L. E. Cram

Toward a Resolution of the Discrepancy between Different Estimators of Star Formation Rate

Different wavelength regimes and methods for estimating the space density of the star formation rate (SFR) result in discrepant values. While it is recognized that ultraviolet (UV) and Hα emission-line data must be corrected for the effects of extinction, the magnitude of the required correction is uncertain. Even when these corrections are made there remains a significant discrepancy between SFRs derived from UV and Hα measurements compared with those derived from far-infrared (FIR) and radio luminosities. Since the FIR-radio–derived SFRs are not affected by extinction and simple corrections to reconcile the UV and Hα measurement with these do not fully account for the discrepancies, a more sophisticated correction may be required. Recent results suggest that at least part of the solution may be a form of extinction that increases with increasing SFR (or luminosity, given the common assumption that SFR is proportional to luminosity). We present an analysis of the effects of a dust reddening dependent on star formation rate applied to estimators of SFR. We show (1) that the discrepancies between Hα and FIR-radio SFR estimates may be explained by such an effect and we present an iterative method for applying the correction and (2) that UV-based estimates of SFR are harder to reconcile with FIR-radio estimates using this method, although the extent of the remaining discrepancy is less than for a non-SFR–dependent correction. Particularly at high redshift, our understanding of extinction at UV wavelengths may require a still more complex explanation.

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, Hα, and far-infrared luminosities. We also show that there are systematic discrepancies between these various indicators, suggesting that the Hα luminosity may underestimate the star formation rate by approximately an order of magnitude when the star formation rate is 20 M☉ yr-1. 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 ≈ 30 Myr.

The Phoenix Deep Survey: The 1.4 GHz Microjansky Catalog

The initial Phoenix Deep Survey (PDS) observations with the Australia Telescope Compact Array have been supplemented by additional 1.4 GHz observations over the past few years. Here we present details of the construction of a new mosaic image covering an area of 4.56 deg2, an investigation of the reliability of the source measurements, and the 1.4 GHz source counts for the compiled radio catalog. The mosaic achieves a 1 σ rms noise of 12 μJy at its most sensitive, and a homogeneous radio-selected catalog of over 2000 sources reaching flux densities as faint as 60 μJy has been compiled. The source parameter measurements are found to be consistent with the expected uncertainties from the image noise levels and the Gaussian source fitting procedure. A radio-selected sample avoids the complications of obscuration associated with optically selected samples, and by utilizing complementary PDS observations, including multicolor optical, near-infrared, and spectroscopic data, this radio catalog will be used in a detailed investigation of the evolution in star formation spanning the redshift range 0 < z < 1. The homogeneity of the catalog ensures a consistent picture of galaxy evolution can be developed over the full cosmologically significant redshift range of interest. The 1.4 GHz mosaic image and the source catalog are available on the World Wide Web; or from the authors by request.

A Comparison of Independent Star Formation Diagnostics for an Ultraviolet-selected Sample of Nearby Galaxies

We present results from a decimetric radio survey undertaken with the Very Large Array as part of a longer term goal to intercompare star formation and dust extinction diagnostics on a galaxy-by-galaxy basis for a representative sample of nearby galaxies. For our survey field, Selected Area 57, star formation rates derived from 1.4 GHz luminosities are compared with earlier nebular emission-line and ultraviolet (UV) continuum diagnostics. We find broad correlations, over several decades in luminosity, between the Hα, UV continuum, and 1.4 GHz diagnostics. However, the scatter in these relations is found to be larger than observational errors, with offsets between the observed relations and those expected assuming constant star formation histories and luminosity-independent extinction models. We investigate the physical origin of the observed relations and conclude that the discrepancies between different star formation diagnostics can only be partly explained by simple models of dust extinction in galaxies. These models cannot by themselves explain all the observed differences, introducing the need for temporally varying star formation histories and/or more complex models of extinction to explain the entire data set.

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 ≈3 deg2 to a 4σ sensitivity of ≥ 100 μJy at 1.4 GHz, we study the nature of faint radio galaxies. About 50 per cent of the detected radio sources are identified with an optical counterpart revealed by CCD photometry to mR=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 per cent 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 (≈ 10 per cent) 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 AV=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 Hα luminosity, in agreement with the result of Benn et al. This suggests that the radio emission of starburst radio galaxies is a good indicator of star formation activity.

OPTICAL AND X-RAY IDENTIFICATION OF FAINT RADIO SOURCES IN THE GOODS CDF-S ADVANCED CAMERA FOR SURVEYS FIELD

We present optical and X-ray identifications for the 64 radio sources in the Great Observatories Origins Deep Survey Chandra Deep Field-South Advanced Camera for Surveys (ACS) field revealed in the Australia Telescope Compact Array 1.4 GHz survey of the Chandra Deep Field-South. Optical identifications are made using the ACS images and catalogs, while the X-ray view is provided by the Chandra X-Ray Observatory 1 Ms observations. Redshifts for the identified sources are drawn from publicly available catalogs of spectroscopic observations and multiband photometric-based estimates. Using this multiwavelength information we provide a first characterization of the faint radio source population in this region. The sample contains a mixture of star-forming galaxies and active galactic nuclei, as identified by their X-ray properties and optical spectroscopy. A large number of morphologically disturbed galaxies are found, possibly related to star formation. In spite of the very deep optical data available in this field, seven of the 64 radio sources have no optical identification to z850 ~ 28 mag. Only one of these is identified in the X-ray.

Archetypal analysis of galaxy spectra

ABSTRACT Archetypal analysis represents each individual member of a set of data vectors as a mixture (aconstrained linear combination)of the pure types or archetypes of the data set. The archetypesare themselves required to be mixtures of the data vectors. Archetypal analysis may be partic-ularlyuseful in analysingdata sets comprisinggalaxyspectra, since each spectrumis, presum-ably, a superposition of the emission from the various stellar populations, nebular emissionsand nuclear activity making up that galaxy, and each of these emission sources corresponds toa potential archetype of the entire data set. We demonstrate archetypal analysis using sets ofcomposite synthetic galaxy spectra, showing that the method promises to be an effective andefficient way to classify spectra. We show that archetypal an alysis is robust in the presence ofvarious types of noise.Key words: methods: data analysis – methods: statistical – galaxies: e volution – galaxies:fundamental parameters – galaxies: stellar content

Chromospheric H-alpha and CA II lines in late-type stars

Observations of H-alpha and Ca II H and K in the chromospheres of 50 main-sequence K and M stars are reported. The photospheric contribution to the integrated core flux in these lines is found to be more important than previously thought, and earlier estimates of the basal flux of cool dwarf stars are revised. The H-alpha data confirm the presence of both an upper and a lower limit to the H-alpha equivalent width attained at any given spectral type. The maximum H-alpha absorption strength decreases gradually toward cooler stars while the maximum saturated quiescent emission strength increases. The Ca II emission strength is related to the strength of the emission or absorption feature at H-alpha, but there is not a one-to-one correlation. The main results may be explained in terms of photoionization and collisional control of the non-LTE H-alpha source function. Several aspects of the structure and heating of the outer atmospheres of dwarf K and M stars are discussed. 60 refs.

The 1.4-GHz and Hα luminosity functions and star formation rates from faint radio galaxies

A sample of over 1000 objects selected from a 1.4 GHz survey made by the Australia Telescope Compact Array (ATCA) is used to study the properties of the faint radio source population. The sample, covering an area of ≈ 3 deg 2 , is 50% complete to 0.2 mJy. Over 50% of the radio sources are found to have optical counterparts brighter then 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 � �

The Phoenix Deep Survey: X-ray properties of faint radio sources

In this paper, we use a 50-ks XMM-Newton pointing overlapping with the Phoenix Deep Survey, a homogeneous radio survey reaching pJy sensitivities, to explore the X-ray properties and the evolution of star-forming galaxies. Multiwavelength ultraviolet, optical and near-infrared photometric data are available for this field and are used to estimate photometric redshifts and spectral types for all radio sources brighter than R = 21.5 mag (a total of 82). Faint radio galaxies with R 3.5σ) is detected in the 0.5-2 keV band, corresponding to a mean flux of 3 x 10 - 1 6 erg s - 1 cm - 2 for both subsamples. This flux translates to mean luminosities of 5 x 10 4 0 and 1.5 x 10 4 1 erg s - 1 for the z = 0.240 and 0.455 subsamples, respectively. Only a marginally significant signal (2.6a) is detected in the 2-8 keV band for the z = 0.455 subsample. This may indicate hardening of the mean X-ray properties of sub-mJy sources at higher redshifts and/or higher luminosities. Alternatively, this may be due to contamination of the z = 0.455 subsample by a small number of obscured active galactic nuclei (AGNs). On the basis of the observed optical and X-ray properties of the faint radio sample, we argue that the stacked signal above is dominated by star formation, with the AGN contamination being minimal. The mean X-ray-to-optical flux ratio and the mean X-ray luminosity of the two subsamples are found to be higher than optically selected spirals and similar to starbursts. We also find that the mean X-ray and radio luminosities of the faint radio sources studied here are consistent with the L X -L 1 . 4 correlation of local star-forming galaxies. Moreover, the X-ray emissivity of sub-mJy sources to z 0.3 is estimated and is found to be elevated compared with local H II galaxies. The observed increase is consistent with X-ray luminosity evolution of the form (1 + z) 3 . Assuming that our sample is indeed dominated by star-forming galaxies, this is direct evidence for evolution of such systems at X-ray wavelengths. Using an empirical X-ray luminosity to star formation rate (SFR) conversion factor, we estimate a global SFR density at z 0.3 of 0.029 ′ 0.007 M O . yr - 1 Mpc - 3 . This is found to be in fair agreement with previous results based on galaxy samples selected at different wavelengths.