HST Images and KPNO Spectroscopy of the Binary Black Hole Candidate SDSS J153636.22+044127.0

TL;DR

This study uses HST imaging and spectroscopy to investigate a candidate binary supermassive black hole, finding no evidence of orbital motion or amplitude changes over 0.75 years, challenging previous binary interpretations.

Contribution

The paper provides high-resolution imaging and spectroscopic data that refute the binary black hole hypothesis for SDSS J153636.22+044127.0, offering new insights into its spectral features.

Findings

No spatial offset of broad line components.

No velocity shift over 0.75 years within 40 km/s.

Spectral features are unique among double-peaked emitters.

Abstract

We present HST WFPC2/PC images and KPNO 4-m longslit spectroscopy of the QSO SDSS J153636.22+044127.0, which we advanced as a candidate binary supermassive black hole. The images reveal a close companion coincident with the radio source identified by Wrobel & Laor (2009). It appears to be consistent with a M_g ~ -21.4 elliptical galaxy, if it is at the QSO redshift. The spectroscopy, however, shows no spatial offset of the red or blue Balmer line subcomponents. The companion is thus not the source of either the red or blue broad line systems; SDSS J153636.22+044127.0 cannot be explained as a chance superposition of objects, or as an ejected black hole. Over the Delta T=0.75 yr difference between the rest frame epochs of the present and SDSS spectroscopy, we find no velocity shift to within 40 km/s, nor any amplitude change in either broad line system. The lack of a shift can be admitted under the binary hypothesis if the implied radial velocity is a larger component of the full orbital velocity than was assumed in our earlier work. A strong test of the binary hypothesis requires yet longer temporal baselines. The lack of amplitude variations is unusual for the alternative explanation of this object as a "double-peaked" emitter; we further argue that SDSS J153636.22+044127.0 has unique spectral features that have no obvious analogue with other members of this class.