Alfalfa Discovery of the Nearby Gas-Rich Dwarf Galaxy Leo P. I. H I Observations

Abstract

The discovery of a previously unknown 21cm HI line source identified as an ultra-compact high velocity cloud in the ALFALFA survey is reported. The HI detection is barely resolved by the Arecibo 305m telescope ∼4 ′ beam and has a narrow HI linewidth (HPFW of 24 km s −1 ). Further HI observations at Arecibo and with the VLA corroborate the ALFALFA HI detection, provide an estimate of the HI radius, ∼ 1 ′ at the 5 × 10 19 cm −2 isophote, and show the cloud to exhibit rotation with an amplitude of ≃ 9.0±1.5 km s −1 . In other papers, Rhode et al. (2013) show the HI source to have a resolved stellar counterpart and ongoing star forming activity, while Skillman et al. (2013) reveal it as having extremely low metallicity: 12 + log(O/H) = 7.16 ± 0.04. The HI mass to stellar mass ratio of the object is found to be 2.6. We use the Tully-Fisher template relation in its baryonic form (McGaugh 2012) to obtain a distance estimate D Mpc = 1.3 +0.9 −0.5 . Additional constraints on the distance are also provided by the optical data of Rhode et al. (2013) and McQuinn et al. (private communication), both indicating a distance in the range of 1.5 to 2.0 Mpc. The three estimates are compatible within their errors. The object appears to be located beyond the dynamical boundaries of, but still in close proximity to the Local Group. Its pristine properties are consistent with the sedate environment of its location. At a nominal distance of 1.75 Mpc, it would have an HI mass of ≃ 1.0 × 10 6 M ⊙ , a stellar mass of ≃ 3.6 × 10 5 M ⊙ , and a dynamical mass within the HI radius of ≃ 1.5 × 10 7 M ⊙ . This discovery supports the idea that optically faint -or altogether dark -low mass halos may be detectable through their non-stellar baryons.

The StEllar Counterparts of COmpact high velocity clouds (SECCO) survey

“…This has indeed been the case for the most compact cloud identified in the survey: Giovanelli et al (2013) and Rhode et al (2013) reveal that this cloud embeds a faint (M V −9.4), low surface brightness (µ V 24.5 mag/arcsec 2 ), star-forming galaxy at a distance of D 1.7 Mpc, Leo P (McQuinn et al 2013). Leo P has a structure, as well as gas and stellar content, that is very similar to Leo T, another recently discovered dwarf galaxy (Irwin et al 2007), which, quoting Ryan-Weber et al (2008) ".…”

“…As discussed in depth by A13, these systems are consistent with a population of LG dwarf galaxies located between 0.25 and 2 Mpc from us. A population of very faint, gas-rich, metal-poor, star-forming dwarf galaxies at relatively short distances from us would not only play a crucial role in the missing satellites problem, but would also provide a direct probe of the effects of feedback and re-ionization (as a function of baryonic and dark mass) on the evolution of baryons within low mass haloes (see Koposov et al 2009;Giovanelli et al 2013;Brown et al 2014, A13).…”

Alfalfa Discovery of the Nearby Gas-Rich Dwarf Galaxy Leo P. V. Neutral Gas Dynamics and Kinematics

“…0 6 × 10 36 Rhode et al (2013) Single-dish S H I (Jy km s −1 ) 1.31 Giovanelli et al (2013) H i mass M H I (M ) 9 .5 × 10 5 Giovanelli et al (2013), this work Stellar mass M (M ) 5 .7 × 10 5 McQuinn et al (2013) One of the primary goals of the ALFALFA blind extragalactic H i survey is to populate the faint end of the H i mass function with statistical confidence . Major follow-up survey programs targeting ALFALFA detections are underway, sampling low-mass, H i-rich systems that both possess and lack obvious stellar counterparts in moderatedepth optical survey data products: SHIELD (Cannon et al 2011a) and the "ultra-compact high velocity cloud" ("UCHVC") sample .…”

“…In this fifth paper in the series of manuscripts about Leo P (papers one through four are Giovanelli et al 2013, Rhode et al 2013, and McQuinn et al 2013, respectively; paper six by Warren et al 2014, presents CARMA CO imaging of Leo P), we study the neutral gas morphology and dynamics of Leo P. We organize this manuscript as follows. In Section 2 we describe the data acquisition, reduction, and analysis.…”

“…While the kinematic major axis angle, systemic velocity, and velocity dispersion are well-constrained by the data, a range of values for the dynamical center, inclination, and rotation velocity were able to return adequate fits to the data. In order to extract dynamical information about Leo P from these data, we thus adopt the value of inclination, i = 60 • , from Giovanelli et al (2013). This inclination is based on both the H i and the optical axial ratios.…”