Near-infrared Variability Detected in the Young Star-Forming Dwarf Galaxy SBS 0335-052E

Abstract

SBS~0335-052E is a young star-forming dwarf galaxy with a total stellar mass of $M_{\ast }\lesssim 10^8{M}_{\odot }$ and an extremely low metallicity ($Z\sim 1/40Z_{\odot }$), which has long been considered to be devoid of an active galactic nucleus (AGN). Here we report the detection of temporal flux variability of SBS~0335-052E in near-infrared (NIR) 3-4\ $\hat{\mathrm{I}}¼$m bands on timescales of several years, showing dimming and brightening of up to 50\% over 14~years, based on archival data from the Wide-field Infrared Survey Explorer. Our spectral energy distribution (SED) fitting of archival ultraviolet (UV)-NIR photometry, including AGN SED models, indicates that the variable NIR emission arises from an edge-on AGN dust torus. The UV-optical emission from the accretion disk is obscured and does not reach us, leading to the dominance of the host galaxy's young stellar population in the UV-optical wavelengths. This analysis favors the presence of a Compton-thick, heavily obscured AGN in SBS~0335-052E, consistent with its observed X-ray weakness. From the SED fitting, we estimate an AGN bolometric luminosity of $L_{\mathrm{bol}}=1.2\times 10^{43}\mathrm{erg}{\mathrm{s}}^{-1}$, which implies a black hole mass of $M_{\mathrm{BH}}\simeq 10^5{M}_{\odot }$ if the AGN is accreting at the Eddington limit. If confirmed, SBS~0335-052E would be the least massive galaxy known to host an AGN, likely harboring an intermediate-mass black hole.

19 pages, 5 figures, accepted for publication in ApJ