Emmanuel Momjian

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⊙.

Discovery of radio emission from the brown dwarf LP944-20

Brown dwarfs are not massive enough to sustain thermonuclear fusion of hydrogen at their centres, but are distinguished from gas-giant planets by their ability to burn deuterium. Brown dwarfs older than ∼10 Myr are expected to possess short-lived magnetic fields and to emit radio and X-rays only very weakly from their coronae. An X-ray flare was recently detected on the brown dwarf LP944-20, whereas previous searches for optical activity (and one X-ray search) yielded negative results. Here we report the discovery of quiescent and flaring radio emission from LP944-20, with luminosities several orders of magnitude larger than predicted by the empirical relation between the X-ray and radio luminosities that has been found for many types of stars. Interpreting the radio data within the context of synchrotron emission, we show that LP944-20 has an unusually weak magnetic field in comparison to active M-dwarf stars, which might explain the previous null optical and X-ray results, as well as the strength of the radio emissions compared to those at X-ray wavelengths.

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS) - I. The UV luminosity function of the central 12 sq. deg

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS) is a complete blind survey of the Virgo cluster covering similar to 40 sq. deg in the far UV (FUV, lambda(eff) = 1539 angstrom, Delta lambda = 442 angstrom) and similar to 120 sq. deg in the near UV (NUV, lambda(eff) = 2316 angstrom, Delta lambda = 1060 angstrom). The goal of the survey is to study the ultraviolet (UV) properties of galaxies in a rich cluster environment, spanning a wide luminosity range from giants to dwarfs, and regardless of prior knowledge of their star formation activity. The UV data will be combined with those in other bands (optical: NGVS; far-infrared - submm: HeViCS; HI: ALFALFA) and with our multizone chemo-spectrophotometric models of galaxy evolution to make a complete and exhaustive study of the effects of the environment on the evolution of galaxies in high density regions. We present here the scientific objectives of the survey, describing the observing strategy and briefly discussing different data reduction techniques. Using UV data already in-hand for the central 12 sq. deg we determine the FUV and NUV luminosity functions of the Virgo cluster core for all cluster members and separately for early-and late-type galaxies and compare it to the one obtained in the field and other nearby clusters (Coma, A1367). This analysis shows that the FUV and NUV luminosity functions of the core of the Virgo clusters are flatter (alpha similar to -1.1) than those determined in Coma and A1367. We discuss the possible origin of this difference.

The spin temperature of high-redshift damped Lyman α systems

We report results from a programme aimed at investigating the temperature of neutral gas in high-redshift damped Lyman α absorbers (DLAs). This involved (1) H I 21 cm absorption studies of a large sample of DLAs towards radio-loud quasars, (2) very long baseline interferometric studies to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallicities and the velocity widths of low-ionization metal lines. Including literature data, our sample consists of 37 DLAs with estimates of the harmonic mean spin temperature Ts. We find a statistically significant (4σ) difference between the Ts distributions in the high-z (z > 2.4) and low-z (z 1.7 have Ts values indicating CNM fractions >20 per cent, comparable to the median value (≈27 per cent) in the Galaxy. We tested whether the H I column density measured towards the optical quasar might be systematically different from that towards the radio core by comparing the H I column densities inferred from H I 21 cm emission studies at different spatial resolutions (≈15 pc-1 kpc) in the Large Magellanic Cloud. The high-resolution NH I values are, on average, larger than the smoothed ones for NH I > 1021 cm-2, but lower than the smoothed NH I estimates for NH I 1. Since all DLAs at z > 1 have angular diameter distances comparable to or larger than those of their background quasars, they have similar efficiency in covering the quasars. We conclude that low covering factors in high-z DLAs cannot account for the observed redshift evolution in spin temperatures.

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.

The Arecibo Galaxy Environment Survey: precursor observations of the NGC 628 group

The Arecibo Galaxy Environment Survey (AGES) is one of several HI surveys utilising the new Arecibo L-band Feed Array (ALFA) fitted to the 305m radio telescope at Arecibo. The survey is specifically designed to investigate various galactic environments to higher sensitivity, higher velocity resolution and higher spatial resolution than previous fully sampled, 21 cm multibeam surveys. The emphasis is on making detailed observations of nearby objects although the large system bandwidth (100 MHz) will allow us to quantify the HI properties over a large instantaneous velocity range. In this paper we describe the survey and its goals and present the results from the precursor observations of a 5 degree x 1 degree region containing the nearby (~10 Mpc) NGC 628 group. We have detected all the group galaxies in the region including the low mass (M{HI}~10^7Mo) dwarf, dw0137+1541 (Briggs, 1986). The fluxes and velocities for these galaxies compare well with previously published data. There is no intra-group neutral gas detected down to a limiting column density of 2x10^{18}cm^{-2}. In addition to the group galaxies we have detected 22 galaxies beyond the NGC 628 group, 9 of which are previously uncatalogued.(Abridged)

The Arecibo Galaxy Environment Survey – II. A H i view of the Abell cluster 1367 and its outskirts

We present 21 cm H i line observations of 5 × 1 deg2 centred on the local Abell cluster 1367 obtained as part of the Arecibo Galaxy Environment Survey. One hundred sources are detected (79 new H i measurements and 50 new redshifts), more than half belonging to the cluster core and its infalling region. Combining the H i data with Sloan Digital Sky Survey (SDSS) optical imaging, we show that our H i selected sample follows scaling relations similar to the ones usually observed in optically selected samples. Interestingly, all galaxies in our sample appear to have nearly the same baryon fraction independently of their size, surface brightness and luminosity. The most striking difference between H i and optically selected samples resides in their large-scale distribution: whereas optical and X-ray observations trace the cluster core very well, in H i there is almost no evidence of the presence of the cluster. Some implications on the determination of the cluster luminosity function and H i distribution for samples selected at different wavelength are also discussed.

A Pilot for a Very Large Array H I Deep Field

High-resolution 21-cm HI deep fields provide spatially and kinematically resolved images of neutral hydrogen at different redshifts, which are key to understanding galaxy evolution across cosmic time and testing predictions of cosmological simulations. Here we present results from a pilot for an HI deep field done with the Karl G. Jansky Very Large Array (VLA). We take advantage of the newly expanded capabilities of the telescope to probe the redshift interval 0 < z < 0.193 in one observation. We observe the COSMOS field for 50 hours, which contains 413 galaxies with optical spectroscopic redshifts in the imaged field of 34′ × 34′ and the observed redshift interval. We have detected neutral hydrogen gas in 33 galaxies in different environments spanning the probed redshift range, including three without a previously known spectroscopic redshift. The detections have a range of HI and stellar masses, indicating the diversity of galaxies we are probing. We discuss the observations, data reduction, results and highlight interesting detections. We find that the VLA’s B-array is the ideal configuration for HI deep fields since its long spacings mitigate RFI. This pilot shows that the VLA is ready to carry out such a survey, and serves as a test for future HI deep fields planned with other SKA pathfinders.

Late-Time Observations of the Afterglow and Environment of GRB 030329

We present Very Long Baseline Interferometry (VLBI) observations 217 days after the {gamma}-ray burst of 2003 March 29. These observations provide further measurements of the size and position of GRB 030329 that are used to constrain the expansion rate and proper motion of this nearby GRB. The expansion rate appears to be slowing down with time, favoring expansion into a constant density interstellar medium, rather than a circumstellar wind with an r{sup -2} density profile. We also present late time Arecibo observations of the redshifted Hi and OH absorption spectra towards GRB 030329. No absorption (or emission) is seen allowing us to place limits on the atomic neutral hydrogen of N{sub H} < 8.5 x 10{sup 20} cm{sup -2}, and molecular hydrogen of N{sub H{sub 2}} < 1.4 x 10{sup 22} cm{sup -2}. Finally, we present VLA limits on the radio polarization from the afterglow of <2% at late times.

The covering factor of high-redshift damped Lyman-α systems

We have used the very long baseline array to image 18 quasars with foreground damped Lyman-αsystems (DLAs) at 327, 610 or 1420 MHz, to measure the covering factor f of each DLA at or near its redshifted H I 21 cm line frequency. Including six systems from the literature, we find that none of 24 DLAs at 0.09 ~ 0.45-1 for the 14 DLAs at z > 1.5 and f ~ 0.41-1 for the 10 systems at z < 1, and consistent covering factor distributions in the two subsamples. The observed paucity of detections of H I 21 cm absorption in high-z DLAs thus cannot be explained by low covering factors and is instead likely to arise due to a larger fraction of warm H I in these absorbers.