arXiv subjects

A. Hopkins

Publications and source records attributed to A. Hopkins.

42 records · Page 3Linked to original sources

arXiv1999

The Phoenix Deep Survey: A Deep Microjansky Radio Survey

The study of the nature of faint radio sources is of great importance since a significant fraction of these objects is thought to be composed of actively star-forming galaxies. Due to the increased sensitivity of radio telescopes, we are now not only able to catalogue large numbers of these sources in the sub-millijansky regime, but also to start the study of the nature of increasingly fainter microjansky sources. This paper presents a new very deep 1.4 GHz radio survey made as a part of the Phoenix Deep Survey, a project aimed to study the nature of the faintest radio sources. With a limiting sensitivity of 45 microJy, this new survey has allowed us to assemble a large number of sources with 1.4 GHz flux densities below 100 microJy. The resulting source counts and the analysis of the optical properties of the faintest radio sources are presented.

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arXiv1999

Microjansky sources at 1.4 GHz

We present a deep 1.4 GHz survey made with the Australia Telescope Compact Array (ATCA), having a background RMS of 9 microJy near the image phase centre, up to 25 microJy at the edge of a 50' field of view. Over 770 radio sources brighter than 45 microJy have been catalogued in the field. The differential source counts in the deep field provide tentative support for the growing evidence that the microjansky radio population exhibits significantly higher clustering than found at higher flux density cutoffs. The optical identification rate on CCD images is approximately 50% to R=22.5, and the optical counterparts of the faintest radio sources appear to be mainly single galaxies close to this optical magnitude limit.

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arXiv1999

The 1.4 GHz and Halpha Luminosity Functions and Star Formation Rates from Faint Radio Galaxies

Using a recently completed survey of faint (sub-mJy) radio sources, selected at 1.4 GHz, a dust-free estimate of the local star formation rate (SFR) is carried out. The sample is 50% complete to 0.2 mJy, with over 50% of the radio sources having optical counterparts brighter than R = 21.5. Spectroscopic observations of 249 optically identified radio sources have been made, using the 2-degree Field (2dF) facility at the Anglo-Australian Telescope (AAT). Redshifts and equivalent widths of several spectral features (e.g., Hαand [OII]3727) sensitive to star formation have been measured and used to identify the star-forming and absorption-line systems. The spectroscopic sample is corrected for incompleteness and used to estimate the 1.4 GHz and Hαluminosity functions (LFs) and luminosity density distributions. The 1.4 GHz LF of the star-forming population has a much steeper faint-end slope (1.85) than that for the ellipticals (1.35). This implies an increasing preponderance of star-forming galaxies among the optically identified (i.e., z < 1) radio sources at fainter flux densities. The HαLF of the faint radio population agrees with published HαLFs derived from local samples selected by Hαemission. This suggests that the star-forming faint radio population is coincident with the Hαselected galaxies. The 1.4 GHz and Hαluminosity densities have been used to estimate the SFRs. The two estimates agree, both giving a SFR density of $0.032 M_\odot yr^{-1} Mpc^{-3}$ in the range z < 1.

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arXiv1999

The Phoenix survey: optical and near-infrared observations of faint radio sources

Using a deep Australia Telescope Compact Array (ATCA) radio survey covering an area of ~3deg^{2} to a 4σsensitivity of \ge 100 \muJy at 1.4GHz, we study the nature of faint radio galaxies. About 50% of the detected radio sources are identified with an optical counterpart revealed by CCD photometry to m_{R}=22.5 mag. Near-infrared (K-band) data are also available for a selected sample of the radio sources, while spectroscopic observations have been carried out for about 40% of the optically identified sample. These provide redshifts and information on the stellar content. Emission-line ratios imply that most of the emission line sources are star-forming galaxies, with a small contribution (\approx 10%) from Sy1/Sy2 type objects. We also find a significant number of absorption line systems, likely to be ellipticals. These dominate at high flux densities (> 1 mJy) but are also found at sub-mJy levels. Using the Balmer decrement we find a visual extinction A_{V}=1.0 for the star-forming faint radio sources. This moderate reddening is consistent with the V-R and R-K colours of the optically identified sources. For emission line galaxies, there is a correlation between the radio power and the Hαluminosity, in agreement with the result of Benn et al. (1993). This suggests that the radio emission of starburst radio galaxies is a good indicator of star-formation activity.

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arXiv1998

A 1.4 GHz Survey of the Southern ELAIS Region

A deep survey of the European Large Area ISO Survey (ELAIS) field in the southern celestial hemisphere (hereinafter S1) has been carried out with the Australia Telescope Compact Array (ATCA) at 1.4 GHz. The S1 region, covering about 4 square degrees, has been also surveyed in the mid- and far-infrared (5-200 micron) with the Infrared Space Observatory (ISO). The radio survey provides uniform coverage of the entire S1 region, with a sensitivity (5 sigma) of 0.4 mJy over the whole area and 0.2 mJy in the center. To this sensitivity, virtually all the radio counterparts of the far-infrared extragalactic ISO sources should be detected. This paper presents a radio sample - complete at the 5 sigma level - consisting of 581 sources detected at 1.4 GHz. Of these radio sources, 349 have peak flux density in the range 0.2-1 mJy, forming a new homogeneous sample of sub-mJy radio sources. Due to its size, depth and multi-waveband coverage, the sample will allow us to study in greater detail the sub-mJy radio source population. The full catalogue will be available from http://athena.ph.ac.uk/

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arXiv1998

Star Formation Rates in Faint Radio Galaxies

The decimetric radio continuum luminosity of a star-forming galaxy appears to be directly proportional to the rate of formation of supernovae in the galaxy. Since decimetric radiation does not suffer significant extinction and is not directive, radio luminosities may thus provide a particularly straightforward way to determine the current rate of star formation. Using a sample of over 700 local galaxies we confirm the utility of the radio luminosity as a measure of star formation rate by showing concordance with the rates predicted by U-band, H-alpha, and far-infrared luminosites. We also show that there are systematic discrepancies between these various indicators, suggesting that the H-alpha luminosity may underestimate the star formation rate by approximately an order of magnitude when the star formation rate is more than 20 solar mass per year. We use this calibration and the measured radio luminosities of sub-mJy radio sources to infer the star formation rate in approximately 60 star-forming galaxies at moderate (z = 0.1) redshifts, both as the actual rate and as the fraction of the existing mass of stars in the galaxy. For some of these objects the inferred current rate of star formation could increase the stellar mass in the galaxy by approximately 10% over an interval of about 30 Myr.

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