• Science Goals
• A Legacy Survey: HI in the Nearby Universe
• The HI Mass Function and the "Missing Satellite Problem"
• Galaxy Evolution and Dynamics within Local Large Scale Structure
• The Extent and Origin of HI Disks
• The Nature of High Velocity Clouds
• A Blind Survey for 21 cm Absorbers at z < 0.06
• A Blind Survey for OH Megamasers at 0.16 < z < 0.25
• Comparison with other surveys

Science Goals

Just as the introduction of wide field CCDs revolutionized the survey capabilities of optical and infrared telescopes, HI line astronomy is undergoing a similar renaissance with the advent of multi-beam receivers on the large single-dish telescopes, enabling blind HI surveys that cover wide areas. In particular, the upgrade of the surface of the Arecibo antenna in the mid-1970's initiated a new era of extragalactic 21 cm HI line studies which exploited the big dish's collecting area and superior ancillary instrumentation (low noise amplifiers; broadband, flexible multi-bit spectrometers). The addition in 2004 of the Arecibo L-band Feed Array (ALFA) has made possible new wide area surveys in galactic, extragalactic and pulsar research. The local extragalactic sky visible to Arecibo is rich, containing the central longitudes of the Supergalactic Plane in and around the Virgo cluster, the main ridge of the Pisces-Perseus Supercluster, and the extensive filaments connecting A1367, Coma and Hercules. With ALFA, the Arecibo legacy of extragalactic HI studies will continue, probing regimes untouched by other surveys and addressing fundamental cosmological questions (the number density, distribution and nature of low mass halos) and issues of galaxy formation and evolution (sizes of HI disks, history of tidal interactions and mergers, low z absorber cross section, origin of dwarf galaxies, nature of high velocity clouds). Here we briefly outline the main science objectives of the E-ALFA wide area high galactic latitude program, the Arecibo Legacy Fast ALFA (ALFALFA) survey. For further technical information, look at our publications page.

2.1 A Legacy Survey: HI in the Nearby Universe     The survey design and its strategy as proposed here have evolved from numerical simulations (c.f. Giovanelli 2003; Masters et al. 2004), in which we use a cosmic density map provided by the PSCz density reconstruction (Branchini et al. 1999) gridded with 0.9375h^-1 Mpc spacing in the inner 60h^-1 Mpc, and at twice that value between 60 and 120h^-1 Mpc; the map is smoothed with a Gaussian filter of 3.2h^-1 Mpc. The density map is complemented by a peculiar velocity map, which allows us to infer more accurate estimates of the distances than those derived purely from redshifts. Two different HIMFs (those derived by Zwaan et al. 1997 and Rosenberg & Schneider 2002; the Zwaan et al. 2003 HIMF has values intermediate between the former two) are used to populate the map with HI "clouds", which we then proceed to "detect". HI sizes and velocity widths are assigned using empirical scaling relations obtained from our own HI survey data and Broeils & Rhee (1997), with realistic scatter and spectral baseline instability. Disk inclinations and pointing offsets are randomized. We have inspected a wide grid of survey parameters, including different scenarios of suppression of gas infall onto low mass halos due to reionization. Experiments made with the single pixel L-narrow receiver in 2003 and preliminary inspection of our recent A1946 precursor observations confirm the efficacy of these simulations in practical observing conditions (Giovanelli et al 2005a; 2005b).

Our early simulations, based on past estimates of the HI Mass Function predicted that ALFALFA would detect more than 16,000 objects. However, the actual areal density currently being achieved by ALFALFA, about 5 sources per square degree, with peaks 10-20 times higher in regions of groups and clusters, now suggests that the full ALFALFA survey may yield 30,000 extragalactic HI sources. We attribute this higher yield to the high quality of the data, made possible by its "minimum intrusion" observing strategy and the specialized data processing routines developed by the ALFALFA team specifically for the survey, including a Fourier domain signal extraction algorithm written by Cornell graduate student Amélie Saintonge (2007a Ph.D. thesis, Cornell University; 2007b).

2.2 The HI Mass Function and the "Missing Satellite Problem"     One of the principal discrepancies between cold dark matter (CDM) theory and current observations revolves around the large difference between the number of dwarf dark matter halos seen around giant halos in numerical simulations based on CDM and the observed dwarf satellite population in the Local Group (Kauffmann et al. 1993; Klypin et al. 1999; Moore et al. 1999b), referred to as the "missing satellite problem". The logarithmic slope of the faint end of the galaxy mass function predicted by CDM simulations is close to the value of α = -1.8 that arises analytically from the Press-Schechter formalism (Press & Schechter 1974; Bardeen et al. 1986). Because the mass function itself is difficult to determine directly, current efforts focus on estimation of the faint end of the optical luminosity function (LF) and, of direct relevance to this proposal, of the HIMF. By determining both, limits can be set on the number of low mass halos containing measurable stellar or gaseous components. The shape of the low mass end of the HIMF and its corollary, the cosmological mass density of HI, are important parameters in the modelling of the formation and evolution of galaxies.

The HIMF is the probability distribution over HI mass of detectable HI line signals in a survey sensitive to the global neutral hydrogen within a system. The most recent estimates of the HIMF have been presented by Zwaan et al. (1997; Z97), Rosenberg & Schneider (2002; RS02), Zwaan et al. (2003; Z03) and Springob et al. (2004). The latter is derived from a compilation of some 9000 optically selected galaxies, further restricted by HI line flux and optical diameter to a complete subsample containing 2200 galaxies. The other determinations are based on blind HI surveys and thus have no bias against the low luminosity and low optical surface brightness galaxies which may be underrepresented in optical galaxy catalogs. The Z03 HIMF is based on the HIPASS survey (Koribalski et al. 2004; Meyer et al. 2004), while the RS02 and Z97 HIMFs are both based on drift scan surveys conducted at Arecibo during the period of its recent upgrade. The faint end slope of those determinations of the HIMF vary between -1.20 and -1.53, yielding extrapolations below M_HI = 10⁷ M_sun that disagree by an order of magnitude, the RS02 HIMF having the steeper slope. All three HI blind surveys sample a lower mass limit just below M_HI = 10⁸ M_sun, for H_° = 70 kms^-1Mpc^-1 (a value that will be assumed throughout, while for Virgo we adopt a distance D = 16 Mpc). No galaxies were detected by RS02 or Z97 with M_HI < 10⁷ M_sun, while 3 are claimed by Z03, and only a small number of detections have M_HI < 10⁸ M_sun.

As pointed out by Kratsov et al. (2004), models of the formation of large scale structure must explain not only the number of satellites found in the Local Group, but also their clustering characteristics: whereas the dSphs are found concentrated with ~300 kpc of their host giant galaxies, the irregulars are spread throughout, both near and far from the giants (Grebel 2004). The origin of this segregation as well as the fundamental differences among the dwarf populations are thus important issues for galaxy formation theories. The possible variation in the HIMF with local galaxy density or velocity dispersion can provide a statistical measure of the impact of environment mechanisms on the gas as galaxies evolve.

Surveys using ALFA will explore two fundamental aspects of the HIMF: its low mass slope, which has a direct bearing on the "missing satellite problem", and its behavior with varying galaxy environment. To date, studies of the possible environmental dependence of the HIMF have been limited to comparisons of the HIMF derived for galaxies in the Virgo cluster with those in the field (Hoffman et al. 1992; Briggs & Rao 1993; RS02; Davies et al. 2004; Gavazzi et al. 2004) but suffer from poor statistics and incompleteness. The results marginally suggest that the HIMF in Virgo is missing the low HI mass dwarfs found in the field or is at least flatter at the faint end than the field HIMF.

The science program of the E-ALFA consortium as illustrated in the E-ALFA white paper has, as one of its main goals, the robust determination of the HIMF over a range of independent volumes characterized by varying cosmic density. No single survey is likely to explore all the relevant parameter space. Achieving adequate detection statistics for objects in the 10⁶ - 10⁸M_sun range requires a balance of survey areal coverage and survey depth in order to sample adequate volume. Studying a wide range of environments likewise necessitates tradeoffs of depth and area. In combination with the deeper surveys proposed under the AGES (Arecibo Galaxy Environments Survey) program, ALFALFA will allow exploration of a wide range of possible HIMF scenarios. It will focus on studies of the lowest mass objects in the very nearby universe (M_HI < 10⁷ M_sun, D < 15 Mpc) within the Local Supercluster but will also explore variations of the global HIMF, suggested by Springob et al. (2004) by sampling, at higher masses, across the range of local densities that characterize the rich clusters like A262, A1367 and Coma, their supercluster filaments and the voids between them.

We emphasize that since the lowest HI masses will be found only very locally, ALFALFA must cover a very large solid angle in order to survey adequate volume at D < 15 Mpc. ALFALFA as proposed will detect between 60 and 300 objects with M_HI < 10^7.5 M_sun depending on whether the HIMF follows Z97 or RS02. Both the legacy aspect and the local volume requirement thus dictate the need to survey 7000 deg².

2.3 Galaxy Evolution and Dynamics within Local Large Scale Structures     The large scale distribution of galaxies in the local universe is concentrated in a structure (Lahav et al. 2000) first recognized by de Vaucouleurs and today designated as the Supergalactic Plane. At its center, the Virgo cluster is the nearest rich cluster to us. Overall, the galaxy distribution in that direction has been shown to trace a filamentary structure (West & Blakeslee 2000; Gavazzi et al. 1999; Solanes et al. 2002) elongated along the line of sight. Galaxies in the cluster core are known to be HI-deficient due to interaction with the hot intracluster gas, while galaxies in the cluster periphery, foreground and background are not. Including its several principal concentrations, the cluster extends about 14° over the sky (Binggeli, Popescu & Tammann 1993). The solid angle subtended by this region samples the highest densities in the local Universe, and thus constitutes the obvious choice for the study of the HIMF in a high density environment. A region of comparable volume but low density, surveyed to comparable sensitivity, is required to provide a reference. The regions with lowest cosmic density at comparable distance are, unsurpringly, in the anti-Virgo direction. Optimization of the Arecibo sky coverage and zenith angle dependence of sensitivity suggests an anti-Virgo region centered near 1.5^h in R.A. and +24° in Dec. This region includes a large section of the largest, nearby cosmic "void"; averaged over a solid angle of ~0.25 sterad, the anti-Virgo region between 0 and 3000 km/s is underdense by a factor of ~6 with respect to the Virgo region in the same distance range. The comparative study of the HI and other properties of the galaxies in these two regions will yield clues on the procesess of environmental influence on galaxy evolution. HI contents will be compared, the dwarf population will traced over wide ranges of cosmic density, and a first truly blind survey for HI tidal remnants will be made.

A wide area Virgo survey will provide a database of unprecedented breadth for the investigation of the origin of gas deficiency in that cluster which will complement nicely the targeted, higher spatial resolution HI line synthesis study of Virgo galaxies currently being undertaken with the Very Large Array (PI: J. Kenney). It will also improve the dynamical understanding of the cluster and its surrounding groups, as well as of the processes associated with the evolution of large-scale structure, by providing a rich redshift data base of low optical luminosity gas-rich dwarfs not only within the cluster core but also in its broad surroundings.

Virgo is the only environment which is both near enough that distance estimates based on secondary methods can distinguish between infall and expansion regimes in the region around the cluster (the so-called "triple-valued region") and also massive enough to possess an extensive, well-populated infall region. In the case of Virgo, the infall domain extends 28° from the center of the cluster, so that a survey that can identify objects at turnaround must cover a very wide area. While the SDSS may provide the required photometric parameters for applications of the Tully-Fisher distance method, its 3 arcsec fibers cannot provide adequate rotational width measures. Thus ALFALFA, in combination with the SDSS database, will provide the basis for a unique study of the galaxy dynamics both in and around the Virgo cluster.

Other groups within the Local Supercluster will also be targets including: the Canes Venatici I Group at about 5 Mpc, the Leo I group at about 10 Mpc, the "groups of dwarfs" (Tully et al. 2002) around UGC 3974 (D = 5.4 Mpc) and NGC 784 (D = 4.4 Mpc), and the Canes Venatici II and Coma I groups at at 10-20 Mpc. There are ~20 additional groups at velocities less than 1000 km/s which we should be able to study in great detail. Models of the structure of the Local Supercluster are being developed by KLM and by IDK and collaborators and will both contribute to and benefit from the ALFALFA survey.

Of particular note, the Leo I (M 96) Group offers an attractive opportunity for exploring both the optical luminosity function and the HIMF in an intermediate density environment. Unlike the Local Group, Leo I is dominated by early type galaxies, yet it is still characterized by a low velocity dispersion. For 19 galaxies with measured redshifts, the dispersion in radial velocity is 130 km/s. Two of the brightest galaxies in Leo I - NGC 3379, and NGC 3384 - are surrounded by a 200 kpc ring of HI gas (Schneider et al. 1983). Two possible scenarios for the origin of this cloud have been proposed. Rood & Williams (1985) suggested that the ring resulted from a collision between NGC 3384 and NGC 3368 some 500 Myr ago. After the discovery of several additional gas features, Schneider (1985) noted that the clouds appear to be stable against tidal disruption and proposed that they instead represent a remnant of the primordial gas cloud from which all of the group members formed. Recently uncovered kinematic signatures suggest that all of the brighter galaxies have been involved in past interactions (Sil'chenko et al. 2003). Thus the Leo I region presents an interesting environment in which to study differences among the low luminosity dwarf populations: a region of low velocity dispersion but containing a local density enhancement that supports the presence of bright E/S0 galaxies. The methodology developed by IDK & VEK to identify nearby dwarfs has already uncovered considerable numbers of faint gas-rich members of other nearby groups (e.g., Karachentseva & Karachentsev 1998; Karachentseva et al. 1999, 2001; Makarov, Karachentsev & Burenkov 2003). To probe the dI population found by Karachentsev & Karachentseva (2004), a very wide field (> 120 square deg), as provided by ALFALFA, must be studied.

2.4 The Extent and Origin of HI Disks     Extended gas disks around galaxies represent a reservoir for future star formation activity. The study of the distribution of HI relative to that of the optical (stellar) disk allows the investigation of the relationship of gas to star formation and the discrimination of models of the origin of the observed truncation of stellar disks at 3 - 5 optical disk scale lengths based on gas density threshholds (Fall & Efstathiou 1989) versus those related to the maximum protogalaxy specific angular momentum (van der Kruit 1979). In contrast to other major wide area surveys such as HIPASS and HIJASS, some 500 gas-rich galaxies will be resolved by ALFA's 3.5 arcmin beam, allowing a quantitative measure of their characteristic HI sizes (Hewitt et al. 1984) and the derivation of the HI diameter function. In combination with optical photometry, ALFALFA will determine the fraction of galaxies with extended gas disks and enable studies of their host galaxies, their environment, morphology and the role of gas in their evolution. Of particular note, we hope to discover more extremely extended gas disks, such as those found in DDO 154 (Krumm & Burstein 1984), NGC 4449 (Bajaja et al. 1994), NGC 2915 (Meurer et al. 1996) and UGC 5288 (van Zee 2004) and extensive tidal features such as those seen in the Leo Triplet (Haynes, Giovanelli & Roberts 1979).

Additionally, ALFALFA will resolve extended HI in the vicinity of the 100 large (D_UGC > 5 arcmin) nearby galaxies that may have been missed by interferometric observations, allowing for a census of the neutral ISM on all spatial scales. In particular, the Arecibo telescope provides an ideal probe of the short spacings missed by the VLA in its more compact configurations. For the ALFALFA parameters t_s = 28 s/beam and channel bandwidth of 5 km/s, the antenna temperature detectable at the 5 σ limit is T_A = 0.13 K; for a source with a spectral width of 25 km/s which fills the beam, this limit corresponds to a minimum detectable column density of N_HI ~6 x 10¹⁸ cm^-2.

The column density regime probed by ALFALFA will characterize the broad-scale emission at the edges of galaxy disks, which are hypothesized to truncate at roughly the same value of N_HI (e.g. Corbelli & Salpeter 1993, Maloney 1993). The existence of a smooth HI component similar to that in DDO 154 (Hoffman et al. 2001) would also extend rotation curves further into the dark matter halo, allowing for more robust determination of the halo shape and concentration to contrast with cold dark matter paradigm predictions on galaxy scales (e.g. Dutton et al. 2004; Barnes et al. 2004).

ALFALFA will provide a census of the abundance and distribution of HI disks, providing the low redshift link to the damped Lyman α (DLA) absorption seen in quasar spectra. At higher redshift, the neutral gas mass traced by DLA absorption makes a greater contribution to the luminous baryonic mass than it does at the current epoch. ALFALFA will provide important clues to such gas disk evolution. While the HIMFs derived from all surveys to date suggest that large galaxies contribute the majority of the local HI mass density, it also seems that massive galaxies do not dominate the cross section for DLA absorption (Rosenberg & Schneider 2001; Rao & Turnshek 1998). Recent observations with the GMRT of resolved low z DLAs similarly has found that the absorption is associated with HI masses less than those characteristic of L* galaxies (Chengalur & Kanekar 2002). To understand the DLA cross section at low redshifts requires study of the population of low mass but HI rich galaxies that are missing from optical catalogs. Followup optical studies of ADBS counterparts by JLR and JJS demonstrate that the earlier survey detected galaxies with absolute magnitudes of -16 at distances of 70 Mpc. ALFALFA should detect such objects in very large numbers, allowing not only robust estimates of their contribution to the local HI cross section, but also a measure of their clustering correlation amplitude and scale.

2.5 The Nature of High Velocity Clouds     In addition to providing important clues on the extents and kinematics of HI gas around other galaxies, ALFALFA will allow a wide area study of gas in and around the Milky Way as a complement to, and in conjunction with, the G-ALFA surveys. In particular, ALFALFA will explore the nature of the local high velocity clouds (HVCs) of neutral hydrogen which may represent gas accretion onto our Galaxy (e.g., Tripp et al. 2003) but which also have been claimed to be more distant, the "missing satellites" in the Local Group (Blitz et al. 1999; Braun & Burton 1999). Previous surveys of HVCs have been of substantially lower resolution (15.5 arcmin at best) and/or were unable to trace the connection between HVCs and Galactic HI emission (Putman et al. 2002; Wakker & van Woerden 1991). ALFALFA will trace important high-velocity structures, such as the northern portions of the Magellanic Stream and Complex C at 4× better resolution than HIPASS. It will also be 8× more sensitive to unresolved small clouds, or ultra-compact HVCs (if any exist with central neutral column density above 10²⁰ cm^-2). This will allow us to determine if HVCs are interacting with a diffuse halo medium (e.g., Brüns et al. 2000; Quilis & Moore 2001) and or if they are bona fide dark matter-dominated Galactic satellites (e.g., Moore et al. 1999a).

The recent discovery of an extended, faint population of HI clouds within 50 kpc of M31 by Thilker et al. (2004) suggests a similar search for clouds around M33. At the Andromeda distance, the Thilker et al. clouds have masses between 10⁵ - 10⁷ M_sun. While Arecibo can not reach as far north as M31, ALFALFA will cover part of the region containing the clouds discovered by Thilker et al. and their possible extension toward the region around M33. Wright's cloud (Wright 1979; Braun & Thilker 2004) was detected easily in our A1946 observations in Aug/Sep 2004.

2.6 A Blind Survey for 21 cm Absorbers at z < 0.06     The background continuum source counts for this work at 1.4 GHz yield 2100 sources brighter than 0.4 Jy and 11400 sources brighter than 0.1 Jy, within the survey area proposed. A recent study by Vermeulen et al. (2004) searching for HI in absorption in compact sources with the WSRT found absorption in 1/3 of the targeted sources and we adopt their results as "typical" (although the redshift range is considerable higher). Similarly, Darling et al. (2004) have detected HI in absorption in an "optically blind" search against continuum sources using the GBT. The HI features found in those surveys show a range of optical depths from τ = 0.16 to τ < 0.001 and exhibit a variety of line profiles with widths as narrow as 10 km/s but more typically! 150 km/s. For a source of 0.5 Jy, a peak absorption of 10 mJy (5σ) corresponds to τ ~0.02. The peak column density is given by N_HI 1.82 × 10¹⁸ T_spin τ_peak ΔV   cm^-2. For T_spin ~ 100K and a velocity width of 100 km/s, τ_peak ~ 0.02 corresponds to N_HI ~ 3.6 x 10²⁰ cm^-2 with the obvious condition that narrower widths would probe lower column densities. Using the values for τ_peak and ΔV given in Table 1 of Vermeulen et al., we estimate that ALFALFA will be be able to detect all but three of the lines found by those authors, assuming a frequency range match. ALFALFA will target low redshift absorbers not associated with the radio sources themselves.

A major difficulty with absorption studies is spectral baseline determination. A method commonly used averages sources of similar strength observed at comparable telescope configuration (possible for the limited azimuth drift mode considered here). We expect that standing waves will be broader than expected HI absorption lines, and that most rfi will be spectrally unresolved. To identify absorbers, we will establish and follow a simple set of rules to assess whether or not a given spectral feature is RFI or real absorption. This aspect of the project will require extra effort, but will yield cosmologically interesting statistics based on such a "blind" HI absorption survey. Among others, JKD, EMM and CMS are interested in pursuing the absorption line study.

2.7 A Blind Survey for OH Megamasers at 0.16 < z < 0.25     OH Megamasers (OHM) are powerful line sources observed in the L band, arising from the nuclear molecular regions in merging galaxy systems. Approximately 100 such sources are known to date, half of which were discovered by JKD's Ph.D dissertation work (e.g. Darling & Giovanelli 2002) at Arecibo. Several of them are observed to have variable spectral features allowing superresolution and insight into the source structure and physics. Observations of OHMs hold the potential for tracing the merger history of the Universe since the sources are associated with merging galaxies.

Comparison with Previous Surveys:

HIPASS and HIJASS cover the same area of sky that is visible at Arecibo, HIPASS south of Dec.= +25°, and HIJASS further to the north. However, in addition to the large increase in sensitivity, ALFA surveys provide 2 direct benefits over the other two: improved angular and velocity resolution. The significant higher angular resolution (FWHM ~3.5arcmin for ALFA versus 12 arcmin for HIJASS and 15.5 arcmin for HIPASS) will help to limit the confusion of sources that plagued those other surveys. The HIPASS follow-up needed is enormous and therefore has been limited to the highest flux sources. It will be years before the sources are followed-up (if ever). An ALFA survey will be able to do science with the survey data directly, without time consuming interferometric follow-up. Additionally, the higher velocity resolution of ALFA will be useful in several ways: First, detecting edge-on galaxies with peak fluxes near the noise limit. The edge of a double peak spectrum is much sharper at higher velocity resolution which should make it easier to automatically detect these sources. Second, the higher velocity resolution will allow more accurate velocity and velocity width measurements, without the need for follow-up. Even the narrowest sources will be detected over several channels. Third, since most rfi is narrow band, the higher frequency resolution will be extremely useful in identifying and excising rfi.

The HIJASS survey has a further serious limitation. Very bad rfi in the frequency band corresponding to cz ~ 4500 - 7500 km/s (within the range of much of the interesting large scale structure e.g., Pisces-Perseus, A1367-Coma-Great Wall). In addition, HIJASS is not scheduled to do any more observing in the Arecibo range (a 4° × 4° region in Virgo and a few other areas have been covered at this point) for the next few years.

The principal advantage that an Arecibo survey will have over previous surveys is depth and the number of independent volumes surveyed. Table B.1 below includes a comparison of the major surveys, including those discussed here. For comparative purposes, the rms noise per beam quoted for each survey has been scaled to a velocity resolution of 18 km/s, the resolution of HIPASS.