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  • The HI Properties of
    Dwarf Galaxies Hosting
    Massive Black Holes

    Introduction

    Supermassive black holes (BHs) live at the heart of essentially all massive galaxies with bulges. BHs power active galactic nuclei (AGN) and are thought to be important agents in the evolution of their hosts. Using optical spectroscopy from the SDSS, we have assembled the largest sample of low mass (dwarf) galaxies hosting massive BHs to date (Reines et al. 2013). These dwarf galaxies have stellar masses comparable to the Magellanic Clouds (\(M_{\rm stellar} ∼ 10^{8.5} − 10^{9.5} M_{\odot}\)) and contain some of the least-massive BHs known in galaxy nuclei (\(M_{\rm BH} ∼ 10^5 M_{\odot}\)). Determining the impact of AGN on thier low-mass host galaxies will help constrain BH feedback and galaxy formation models at all mass scales.

    We propose new 21 cm neutral hydrogen (HI) emission observations to study the properties of the least-massive galaxies known to contain massive BHs. By examining BH to HI scaling relations at the sparsely-populated low-mass end, these new observations will enable us to inspect the correlation of gas content and dynamical masses to the BH and galaxy properties of our BH hosts. In order to separate the effects of AGN feedback from environmental effects, we will compare our isolated dwarf galaxies that host BHs against the isolated control sample of Bradford, et al., (in prep.). The results of our study will provide vital information on the formation and evolution of the smallest galaxies hosting massive BHs as well as the effect of AGN feedback on the cold HI gas content of low mass galaxies.

    Figure 1: BPT [OIII]/H\(\beta\) vs. [NII]/H\(\alpha\) narrow-line diagnostic diagram for all BH host candidates. All BPT AGN and broad line candidates are shown in grey, while the specific BH hosts in the target selection window for Arecibo are in black.

    Background

    Until recently, very few dwarf galaxies were known to host massive BHs. The prototypical examples are NGC 4395, a dwarf spiral, and Pox 52, a dwarf elliptical. Both galaxies exhibit clear AGN signatures and the BH masses are estimated to be \(M_{\rm BH} = 3 \times 10^5 M_{\odot}\)(e.g., Filippenko & Ho 2003; Barth et al. 2004; Peterson et al. 2005). Reines et al. (2011) found multi-wavelength evidence for the first example of a massive BH in a dwarf starburst galaxy, Henize 2-10, including Chandra and VLA point sources. Follow-up VLBI observations reveal parsec-scale non-thermal radio emission from the active nucleus (Reines & Deller 2012). Systematic searches for active low-mass BHs have revealed more than 200 galaxies with \(M_{\rm BH} \leq 10^{6.5} M_{\odot}\) (Greene & Ho 2007; Dong X.-B. et al. 2012), however, the vast majority of the galaxies in these samples are larger and more massive than typical dwarf galaxies (Greene et al. 2008; Jiang et al. 2011; Dong,X.-B. et al. 2012).

    We have undertaken the first systematic search for AGN in dwarf galaxies with stellar masses less than or comparable to the Large Magellanic Cloud (LMC), increasing the number of known dwarfs hosting massive BHs by an order of magnitude (Reines et al. 2013). Starting with a sample of ~ 25, 000 emission-line galaxies in the SDSS DR8 spectroscopic catalog with stellar masses \(M_{\rm stellar} \lesssim 3 \times 10^9 M_{\odot}\) (~ LMC) and redshifts \(z \lesssim 0.055\), we have undertaken the first systematic search for AGN in dwarf galaxies with stellar masses less than or comparable to the Large Magellanic Cloud (LMC). In Reines et al. 2013, we presented 151 dwarfs galaxies exhibiting optical signitures of AGN, increasing the number of known dwarfs hosting massive BHs by an order of magnitude. Most of these galaxies fall in the AGN or composite region of the [OIII]/H\(\beta\) vs. [NII]/H\(\alpha\) (i.e., BPT) diagnostic diagram, indicating that the gas is (at least partially) photoionized by an accreting BH (see Figure X).

    AGN feedback is thought to be responsible for quenching star formation in massive galaxies and BH outflows have been invoked to explain star formation triggering in a dense interstellar medium. However, the role of BH feedback in low-mass galaxies is unconstrained and has hitherto been neglected in models of galaxy formation and evolution. For the first time, we now have a large sample of AGN in dwarf galaxies and can begin to investigate the impact of BH feedback in these objects. Neutral hydrogen observations are an important first step, providing valuable measurements of the global HI gas content with the galaxies.