Rebecca A. Koopmann

THE ARECIBO LEGACY FAST ALFA SURVEY. I. SCIENCE GOALS, SURVEY DESIGN, AND STRATEGY

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map ~7000 deg2 of the high Galactic latitude sky visible from Arecibo, providing a H I line spectral database covering the redshift range between -1600 and 18,000 km s-1 with ~5 km s-1 resolution. Exploiting Arecibos large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H I mass function in the local universe and will provide a census of H I in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide-area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local Supercluster and nearby superclusters, allow measurement of the H I diameter function, and enable a first wide-area blind search for local H I tidal features, H I absorbers at z < 0.06, and OH megamasers in the redshift range 0.16 < z < 0.25. Although completion of the survey will require some 5 years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique that samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large-scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic H I line sources out to z ~ 0.06, including several hundred with H I masses M < 107.5 M⊙.

The Arecibo Legacy Fast ALFA Survey: the α.40 H I source catalog, its characteristics and their impact on the derivation of the H I mass function

We present a current catalog of 21 cm H I line sources extracted from the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey over ~2800 deg^2 of sky: the α.40 catalog. Covering 40% of the final survey area, the α.40 catalog contains 15,855 sources in the regions 07^h30^m < R.A. < 16^h30^m, +04° < decl. <+16°, and +24° < decl. <+28° and 22^h < R.A. < 03^h, +14° < decl. <+16°, and +24° < decl. < + 32°. Of those, 15,041 are certainly extragalactic, yielding a source density of 5.3 galaxies per deg^2, a factor of 29 improvement over the catalog extracted from the H I Parkes All-Sky Survey. In addition to the source centroid positions, H I line flux densities, recessional velocities, and line widths, the catalog includes the coordinates of the most probable optical counterpart of each H I line detection, and a separate compilation provides a cross-match to identifications given in the photometric and spectroscopic catalogs associated with the Sloan Digital Sky Survey Data Release 7. Fewer than 2% of the extragalactic H I line sources cannot be identified with a feasible optical counterpart; some of those may be rare OH megamasers at 0.16 < z < 0.25. A detailed analysis is presented of the completeness, width-dependent sensitivity function and bias inherent of the α.40 catalog. The impact of survey selection, distance errors, current volume coverage, and local large-scale structure on the derivation of the H I mass function is assessed. While α.40 does not yet provide a completely representative sampling of cosmological volume, derivations of the H I mass function using future data releases from ALFALFA will further improve both statistical and systematic uncertainties.

Hα Morphologies and Environmental Effects in Virgo Cluster Spiral Galaxies

We describe the various Hα morphologies of Virgo Cluster and isolated spiral galaxies and associate the Hα morphologies with the types of environmental interactions that have altered the cluster galaxies. The spatial distributions of Hα and R-band emission are used to divide the star formation morphologies of the 52 Virgo Cluster spiral galaxies into several categories: normal (37%), anemic (6%), enhanced (6%), and (spatially) truncated (52%). Truncated galaxies are further subdivided on the basis of their inner star formation rates into truncated/normal (37%), truncated/compact (6%), truncated/anemic (8%), and truncated/enhanced (2%). The fraction of anemic galaxies is relatively small (6%-13%) in both environments, suggesting that starvation is not a major factor in the reduced star formation rates of Virgo spiral galaxies. The majority of Virgo spiral galaxies have their Hα disks truncated (52%), whereas truncated Hα disks are rarer in isolated galaxies (12%). Most of the Hα-truncated galaxies have relatively undisturbed stellar disks and normal to slightly enhanced inner disk star formation rates, suggesting that intracluster medium-interstellar medium (ICM-ISM) stripping is the main mechanism causing the reduced star formation rates of Virgo spiral galaxies. Several of the truncated galaxies are peculiar, with enhanced central star formation rates, disturbed stellar disks, and barlike distributions of luminous H II complexes inside the central 1 kpc but no star formation beyond, suggesting that recent tidal interactions or minor mergers have also influenced their morphology. Two highly inclined Hα-truncated spiral galaxies have numerous extraplanar H II regions and are likely in an active phase of ICM-ISM stripping. Several spiral galaxies have one-sided Hα enhancements at the outer edge of their truncated Hα disks, suggesting modest local enhancements in their star formation rates due to ICM-ISM interactions. Low-velocity tidal interactions and perhaps outer cluster H I accretion seem to be the triggers for enhanced global star formation in four Virgo galaxies. These results indicate that most Virgo spiral galaxies experience ICM-ISM stripping, many experience significant tidal effects, and many experience both.

An Atlas of Hα and R Images and Radial Profiles of 63 Bright Virgo Cluster Spiral Galaxies

Narrowband Hα and broadband R images and radial profiles are presented for 63 bright spiral galaxies in the Virgo Cluster. The sample is complete for Sb-Scd galaxies with B ≤ 12 and inclination ≤75°. Isophotal radii, disk scale lengths, concentration parameters, and integrated fluxes are derived for the sample galaxies.

Massive Star Formation Rates and Radial Distributions from Hα Imaging of 84 Virgo Cluster and Isolated Spiral Galaxies

The massive star formation properties of 55 Virgo Cluster and 29 isolated S0-Scd bright (MB ≤ -18) spiral galaxies are compared via analyses of R and Hα surface photometry and integrated fluxes as functions of Hubble type and central R light concentration (bulge-to-disk ratio). In the median, the total normalized massive star formation rates (NMSFRs) in Virgo Cluster spiral galaxies are reduced by factors of up to 2.5 compared with isolated spiral galaxies of the same type or concentration, with a range from enhanced (up to 2.5 times) to strongly reduced (up to 10 times). Within the inner 30% of the optical disk, Virgo Cluster and isolated spiral galaxies have similar ranges in NMSFRs, with similar to enhanced (up to 4 times) median NMSFRs for Virgo galaxies. NMSFRs in the outer 70% of the optical disk are reduced in the median by factors of up to 9 for Virgo Cluster spiral galaxies, with more severely reduced star formation at progressively larger disk radii. Thus, the reduction in total star formation of Virgo Cluster spiral galaxies is caused primarily by spatial truncation of the star-forming disks. The correlation between H I deficiency and R light central concentration is much weaker than the correlation between H I deficiency and Hubble type. The previously observed systematic difference in H I spatial distributions and kinematics between early- and late-type spiral galaxies in the Virgo Cluster is at least partially due to the misleading classification of stripped spiral galaxies as early types. Intracluster medium-interstellar medium stripping of the gas from spiral galaxies is likely responsible for the truncated star-forming disks of Virgo Cluster spiral galaxies. This effect may be responsible for a significant part of the morphology-density relationship, in that a large fraction of Virgo Cluster galaxies classified as Sa galaxies are H I-deficient galaxies with truncated star-forming disks rather than galaxies with large bulge-to-disk ratios.

Ongoing Gas Stripping in the Virgo Cluster Spiral Galaxy NGC 4522

The Virgo Cluster galaxy NGC 4522 is one of the best spiral candidates for intracluster medium–interstellar medium (ICM-ISM) stripping in action. Optical broadband and Hα images from the WIYN telescope of the highly inclined galaxy reveal a relatively undisturbed stellar disk and a peculiar distribution of Hα emission. Ten percent of the Hα emission arises from extraplanar H II regions which appear to lie within filamentary structures ≥3 kpc long above one side of the disk. The filaments emerge from the outer edge of a disk of bright Hα emission which is abruptly truncated beyond 0.35R25. Together the truncated Hα disk and extraplanar Hα filaments are reminiscent of a bow shock morphology, which strongly suggests that ISM of NGC 4522 is being stripped by the gas pressure of the ICM. The galaxy has a line-of-sight velocity of 1300 km s-1 with respect to the mean Virgo Cluster velocity and thus is expected to experience a strong interaction with the intracluster gas. The existence of H II regions apparently located above the disk plane suggests that star formation is occurring in the stripped gas, and that newly formed stars will enter the galaxy halo and/or intracluster space. The absence of H II regions in the disk beyond 0.35R25 and the existence of H II regions in the stripped gas suggest that even molecular gas has been effectively removed from the disk of the galaxy.

The HI content of early-type galaxies from the ALFALFA survey - I. Catalogued HI sources in the Virgo cluster

Aims. We are using the Arecibo Legacy Fast ALFA survey (ALFALFA), which is covering 17% of the sky at 21 cm, to study the HI content of early-type galaxies (ETG) in an unbiased way. The aim is to get an overall picture of the hot, warm and cold ISM of ETG, as a function of galaxy mass and environment, to understand its origin and fate, and to relate it to the formation and evolution history of these objects. Methods. This paper deals with the first part of our study, which is devoted to the 8-16 deg. declination strip in the Virgo cluster. In this sky region, using the Virgo Cluster Catalogue (VCC), we have defined an optical sample of 939 ETG, 457 of which are brighter than the VCC completeness limit at B T = 18.0. We have correlated this optical sample with the catalogue of detected HI sources from ALFALFA. Results. Out of the 389 ETG from the VCC with B T < 18.0, outside the 1 deg. region of poor HI detection around M 87, and corrected for background contamination of VCC galaxies without a known radial velocity, only 9 galaxies (2.3%) are detected in HI with a completeness limit of 3.5 and 7.6 x 10 7 M ⊙ of HI for dwarf and giant ETG, respectively. In addition 4 VCC ETG with fainter magnitudes are also detected. Our HI detection rate is lower than previously claimed. The majority of the detected ETG appear to have peculiar morphology and to be located near the edges of the Virgo cluster. Conclusions. Our preliminary conclusion is that cluster ETG contain very little neutral gas, with the exceptions of a few peculiar dwarf galaxies at the edge of the ETG classification and of very few larger ETG, where the cold gas could have a recent external origin.

THE ARECIBO LEGACY FAST ALFA SURVEY. II. RESULTS OF PRECURSOR OBSERVATIONS

In preparation for the full Arecibo Legacy Fast ALFA (ALFALFA) extragalactic H I survey, precursor observations were carried out in 2004 August–September with the seven-beam Arecibo L-band Feed Array (ALFA) receiver system and the Wideband Arecibo Pulsar Processor spectral processors. While these observations were geared mainly at testing and debugging survey strategy, hardware, and software, approximately 48 hr of telescope time yielded science-quality data. The efficiency of system usage (allowing for minor malfunctions and the impact of radio-frequency interference) during that time was 75%. From those observations, an initial list of 730 tentative detections of varying degrees of reliability was extracted. Ninety-eight high signal-to-noise ratio candidates were deemed to be bona fide H I line detections. To test our ability to discriminate cosmic signals from radio-frequency interference and noise, 165 candidates ranging in reliability likelihood were reobserved with the single-beam L-band wide system at Arecibo in 2005 January–February. Of those, 41% were confirmed as real. We present the results of both the ALFA and the single-beam observations for the sample of 166 confirmed H I sources, as well as our assessment of their optical counterparts. Of the 166 sources, 62 coincided with previously known H I sources, while optical redshifts were available for an additional 18 galaxies; thus, 52% of the redshifts reported here were previously unknown. Of the 166 H I detections, 115 are identified with previously cataloged galaxies of either known or unknown redshift, leaving 51 objects identified for the first time. Because of the higher sensitivity of the Arecibo system, fewer than 10% of the 166 H I sources would have been detected by a HIPASS-like survey of the same region. Three of the objects have H I masses less than 107 M⊙. The full ALFALFA survey, which commenced in 2005 February, should detect more than 100 times as many objects of similarly low H I mass over the next 5 years.

A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals

The scale lengths of the old stars and ionized gas distributions are compared for similar samples of Virgo Cluster members and field spiral galaxies via Hα and broad R-band surface photometry. While the R-band and Hα scale lengths are, on average, comparable for the combined sample, we find significant differences between the field and cluster samples. While the Hα scale lengths of the field galaxies are a factor of 1.14 ± 0.07 longer, on average, than their R-band scale lengths, the Hα scale lengths of Virgo Cluster members are, on average, 20% smaller than their R-band scale lengths. Furthermore, in Virgo, the scale length ratios are correlated with the size of the star-forming disk: galaxies with smaller overall Hα extents also show steeper radial falloff of star formation activity. At the same time, we find no strong trends in scale length ratio as a function of other galaxy properties, including galaxy luminosity, inclination, morphological type, central R-band light concentration, or bar type. Our results for Hα emission are similar to other results for dust emission, suggesting that Hα and dust have similar distributions. The environmental dependence of the Hα scale length places additional constraints on the evolutionary process(es) that cause gas depletion and a suppression of the star formation rate in clusters of galaxies.

HI content and other structural properties of galaxies in the Virgo cluster from the Arecibo Legacy Fast ALFA Survey

We report the results of an HI blind survey of 80deg^2 of the Virgo cluster, based on the 08deg 10^7.5-8 M_sun; vi) less than 1% of early-type galaxies contain neutral hydrogen with M_HI>10^7.5-8 M_sun (di Serego Alighieri et al. 2007).