arXiv subjects

Rachana Bhatawdekar

Publications and source records attributed to Rachana Bhatawdekar.

98 records · Page 6Linked to original sources

arXiv2022

First Insights into the ISM at $z>8$ with JWST: Possible Physical Implications of a High [O III]$\mathbf{λ4363}$/[O III]$\mathbf{λ5007}$

We present a detailed analysis of the rest-frame optical emission line ratios for three spectroscopically confirmed galaxies at $z>7.5$. The galaxies were identified in the James Webb Space Telescope (JWST) Early Release Observations field SMACS J0723.3$-$7327. By quantitatively comparing Balmer and oxygen line ratios of these galaxies with various low-redshift "analogue" populations (e.g. Green Peas, Blueberries, etc.), we show that no single analogue population captures the diversity of line ratios of all three galaxies observed at $z>7.5$. We find that S06355 at $z=7.67$ and S10612 at $z=7.66$ are similar to local Green Peas and Blueberries. In contrast, S04590 at $z=8.50$ appears to be significantly different from the other two galaxies, most resembling extremely low-metallicity systems in the local Universe. Perhaps the most striking spectral feature in S04590 is the curiously high [O III] $\lambda4363$/[O III] $\lambda5007$ ratio (RO3) of $0.048$ (or $0.055$ when dust-corrected), implying either extremely high electron temperatures, $\sim3\times10^4$ K, or gas densities $>10^4\ {\rm cm^{-3}}$. Observed line ratios indicate that this galaxy is unlikely to host an AGN. Using photoionization modelling, we show that the inclusion of high-mass X-ray binaries or a high cosmic ray background in addition to a young, low-metallicity stellar population can provide the additional heating necessary to explain the observed high RO3 while remaining consistent with other observed line ratios. Our models represent a first step at accurately characterising the dominant sources of photoionization and heating at very high redshifts, demonstrating that non-thermal processes may become important as we probe deeper into the Epoch of Reionization.

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arXiv2022

A Long Time Ago in a Galaxy Far, Far Away: A Candidate z ~ 12 Galaxy in Early JWST CEERS Imaging

We report the discovery of a candidate galaxy with a photo-z of z~12 in the first epoch of the JWST Cosmic Evolution Early Release Science (CEERS) Survey. Following conservative selection criteria we identify a source with a robust z_phot = 11.8^+0.3_-0.2 (1-sigma uncertainty) with m_F200W=27.3, and >7-sigma detections in five filters. The source is not detected at lambda < 1.4um in deep imaging from both HST and JWST, and has faint ~3-sigma detections in JWST F150W and HST F160W, which signal a Ly-alpha break near the red edge of both filters, implying z~12. This object (Maisie's Galaxy) exhibits F115W-F200W > 1.9 mag (2-sigma lower limit) with a blue continuum slope, resulting in 99.6% of the photo-z PDF favoring z > 11. All data quality images show no artifacts at the candidate's position, and independent analyses consistently find a strong preference for z > 11. Its colors are inconsistent with Galactic stars, and it is resolved (r_h = 340 +/- 14 pc). Maisie's Galaxy has log M*/Msol ~ 8.5 and is highly star-forming (log sSFR ~ -8.2 yr^-1), with a blue rest-UV color (beta ~ -2.5) indicating little dust though not extremely low metallicity. While the presence of this source is in tension with most predictions, it agrees with empirical extrapolations assuming UV luminosity functions which smoothly decline with increasing redshift. Should followup spectroscopy validate this redshift, our Universe was already aglow with galaxies less than 400 Myr after the Big Bang.

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arXiv2022

Hubble Asteroid Hunter: II. Identifying strong gravitational lenses in HST images with crowdsourcing

The Hubble Space Telescope (HST) archives constitute a rich dataset of high resolution images to mine for strong gravitational lenses. While many HST programs specifically target strong lenses, they can also be present by coincidence in other HST observations. We aim to identify non-targeted strong gravitational lenses in almost two decades of images from the ESA it Hubble Space Telescope archive (eHST), without any prior selection on the lens properties. We used crowdsourcing on the Hubble Asteroid Hunter (HAH) citizen science project to identify strong lenses, alongside asteroid trails, in publicly available large field-of-view HST images. We visually inspected 2354 objects tagged by citizen scientists as strong lenses to clean the sample and identify the genuine lenses. We report the detection of 252 strong gravitational lens candidates, which were not the primary targets of the HST observations. 198 of them are new, not previously reported by other studies, consisting of 45 A grades, 74 B grades and 79 C grades. The majority are galaxy-galaxy configurations. The newly detected lenses are, on average, 1.3 magnitudes fainter than previous HST searches. This sample of strong lenses with high resolution HST imaging is ideal to follow-up with spectroscopy, for lens modelling and scientific analyses. This paper presents an unbiased search of lenses, which enabled us to find a high variety of lens configurations, including exotic lenses. We demonstrate the power of crowdsourcing in visually identifying strong lenses and the benefits of exploring large archival datasets. This study shows the potential of using crowdsourcing in combination with artificial intelligence for the detection and validation of strong lenses in future large-scale surveys such as ESA's future mission Euclid or in JWST archival images.

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arXiv2021

UV Spectral-Slopes at $z=6-9$ in the Hubble Frontier Fields: Lack of Evidence for Unusual or Pop III Stellar Populations

We present new measurements of the UV spectral slope $β$ for galaxies at $z=6-9$ in the Frontier Fields cluster MACSJ0416.1-2403 and its parallel field, to an unprecedented level of low stellar mass. We fit synthetic stellar population models to the observed spectral energy distribution and calculate $β$ by fitting a power law to the best-fit spectrum. With this method, we report the derivation of rest-frame UV colours of galaxies for the Frontier Fields program extending out to $z=9$, probing magnitudes as faint as $M\mathrm{_{UV}=-13.5}$ at $z=6$. We find no significant correlation between $β$ and rest-frame UV magnitude $M_{1500}$ all redshifts, but we do find a strong correlation between $β$ and stellar mass with lower mass galaxies exhibiting bluer UV slopes. At $z=7$ the bluest median value of our sample is redder than previously reported values in the literature, whereas at $z=9$ our bluest data point has a median value of $β=-2.63_{-0.43}^{+0.52}$. Thus, we find no evidence for extreme stellar populations at $z>6$. We also observe a strong correlation between $β$ and SFR, such that galaxies with low SFRs exhibit bluer slopes. Additionally, there exists a star formation main sequence up to $z = 9$ with SFRs correlating with stellar mass. All of these relations show that $β$ values correlate with a process that drives both the overall SFR and stellar mass assembly. Furthermore, we observe no trend between $β$ and specific SFR, suggesting that $β$ is getting set by a global process driven by the scale of the galaxy.

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arXiv2019

Unbiased Differential Size Evolution and the Inside-Out Growth of Galaxies in the Deep CANDELS GOODS Fields at $1 \leq z \leq 7$

We present a size analysis of a sample of $\sim$ 49,000 galaxies from the CANDELS GOODS North and South fields using redshift-independent relative surface brightness metrics to determine an unbiased measure of the differential size evolution of galaxies at $1 \leq z \leq 7$. We introduce a novel method of removing foreground objects from distant galaxy ($z > 3$) images that makes use of the Lyman-break at 912Å, in what we call `2-D Lyman-Break Imaging'. The images used are in the rest-frame optical at $z < 3$ and progressively bluer bands at $z > 3$. They are therefore subject to K-correction and cosmological dimming effects which are tested and corrected for. We separately consider a mass-selected sample (with masses in the range 10$^9$M$_{\odot}$$\leq$M$_*$$\leq$10$^{10.5}$M$_{\odot}$) and a number density selected sample (using a constant number density of $n = 1\times10^{-4}$Mpc$^{-3}$). Instead of utilising the commonly used, but potentially biased, effective radii for size measurements, we measure the redshift-independent Petrosian radius, defined by the parameter $η$, for each galaxy for three values of $η$ and use this as a proxy for size. The evolution of the measured radii can be described by a power-law of the form $R_{Petr} = α(1+z)^β$kpc where $β< 0$. We find that the outer radius increases more rapidly, suggesting that as a galaxy grows mass is added to its outer regions via an inside-out growth. This growth is stronger for the number density selected sample, with a growth rate of nearly three in the outer radii compared to the inner. We test and confirm these results using a series of image simulations.

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arXiv2019

Gravitational Waves from Black Holes in Merging Ultra-Dwarf Galaxies

The origin of the black-hole:black-hole mergers discovered through gravitational waves with for example the LIGO/Virgo collaboration are a mystery. We investigate the idea that some of these black holes originate from the centers of extremely low-mass ultra-dwarf galaxies that have merged together in the distant past at $z>1$. Extrapolating the central black hole to stellar mass ratio suggests that the black holes in these mergers could arise from galaxies of masses $\sim 10^{5} - 10^{6}$ M$_{\odot}\,$. We investigate whether these galaxies merge enough, or too much, to be consistent with the observed GW rate of $\sim 9.7-101$ Gpc$^{-3}$ yr$^{-1}$ using the latest LIGO/Virgo results. We show that in the nearby universe the merger rate and number densities of ultra-dwarf galaxies are too low, by an order or magnitude, to produce these black hole mergers. However, by considering that the merger fraction, merger-time scales, and the number densities of low-mass galaxies all conspire at $z>1-1.5$ to increase the merger rate for these galaxies at higher redshifts we argue that it is possible that some of the observed GW events arise from BHs in the centers of low-mass galaxies. The major uncertainty in this calculation is the dynamical time-scales for black holes in low-mass galaxies. Our results however suggest a very long BH merger time-scale of 4-7 Gyr, consistent with an extended black hole merger history. Further simulations are needed to verify this possibility, however our theory can be tested by searching for host galaxies of gravitational wave events. Results from these searches would will put limits on dwarf galaxy mergers and/or the presence and formation mechanisms of black holes through PopIII stars in the lowest mass galaxies.

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arXiv2019

Evolution of the galaxy stellar mass functions and UV luminosity functions at $z=6-9$ in the Hubble Frontier Fields

We present new measurements of the evolution of the galaxy stellar mass functions (GSMF) and UV luminosity functions (UV LF) for galaxies from $z=6-9$ within the Frontier Field cluster MACSJ0416.1-2403 and its parallel field. To obtain these results, we derive the stellar masses of our sample by fitting synthetic stellar population models to their observed spectral energy distribution with the inclusion of nebular emission lines. This is the deepest and farthest in distance mass function measured to date and probes down to a level of M$_{*} = 10^{6.8}M_{\odot}$. The main result of this study is that the low-mass end of our GSMF to these limits and redshifts appears to become steeper from $-1.98_{-0.07}^{+0.07}$ at $z=6$ to $-2.38_{-0.88}^{+0.72}$ at $z=9$, steeper than previously observed mass functions at slightly lower redshifts, and we find no evidence of turnover in the mass range probed. We furthermore demonstrate that the UV LF for these system also appears to show a steepening at the highest redshifts, without any evidence of turnover in the luminosity range probed. Our $M_{\mathrm{UV}}-M_{*}$ relation exhibit shallower slopes than previously observed and are in accordance with a constant mass-to-light ratio. Integrating our GSMF, we find that the stellar mass density increases by a factor of $\sim15_{-6}^{+21}$ from $z=9$ to $z=6$. We estimate the dust-corrected star formation rates (SFRs) to calculate the specific star formation rates ($\mathrm{sSFR}=\mathrm{SFR/M_{*}}$) of our sample, and find that for a fixed stellar mass of $5\times10^{9}M_{\odot}$, sSFR $\propto(1+z)^{2.01\pm0.16}$. Finally, from our new measurements, we estimate the UV luminosity density ($ρ_{\textrm{UV}}$) and find that our results support a smooth decline of $ρ_{\textrm{UV}}$ towards high redshifts.

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arXiv2018

MUSE spectroscopy and deep observations of a unique compact JWST target, lensing cluster CLIO

We present the results of a VLT MUSE/FORS2 and Spitzer survey of a unique compact lensing cluster CLIO at z = 0.42, discovered through the GAMA survey using spectroscopic redshifts. Compact and massive clusters such as this are understudied, but provide a unique prospective on dark matter distributions and for finding background lensed high-z galaxies. The CLIO cluster was identified for follow up observations due to its almost unique combination of high mass and dark matter halo concentration, as well as having observed lensing arcs from ground based images. Using dual band optical and infra-red imaging from FORS2 and Spitzer, in combination with MUSE optical spectroscopy we identify 89 cluster members and find background sources out to z = 6.49. We describe the physical state of this cluster, finding a strong correlation between environment and galaxy spectral type. Under the assumption of a NFW profile, we measure the total mass of CLIO to be M$_{200} = (4.49 \pm 0.25) \times 10^{14}$ M$_\odot$. We build and present an initial strong-lensing model for this cluster, and measure a relatively low intracluster light (ICL) fraction of 7.21 $\pm$ 1.53% through galaxy profile fitting. Due to its strong potential for lensing background galaxies and its low ICL, the CLIO cluster will be a target for our 110 hour JWST 'Webb Medium-Deep Field' (WMDF) GTO program.

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