activity could have contributed to the epoch of reionisation, and that local analogues are useful to derive a complete multi-wavelength picture of black hole growth in high redshift low luminosity AGN.

astro-ph.GA↗

arXiv2024

An Intermediate Mass Black Hole Hidden Behind Thick Obscuration

Recent models suggest approximately half of all accreting supermassive black holes (SMBHs; $M_{\rm BH}$ $\gtrsim$ 10$^{5}$ M$_{\odot}$) are expected to undergo intense growth phases behind Compton-thick ($N_{\rm H}$ $>$ 1.5 $\times$ 10$^{24}$ cm$^{-2}$) veils of obscuring gas. However, despite being a viable source for the seeding of SMBHs, there are currently no examples known of a Compton-thick accreting intermediate mass black hole (IMBH; $M_{\rm BH}$ $\sim$ 10$^{2}$ $-$ 10$^{5}$ M$_{\odot}$). We present a detailed X-ray spectral analysis of IC 750 $-$ the only AGN to-date with a precise megamaser-based intermediate mass $<$ 10$^{5}$ M$_{\odot}$. We find the equivalent width of neutral 6.4 keV Fe K$α$ to be 1.9$^{+2.2}_{-1.0}$ keV via phenomenological modelling of the co-added 177 ks Chandra spectrum. Such large equivalent widths are seldom produced by processes other than fluorescence from dense obscuration. We fit three physically-motivated X-ray spectral models to infer a range of possible intrinsic 2$-$10 keV luminosity posteriors that encompass the systematic uncertainties associated with a choice of model. Despite a wide range of predicted intrinsic 2$-$10 keV luminosities between $\sim$ 10$^{41}$ and 10$^{43}$ erg s$^{-1}$, all three models agree that IC 750 has a Compton-thick line-of-sight column density to $>$ 99\% confidence. Compton-thick obscuration is well-documented to impinge substantial bias on the pursuit of SMBH AGN. Our results thus provide the first indication that Compton-thick obscuration should also be properly considered to uncover and understand the IMBH population in an unbiased manner.

astro-ph.HE↗

arXiv2024

Radio properties of Green Pea galaxies

Green Peas (GPs) are young, compact, star-forming dwarf galaxies, and local (z~0.3) analogues of the early galaxies (z>6) considered to be mainly responsible for the reionisation of the Universe. Recent X-ray observations of GPs have detected high excess emission which cannot be accounted for by star formation alone, and implies presence of an active galactic nucleus (AGN). We employ radio observations to study the radio properties of GPs, build their radio spectral energy distributions, and verify the presence of AGNs. We performed new radio observations of three GPs with the Karl G. Jansky Very Large Array (JVLA) in the L, C, and X bands (1.4, 6 and 10 GHz resp.), supplemented by the data from archival observations and large radio surveys. We also analysed the archival radio data for a larger sample of GPs and blueberry (BBs) galaxies, which are lower-mass and lower-redshift analogues of the GPs. To understand the significance of the radio observations, we assess the detectability of these sources, and compare the detected radio luminosities with the expectations from theoretical and empirical relations. Two of the three targeted GPs were strongly detected ($>10σ$) in the JVLA observations and their fluxes are consistent with star formation, while the third source was undetected. Although a large fraction (~75%) of the sources from the larger archival sample of GPs and BBs could be detected with archival surveys, only a small number (<40%) are detected and their radio luminosity is significantly lower than the expectation from empirical relations. Our results show that the majority of the dwarf galaxy sample is highly underluminous. Especially towards the lower end of galaxy mass and star formation rate (SFR), the radio luminosity-SFR relation deviates from the empirical relations, suggesting that the relations established for larger galaxies may not hold towards the low-mass end.

astro-ph.GA↗

arXiv2023

Dusty plasma in active galactic nuclei

Since many years we know that dust in the form of the dusty-molecular torus is responsible for the obscuration in active galactic nuclei (AGN) at large viewing angles and thus for the classification of AGN. Recently, we gained some observational and theoretical insight into geometry of the region and the role of the dust in the dynamics of the outflow and failed winds. We will briefly touch on all these aspects, including our dust-based model (FRADO - Failed Radiatively Accelerated Dusty Outflow) of the formation of the Balmer lines in AGN.

astro-ph.GA↗

arXiv2022

Do Radio Active Galactic Nuclei Reflect X-ray Binary Spectral States?

Over recent years there has been mounting evidence that accreting supermassive black holes in active galactic nuclei (AGN) and stellar mass black holes have similar observational signatures: thermal emission from the accretion disk, X-ray corona, and relativisitic jets. Further, there have been investigations into whether or not AGN have spectral states similar to that of X-ray binaries (XRBs) and what parallels can be drawn between the two using a hardness-intensity diagram (HID). To address whether AGN jets might be related to accretion states as in XRBs, we explore whether populations of radio-AGN classified according to their radio jet morphology (Fanaroff-Riley classes I and II; FR I and II), excitation class (HERG and LERG), and radio jet linear extent (compact to giant) occupy different and distinct regions of the AGN hardness-intensity diagram (total luminosity vs. hardness). We do this by cross correlating 15 catalogs of radio galaxies with the desired characteristics from the literature with XMM Newton and Swift X-ray and ultraviolet (UV) source catalogs. We calculate the luminosity and hardness from the X-ray and UV photometry, place the sources on the AGN hardness-intensity diagram, and search for separation of populations and analogies with the XRB spectral state HID. We find that (a) FR Is and IIs, (b) HERGs and LERGs, (c) FR I-LERGs and FR II-HERGs occupy distinct areas of the HID at a statistically significant level (p-value < 0.05) and no clear evidence for population distinction between the different radio jet linear extents. The separation between FR I-LERG and FR II-HERG populations is the strongest in this work. Our results indicate that radio-loud AGN occupy distinct areas of the HID depending on the morphology and excitation class, showing strong similarities to XRBs.

astro-ph.GA↗

arXiv2017

Molecular gas in the immediate vicinity of Sgr A* seen with ALMA

We report serendipitous detections of line emission with ALMA in band 3, 6, and 7 in the central parsec of the Galactic center at an up to now highest resolution (<0.7''). Among the highlights are the very first and highly resolved images of sub-mm molecular emission of CS, H13CO+, HC3N, SiO, SO, C2H, and CH3OH in the immediate vicinity (~1'' in projection) of Sgr A* and in the circumnuclear disk (CND). The central association (CA) of molecular clouds shows three times higher CS/X (X: any other observed molecule) luminosity ratios than the CND suggesting a combination of higher excitation - by a temperature gradient and/or IR-pumping - and abundance enhancement due to UV- and/or X-ray emission. We conclude that the CA is closer to the center than the CND is and could be an infalling clump consisting of denser cloud cores embedded in diffuse gas. Moreover, we identified further regions in and outside the CND that are ideally suited for future studies in the scope of hot/cold core and extreme PDR/XDR chemistry and consequent star formation in the central few parsecs.

astro-ph.GA↗

arXiv2017

Approaching hell's kitchen: Molecular daredevil clouds in the vicinity of Sgr A*

We report serendipitous detections of line emission with the Atacama Large Millimeter/submillimeter Array (ALMA) in band 3, 6, and 7 in the central parsec down to within 1" around Sgr A* at an up to now highest resolution (<0.5") view of the Galactic Center (GC) in the sub-millimeter (sub-mm) domain. From the 100 GHz continuum and the H39αemission we obtain a uniform electron temperature around 6000 K for the minispiral. The spectral index of Sgr A* is ~ 0.5 at 100 - 250 GHz and ~ 0.0 at 230 - 340 GHz. The bright sources in the center show spectral indices around -0.1 implying Bremsstrahlung emission, while dust emission is emerging in the minispiral exterior. Apart from CS, which is most widespread in the center, H13CO+, HC3N, SiO, SO, C2H, CH3OH, 13CS and N2H+ are also detected. The bulk of the clumpy emission regions is at positive velocities and in a region confined by the minispiral northern arm, bar and the sources IRS 3 and 7. Although partly spatially overlapping with the radio recombination line (RRL) emission at same negative velocities, the relation to the minispiral remains unclear. A likely explanation is an infalling clump consisting of denser cloud cores embedded in diffuse gas. The central association of clouds (CA) shows three times higher CS/X (X: any other observed molecule) ratios than the circumnuclear disk (CND) suggesting a combination of higher excitation, by a temperature gradient and/or IR-pumping, and abundance enhancement due to UV- and/or X-ray emission. Hence, we conclude that this CA is closer to the center than the CND is to the center. Moreover, we find molecular emission at velocities up to 200 km s-1. ...

astro-ph.GA↗