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Register a new userFirst Results from the HI Jodrell All Sky Survey: Inclination-Dependent Selection Effects in a 21-cm Blind Survey
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R. H. Lang
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Details are presented of the HI Jodrell All Sky Survey (HIJASS). HIJASS is a blind neutral hydrogen (HI) survey of the northern sky, being conducted using the multibeam receiver on the Lovell Telescope at Jodrell Bank. HIJASS covers the velocity range -3500 km/s to 10000 km/s with a velocity resolution of 18.1 km/s and a spatial positional accuracy of ~2.5 arcmin. Thus far about 1115 sq deg have been surveyed. We describe the methods of detecting galaxies within the HIJASS data and of measuring their HI parameters. The properties of the resulting HI-selected sample of galaxies are described. Of the 222 sources so far confirmed, 170 (77 per cent) are clearly associated with a previously catalogued galaxy. A further 23 sources (10 percent) lie close (within 6 arcmin) to a previously catalogued galaxy for which no previous redshift exists. A further 29 sources (13 per cent) do not appear to be associated with any previously catalogued galaxy. The distributions of peak flux, integrated flux, HI mass and cz are discussed. We show, using the HIJASS data, that HI self-absorption is a significant, but often overlooked, effect in galaxies with large inclination angles to the line of sight. Properly accounting for it could increase the derived HI mass density of the local Universe by at least 25 per cent. The effect this will have on the shape of the HI Mass Function will depend on how self-absorption affects galaxies of different morphological types and HI masses. We also show that galaxies with small inclinations to the line of sight may also be excluded from HI-selected samples, since many such galaxies will have observed linewidths which are too narrow for them to be distinguished from narrow-band radio frequency interference.
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We report a Giant Metrewave Radio Telescope (GMRT) survey for associated HI 21-cm absorption from 50 active galactic nuclei (AGNs), at $z approx 0.04 - 3.01$, selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. Clean spectra were obtained towards 40 sources, yielding two new absorption detections, at $z = 0.229$ towards TXS 0003+380 and $z = 0.333$ towards TXS 1456+375, besides confirming an earlier detection, at $z = 1.277$ towards TXS 1543+480. There are 92 CJF sources, at $0.01 lesssim z lesssim 3.6$, with searches for associated HI 21-cm absorption, by far the largest uniformly-selected AGN sample with searches for such absorption. We find weak ($approx 2sigma$) evidence for a lower detection rate of HI 21-cm absorption at high redshifts, with detection rates of $28^{+10}_{-8}$% and $7^{+6}_{-4}$% in the low-$z$ ($z < z_{rm med}$) and high-$z$ ($z > z_{rm med}$) sub-samples, respectively. We use two-sample tests to find that the strength of the HI 21-cm absorption in the AGNs of our sample depends on both redshift and AGN luminosity, with a lower detection rate and weaker absorption at high redshifts and high ultraviolet/radio AGN luminosities. Unfortunately, the luminosity bias in our sample, with high-luminosity AGNs arising at high redshifts, implies that it is not currently possible to identify whether redshift evolution or AGN luminosity is the primary cause of the weaker absorption in high-$z$, high-luminosity AGNs. We find that the strength of HI 21-cm absorption does not depend on AGN colour, suggesting that dust extinction is not the main cause of reddening in the CJF sample.
High-velocity clouds (HVCs) are neutral or ionised gas clouds in the vicinity of the Milky Way that are characterised by high radial velocities inconsistent with participation in the regular rotation of the Galactic disc. Previous attempts to create a homogeneous all-sky HI map of HVCs have been hampered by a combination of poor angular resolution, limited surface brightness sensitivity and suboptimal sampling. Here, a new and improved HI map of Galactic HVCs based on the all-sky HI4PI survey is presented. The new map is fully sampled and provides significantly better angular resolution (16.2 versus 36 arcmin) and column density sensitivity (2.3 versus 3.7 * 10^18 cm^-2 at the native resolution) than the previously available LAB survey. The new HVC map resolves many of the major HVC complexes in the sky into an intricate network of narrow HI filaments and clumps that were not previously resolved by the LAB survey. The resulting sky coverage fraction of high-velocity HI emission above a column density level of 2 * 10^18 cm^-2 is approximately 15 per cent, which reduces to about 13 per cent when the Magellanic Clouds and other non-HVC emission are removed. The differential sky coverage fraction as a function of column density obeys a truncated power law with an exponent of -0.93 and a turnover point at about 5 * 10^19 cm^-2. HI column density and velocity maps of the HVC sky are made publicly available as FITS images for scientific use by the community.
We analyze the absolute magnitude (M_r) and color (u-r) of low redshift (z<0.06) galaxies in the Sloan Digital Sky Survey Data Release 6. galaxies with nearly exponential profiles (Sloan parameter fracDeV < 0.1) fall on the blue sequence of the color - magnitude diagram; if, in addition, these exponential galaxies have M_r < -19, they show a dependence of u-r color on apparent axis ratio q expected for a dusty disk galaxy. By fitting luminosity functions for exponential galaxies with different values of q, we find that the dimming is well described by the relation Delta M_r = 1.27 (log q)^2, rather than the Delta M = C log q law that is frequently assumed. When the absolute magnitudes of bright exponential galaxies are corrected to their face-on value, M_r^f = M_r - Delta M_r, the average u-r color is linearly dependent on M_r^f for a given value of q. Nearly face-on exponential galaxies (q > 0.9) have a shallow dependence of mean u-r color on M_r^f (0.096 magnitudes redder for every magnitude brighter). By comparison, nearly edge-on exponential galaxies (q < 0.3) are 0.265 magnitudes redder for every magnitude brighter. When the dimming law Delta M = 1.27 (log q)^2 is used to create an inclination-corrected sample of bright exponential galaxies, their apparent shapes are confirmed to be consistent with a distribution of mildly non-circular disks, with median short-to-long axis ratio gamma = 0.22 and median disk ellipticity epsilon = 0.08.
The HI line at 21 cm is a tracer of circumstellar matter around AGB stars, and especially of the matter located at large distances (0.1-1 pc) from the central stars. It can give unique information on the kinematics and on the physical conditions in the outer parts of circumstellar shells and in the regions where stellar matter is injected into the interstellar medium. However this tracer has not been much used up to now, due to the difficulty of separating the genuine circumstellar emission from the interstellar one. With the Nancay Radiotelescope we are carrying out a survey of the HI emission in a large sample of evolved stars. We report on recent progresses of this long term programme, with emphasis on S-type stars.
We describe The HI Nearby Galaxy Survey (THINGS), the largest program ever undertaken at the VLA to perform 21-cm HI observations of the highest quality (~7, <= 5 km/s resolution) of nearby galaxies. The goal of THINGS is to investigate key characteristics related to galaxy morphology, star formation and mass distribution across the Hubble sequence. A sample of 34 objects with distances between 3 and 10 Mpc will be observed, covering a wide range of evolutionary stages and properties. Data from THINGS will complement SINGS, the Spitzer Infrared Nearby Galaxy Survey. For the THINGS sample, high-quality observations at comparable resolution will thus be available from the X-ray regime through to the radio part of the spectrum. THINGS data can be used to investigate issues such as the small-scale structure of the ISM, its three-dimensional structure, the (dark) matter distribution and processes leading to star formation. To demonstrate the quality of the THINGS data products, we present some prelimary HI maps here of four galaxies from the THINGS sample.
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