arXiv subjects

M. D. Gregg

Publications and source records attributed to M. D. Gregg.

45 records · Page 3Linked to original sources

arXiv1998

The Fornax Spectroscopic Survey: The Number of Unresolved Compact Galaxies

We describe a sample of thirteen bright (18.5<Bj<20.1) compact galaxies at low redshift (0.05<z<0.21) behind the Fornax Cluster. These galaxies are unresolved on UK Schmidt sky survey plates, so would be missing from most galaxy catalogs compiled from this material. The objects were found during initial observations of The Fornax Spectroscopic Survey. This project is using the Two-degree Field spectrograph on the Anglo-Australian Telescope to obtain spectra for a complete sample of all 14000 objects, stellar and non-stellar, with 16.5<Bj<19.7, in a 12 square degree area centered on the Fornax cluster of galaxies. The surface density of compact galaxies with magnitudes 16.5<Bj<19.7 is 7+/-3 /sq.deg., representing 2.8+/-1.6% of all local (z<0.2) galaxies to this limit. There are 12+/-3 /sq.deg. with 16.5<Bj<20.2. They are luminous (-21.5<Mb<-18.0, for H0=50 km/s/mpc) and most have strong emission lines (H alpha equivalent widths of 40-200 A) and small sizes typical of luminous HII galaxies and compact narrow emission line galaxies. Four out of thirteen have red colors and early-type spectra, so are unlikely to have been detected in any previous surveys.

astro-ph↗

arXiv1998

Found: High Surface Brightness Compact Galaxies

We are using the 2dF spectrograph to make a survey of all objects (`stars' and `galaxies') in a 12 sq.deg region towards the Fornax cluster. We have discovered a population of compact emission-line galaxies unresolved on photographic sky survey plates and therefore missing in most galaxy surveys based on such material. These galaxies are as luminous as normal field galaxies. Using H-alpha to estimate star formation they contribute at least an additional 5 per cent to the local star formation rate.

astro-ph↗

arXiv1998

The Discovery of the Smallest Blue Compact Dwarf Galaxies

In a spectroscopic survey of the Fornax cluster to Bj=17.5 using the FLAIR spectrograph on the UK Schmidt Telescope we have discovered seven new compact dwarf cluster members. These were previously thought to be giant background spirals. These new members are among the most compact, high surface brightness dwarf galaxies known with absolute magnitudes of M_B=-14 and scale lengths of alpha=400 pc. One in particular may be the first high (normal) surface brightness dwarf spiral discovered. Three of the new dwarfs are blue compact dwarfs (BCDs); their inclusion in the cluster increases the faint end of the BCD luminosity function by a factor of 2 or more. We extended the survey 2.5 mag fainter with the 2dF spectrograph and in our first field found 7 bright emission line galaxies beyond the Fornax cluster which were unresolved on the sky survey plates. Galaxies of this type would be missed in most existing galaxy surveys.

astro-ph↗

arXiv1998

A Complete 2dF Survey of Fornax

We are using the 2dF spectrograph on the Anglo-Australian Telescope to obtain spectra for a complete sample of all 14000 objects with 16.5<B<19.7 in a 12 square degree area centred on the Fornax cluster. The aims of this project include the study of dwarf galaxies in the cluster (both known low surface brightness objects and putative normal surface brightness dwarfs) and a comparison sample of background field galaxies. We will also measure quasars, any previously unrecognised compact galaxies and a large sample of Galactic stars. Here we present initial results from the first 680 objects observed, including the discovery of a number of dwarf galaxies in the cluster more compact than any previously known.

astro-ph↗

arXiv1998

High surface brightness dwarf galaxies in the Fornax cluster

We describe a search for compact dwarf galaxies in the Fornax cluster using the FLAIR spectrograph on the UK Schmidt Telescope. We measured radial velocities of 453 compact galaxies brighter than Bt=17.3 and found seven new compact dwarf cluster members that were not classified in previous surveys as members of the cluster. These are amongst the most compact, high surface brightness dwarf galaxies known. The inclusion of these galaxies in the cluster does not change the total luminosity function significantly but they are important because of their extreme nature; one in particular appears to be a high (normal) surface brightness dwarf spiral. Three of the new dwarfs have strong emission lines and appear to be blue compact dwarfs (BCDs), doubling the number of confirmed BCDs in the cluster. We also determined that none of the compact dwarf elliptical (M32-like) candidates are in the cluster, down to an absolute magnitude Mb=-13.2. We have investigated the claim of Irwin et al. (1990) that there is no strong relation between surface brightness and magnitude for the cluster members and find some support for this for the brighter galaxies (Bt<17.3) but fainter galaxies still need to be measured.

astro-ph↗

arXiv1996

The Angular Two-Point Correlation Function for the FIRST Radio Survey

The FIRST (Faint Images of the Radio Sky at Twenty Centimeters) survey now covers 1550 square degrees of sky where $07^{h}16 < α< 17^{h}40$ and $28^{\circ}.3 < δ< 42^{\circ}$. This yields a catalog of 138,665 sources above the survey threshold of 1 mJy, about one third of which are in double-lobed and multi-component sources. We have used these data to obtain the first high-significance measurement of the two-point angular correlation for a deep radio sample. We find that the correlation function between $0.02^{\circ}$ and $2^{\circ}$ is well fitted by a power law of the form $Aθ^γ$ where $A\approx 3\times 10^{-3}$ and $γ\approx -1.1$. On small scales ($θ<0.2^{\circ}$), double and multi-component sources are shown to have a larger clustering amplitude than that of the whole sample. Sources with flux densities below 2 mJy are found to have a shallower slope than that obtained for the whole sample, consistent with there being a significant contribution from starbursting galaxies at these faint fluxes. The cross-correlation of radio sources and Abell clusters is determined. A preliminary approach to inferring spatial information is outlined.

astro-ph↗

arXiv1995

An HI survey of the Bootes Void. I. The Data

We present the results of a neutral hydrogen survey of the Bootes void carried out with the VLA in D-array. The survey covers $\sim 1100$ Mpc$^{3}$, about 1\% of the volume of the void as defined by Kirshner \etal 1987. We observed 24 fields, centered on known void galaxies; 16 of these were detected in HI. Eighteen uncataloged companion galaxies were discovered directly in the HI line at distances of $45\sec$ to $14.5\min$ from the target galaxies. We also present the results of follow-up optical imaging observations and discovery of one additional Bootes void galaxy, found through spectroscopy of a number of apparent companions to known void members. Our angular resolution is $\sim 1\min$ (45 kpc) \footnote{Throughout this paper, we have assumed ${H_{0}}$ = ${\rm 100\,km\,s^{-1}Mpc^{-1}}$.}, each field has a size of $\sim 1°$ (2.7 Mpc). The detected HI masses range from $8\times 10^{8}$ to $1\times 10^{10} \msol$. Typically our $2σ$ HI column density sensitivity is $2\times 10^{19} {\rm cm^{-2}}$. The radio and optical data are analyzed and discussed in the following companion article (Paper~2, Szomoru, van Gorkom, Gregg and Strauss 1996).

astro-ph↗

arXiv1995

An HI survey of the Bootes Void. II. The Analysis

We discuss the results of a VLA HI survey of the Bootes void and compare the distribution and HI properties of the void galaxies to those of galaxies found in a survey of regions of mean cosmic density. The Bootes survey covers 1100 Mpc$^{3}$, or $\sim$ 1\% of the volume of the void and consists of 24 cubes of typically 2 Mpc * 2 Mpc * 1280 km/s, centered on optically known galaxies. Sixteen targets were detected in HI; 18 previously uncataloged objects were discovered directly in HI. The control sample consists of 12 cubes centered on IRAS selected galaxies with FIR luminosities similar to those of the Bootes targets and located in regions of 1 to 2 times the cosmic mean density. In addition to the 12 targets 29 companions were detected in HI. We find that the number of galaxies within 1 Mpc of the targets is the same to within a factor of two for void and control samples, and thus that the small scale clustering of galaxies is the same in regions that differ by a factor of $\sim$ 6 in density on larger scales. A dynamical analysis of the galaxies in the void suggests that on scales of a few Mpc the galaxies are gravitationally bound, forming interacting galaxy pairs, loose pairs and loose groups. One group is compact enough to qualify as a Hickson compact group. The galaxies found in the void are mostly late-type, gas rich systems. A careful scrutiny of their HI and optical properties shows them to be very similar to field galaxies of the same morphological type. This, combined with our finding that the small scale clustering of the galaxies in the void is the same as in the field, suggests that it is the near environment that mostly affects the evolution of galaxies.

astro-ph↗

arXiv1995

The Angular Two Point Correlation Function for the FIRST Radio Survey - Initial Results

The angular two-point correlation function is calculated for the first 300 square degrees of the FIRST radio survey. Results for various subsamples are also obtained. Double-lobed sources are shown to have a higher clustering amplitude than the sample as a whole. Small differences in the correlation function from one region of the sample to another and results of various flux cuts are discussed.

astro-ph↗