Wim van Driel

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⊙.

A Dark Hydrogen Cloud in the Virgo Cluster

VIRGOHI 21 is an H I source detected in the Virgo Cluster survey of Davies et al. that has a neutral hydrogen mass of 108 M and a velocity width of ?V20 = 220 km s-1. From the Tully-Fisher relation, a galaxy with this velocity width would be expected to be 12 mag or brighter; however, deep CCD imaging has failed to turn up a counterpart down to a surface brightness level of 27.5 B mag arcsec-2. The H I observations show that it is extended over at least 16 kpc, which, if the system is bound, gives it a minimum dynamical mass of ~1011 M and a mass-to-light ratio of Mdyn/LB > 500 M/L. If it is tidal debris, then the putative parents have vanished; the remaining viable explanation is that VIRGOHI 21 is a dark halo that does not contain the expected bright galaxy. This object was found because of the low column density limit of our survey, a limit much lower than that achieved by all sky H I surveys such as those carried out at Parkes and Jodrell Bank. Further such sensitive surveys might turn up a significant number of the dark matter halos predicted by galaxy formation models.

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.

21 cm Synthesis Observations of VIRGOHI 21?A Possible Dark Galaxy in the Virgo Cluster

Many observations indicate that dark matter dominates the extragalactic universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously, we have suggested that VIRGOHI 21, a 21 cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line width (~200 km s(-1)) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ~10(8) yr ago with a perturber of ~10(11) M☉. This we take as additional evidence for the massive nature of VIRGOHI 21, as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the H I and find none down to an I-band surface brightness limit of 31.1 ± 0.2 mag arcsec(-2).

A search for low surface brightness dwarf galaxies in different environments

According to the cold dark matter (CDM) hierarchical clustering theory of galaxy and large-scale structure formation, there should be numerous low-mass dark matter haloes present in the Universe today. If these haloes contain sufficient stars, they should be detectable as low-luminosity stellar systems or dwarf galaxies. We have previously described a new detection method for faint low surface brightness objects and we have shown that there are relatively large numbers of very faint dwarf galaxies in the nearby Virgo cluster. In this paper, we present results from a similar survey carried out on the Millennium Galaxy strip, which runs along the celestial equator and samples a very different galaxy environment. We show that the dwarf-to-giant galaxy number ratio along this strip ranges from 0.7 : 1 to, at most, 6 : 1, corresponding to a flat luminosity function (α≈−0.8 to −1.0). This is very different to our value of 20 : 1 for the Virgo cluster. There is no population of low surface brightness dwarf galaxies in the field that have gone undetected by the redshift surveys. This result is exactly opposite to what CDM models predict for the environmental dependence of the dark matter mass function, which is that there are proportionally more small dark matter haloes in lower-density environments.

ΛCDM SATELLITES AND H i COMPANIONS—THE ARECIBO ALFA SURVEY OF NGC 2903

We have conducted a deep, complete H I survey, using Arecibo/Arecibo L-band Feed Array (ALFA), of a field centered on the nearby, isolated galaxy, NGC 2903, which is similar to the Milky Way in its properties. The field size was 150 kpc × 260 kpc and the final velocity range spanned from 100 to 1133 km s–1. The ALFA beams have been mapped as a function of azimuth and cleaned from each azimuth-specific cube prior to forming final cubes. The final H I data are sensitive down to an H I mass of 2 × 105 M ☉ and column density of 2 × 1017 cm–2 at the 3 σ 2 δ V level, where σ is the rms noise level and δ V is the velocity resolution. NGC 2903 is found to have an H I envelope that is larger than previously known, extending to at least three times the optical diameter of the galaxy. Our search for companions yields one new discovery with an H I mass of 2.6 × 106 M ☉. The companion is 64 kpc from NGC 2903 in projection, is likely associated with a small optical galaxy of similar total stellar mass, and is dark matter dominated, with a total mass >108 M ☉. In the region surveyed, there are now two known companions: our new discovery and a previously known system that is likely a dwarf spheroidal, lacking H I content. If H I constitutes 1% of the total mass in all possible companions, then we should have detected 230 companions, according to Λ cold dark matter (ΛCDM) predictions. Consequently, if this number of dark-matter clumps are indeed present, then they contain less than 1% H I content, possibly existing as very faint dwarf spheroidals or as starless, gasless dark-matter clumps.

The Photometric and Kinematic Structure of Face-on Disk Galaxies. I. Sample Definition, Hα Integral Field Spectroscopy, and H I Line Widths

We present a survey of the photometric and kinematic properties of 39 nearby, nearly face-on disk galaxies. Our approach exploits echelle-resolution integral-field spectroscopy of the Hα regions, obtained with DensePak on the WIYN 3.5 m telescope Bench Spectrograph. These data are complemented by H I line profiles observed with the Nancay radio telescope for 25 of these sample galaxies. Twelve additional line widths are available for sample galaxies from the literature. In this paper, we introduce the goals of this survey, define the sample selection algorithm, and amass the integral field spectroscopic data and H I line widths. We establish spatially integrated Hα line widths for the sample. We test the veracity of these spatially integrated line profiles by convolving narrowband imaging data with velocity field information for one of the sample galaxies, PGC 38268, and also by comparing to H I line profiles. We find H I and Hα line profiles to be similar in width but different in shape, indicating that we are observing different spatial distributions of ionized and neutral gas in largely axisymmetric systems with flat outer rotation curves. We also find vertical velocity dispersions of the ionized disk gas within several disk scale lengths have a median value of 18 km s-1 and an 80% range of 12-26 km s-1. This is only a factor of ~2 larger than what is observed for neutral atomic and molecular gas. With standard assumptions for intrinsic and thermal broadening for Hα, this translates into a factor of 3 range in turbulent velocities, between 8 and 25 km s-1.

Radio Quiet Zones and the Square Kilometre Array

The goal of the Square Kilometre Array is to increase the sensitivity of radio telescopes a hundred-fold in the frequency range of ∼70 MHz to 10 GHz. In order to exploit its tremendous scientific capabilities it should be made as robust as possible against unwanted man-made Radio Frequency Interference (RFI). An essential part in this RFI protection scheme is the establishment of Radio Quiet Zones around the instrument, in which regulatory and other legislative measures will ensure acceptable levels of unwanted emissions that could degrade the quality of the astronomical data. An overview will be given of the principles underlying the establishment of Radio Quiet Zones in general, and their application to the Square Kilometre Array case.

Neutral Hydrogen in Galaxies from Low to High Redshift

HIspectral stacking techniques have recently been used to isolate the average signal from coadded low signal-to-noise observations of intermediate redshift galaxies. We exploit these methods to extract HIinformation for galaxies which were classified as non-detections (i.e. 3s) in the Nancay Interstellar Baryon Legacy Extragalactic Survey (NIBLES). We present the average HIcontent of the non-detected galaxies and investigate them further by selecting subsamples according to a colour cut corresponding to different morphologies with the aim to learn more about the HIdistribution in galaxies in the Local Volume. The use of these techniques is also extended to outline possible survey scenarios for a deep HIsurvey using MeerKAT. Future radio surveys aiming to quantify the average HIcontent of the universe over cosmic time will rely on HIstacking and co-adding techniques in order to push to the lowest mass limits achievable within reasonable observing timescales. We present preliminary stacking results based on simulated data for three survey scenarios using MeerKAT.

The Arecibo galaxy environment survey - a sensitive survey for neutral hydrogen in the local Universe

R. F. Minchin∗a, R. Auldb, L. Corteseb, J. I. Daviesb, E. Momjianc, R. Taylorb, B. Catinellad, P. A. Henninge, S. Linder f , E. Mullerg, K. L. O’Neilh, J. Rosenbergi, S. Sabatini j, S. E. Schneiderk, M. D. Stagek, W. van Driell a Arecibo Observatory b Cardiff University c NRAO Socorro d MPIfA Garching e University of New Mexico f University of Hamburg g ATNF h NRAO Green Bank i George Mason University j Osservatorio Astronomico di Roma k University of Massachusetts l Observatoire de Paris