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 presented here were also identified by Saul et al. (2012) as being likely galaxies because they cannot be easily associated with known high velocity cloud (HVC) complexes or Galactic gas in position-velocity space.

Optical Imaging


We performed follow-up optical imaging of 22 of the clouds from Grcevich (2013). These observations were performed with the pODI instrument on the WIYN Telescope in the \(g\) and \(r\)-band filters, with integration times of 600-1200 sec per filter per target. Standard imaging reductions were performed by the ODI Portal, Pipeline, and Archive facility. These include bias subtraction, flat-fielding, and alignment of individual Orthogonal Transfer Array (OTA) cells into chips. The SWarp program (Bertin et al., 2002) was used to combine the individual exposures, and DAOPHOT (Stetson, 1987) was used to perform PSF-fitting photometry on stars in the field.

Most of the clouds did not have optical counterparts with morphologies like nearby galaxies within the \(\sim 4'\) GALFA-HI beam. Those in the Sloan Digital Sky Survey (SDSS, Ahn et al., 2014) footprint show neither diffuse features like the galaxies described below, nor point source overdensities to the limit of the DR 10 catalog. Similarly, our deeper pODI imaging showed neither overdensities nor Red Giant Branch (RGB) features in the color-magnitude diagrams (CMD) down to \(r \lesssim 24\) (an RGB tip distance \( > 3 \, {\rm Mpc}\)) for any of the targets we observed other than the two described below.

Only two objects showed nearby dwarf galaxy-like optical counterparts within the GALFA-HI beam. The pODI images of these two candidates are shown in the upper panels of Figure \ref{fig:images}. They are also visible in the SDSS, although the SDSS catalog incompletely deblends them into a mix of stars and galaxies. Also shown in the lower panels of Figure \ref{fig:images} are images from the GALEX All-sky Imaging Survey (AIS, Morrissey et al., 2007).

The morphology of the objects in these images and presence of detectable UV flux is consistent with both being dwarf (irregular) galaxies. Additionally, the presence of such point sources resolved in ground-based imaging implies that the galaxies are relatively nearby (\(\lesssim 10\) Mpc). In particular, Pisces A (left panel of Figure \ref{fig:images}) shows point sources resolved enough to generate a CMD. We discuss this further in Section \ref{sec:distance} in the context of providing a distance estimate.

While the centroid of the optical (and GALEX) objects are offset by \(30-40"\) from the emission, this is well within the \(4'\) uncertainty from the GALFA-HI beam. All other optical counterparts within the beam are less likely to be associated with the ; they either appear stellar or are consistent with being distant background galaxies (and hence at too high a redshift to match the ). Furthermore, the H\(\alpha\) emission discussed in the next section is clearly associated with these optical conterparts, and its velocity is consistent with the , confirming the association between the optical objects and the cloud.