A Deeply Buried Narrow-Line Seyfert 1 Nucleus Uncovered in Scattered Light

Abstract

We present spectropolarimetric and spectrophotometric observations of the peculiar active galactic nucleus (AGN) SDSS J120300.19+162443.7 (hereafter J1203+1624) at $z=0.1656$. Its optical total flux spectra clearly show broad emission lines (BELs) in H$\hat{I}\pm$ and H${\hat{I}}^2$. After removal of narrow emission lines (NELs), the full width at half maximum of the lorentzian BEL is $FWH{M}_{\mathrm{BEL}}\approx 1,270$ km s${}^{-1}$, fulfilling the conventional definition of a narrow line Seyfert 1 (NLS1) galaxy. However, its NELs are unprecedentedly strong when compared to type 1AGNs. This, together with its large MIR excess ($g-W_4=13.172$ mag), implies that the observer and the narrow emission line region (NELR) might see a different ionization continuum. Our optical spectropolarimetry confirms its type 2 nature by detecting a polarized blue continuum and Balmer BELs ($FWH{M}_{\mathrm{Polarized}\mathrm{BEL}}\approx 1,183$ km s${}^{-1}$), with a high polarization degree of $>20$% in the blue wing. The mass and Eddington rate of the central black hole is estimated based on both transmitted and scattered AGN radiation, which is $M_{\bullet }<2.9\times 10^7{M}_{\odot }$ and $L_{\mathrm{bol}}/L_{\mathrm{Edd}}>1.5$. Severe extinction of the AGN emission also enables a clear view of the compact host galaxy. Discovery of J1203+1624 suggests that NLS1 follows AGN unfication scheme, studying of its analogs could blaze a new trail for exploring the connection between black hole growth and star formation in the host galaxy. Interesting features of J1203+1624 like the peculiar NELs, inflowing scattering clouds within sublimation radius are worth detailed follow-ups in the future.

9 pages, 6 figures