Phoenix Deep Survey Research Articles

Radio variability in the Phoenix Deep Survey at 1.4 GHz

We use archival data from the Phoenix Deep Survey to investigate the variable radio source population above 1mJy/beam at 1.4GHz. Given the similarity of this survey to other such surveys we take the opportunity to investigate the conflicting results which have appeared in the literature. Two previous surveys for variability conducted with the Very Large Array (VLA) achieved a sensitivity of 1mJy/beam. However, one survey found an areal density of radio variables on timescales of decades that is a factor of ~4 times greater than a second survey which was conducted on timescales of less than a few years. In the Phoenix deep field we measure the density of variable radio sources to be $\rho =0.98\mathrm{deg}^{-2}$ on timescales of 6 months to 8 years. We make use of WISE infrared cross-ids, and identify all variable sources as an AGN of some description. We suggest that the discrepancy between previous VLA results is due to the different time scales probed by each of the surveys, and that radio variability at 1.4 GHz is greatest on timescales of 2 - 5 years.

THE PHOENIX DEEP SURVEY: EXTREMELY RED GALAXIES AND CLUSTER CANDIDATES

We present the results of a study of a sample of 375 extremely red galaxies (ERGs) in the Phoenix Deep Survey, 273 of which constitute a subsample which is 80% complete to Ks = 18.5 over an area of 1160 arcmin2. The angular correlation function for ERGs is estimated, and the association of ERGs with faint radio sources explored. We find tentative evidence that ERGs and faint radio sources are associated at z 0.5. A new overdensity-mapping algorithm has been used to characterize the ERG distribution, and identify a number of cluster candidates, including a likely cluster containing ERGs at 0.5 4 criterion is more efficient than R – Ks > 5 at selecting dusty star-forming galaxies, rather than passively evolving ERGs.

The Phoenix Deep Survey: the star formation rates and the stellar masses of extremely red objects

We estimate the star-formation rates and the stellar masses of the Extremely Red Objects (EROs) detected in a � 180arcmin 2 Ks-band survey (Ks � 20mag). This sample is complemented by sensitive 1.4GHz radio observations (12µJy 1� rms) and multiwaveband photometric data (UBV RIJ) as part of the Phoenix Deep Survey. For bright K 4mag; total of 177) we use photometric methods to discriminate dust-enshrouded active systems from earlytype galaxies and to constrain their redshifts. Radio stacking is then employed to estimate mean radio flux densities of � 8.6 (3�) and 6.4µJy (2.4�) for the dusty and early-type subsamples respectively. Assuming that dust enshrouded active EROs are powered by star-formation the above radio flux density at the median redshift of z = 1 translates to a radio luminosity of L1.4 = 4.5 × 10 22 W/Hz and a star-formation rate of SFR = 25M⊙ yr −1 . Combining this result with photometric redshift estimates we find a lower limit to the star-formation rate density of 0.02±0.01M⊙ yr −1 Mpc −3 for the K 5 × 10 10 M⊙. We also find that EROs represent a sizable fraction (about 50 per cent) of the number density of galaxies more massive than M = 5 × 10 10 M⊙ at z � 1, with almost equal contributions from dusty and early type systems. Similarly, we find that EROs contribute about half of the mass density of the Universe at z � 1 (with almost equal contributions from dusty and early types), after taking into account incompleteness because of the magnitude limit K = 19.5mag.

The Phoenix Deep Survey: Spectroscopic Catalog

The Phoenix Deep Survey is a multiwavelength survey based on deep 1.4 GHz radio imaging, reaching well into the sub-100 μJy level. One of the aims of this survey is to characterize the submillijansky radio population, exploring its nature and evolution. In this paper we present the catalog and results of the spectroscopic observations aimed at characterizing the optically bright (R 21.5 mag) counterparts of faint radio sources. Of 371 sources with redshift determination, 21% have absorption lines only, 11% show active galactic nucleus signatures, 32% are star-forming galaxies, 34% show narrow emission lines that do not allow detailed spectral classification (owing to poor signal-to-noise ratio and/or lack of diagnostic emission lines), and the remaining 2% are identified with stars. For the star-forming galaxies with a Balmer decrement measurement we find a median extinction of AHα = 1.9 mag, higher than that of optically selected samples. This is a result of the radio selection, which is not biased against dusty systems. Using the available spectroscopic information, we estimate the radio luminosity function of star-forming galaxies in two independent redshift bins at z ≈ 0.1 and 0.3, respectively. We find direct evidence for strong luminosity evolution of these systems consistent with L1.4 GHz ∝ (1 + z)2.7.

The Phoenix Deep Survey: The Clustering and Environment of Extremely Red Objects

In this paper we explore the clustering properties and environment of the extremely red objects (EROs; I - K > 4 mag) detected in a ≈180 arcmin2 deep (Ks ≈ 20 mag) Ks-band survey of a region within the Phoenix Deep Survey, an ongoing multiwavelength program aiming to investigate the nature and evolution of faint radio sources. Using our complete sample of 289 EROs brighter than Ks = 20 mag, we estimate a statistically significant (≈3.7 σ) angular correlation function signal with amplitude Aw = 8.7 × 10-3 (assuming w(θ) = Awθ-0.8, with θ in degrees), consistent with earlier work based on smaller samples. This amplitude suggests a clustering length in the range ro = 12-17 h-1 Mpc, implying that EROs trace regions of enhanced density. Using a novel method, we further explore the association of EROs with galaxy overdensities by smoothing the K-band galaxy distribution using the matched filter algorithm of Postman et al. (1996) and then cross-correlating the resulting density maps with the ERO positions. Our analysis provides direct evidence that EROs are associated with overdensities at redshifts z 1. We also exploit the available deep radio 1.4 GHz data (limiting flux 60 μJy) to explore the association of EROs and faint radio sources and whether the two populations trace similar large-scale structures. Cross-correlation of the two samples (after excluding 17 EROs with radio counterparts) gives a 2 σ signal only for the subsample of high-z radio sources (z > 0.6). Although the statistics are poor, this suggests that it is the high-z radio subsample that traces similar structures with EROs.

The Phoenix Deep Survey: Optical and Near‐infrared Imaging Catalogs

The Phoenix Deep Survey is a multiwavelength galaxy survey based on deep 1.4 GHz radio imaging. The primary goal of this survey is to investigate the properties of star formation in galaxies and to trace the evolution in those properties to a redshift z = 1, covering a significant fraction of the age of the universe. By compiling a sample of star-forming galaxies based on selection at radio wavelengths we eliminate possible biases due to dust obscuration, a significant issue when selecting objects at optical and ultraviolet wavelengths. In this paper, we present the catalogs and results of deep optical (UBVRI) and near-infrared (Ks) imaging of the deepest region of the existing decimetric radio imaging. The observations and data processing are summarized and the construction of the optical source catalogs described, together with the details of the identification of candidate optical counterparts to the radio catalogs. Based on our UBVRIKs imaging, photometric redshift estimates for the optical counterparts to the radio detections are explored.

The Phoenix Deep Survey: the radio properties of the hard X-ray-selected sample

The radio properties of hard (2-8 keV) X-ray-selected sources are explored by combining a single 50-ks XMM-Newton pointing with the ultradeep and homogeneous Phoenix radio (1.4-GHz) survey. A total of 43 sources are detected above the X-ray flux limit f X (2-8 keV) = 7.7 x 10 -15 erg s -1 cm -2 , with 14 of them exhibiting radio emission above 40 μJy (3σ). The X-ray-radio matched population lies in the borderline between radio-loud and radio-quiet active galactic nuclei (AGNs) and comprises sources with both soft and hard X-ray spectral properties, suggesting both obscured and unobscured systems. The spectroscopically identified subsample (with a total of six X-ray-radio matches) comprises narrow emission-line AGNs (four) with hard X-ray spectral properties and broad line sources (two) with soft X-ray spectra. We find evidence that the fraction of X-ray-radio matches increases from 20 per cent for sources with a rest-frame column density of N H 10 22 cm -2 their combined spectrum exhibits a soft X-ray component that may be associated with star formation activity, although other possibilities cannot be excluded. We also find that radio-emitting AGNs make up approximately 13-20 per cent of the hard-band X-ray background depending on the adopted normalization.

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

In this paper we use a 50ks XMM-Newton pointing overlapping with the Phoenix Deep Survey, a homogeneous radio survey reaching muJy sensitivities, to explore the X-ray properties and the evolution of star-forming galaxies. UV, optical and NIR photometry is available and is used to estimate photometric redshifts and spectral types for radio sources brighter than R=21.5mag (total of 82). Sources with R 3.5sigma) is detected in the 0.5-2keV band corresponding to a mean flux of ~3e-16cgs for both subsamples. This flux translates to mean luminosities of ~5e40 and 1.5e41cgs for the z=0.240 and 0.455 subsamples respectively. Only a marginally significant signal (2.6sigma) is detected in the 2-8keV band for the z=0.455 subsample. We argue that the stacked signal above is dominated by star-formation. The mean L_X/L_B ratio and the mean L_X of the two subsamples are found to be higher than optically selected spirals and similar to starbursts. We also find that the mean L_X and L_1.4 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 found to be elevated compared to local HII galaxies. The observed increase is consistent with L_X evolution of the form (1+z)^3. Assuming that our sample is indeed dominated by starbursts this is direct evidence for evolution of such systems at X-ray wavelengths. Using an empirical L_X to SFR conversion we estimate a global SFR density at z~0.3 of \~0.029M_o/yr/Mpc in agreement with previous studies.

Dependence of Dust Obscuration on Star Formation Rates in Galaxies

Many investigations of star formation rates (SFRs) in galaxies have explored details of dust obscuration, with a number of recent analyses suggesting that obscuration appears to increase in systems with high rates of star formation. To date, these analyses have been primarily based on nearby (z ≤ 0.03) or UV-selected samples. Using 1.4 GHz imaging and optical spectroscopic data from the Phoenix Deep Survey, the SFR-dependent obscuration is explored. The use of a radio-selected sample shows that previous studies exploring SFR-dependent obscurations have been biased against obscured galaxies. The observed relation between obscuration and SFR is found to be an unsuitable obscuration measure for individual galaxies. Nevertheless, it is shown to be successful as a first-order correction for large samples of galaxies when no other measure of obscuration is available, out to intermediate redshifts (z ≈ 0.8).

The Phoenix Deep Survey: The 1.4 GH[CLC]z[/CLC] 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.

Galaxy evolution from a microJansky radio survey

A strong evolution of galaxies is observed for 0<z<1, as evidenced by an increase of almost an order of magnitude in the galaxy star-formation rate density. However, it is known that dust obscuration has affected our understanding of galaxy evolution over this significant fraction of the age of the Universe. In order to study galaxy evolution free from dust induced biases, an ultra deep radio survey – the Phoenix Deep Survey – was initiated. With a detection limit of 60μJy, this homogeneous survey, complemented with multiwavelength (photometric and spectroscopic) observations, is being used to build a consistent picture of galaxy evolution. The ultra-deep radio source counts are presented, and interpreted using luminosity function evolutionary models. The discovery of extremely dusty galaxies from this survey, along with the clustering properties of the sub-mJy radio population, are also discussed.

ISO Observations of Faint Radio Sources

Deep radio surveys have revealed, below the millijansky level, a population of actively starforming galaxies, undergoing strong luminosity evolution as found from the observed radio source counts. A random sample of 65 sub-mJy radio sources, selected from the Phoenix Deep Survey, was observed at mid and far-infrared wavelengths using the Infrared Space Observatory. Nine sources were detected at both far-infrared and at least one of the mid-infrared wavelengths. A preliminary analysis of the infrared properties of these faint radio sources is carried out by modeling their infrared Spectral Energy Distribution. The radio vs. far-infrared relation, for these faint radio objects, is also presented.

A Complete Microjansky Radio Survey

The Phoenix Deep Survey is an ongoing multi-wavelength survey of a 2° diameter field aimed at studying the properties of the sub-mJy and μJy radio population. Here, we present the latest 1.4 GHz observations of this field. The new data, reaching a 5 σ flux level of 45 μJy at the centre of a 50′ diameter field, comprise more than 700 sources with flux densities less than 1 mJy (187 of which have S1.4 < 100 μJy). This provides one of the deepest radio (1.4 GHz) surveys currently available. The 1.4 GHz source counts are presented and show a flattening down to the 50 μJy level. At flux densities around 300 μJy there are indications that the sources detected may exhibit higher clustering than those observed at higher flux levels. This suggests that deep radio surveys could be useful for studies of large-scale structure but it also presents a warning for the representativity of sources in deep pencil-beam radio surveys. The study of the optical counterparts of the μJy population seems to indicate that the median R magnitude starts to decrease below 100 μJy. Spectroscopic classification of a sample of sources in this survey confirms the trend for an increasing fraction of star-forming galaxies over other systems down to ∼ 100 μJy.

Microjansky Radio Sources in DC 0107−46 (Abell 2877)

The cluster DC 0107-46 (Abell 2877) lies within the Phoenix Deep Survey, made at 1.4 GHz with the Australia Telescope Compact Array. Of 89 known optical cluster members, 70 lie within the radio survey area. Of these 70 galaxies, 15 (21%) are detected, with luminosities as faint as 1.0 × 1020 W Hz-1. Spectroscopic observations are available for 14/15 of the radio-detected cluster galaxies. Six galaxies show only absorption features and are typical low-luminosity active galactic nucleus (AGN) radio sources. One galaxy hosts a Seyfert 2 nucleus, two are star-forming galaxies, and the remaining five may be star-forming galaxies, AGNs, or both.

The Phoenix Deep Survey: 1.4-GHz source counts

We present the first results from the Phoenix Deep Survey, a multiwavelength survey of a 2° diameter region. Observations in the radio continuum at 1.4 GHz carried out with the Australia Telescope Compact Array are described. The catalogue of over 1000 radio sources compiled from these observations is analysed, and the source counts are presented. We model the observational source counts using a two-population model and published luminosity functions for these populations. Upon invoking luminosity and density evolution, we find that a luminosity evolution model best fits the radio observations, consistent with earlier work. The redshift distribution of the two galaxy populations investigated is also modelled and discussed.