We present the catalog of HI sources extracted from the ongoing Arecibo Legacy Fast ALFA (ALFALFA) extragalactic HI line survey, found within the sky region bounded by 9h36m < RA < 11h36m and +08deg < DEC < +12deg. The HI catalog presented here for this 118-deg^2 region is combined with ones derived from surrounding regions also covered by the ALFALFA survey to examine the large scale structure in the complex Leo region. Because of the combination of wide sky coverage and superior sensitivity, spatial and spectral resolution, the ALFALFA HI catalog of the Leo region improves significantly on the numbers of low HI mass sources as compared with those found in previous HI surveys. The HI mass function of the Leo I group presented here is dominated by low-mass objects: 45 of the 65 Leo I members have M_HI < 10^8 Msun, yielding tight constraints on the low-mass slope of the Leo I HI mass function. The best-fit slope is alpha < -1.41 + 0.2 - 0.1. A direct comparison between the ALFALFA HI line detections and an optical search of the Leo I region proves the advantage of the ALFALFA strategy in finding low mass, gas-rich dwarfs. These results suggest the existence of a significant population of low surface brightness, gas-rich, yet still very low HI mass galaxies, and may reflect the same type of morphological segregation as is seen in the Local Group. While the low mass end slope of the Leo I HI mass function is steeper than that determined for luminosity functions of the group, the slope still falls short of the values predicted by simulations of structure formation in the LCDM paradigm.
We present a fourth catalog of HI sources from the Arecibo Legacy Fast ALFA (ALFALFA) Survey. We report 541 detections over 136 deg2, within the region of the sky having 22h < R.A. < 03h and 24 deg < Dec. < 26 deg . This complements a previous catalog in the region 26 deg < Dec. < 28 deg (Saintonge et al. 2008). We present here the detections falling into three classes: (a) extragalactic sources with S/N > 6.5, where the reliability of the catalog is better than 95%; (b) extragalactic sources 5.0 < S/N < 6.5 and a previously measured optical redshift that corroborates our detection; or (c) Hig…
The Arecibo Legacy Fast ALFA (ALFALFA) survey has completed source extraction for 40% of its total sky area, resulting in the largest sample of HI-selected galaxies to date. We measure the HI mass function from a sample of 10,119 galaxies with 6.2 < log (M_HI/M_Sun) < 11.0 and with well-described mass errors that accurately reflect our knowledge of low-mass systems. We characterize the survey sensitivity and its dependence on profile velocity width, the effect of large-scale structure, and the impact of radio frequency interference in order to calculate the HIMF with both the 1/Vmax and 2DSWML…
Making use of HI 21 cm line measurements from the ALFALFA survey (alpha.40) and photometry from the Sloan Digital Sky Survey (SDSS) and GALEX, we investigate the global scaling relations and fundamental planes linking stars and gas for a sample of 9417 common galaxies: the alpha.40-SDSS-GALEX sample. In addition to their HI properties derived from the ALFALFA dataset, stellar masses (M_*) and star formation rates (SFRs) are derived from fitting the UV-optical spectral energy distributions. 96% of the alpha.40-SDSS-GALEX galaxies belong to the blue cloud, with the average gas fraction f_HI = M_…
Leo P is a low-luminosity dwarf galaxy discovered through the blind HI Arecibo Legacy Fast ALFA (ALFALFA) survey. The HI and follow-up optical observations have shown that Leo P is a gas-rich dwarf galaxy with both active star formation and an underlying older population, as well as an extremely low oxygen abundance. Here, we measure the distance to Leo P by applying the tip of the red giant branch (TRGB) distance method to photometry of the resolved stellar population from new Large Binocular Telescope (LBT) V and I band imaging. We measure a distance modulus of 26.19 (+0.17/-0.50) mag corres…
We present KPNO 4-m and LBT/MODS spectroscopic observations of an HII region in the nearby dwarf irregular galaxy Leo P discovered recently in the Arecibo ALFALFA survey. In both observations, we are able to accurately measure the temperature sensitive [O III] 4363 Angstrom line and determine a "direct" oxygen abundance of 12 + log(O/H) = 7.17 +/- 0.04. Thus, Leo P is an extremely metal deficient (XMD) galaxy, and, indeed, one of the most metal deficient star-forming galaxies ever observed. For its estimated luminosity, Leo P is consistent with the relationship between luminosity and oxygen ab…
The Arecibo L-band Feed Array (ALFA) is being used to conduct a low-Galactic latitude survey, to map the distribution of galaxies and large-scale structures behind the Milky Way through detection of galaxies' neutral hydrogen (HI) 21-cm emission. This Zone of Avoidance (ZOA) survey finds new HI galaxies which lie hidden behind the Milky Way, and also provides redshifts for partially-obscured galaxies known at other wavelengths. Before the commencement of the full survey, two low-latitude precursor regions were observed, totalling 138 square degrees, with 72 HI galaxies detected. Detections thr…
The Arecibo L-Band Feed Array Zone of Avoidance (ALFA ZOA) Survey has discovered a nearby galaxy, ALFA ZOA J1952+1428, at a heliocentric velocity of +279 km s-1. The galaxy was discovered at low Galactic latitude by 21-cm emission from neutral hydrogen (Hi). We have obtained follow-up observations with the EVLA and the 0.9-m SARA optical telescope. The Hi distribution overlaps an uncataloged, potential optical counterpart. The Hi linear size is 1.4 kpc at our adopted distance of D = 7 Mpc, but the distance estimate is uncertain as Hubble's law is unreliable at low recessional velocities. The o…
The ongoing Arecibo Legacy Fast ALFA (ALFALFA) survey is a wide-area, extragalactic HI-line survey conducted at the Arecibo Observatory. Sources have so far been extracted over ~3,000 sq.deg of sky (40% of its final area), resulting in the largest HI-selected sample to date. We measure the space density of HI-bearing galaxies as a function of their observed velocity width (uncorrected for inclination) down to w = 20 km/s, a factor of 2 lower than the previous generation HI Parkes All-Sky Survey. We confirm previous results that indicate a substantial discrepancy between the observational distr…
In this paper we describe results from the Arecibo Galaxy Environments Survey (AGES). The survey reaches column densities of ~3x10^18 cm^-2 and masses of ~10^7 M_O, over individual regions of order 10 sq deg in size, out to a maximum velocity of 18,000 km s^-1. Each surveyed region is centred on a nearby galaxy, group or cluster, in this instance the NGC7448 group. Galaxy interactions in the NGC7448 group reveal themselves through the identification of tidal tails and bridges. We find ~2.5 times more atomic gas in the inter-galactic medium than in the group galaxies. We identify five new dwarf…
We present a current catalog of 21 cm HI line sources extracted from the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey over ~2800 square degrees of sky: the alpha.40 catalog. Covering 40% of the final survey area, the alpha.40 catalog contains 15855 sources in the regions 07h30m < R.A. < 16h30m, +04 deg < Dec. < +16 deg and +24 deg < Dec. < +28 deg and 22h < R.A. < 03h, +14 deg < Dec. < +16 deg and +24 deg < Dec. < +32 deg. Of those, 15041 are certainly extragalactic, yielding a source density of 5.3 galaxies per square degree, a factor of 29 improvement over the catalog extra…
We present results from ground-based optical imaging of a low-mass dwarf galaxy discovered by the ALFALFA 21-cm HI survey. Broadband (BVR) data obtained with the WIYN 3.5-m telescope at Kitt Peak National Observatory (KPNO) are used to construct color-magnitude diagrams of the galaxy's stellar population down to V_0 ~ 25. We also use narrowband H-alpha imaging from the KPNO 2.1-m telescope to identify an HII region in the galaxy. We use these data to constrain the distance to the galaxy to be between 1.5 and 2.0 Mpc. This places Leo P within the Local Volume but beyond the Local Group. Its pro…
Galaxies detected in the 21-cm line of neutral hydrogen (HI) from the on-going Arecibo Legacy Fast ALFA (ALFALFA) blind extragalactic HI survey have been cross-correlated with Sloan Digital Sky Survey Data Release 7 (Abazajian et al. 2009) in order to define a reference sample of HI content in regions of low galactic density. This observational sample will be used in the future to derive new standards of normal atomic gas content that allow a statistical investigation of the HI properties of galaxies in differing environments of the local universe. As a previous step, we compare here morpholog…
We present the results of a pilot survey for neutral hydrogen (HI) 21 cm absorption in the Arecibo Legacy Fast Arecibo L-Band Feed Array (ALFALFA) Survey. This project is a wide-area "blind" search for HI absorption in the local universe, spanning -650 km/s < cz < 17,500 km/s and covering 517.0 square degrees (7% of the full ALFALFA survey). The survey is sensitive to HI absorption lines stronger than 7.7 mJy (8983 radio sources) and is 90% complete for lines stronger than 11.0 mJy (7296 sources). The total redshift interval sensitive to all damped Lyman alpha (DLA) systems (N_HI >= 2x10^20 cm…
We measure the HI mass function (HIMF) and velocity width function (WF) across environments over a range of masses $7.2<\log(M_{HI}/M_{\odot})<10.8$, and profile widths $1.3\log(km/s)<\log(W)<2.9\log(km/s)$, using a catalog of ~7,300 HI-selected galaxies from the ALFALFA Survey, located in the region of sky where ALFALFA and SDSS (Data Release 7) North overlap. We divide our galaxy sample into those that reside in large-scale voids (void galaxies) and those that live in denser regions (wall galaxies). We find the void HIMF to be well fit by a Schechter function with normalization $Φ^*=(1.37\pm…
We investigate the growth rate of structures in the local Universe. For this, we use as a cosmological tracer the HI line extra-galactic sources from the Arecibo Legacy Fast ALFA (ALFALFA) survey to obtain a measurement of the normalized growth rate parameter, $f σ_{8}$, considered a powerful tool to constrain alternative models of gravity. For these analyses, we calculate the Local Group velocity due to the matter structures distribution in the ALFALFA catalogue and compare it with the Local Group velocity relative to the Cosmic Microwave Background frame to obtain the velocity scale paramete…
The "ALFA Ultra Deep Survey" (AUDS) is an ongoing 21-cm spectral survey with the Arecibo 305m telescope. AUDS will be the most sensitive blind survey undertaken with Arecibo's 300 MHz Mock spectrometer. The survey searches for 21-cm HI line emission at redshifts between 0 and 0.16. The main goals of the survey are to investigate the HI content and probe the evolution of HI gas within that redshift region. In this paper, we report on a set of precursor observations with a total integration time of 53 hours. The survey detected a total of eighteen 21-cm emission lines at redshifts between 0.07 a…
The Arecibo L-Band Feed Array Zone of Avoidance (ALFA ZOA) Deep Survey is the deepest and most sensitive blind Hi survey undertaken in the ZOA. ALFA ZOA Deep will cover about 300 square degrees of sky behind the Galactic plane in both the inner (30 deg < l < 75 deg; b < |2 deg|) and outer (175 deg < l < 207 deg; -2 deg < b < +1 deg) Galaxy, using the Arecibo Radio Telescope. First results from the survey have found 61 galaxies within a 15 square degree area centered on l = 192 deg and b = -2 deg. The survey reached its expected sensitivity of rms = 1 mJy at 9 km/s channel resolution, and is sh…
We present Halpha3 (acronym for Halpha-alpha-alpha), an Halpha narrow-band imaging survey of ~400 galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Local Supercluster, including the Virgo cluster. By using hydrogen recombination lines as a tracer of recent star formation, we aim to investigate the relationships between atomic neutral gas and newly formed stars in different environments (cluster and field), morphological types (spirals and dwarfs), and over a wide range of stellar masses (~10^7.5-10^11.5 Msun). We image in Halpha+[NII] all the galaxies that contain …
The 40% Arecibo Legacy Fast ALFA (ALFALFA) survey catalog (α.40) of approximately 10,150 HI-selected galaxies is used to analyze the clustering properties of gas-rich galaxies. By employing the Landy-Szalay estimator and a full covariance analysis for the two-point galaxy-galaxy correlation function, we obtain the real-space correlation function and model it as a power law, ξ(r) = (r/r_0)^(-γ), on scales less than 10 h^{-1} Mpc. As the largest sample of blindly HI-selected galaxies to date, α.40 provides detailed understanding of the clustering of this population. We find γ= 1.51 +/- 0.09 and …
We combine information from the clustering of HI galaxies in the 100% data release of the Arecibo Legacy Fast ALFA survey (ALFALFA), and from the HI content of optically-selected galaxy groups found in the Sloan Digital Sky Survey (SDSS) to constrain the relation between halo mass $M_h$ and its average total HI mass content $M_{\rm HI}$. We model the abundance and clustering of neutral hydrogen through a halo-model-based approach, parametrizing the $M_{\rm HI}(M_h)$ relation as a power law with an exponential mass cutoff. To break the degeneracy between the amplitude and low-mass cutoff of the…
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). 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 x 10^19 cm^-2 isophote, and show the cloud to exhibit rotation with an amplitude of ~9.0 +/- 1.5 km/s. In other papers, Rhode et al. (2013) show the HI source to have a resolved stellar count…
We analyze a sample of galaxies with stellar masses greater than $10^{10} M_{\odot}$ and with redshifts in the range $0.025<z<0.05$ for which HI mass measurements are available from the GALEX Arecibo SDSS Survey (GASS) or from the Arecibo Legacy Fast ALFA survey (ALFALFA). At a given value of $M_*$, our sample consists primarily of galaxies that are more HI-rich than average. We constructed a series of three control samples for comparison with these HI-rich galaxies. As expected, HI-rich galaxies differ strongly from galaxies of same stellar mass that are selected without regard to HI content.…
We use a sample of ~6000 galaxies detected by the Arecibo Legacy Fast ALFA (ALFALFA) 21cm survey, to measure the clustering properties of HI-selected galaxies. We find no convincing evidence for a dependence of clustering on the galactic atomic hydrogen (HI) mass, over the range M_HI ~ 10^{8.5} - 10^{10.5} M_sun. We show that previously reported results of weaker clustering for low-HI mass galaxies are probably due to finite-volume effects. In addition, we compare the clustering of ALFALFA galaxies with optically selected samples drawn from the Sloan Digital Sky Survey (SDSS). We find that HI-…
We present a catalog of 59 ultra-compact high velocity clouds (UCHVCs) extracted from the 40% complete ALFALFA HI-line survey. The ALFALFA UCHVCs have median flux densities of 1.34 Jy km/s, median angular diameters of 10', and median velocity widths of 23 km/s. We show that the full UCHVC population cannot easily be associated with known populations of high velocity clouds. Of the 59 clouds presented here, only 11 are also present in the compact cloud catalog extracted from the commensal GALFA-HI survey, demonstrating the utility of this separate dataset and analysis. Based on their sky distri…
We present the analysis of the HI content of a sample of early-type galaxies (ETGs) in low-density environments (LDEs) using the data set provided by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We compare their properties to the sample in the Virgo cluster that we studied in a previous paper (di Serego Alighieri et al. 2007, Paper I). We have selected a sample of 62 nearby ETGs (V< 3000 km/s) in an area of the sky where the ALFALFA data are already available (8h<RA<16h, 4 deg<DEC<16deg), avoiding the region of the Virgo cluster. Among these, 39 have absolute B magnitudes fainter than M_B = …
We use a stacking technique to measure the average HI content of a volume-limited sample of 1871 AGN host galaxies from a parent sample of galaxies selected from the SDSS and GALEX imaging surveys with stellar masses greater than 10^10 M_sun and redshifts in the range 0.025<z<0.05. HI data are available from the Arecibo Legacy Fast ALFA (ALFALFA) survey. In previous work, we found that the HI gas fraction in galaxies correlates most strongly with the combination of optical/UV colour and stellar surface mass density. We therefore build a control sample of non-AGN matched to the AGN hosts in the…
We present 21 cm observations of a 10 $\times$ 2 degree region in the Virgo cluster, obtained as part of the Arecibo Galaxy Environment Survey. 289 sources are detected over the full redshift range (-2,000 $<$ $v$$_{hel}$ $<$ + 20,000 km/s) with 95 belonging to the cluster ($v$$_{hel}$ $<$ 3,000 km/s). We combine our observations with data from the optically selected Virgo Cluster Catalogue (VCC) and the Sloan Digital Sky Survey (SDSS). Most of our detections can be clearly associated with a unique optical counterpart, and 30% of the cluster detections are new objects fainter than the VCC opti…
We present 21 cm observations of a 5 x degree region in the Virgo cluster, obtained as part of the Arecibo Galaxy Environment Survey. 13 cluster members are detected, together with 36 objects in the background. We compare and contrast the results from this area with a larger 10 x degree region. We combine the two data sets to produce an HI mass function, which shows a higher detection rate at low masses (but finds fewer massive galaxies) than less sensitive wider-area surveys, such as ALFALFA. We find that the HI-detected galaxies are distributed differently to the non-detections, both spatial…
It is well known that both the star formation rate and the cold gas content of a galaxy depend on the local density out to distances of a few Megaparsecs. In this paper, we compare the environmental density dependence of the atomic gas mass fractions of nearby galaxies with the density dependence of their central and global specific star formation rates. We stack HI line spectra extracted from the Arecibo Legacy Fast ALFA survey centered on galaxies with UV imaging from GALEX and optical imaging/spectroscopy from SDSS. We use these stacked spectra to evaluate the mean atomic gas mass fraction …
We present Arecibo Legacy Fast ALFA (ALFALFA) and follow-up Giant Metrewave Radio Telescope (GMRT) HI observations of the gas-rich interacting group NGC 3166/9. The sensitive ALFALFA data provide a complete census of HI-bearing systems in the group while the high-resolution GMRT data elucidate their origin, enabling one of the first unbiased physical studies of gas-rich dwarf companions and the subsequent identification of second generation, tidal dwarf galaxies in a nearby group. The ALFALFA maps reveal an extended HI envelope around the NGC 3166/9 group core, which we mosaic at higher resolu…
We present the analysis of Halpha3, an Halpha imaging survey of 409 galaxies selected from the HI Arecibo ALFALFA Survey in the Local Supercluster, including the Virgo cluster. We explore the relations between the stellar mass, the HI mass and the current, massive SFR of nearby galaxies in the Virgo cluster and we compare them with those of isolated galaxies in the Local Supercluster, disentangling the role of the environment in shaping the star formation properties of galaxies at the present cosmological epoch. We investigate the relationships between atomic neutral gas and newly formed stars…
We report results from a study of the HI content and stellar properties of nearby galaxies detected by the Arecibo Legacy Fast ALFA blind 21-cm line survey and the Sloan Digital Sky Survey in two declination strips covering a total area of 9 hr X 16 deg. Our analysis seeks to assemble a control sample of galaxies suitable for providing absolute measures of the HI content of gaseous objects. From a database of ~15,000 HI detections, we have assembled three samples of gas-rich galaxies expected to show little or no evidence of interaction with their surroundings that could provide adequate HI st…
This is the second paper of two reporting results from a study of the HI content and stellar properties of nearby galaxies detected by the Arecibo Legacy Fast ALFA blind 21-cm line survey and the Sloan Digital Sky Survey in a 2160 deg^2 region covered by both surveys. We apply strategies of multivariate data analysis to a complete HI flux-limited subset of 1624 objects extracted from the control sample of HI emitters assembled by Toribio et al. (2011a) in order to: i) investigate the correlation structure of the space defined by an extensive set of observables describing gas-rich systems; ii) …
We combine data from the Sloan Digital Sky Survey (SDSS) and the Arecibo Legacy Fast ALFA Survey (ALFALFA) to establish an empirical model for the HI gas content within dark matter halos. A cross-match between our SDSS DR7 galaxy group sample and the ALFALFA HI sources provides a catalog of 16,520 HI-galaxy pairs within 14,270 galaxy groups (halos). Using these matched pairs, we model the HI gas mass distributions within halos using two components: 1) {\it in situ} galaxy relations that involve the HI masses, colors $({\rm g-r})$ and stellar masses 2) an {\it ex situ} dependence of the HI mass…
We present first results from the "Survey of HI in Extremely Low-mass Dwarfs" (SHIELD), a multi-configuration EVLA study of the neutral gas contents and dynamics of galaxies with HI masses in the 10^6-10^7 Solar mass range detected by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We describe the survey motivation and concept demonstration using VLA imaging of 6 low-mass galaxies detected in early ALFALFA data products. We then describe the primary scientific goals of SHIELD and present preliminary EVLA and WIYN 3.5m imaging of the 12 SHIELD galaxies. With only a few exceptions, the neutral ga…
We use both an HI-selected and an optically-selected galaxy sample to directly measure the abundance of galaxies as a function of their "baryonic" mass (stars + atomic gas). Stellar masses are calculated based on optical data from the Sloan Digital Sky Survey (SDSS) and atomic gas masses are calculated using atomic hydrogen (HI) emission line data from the Arecibo Legacy Fast ALFA (ALFALFA) survey. By using the technique of abundance matching, we combine the measured baryonic function (BMF) of galaxies with the dark matter halo mass function in a LCDM universe, in order to determine the galact…
We present an analysis of the neutral hydrogen (HI) content and distribution of galaxies in groups as a function of their parent dark matter halo mass. The Arecibo Legacy Fast ALFA survey alpha.40 data release allows us, for the first time, to study the HI properties of over 740 galaxy groups in the volume of sky common to the SDSS and ALFALFA surveys. We assigned ALFALFA HI detections a group membership based on an existing magnitude/volume-limited SDSS DR7 group/cluster catalog. Additionally, we assigned group "proximity" membership to HI detected objects whose optical counterpart falls belo…
We present new narrow-band (H$α$ and [OIII]) imagings and optical spectrophotometry of HII regions for a gas-rich low surface brightness irregular galaxy, KKR 17. The central surface brightness of the galaxy is $μ_0(B)$ = 24.15 $\pm$0.03 mag~sec$^{-2}$. The galaxy was detected by \emph{Arecibo Legacy Fast ALFA survey} (ALFALFA), and its mass is dominated by neutral hydrogen (HI) gas. In contrast, both the stellar masses of the bright HII and diffuse stellar regions are small. In addition, the fit to the spectral energy distribution to each region shows the stellar populations of HII and diffus…
We present HI 21cm observations of 1194 galaxies out to a redshift of 10,000 km/s selected as inclined spirals (i>60deg) from the 2MASS Redshift Survey. These observations were carried out at the National Radio Astronomy Observatory Robert C. Byrd Green Bank Telescope (GBT). This observing program is part of the 2MASS Tully-Fisher (2MTF) survey. This project will combine HI widths from these GBT observations with those from further dedicated observing at the Parkes Telescope, from the ALFALFA survey at Arecibo, and S/N>10 and spectral resolution, v_res < 10km/s published widths from a variety …
