We present a sample of 115 very low optical surface brightness, highly extended, H i-rich galaxies carefully selected from the ALFALFA survey that have similar optical absolute magnitudes, surface brightnesses, and radii to recently discovered "ultra-diffuse" galaxies (UDGs). However, these systems are bluer and have more irregular morphologies than other UDGs, are isolated, and contain significant reservoirs of H i. We find that while these sources have normal star formation rates for H i selected galaxies of similar stellar mass, they have very low star formation efficiencies. We further present deep optical and H i synthesis follow up imaging of three of these H i-bearing ultra-diffuse sources. We measure H i diameters extending to ∼40 kpc, but note that while all three sources have large H i diameters for their stellar mass, they are consistent with the H i mass -H i radius relation. We further analyze the H i velocity widths and rotation velocities for the unresolved and resolved sources respectively, and find that the sources appear to inhabit halos of dwarf galaxies. We estimate spin parameters, and suggest that these sources may exist in high spin parameter halos, and as such may be potential H i-rich progenitors to the ultra-diffuse galaxies observed in cluster environments.

We characterize the radial density, metallicity and flattening profile of the Milky Way's stellar halo, based on the large sample of spectroscopically confirmed giant stars from SDSS/SEGUE-2 (Xue et al. 2014), spanning galactocentric radii 10 kpc ≤ r GC ≤ 80 kpc. After excising stars that were algorithmically attributed to apparent halo substructure (including the Sagittarius stream) the sample has 1757 K giants, with a typical metallicity precision of 0.2 dex and a mean distance accuracy of 16%. Compared to blue horizontal branch stars or RR Lyrae variables, giants are more readily understood tracers of the overall halo star population, with less bias in age or metallicity. The well-characterized selection function of the sample enables forward modeling of those data, based on ellipsoidal stellar density models, ν * (R, z), with Einasto profiles and (broken) power laws for their radial dependence, combined with a model for the metallicity gradient and the flattening profile. Among models with constant flattening, these data are reasonably well fit by an Einasto profile of n = 3.1±0.5 with an effective radius r eff = 15 ± 2 kpc and a flattening of q = 0.7 ± 0.02; or comparably well by an equally flattened broken power law, with radial slopes of α in = 2.1 ± 0.3 and α out = 3.8 ± 0.1, with a break radius of r break = 18 ± 1 kpc; this is largely consistent with earlier work. We find a modest but significant metallicity gradient within the 'outer' stellar halo, [Fe/H] decreasing outward. If we allow for a variable flattening q = f (r GC ), we find the distribution of halo giants to be considerably more flattened at small radii, q(10 kpc) = 0.55 ± 0.02, compared to q(> 30kpc) = 0.8 ± 0.03. Remarkably, the data are then very well fit by a single power law with index of 4.2 ± 0.1 on the variable r q ≡ R 2 + (z/q(r)) 2 .

We report the discovery and follow-up observations of a system of three objects identified by the ALFALFA extragalactic HI survey, cataloged as (almost) dark extragalactic sources, i.e., extragalactic HI detections with no discernible counterpart in publicly available, wide-field, imaging surveys. We have obtained deep optical imaging with WIYN pODI and HI synthesis maps with WSRT of the HI1232+20 system. The source with the highest HI flux has a newly discovered ultra-low surface brightness (LSB) optical counterpart associated with it, while the other two sources have no detected optical counterparts in our images. Our optical observations show that the detected LSB optical counterpart has a peak surface brightness of ∼26.4 mag/arcsec 2 in g , which is exceptionally faint. This source (AGC 229385) has the largest accurately measured HI mass-to-light ratio of an isolated object: M HI /L g =46 M /L , and has an HI mass of 7.2×10 8 M . The other two HI sources (with HI masses 2.0×10 8 and 1.2×10 8 M ) without optical counterparts have upper limit surface brightnesses of 27.9 and 27.8 mag/arcsec 2 in g , and lower limits on their gas mass-to-light ratio of M HI /L g >57 and >31 M /L . This system lies relatively close in projection to the Virgo Cluster, but velocity flow models indicate that it is located at 25 Mpc, substantially beyond Virgo. The system appears to be quite isolated, with no known object closer than ∼500 kpc. These HI sources may represent both sides of the threshold between "dark" star-less galaxies and galaxies with stellar populations. We discuss a variety of possible formation scenarios for the HI1232+20 system.

We statistically quantify the amount of substructure in the Milky Way stellar halo using a sample of 4568 halo K giant stars at Galactocentric distances ranging over 5-125 kpc. These stars have been selected photometrically and confirmed spectroscopically as K giants from the Sloan Digital Sky Survey's SEGUE project. Using a position-velocity clustering estimator (the 4distance) and a model of a smooth stellar halo, we quantify the amount of substructure in the halo, divided by distance and metallicity. Overall, we find that the halo as a whole is highly structured. We also confirm earlier work using BHB stars which showed that there is an increasing amount of substructure with increasing Galactocentric radius, and additionally find that the amount of substructure in the halo increases with increasing metallicity. Comparing to resampled BHB stars, we find that K giants and BHBs have similar amounts of substructure over equivalent ranges of Galactocentric radius. Using a friends-of-friends algorithm to identify members of -2individual groups, we find that a large fraction (∼33%) of grouped stars are associated with Sgr, and identify stars belonging to other halo star streams: the Orphan Stream, the Cetus Polar Stream, and others, including previously unknown substructures. A large fraction of sample K giants (more than 50%) are not grouped into any substructure. We find also that the Sgr stream strongly dominates groups in the outer halo for all except the most metal-poor stars, and suggest that this is the source of the increase of substructure with Galactocentric radius and metallicity.

We present neutral hydrogen (H i) imaging observations with the Westerbork Synthesis Radio Telescope of AGC198606, an H i cloud discovered in the ALFALFA 21cm survey. This object is of particular note as it is located 16 km s −1 and 1.• 2 from the gas-bearing ultrafaint dwarf galaxy Leo T while having a similar H i linewidth and approximately twice the flux density. The H i imaging observations reveal a smooth, undisturbed H i morphology with a full extent of 23 × 16 at the 5 × 10 18 atoms cm −2 level. The velocity field of AGC198606 shows ordered motion with a gradient of ∼25 km s −1 across ∼20 . The global velocity dispersion is 9.3 km s −1 with no evidence for a narrow spectral component. No optical counterpart to AGC198606 is detected. The distance to AGC198606 is unknown, and we consider several different scenarios: physical association with Leo T, a minihalo at a distance of ∼150 kpc based on the models of Faerman et al. (2013, ApJ, 777, 119), and a cloud in the Galactic halo. At a distance of 420 kpc, AGC198606 would have an H i mass of 6.2 × 10 5 M , an H i radius of 1.4 kpc, and a dynamical mass within the H i extent of 1.5 × 10 8 M .