arXiv subjects

A. Upadhyaya

Publications and source records attributed to A. Upadhyaya.

4 recordsLinked to original sources

arXiv2024

Witnessing an extreme, highly efficient galaxy formation mode with resolved Ly$α$ and LyC emission

J1316+2614 at z=3.613 is the UV-brightest ($M_{UV}$ = -24.7) and strongest Lyman continuum (LyC, $f_{esc}^{LyC} \approx$ 90%) emitting star-forming galaxy known, showing also signatures of inflowing gas from its blue-dominated Ly$α$ profile. Here, we present high-resolution imaging with the HST and VLT of the LyC, Ly$α$, rest-UV, and optical emission of J1316+2614. Detailed analysis of the LyC and UV light distributions reveals compact yet resolved profiles, with LyC and UV morphologies showing identical half-light radii of $\simeq$ 220 pc. The continuum-subtracted Ly$α$ emission reveals an extended filamentary structure of $\simeq$ 6.0 kpc oriented south-north with only weak/residual flux within the stellar core, suggesting a Ly$α$ "hole". J1316+2614 presents remarkably high SFR and stellar mass surface densities of log($Σ_{SFR}$ [$M_{\odot}$/yr/kpc^2]) = 3.47$\pm$0.11 and log($Σ_{M}$ [$M_{\odot}$/pc^2]) = 4.20$\pm$0.06, respectively, which are among the highest observed in star-forming galaxies. Our findings indicate that J1316+2614 is a powerful, young, and compact starburst, leaking significant LyC photons due to the lack of gas and dust within the starburst. We explore the conditions for gas expulsion using a simple energetic balance and find that, given the strong binding force in J1316+2614, a high star formation efficiency ($ε_{SF} \geq 0.7$) is necessary to remove the gas and explain its exposed nature. Our results thus suggest a close link between high $ε_{SF}$ and high $f_{esc}^{LyC}$. This high efficiency can also naturally explain the remarkably high SFR, UV-luminosity, and efficient mass growth of J1316+2614, where at least 62% of its mass formed in the last 6 Myr. J1316+2614 may exemplify an intense, feedback-free starburst with a high $ε_{SF}$, similar to those proposed for UV-bright galaxies at high redshifts. (ABRIDGED)

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arXiv2024

Evidence for Very Massive Stars in extremely UV-bright star-forming galaxies at $z \sim 2.2-3.6$

We present a comprehensive analysis of the presence of very massive stars (VMS > $100 M_{\odot}$) in the integrated spectra of 13 UV-bright star-forming galaxies at $2.2 \lesssim z \lesssim 3.6$ taken with the Gran Telescopio Canarias (GTC). These galaxies have very high UV absolute magnitudes ($M_{\rm UV} \simeq -24$), intense star formation (SFR $ \simeq 100-1000$ $M_{\odot}$ yr$^{-1}$), and metallicities in the range of 12+log(O/H) $\simeq8.10-8.50$ inferred from strong rest-optical lines. The GTC rest-UV spectra reveal spectral features indicative of very young stellar populations with VMS, such as strong P-Cygni line profiles in the wind lines N~{\sc v} $λ1240$ and C~{\sc iv} $λ1550$ along with intense and broad He~{\sc ii} $λ1640$ emission with $EW_{0}$ $\simeq 1.40-4.60$ Å, and FWHM $\simeq 1150-3170$ $km \ s^{-1}$. A Comparison with known VMS-dominated sources and typical galaxies without VMS reveals that some UV-bright galaxies closely resemble VMS-dominated clusters (e.g., R136 cluster). The presence of VMS is further supported by a quantitative comparison of the observed strength of the He~{\sc ii} emission with population synthesis models with and without VMS, where models with VMS are clearly preferred. Employing an empirical threshold for $EW_{0}$ (\heii) $\geq 3.0$ Å, along with the detection of other VMS-related spectral profiles (N~{\sc iv} $λ1486, 1719$), we classify nine out of 13 UV-bright galaxies as VMS-dominated sources. This high incidence of VMS-dominated sources in the UV-bright galaxy population ($\approx 70\%$) contrasts significantly with the negligible presence of VMS in typical $L_{\rm UV}^{*}$ LBGs at similar redshifts ($<1\%$). Our results thus indicate that VMS are common in UV-bright galaxies, suggesting a different initial mass function (IMF) with upper mass limits between $175 M_{\odot}$ and $475 M_{\odot}$.

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arXiv2023

Clues on the presence and segregation of very massive stars in the Sunburst Lyman-continuum cluster at z=2.37

We report the identification of very massive stars (VMS; mass $> 100$\,\msun) that may be segregated in the center of the young massive star cluster at $z$=2.37 hosted in the lensed galaxy called {\tt Sunburst} galaxy. This result is based on two pieces of evidence: (1) VLT/MUSE spectra of several multiple images of the same star cluster show key spectral signatures of VMS, such as the \heii\ broad emission, \nivblue\ emission, and an \niv\ P-Cygni profile. In particular, \heii\ is broad ($\sim1610\pm300$ \kms), with an equivalent width of 3Å,\ and asymmetric profile. These features require an extremely young ($\sim2.5$ Myr) stellar population component in which the masses of the stars exceed 100~\msun. When a Salpeter initial mass function and BPASS models for normal massive stars are assumed, the observed spectral features require $\sim$400 VMS. (2) The same star cluster is detected at a signal-to-noise ratio of~$\sim100$ in the Lyman continuum domain ($λ< 900$Å). The Lyman continuum emission emerges from a region with a radius that is at least twice smaller than what is observed at 1700Å~(independently of magnification) and is located in the center of the cluster. After delensing, the effective radii in absolute scales are R$_{\tt eff}[{\tt LyC}]\sim4.7 \pm 1.5$ pc and R$_{\tt eff}[1700]= 7.8 \pm 1.4$ pc. The Lyman continuum radiation is mainly produced by hot and massive stars, which implies that their spatial distribution (including that of VMS) is preferentially more confined in the central parts of the cluster. Approximately 400 VMS hosted by a cluster of $\sim 10^7$ \msun\ produce $\sim$15\% of the escaping Lyman continuum photons, and the remaining photons are produced by other massive early-type stars.

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arXiv2022

An extreme blue nugget, UV-bright starburst at z=3.613 with ninety per cent of Lyman continuum photon escape

We present the discovery and analysis of J1316+2614 at z=3.6130, a UV-bright star-forming galaxy ($M_{\rm UV} \simeq -24.7$) with large escape of Lyman continuum (LyC) radiation. J1316+2614 is a young ($\simeq 10$ Myr) star-forming galaxy with $SFR \simeq 500 M_{\odot}$ yr$^{-1}$ and a starburst mass of log($M_{\star}/M_{\odot}) \simeq 9.7$. It shows a very steep UV continuum, $β_{\rm UV} \simeq -2.59 \pm 0.05$, consistent with residual dust obscuration, $E(B-V)\simeq 0$. LyC emission is detected with high significance ($\simeq 17 σ$) down to $830$Å, for which a very high relative (absolute) LyC escape fraction $f_{\rm esc} \rm (LyC) \simeq 0.92$ ($\simeq 0.87$) is inferred. The contribution of a foreground or AGN contamination to the LyC signal is discussed but is unlikely. J1316$+$2614 is the most powerful ionizing source known among the star-forming galaxy population, both in terms of production ($Q_{\rm H} \approx 10^{56}$ s$^{-1}$) and escape of ionizing photons ($f_{\rm esc} \rm (LyC) \approx 0.9$). Nebular emission in Ly$α$, H$β$, and other rest-frame optical lines are detected, but these are weak ($EW_{0} \rm [Hβ] \simeq 35$Å), with their strengths reduced roughly by $\simeq 90\%$. J1316+2614 is the first case known where the effect of large escape of ionizing photons on the strength of nebular lines and continuum emission is clearly observed. Gas inflows are detected in J1316+2614 from the blue-dominated peak Ly$α$ emission (with a blue-to-red peak line ratio $I_{\rm blue}/I_{\rm red} \simeq 3.7$) and redshifted ISM absorption ($\simeq 100$ km s$^{-1}$). Our results suggest that J1316+2614 is undergoing a gas compaction event, possibly representing a short-lived phase in the evolution of massive and compact galaxies, where strong gas inflows have triggered an extreme star formation episode and nearly $100\%$ LyC photons are escaping.

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