Searching for the Nearest Extragalactic Binary Black Hole: A Spectroscopic Study of NGC 4736
Loading...
Date
2015
Authors
Gustafsson, Annika
Journal Title
Journal ISSN
Volume Title
Publisher
University of Oregon
Abstract
Maoz et al. (1995,1996) concluded that the nearby galaxy NGC 4736 is in the late stages
of a merger event. After further investigation, Maoz et al. (2005) observed variability
in the nuclear region of NGC 4736, revealing a second unknown source of radiation in
the nucleus. Since merging systems are an ideal location to search for binary black
holes (BBH), we hypothesized that the second source could be a second black hole,
making this a potential BBH system. Observational evidence for the existence of BBH
remains sparse, even though BBH are predicted by many theories. To date, only NGC
6240 (Komossa et al., 2003) and Arp 299 (Ballo et al., 2004) have been discovered as
merging galaxies with two active galactic nuclei (AGN). In 2008, NGC 4736 was
observed with the Gemini-North telescope. We can classify the nature of the unknown
source by looking at the optical line ratios following Ho et al. (1997). High signal-tonoise
spectra of the unknown source displayed strong emission of [SII] and [NII], but
extremely weak [OIII] emission. The unknown source has a calculated [NII]/[Hα] ratio
of 1.37 and an upper limit of 0.6 for the [OIII]/[Hβ] ratio. Placing the unknown source
on the BPT-NII diagram (Baldwin et al., 1981), we tentatively conclude that it is a
second black hole potentially making NGC 4736 the nearest BBH system. The result will
enable future observations of a low-luminosity system in extremely late stages of
merging, which will be a significant step forward in validating models of galaxy mergers
and AGN activity that further our understanding of galaxy formation and evolution.
Description
24 pages
Keywords
Galaxy NGC 4736, Black holes, Spectroscopy, Galaxy mergers
Citation
Gustafsson, A. (2015). Searching for the Nearest Extragalactic Binary Black Hole: A Spectroscopic Study of NGC 4736. Oregon Undergraduate Research Journal, 9(1). doi:10.5399/uo/ourj.9.1.5