Searching for the Nearest Extragalactic Binary Black Hole: A Spectroscopic Study of NGC 4736
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In 1995 and 1996, Maoz et al, concluded that the nearby galaxy NGC 4736 (d==I6 million light years) is in the late stages of a merger event. After further investigation, in 2005, Maoz et al, observed UV variability in the nuclear region of NGC 4736, revealing a second unknown source in the nucleus. Since late stage mergers are an ideal location to search for binary black holes (BBH), members of our team hypothesized that the second source could be a second black hole, making this a potential BBH system. This is important since observational evidence for their existence remains sparse, even though BBH are predicted by many theories and potentially plan an important role in galaxy evolution. To date, only two systems have been categorized as mergers with dual active galactic nuclei (AGN), NGC 6240 (Komossa et al., 2003) and Arp 299 (Balla et al., 2004). In January of 2008, NGC 4736 was observed with the GMOS-N instrument on Gemini North. Optical longslit spectra of the nuclear region were obtained with spatial resolution of 0. 1454"/pixel and a spectral resolution of R-1700. At this resolution, the two nuclear sources are spatially resolved at a projected separation of 2.5". As a result, we can classify the nature of the second source by looking at the optical line ratios following Ho et al. ( 1997). High signal-to-noise spectra of the unknown source displayed strong emission of [SII] and [NII], but an extremely weak  emission line. The unknown source has a calculated [NII]/[Ha] ratio of 1.37 and an upper limit of0.6 for the [0111)/[HB] ratio. Placing the unknown source on the BPT-Nll diagram (Baldwin et al., I 98 I), we Tentatively conclude that it is a low-luminosity second black hole potentially making NGC 4736 the nearest BBH system. The result will enable future high-spectral and spatial resolution 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.