arXiv subjects

Arpita Ganguly

Publications and source records attributed to Arpita Ganguly.

4 recordsLinked to original sources

arXiv2026

Persephone's Torch: A 15th Magnitude Quadruply-Lensed Quasar From the Couch Discovered with SPHEREx and the LBT

Here we report the spectroscopic and geometric confirmation of an extremely bright ($i=14.77$) and compact (Einstein radius of $\sim0.45''$) quadruply-lensed quasar at $z=2.22$, J1330$-$0905, which we dub Persephone's Torch. The system had been previously selected as a candidate lensed quasar based on large-area survey data; here we confirm its quasar nature and redshift using public spectrophotometry from the SPHEREx mission, a.k.a. "from the couch". Adaptive optics imaging with LBT/LUCI resolves four images in a "circular kite" configuration. The system is the brightest gravitationally-lensed quasar system ever found. While an elliptical power-law mass distribution plus external shear accurately reproduces the locations of the images and lensing galaxy, and predicts a total magnification of $\sim56$, the brightnesses of the lensed images present highly anomalous flux ratios. Together with short time delays between images ($\leq 2$ days), this makes Persephone's Torch a promising candidate for future microlensing studies. Our discovery highlights the potential of SPHEREx full-sky infrared spectrophotometry to uncover extraordinarily bright objects that have otherwise been overlooked.

astro-ph.GA↗

arXiv2026

Three Hundred Quasars from the Couch: A first look at high-redshift quasar discovery with SPHEREx

Photometric selection of luminous high-redshift ($z\gtrsim4$) quasars is plagued by contamination from numerous low-mass Galactic stars, reddened lower-redshift quasars, as well as compact luminous red galaxies. Confirmation of these rare objects thus requires extensive spectroscopic campaigns on 4 and 8-meter-class telescopes with relatively low success rates. Here we demonstrate the utility of SPHEREx spectrophotometric survey data for quasar confirmation with no ground-based follow-up required, "from the couch," applied to candidates from a purposefully simplistic photometric and astrometric Gaia+WISE selection down to low Galactic latitudes ($|b|\geq8^\circ$). Primarily from the detection of their strong broad H$α$ emission lines, we discover 87 new luminous $4.0 < z < 5.7$ quasars with median $M_\text{1450} = -27.5$, including 19 quasars at $z>5$, and recover 219 previously published quasars at $z>4$. We validate our SPHEREx selection with a 100% confirmation rate in ground-based spectroscopic follow-up of 29 of our new $z>4$ quasars, including 11 unpublished archival spectra. We also discover 203 additional lower-redshift quasars at $0.3 < z < 4$, consisting primarily of relatively rare highly-reddened and strong broad-absorption-line objects that are likely missed by traditional quasar surveys. Finally, we show that the Ly$α$ absorption breaks and H$α$ lines of luminous quasars are already detectable at redshifts $5.7\lesssim z\lesssim6.5$ after the completion of only the first of four all-sky surveys to be performed by SPHEREx during its planned two-year mission.

astro-ph.GA↗

arXiv2026

A Cosmic Miracle: A Remarkably Luminous Galaxy at $z_{\rm{spec}}=14.44$ Confirmed with JWST

JWST has revealed a stunning population of bright galaxies at surprisingly early epochs, $z>10$, where few such sources were expected. Here we present the most distant example of this class yet -- MoM-z14, a luminous ($M_{\rm{UV}}=-20.2$) source in the COSMOS field at $z_{\rm{spec}}=14.44^{+0.02}_{-0.02}$ that expands the observational frontier to a mere 280 million years after the Big Bang. The redshift is confirmed with NIRSpec/PRISM spectroscopy through a sharp Lyman-$α$ break and $\sim3σ$ detections of five rest-UV emission lines. The number density of bright $z_{\rm{spec}}\sim14-15$ sources implied by our "Mirage or Miracle" survey spanning $\sim350$ arcmin$^{2}$ is $>100\times$ larger ($182^{+329}_{-105}\times$) than pre-JWST consensus models. The high EWs of UV lines ($\sim15{-}35$ Å) signal a rising star-formation history, with a $\sim10\times$ increase in the last 5 Myr ($\rm{SFR_{\rm{5Myr}}}/\rm{SFR_{\rm{50Myr}}}=9.9^{+3.0}_{-5.8}$). The source is extremely compact (circularized $r_{\rm{e}} = 74^{+15}_{-12}$ pc), and yet elongated ($b/a=0.25^{+0.11}_{-0.06}$), suggesting an AGN is not the dominant source of UV light. The steep UV slope ($β=-2.5^{+0.2}_{-0.2}$) implies negligible dust attenuation and a young stellar population. The absence of a strong damping wing provides tentative evidence that the immediate surroundings of MoM-z14 may be partially ionized at a redshift where virtually every reionization model predicts a $\sim100\%$ neutral fraction. The nitrogen emission and highly super-solar [N/C]$>1$ hint at an abundance pattern similar to local globular clusters that may have once hosted luminous supermassive stars. Since this abundance pattern is also common among the most ancient stars born in the Milky Way, we may be directly witnessing the formation of such stars in dense clusters, connecting galaxy evolution across the entire sweep of cosmic time.

astro-ph.GA↗

arXiv2025

Stellar Morphology of Optically Dark or Faint Galaxies at $z>3$ with JWST

JWST offers an unprecedented view of optically dark or faint galaxies (OFGs), previously missed by HST. They are likely massive, heavily dust-obscured star-forming galaxies (SFGs) that substantially contribute to the cosmic SFR density at $z>$3. To identify drivers of their high dust attenuation and their role in early universe galaxy evolution, we analyse the stellar morphology of 65 OFGs (from 1892 SFGs at 3$<z<$4) using NIRCam/F444W imaging from the PRIMER and CEERS fields. We study correlations between dust attenuation ($A_v$) and galaxy properties, like stellar mass, size, and orientation, and compare scaling relations between OFGs and typical SFGs. We find that OFGs are ~8-9 times more massive and ~4 times more dust attenuated than the parent sample. Structurally, OFGs resemble parent SFGs in median $R_e$ and median $Σ_{R_e}$ but may be slightly rounder on average. While $A_v$ strongly correlates with stellar mass, it does not show significant dependence on stellar mass-normalised effective radius and stellar mass surface density, Sérsic index, axis ratio, or SFR surface density. The mass-size and mass-surface density relations place OFGs as a higher-mass extension of SFGs, with no concrete proof of evolutionary differences between them. This suggests that OFGs are heavily dust-obscured primarily due to their high stellar masses, which facilitates dust production and retention, with older stellar populations likely contributing as well. Although some OFGs exhibit high $Σ_\mathrm{R_e}$ and occupy regions of the mass-size plane similar to quiescent galaxies, the overall sample is not representative of this. Their current structures resemble typical SFGs, with no concrete signs of rapid compaction. Diversity in their physical properties shows that OFGs span a range of evolutionary states with few showing reduced star formation, while most remain actively star-forming.

astro-ph.GA↗