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  • Two Local Volume Dwarf Galaxies Discovered in 21 cm Emission: Pisces A and B




    The properties of faint dwarf galaxies at or beyond the outer reaches of the Local Group (\(1-5\) Mpc) probe the efficiency of environmentally driven galaxy formation processes and provide direct tests of cosmological predictions (e.g., Klypin et al., 1999; Moore et al., 1999; Strigari et al., 2008; Kravtsov, 2010; Kirby et al., 2010; Boylan-Kolchin et al., 2011; Pontzen et al., 2012; Geha et al., 2013). However, searches for faint galaxies suffer from strong luminosity and surface brightness biases that render galaxies with \(L_V \lesssim 10^6 \, L_\odot\) difficult to detect beyond the Local Group (Tollerud et al., 2008; Walsh et al., 2009; Hargis et al., 2014). Because of these biases, searching for nearby dwarf galaxies with methodologies beyond the standard optical star count methods are essential.

    This motivates searches for dwarf galaxies using the 21 cm emission line of neutral hydrogen (). While such searches cannot identify passive dwarf galaxies like most Local Group satellites, which lack (Grcevich et al., 2009; Spekkens et al., 2014), they have the potential to find gas-rich, potentially starforming dwarf galaxies. This is exemplified by the case of the Leo P dwarf galaxy, found first in and later confirmed via optical imaging (Giovanelli et al., 2013; Rhode et al., 2013).

    Here we describe two faint dwarf galaxies identified via emission in the first data release of the Galactic Arecibo L-band Feed Array (GALFA-HI) survey (Peek et al., 2011). As described below, they are likely within the Local Volume (\(<10\) Mpc) but just beyond the Local Group (\(\gtrsim 1\) Mpc), so we refer to them as Pisces A and B. This paper is organized as follows: in Section \ref{sec:data}, we present the data used to identify these galaxies. In Section \ref{sec:distance}, we consider possible distance scenarios, while in Section \ref{sec:conc} we provide context and some conclusions. Where relevant, we adopt a Hubble constant of \(H_0=69.3 \; {\rm km \; s}^{-1}{\rm Mpc}^{-1}\) from WMAP9 (Hinshaw et al., 2013).



    The two galaxies we report on here were identified initially as cold clouds with possibly galaxy-like properties in DR1 of the GALFA-HI survey (Peek et al., 2011). Confirmation of these clouds as galaxies required additional optical imaging and spectroscopy, which we describe below.



    GALFA-HI was performed with the Arecibo Observatory 305-m telescope, using the ALFA feed array and the GALSPECT spectrometer. GALFA-HI DR1 (Peek et al., 2011) includes velocities \(|V_{\rm LSR}| < 650 {\, {\rm km}\, {\rm s}^{-1}}\), covers 7520 square degrees of sky from \(\delta = -1^\circ\) to \(+38^\circ\), has a channel spacing of \(0.2 {\, {\rm km}\, {\rm s}^{-1}}\), and a spatial resolution of \(4'\). The sensitivity of DR1 varies with position, but the majority of the objects cataloged would have \(M_{\rm HI} < 10^6 \, M_{\odot}\) if at \(1 \, {\rm Mpc}\). The two candidate dwarfs were first found in a GALFA-HI DR1 catalog that identified clouds with sizes \(<20 '\) and velocity \({\rm FWHMs} < 35 {\, {\rm km}\, {\rm s}^{-1}}\) (Saul et al., 2012). From the Saul et al. (2012) sample of 1964 clouds, Grcevich (2013) identified 51 candidate galaxies with fluxes and sizes similar to the known gas-rich Local Group dwarf galaxies (particularly Leo T). The two candidates pre