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 $M_{*} \lesssim 10^{8}~M_{\odot}$ and an extremely low metallicity ($Z \sim 1/40~Z_{\odot}$), 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\ ${\rm μ}$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 $L_{\rm bol} = 1.2\times10^{43}\ {\rm erg\ s^{-1}}$, which implies a black hole mass of $M_{\rm BH} \simeq 10^{5}\ M_\odot$ 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.

Figures (5)

• Figure 1: WISE W1 and W2 band light curves of SBS 0335-052E. Blue and red circles indicate the $W1$ and $W2$ band magnitudes, respectively. Crosses denote the light curve data before correction for systematic offsets.
• Figure 2: X-ray-to-radio SED of SBS 0335-052E. The red squares and the black horizontal lines indicate the photometry of SBS 0335-052E (Table \ref{['tab:photometry']}). The red line at $\nu \lesssim 10^{16}$ Hz represents the best-fit CIGALE SED model. The contributions from dust in the host galaxy, the AGN dust torus, AGN polar dust, the AGN accretion disk, and the stellar population are shown by the magenta, yellow, orange, green, and blue curves, respectively (Section \ref{['sec:AGN_scenario']}). The dotted black line indicates the intrinsic X-ray spectrum predicted from the best-fit SED (Section \ref{['sec:appendix']}). The red dotted and grey solid lines present the net transmitted X-ray spectrum calculated from the intrinsic X-ray spectrum with the hydrogen column density of $\log (N_{\rm H}/{\rm cm^{-2}})=23.2$ (with $Z=Z_{\odot}$) and $24.8$ (with $Z=Z_{\odot}/40$), respectively. The red shade indicates the net transmitted AGN X-ray spectrum range allowed if we take $\log (N_{\rm H}/{\rm cm^{-2}})=24.8$ (with $Z=Z_{\odot}/40$) as the lower limit of $N_{\rm H}$. The purple curve indicates the model of the unobscured galaxy's X-ray emission, and the red solid curve at $\gtrsim 10^{16}$ Hz represents the sum of the unobscured galaxy X-ray emission and the transmitted X-ray emission for the hydrogen column density of $\log (N_{\rm H}/{\rm cm^{-2}})=24.8$ (with $Z=Z_{\odot}/40$).
• Figure 3: WISE W1 and W2 band fluxes of SBS 0335-052E (Circles). The solid line is the best-fit linear regression line ($y=\alpha + \beta \times x$), and the dot-dashed lines indicate the $16-84$% percentile range of the regression. The dashed line shows a reference slope of $\beta = 0.86$, which is a typical flux–flux slope for unobscured AGNs 2022MNRAS.516.2876M.
• Figure 4: Keck/LRIS optical spectrum of SBS 0335-052E obtained in 2021 (black line). The solid cyan, solid purple, and dashed red curves represent the best-fit emission lines of the narrow, medium, and broad components, respectively. The blue curve denotes the total fluxes of the best-fit emission lines. The grey shade indicates the spectrum contaminated by night sky emissions, which are masked out in our emission-line model fitting. The dashed grey line represents the best-fit continuum emission. The grey line shows the VLT/FORS optical spectrum taken in 2002 reported by 2009AA...503...61I, scaled so that its flux at 6500 Å matches that of the Keck/LRIS spectrum for easy comparison.
• Figure 5: Stellar mass to BH mass relation. The red star mark represents the BH mass $M_{\rm BH}$ (Equation \ref{['eqn:eddington_luminosity']}) and the total stellar mass $M_{*}$ of SBS 0335-052E, inferred from our best-fit SED. The downward arrow indicates the upper limit of the BH mass derived from the spectral fitting for the emission line (Equation \ref{['eqn:mass_estimator']}). The black crosses are the BH masses and total stellar masses of local broad-line dwarf galaxies taken from 2015ApJ...813...82R. The blue and green circles represent the least-massive local AGNs RGG 118 2015ApJ...809L..14B and POX 52 2008ApJ...686..892T, respectively. The magenta, orange, and purple circles denote JWST-detected broad-line galaxies at $z \gtrsim 4$ identified by 2023arXiv230200012K, 2023AA...677A.145U, and 2023arXiv230311946H, respectively, where we omit two objects of 2023arXiv230200012K from the data points of 2023arXiv230311946H. The black solid line and the grey shade show the best-fit line and $1\sigma$ error of the local relation in the range of $M_{\rm BH}=10^5-10^{8.5} M_\odot$ provided by 2015ApJ...813...82R. The dotted lines present the sum of intrinsic scatter and measurement uncertainty in the local relation.