A Gravitationally Lensed Low-Luminosity AGN From The Cosmic Noon

Abstract

Strong gravitational lensing provides a unique tool of studying intrinsically faint high-redshift active galactic nuclei (AGN). In particular, low-luminosity AGN (LLAGN) can probe black-hole growth in low-mass galaxies that current X-ray surveys cannot otherwise access. We investigate the nature and physical properties of a faint X-ray source triply imaged by the galaxy cluster SDSS J2243$-$0935, and determine whether the source hosts an AGN. We combine archival Hubble Space Telescope imaging and Chandra X-ray observations with the published strong-lensing models of SDSS J2243$-$0935. We constrain the source redshift geometrically, derive magnification maps, and infer the physical properties of the host galaxy. The geometric lensing analysis gives a source redshift of $z=2.14_{-0.17}^{+0.12}$, consistent with the photometric redshift estimate, and a total magnification of $\mu =78\pm 7$. The source is triply lensed and detected only in the HST F160W band, with an intrinsic magnitude of $m_{\mathrm{AB}}=28.2\pm 0.2$. The rest-frame optical emission implies a stellar mass of $8.6\lesssim \log (M_{\star }/M_{\odot })\lesssim 9.4$. The unobscured star formation rate is constrained to ${\mathrm{SFR}}_{\mathrm{UV}}<0.009M_{\odot }{\mathrm{yr}}^{-1}$, although substantial dust obscuration remains possible. Chandra detects X-ray point-source counterparts to all three HST-detected lensed images. The combined X-ray spectrum contains $100\pm 11$ net counts and is consistent with a moderately absorbed power law, with $\Gamma \simeq 1.48$ and $N_{\mathrm{H}}=(5.5\pm 4.4)\times 10^{22}{\mathrm{cm}}^{-2}$. After correcting for gravitational magnification, we obtain an intrinsic rest-frame $2$--$10$ keV luminosity of $L_{\mathrm{X}}=1.2\pm 0.3\times 10^{42}\mathrm{erg}{\mathrm{s}}^{-1}$. The X-ray emission is likely dominated by an AGN hosted by a low-mass galaxy at $z\simeq 2.14$, making this system a strongly lensed LLAGN at cosmic noon (abridged).

arXiv comment6 pages, 3 figures, submitted