A Deep Survey of HI-Selected Galaxies: The Sample and the Data

In a 21 cm neutral hydrogen survey of approximately 55 sq deg out to a redshift of cz=8340 km/s, we have identified 75 extragalactic HI sources. These objects comprise a well-defined sample of extragalactic sources chosen by means that are independent of optical surface brightness selection effects. In this paper we describe the Arecibo survey procedures and HI data, follow-up VLA HI observations made of several unusual sources, and Kitt Peak B-, R-, and I-band photometry for nearly all of the galaxies. We have also gathered information for some of the optically detected galaxies within the same search volume. We examine how samples generated by different types of search techniques overlap with selection by HI flux. Only the least massive HI object, which is among the lowest mass HI sources previously found, does not have a clear optical counterpart, but a nearby bright star may hide low surface brightness emission. However the newly-detected systems do have unusual optical properties. Most of the 40 galaxies that were not previously identified in magnitude-limited catalogs appear to be gas-dominated systems, and several of these systems have HI mass-to-light ratios among the largest values ever previously found. These gas-dominated objects also tend to have very blue colors, low surface brightnesses, and no central bulges, which correlate strongly with their relative star-to-gas content.Comment: 48 pages, 10 figures, Figure 3 included as 3 separate JPG images. To appear in Ap J Supplement

Viewing the 21 cm sky : a slice of the neutral hydrogen universe.

Among the most fundamental problems in extragalactic astronomy are understanding the processes which led to galaxy formation and establishing the existence and nature of dark matter in the Universe. Before any attempt can be made to solve either of these problems, an accurate and thorough knowledge of the true population of galaxies is required. Unfortunately, there is mounting evidence that our knowledge of extragalactic space may be critically incomplete. This deficiency is due primarily to our dependence on galaxy statistics that are largely derived from observations at optical wavelengths. These statistics may include only a sub-population of extragalactic objects which have properties easily detected in that narrow wavelength range, rather than the true population which may have a more diverse range of properties. To steer ourselves away from this possible optical bias, and thus to obtain a more well-rounded census of galaxy sizes, types, and locations, we have conducted a large-scale, unbiased survey for atomic hydrogen (HI) at 21cm. Because the strength of 21cm emission does not depend on the same forces which drive optical emission, this survey allows us to not only augment the compiled database of galaxies by describing the properties of previously cataloged objects we detect, but more importantly search for new types of objects which have historically remained undetected due to optical biases. Using the NAIC Arecibo Telescope, we have systematically searched a slice of extragalactic space known to contain 48 cataloged galaxies. Our survey has re-detected 38 by their 21cm emission, and has failed to detect 10. We have also detected an additional 41 previously unknown objects. We present atomic hydrogen and optical data for all objects and demonstrate that the newly discovered galaxies represent a population which differs distinctly from the cataloged galaxies in having lower overall luminosities and masses, larger relative HI gas contents, and bluer colors. In addition we show that several extremely low-mass, low luminosity galaxies discovered within 5 Mpc of the Milky Way imply the presence of thousands of similar objects throughout the slice search region. These low-mass objects could, in their great numbers, represent a significant fraction of the total integrated mass of all galaxies in the region. As such, they could have a profound influence on the distribution and evolution processes of all extragalactic objects

A search for Low Surface Brightness galaxies in the near-infrared I. Selection of the sample

A sample of about 3,800 Low Surface Brightness (LSB) galaxies was selected using the all-sky near-infrared (J, H and K_s-band) 2MASS survey. The selected objects have a mean central surface brightness within a 5 arcsec radius around their centre fainter than 18 mag/sq.arcsec in the K_s band, making them the lowest surface brightness galaxies detected by 2MASS. A description is given of the relevant properties of the 2MASS survey and the LSB galaxy selection procedure, as well as of basic photometric properties of the selected objects. The latter properties are compared to those of other samples of galaxies, of both LSBs and `classical' high surface brightness (HSB) objects, which were selected in the optical. The 2MASS LSBs have a (B_T_c)-(K_T) colour which is on average 0.9 mag bluer than that of HSBs from the NGC. The 2MASS sample does not appear to contain a significant population of red objects.Comment: accepted for publication in Astronomy and Astrophysics on 24/2/2003; 62 page

The NGC 672 and NGC 784 Galaxy Groups: Evidence for Galaxy Formation and Growth Along a Nearby Dark Matter Filament

(Abridged): We present U, B, V, R, I, H-alpha and NUV photometry of 14 galaxies in the very local Universe (within 10 Mpc that are dwarf irregular galaxies (dIrr), are at low redshift (51<v<610 km/s), and appear as a six degree long linear filament.. We examine the star formation (SF) properties of individual objects with the current SF rate (SFR) derived directly from the H-alpha line flux and compare the multi-band photometry with results of galaxy evolution assuming short SF bursts separated by long quiescence periods. Most objects contain at least one "old" stellar population (>1-10 Gyr) and one "young" population (<30 Myr) with the recent SF bursts occurring a few to a few 10s of Myr ago, arguing for synchronicity in star formation in these objects. We propose that the ~synchronous star formation in all objects is caused by the accretion of cold gas from intergalactic space onto dark matter haloes arranged along a filament threading the void where these dwarf galaxies reside and point out this galaxy sample as an ideal target to study hierarchical clustering and galaxy formation among very nearby objects.Comment: 29 pages, five figures. MNRAS, in press. This version with no galaxy images to reduce file size. A full version (.pdf) can be downloaded cia anonymous ftp from ftp://wise-gate.tau.ac.il/ftp/pub/noah/AdiPaper_accepted.pd

HI Selected Galaxies in the Sloan Digital Sky Survey I: Optical Data

We present the optical data for 195 HI-selected galaxies that fall within both the Sloan Digital Sky Survey (SDSS) and the Parkes Equatorial Survey (ES). The photometric quantities have been independently recomputed for our sample using a new photometric pipeline optimized for large galaxies, thus correcting for SDSS's limited reliability for automatic photometry of angularly large or low surface brightness (LSB) galaxies. We outline the magnitude of the uncertainty in the SDSS catalog-level photometry and derive a quantitative method for correcting the over-sky subtraction in the SDSS photometric pipeline. The main thrust of this paper is to present the ES/SDSS sample and discuss the methods behind the improved photometry, which will be used in future scientific analysis. We present the overall optical properties of the sample and briefly compare to a volume-limited, optically-selected sample. Compared to the optically-selected SDSS sample (in the similar volume), HI-selected galaxies are bluer and more luminous (fewer dwarf ellipticals and more star formation). However, compared to typical SDSS galaxy studies, which have their own selection effects, our sample is bluer, fainter and less massive.Comment: 14 pages, 8 Figures, accepted for publication in AJ. Complete tables will be available in the AJ electronic version and on the Vizier sit

A search for Low Surface Brightness galaxies in the near-infrared II. Arecibo HI line observations

A total of 367 Low Surface Brightness galaxies detected in the 2MASS all-sky near-infrared survey have been observed in the 21 cm HI line using the Arecibo telescope. All have a K_s-band mean central surface brightness, measured within a 5 arcsec radius, fainter than 18 mag/arcsec^(2). We present global HI line parameters for the 107 clearly detected objects and the 21 marginal detections, as well as upper limits for the undetected objects. The 107 clear detections comprise 15 previously uncatalogued objects and 36 with a PGC entry only.Comment: Accepted for publication by Astronomy and Astrophysics on 30 April 2003, 34 page

Non-confirmation of reported HI clouds without optical counterparts in the Hercules Cluster

21 cm HI line observations were made with the Arecibo Gregorian telescope of 9 HI clouds in the Hercules Cluster which were reported as tenative detections in a VLA HI study of the cluster (Dickey 1997) and for which our deep CCD imaging failed to find any optical counterparts. No sensitive observations could be made of one of these (sw-174) due to the presence of a close-by strong continuum source. The other 8 tentative HI detections were not reconfirmed by the Arecibo HI measurements. The CCD images did reveal faint, low surface brightness counterparts near the centres of two other VLA HI sources invisible on the Palomar Sky Survey, sw-103 and sw-194.Comment: 7 pages, accepted for Astronomy and Astrophysics on 8 Nov. 2002; new references added on 23 Dec. 200

Correlations among the properties of galaxies found in a blind HI survey, which also have SDSS optical data

We have used the Parkes Multibeam system and the Sloan Digital Sky Survey (SDSS) to assemble a sample of 195 galaxies selected originally from their HI signature to avoid biases against unevolved or low surface brightness objects. For each source 9 intrinsic properties are measured homogeneously, as well as inclination and an optical spectrum. The sample, which should be almost entirely free of either misidentification or confusion, includes a wide diversity of galaxies ranging from inchoate, low surface brightness dwarfs to giant spirals. Despite this diversity there are 5 clear correlations among their properties. They include a common dynamical mass-to-light ratio within their optical radii, a correlation between surface-brightness and Luminosity and a common HI surface-density. Such correlation should provide strong constrains on models of galaxy formation and evolution.Comment: 17 pages, 22 figures, submitted to MNRA

The HIPASS Catalogue: III - Optical Counterparts & Isolated Dark Galaxies

We present the largest catalogue to date of optical counterparts for HI radio-selected galaxies, Hopcat. Of the 4315 HI radio-detected sources from the HI Parkes All Sky Survey (Hipass) catalogue, we find optical counterparts for 3618 (84%) galaxies. Of these, 1798 (42%) have confirmed optical velocities and 848 (20%) are single matches without confirmed velocities. Some galaxy matches are members of galaxy groups. From these multiple galaxy matches, 714 (16%) have confirmed optical velocities and a further 258 (6%) galaxies are without confirmed velocities. For 481 (11%), multiple galaxies are present but no single optical counterpart can be chosen and 216 (5%) have no obvious optical galaxy present. Most of these 'blank fields' are in crowded fields along the Galactic plane or have high extinctions. Isolated 'Dark galaxy' candidates are investigated using an extinction cut of ABj < 1 mag and the blank fields category. Of the 3692 galaxies with an ABj extinction < 1 mag, only 13 are also blank fields. Of these, 12 are eliminated either with follow-up Parkes observations or are in crowded fields. The remaining one has a low surface brightness optical counterpart. Hence, no isolated optically dark galaxies have been found within the limits of the Hipass survey.Comment: 10 pages, 15 figures, MNRAS (in press