FIRST Radio Sources Research Articles

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). I. The Optical Counterparts of FIRST Radio Sources

Abstract We report the result of optical identifications of FIRST radio sources with the Hyper Suprime-Cam Subaru Strategic Program survey (HSC-SSP). The positional cross-match within 1″ between the FIRST and HSC-SSP catalogs (i ≲ 26) produced more than 3600 optical counterparts in the 156 deg2 of the HSC-SSP field. The matched counterparts account for more than 50% of the FIRST sources in the search field, which substantially exceed previously reported fractions of SDSS counterparts (i ≲ 22) of ∼30%. Among the matched sample, 9% are optically unresolved sources such as radio-loud quasars. The optically faint (i > 21) radio galaxies (RGs) show that the fitting linear function of the 1.4 GHz source counts has a slope that is flatter than that of the bright RGs, while optically faint radio quasars show a slope steeper than that of bright radio quasars. The optically faint RGs show a flat slope in the i-band number counts down to 24 mag, implying either less massive or distant radio-active galactic nuclei (AGNs) beyond 24 mag. The photometric redshift and the comparison of colors with the galaxy models show that most of the matched RGs are distributed at redshifts from 0 to 1.5. The optically faint sample includes the high radio-loudness sources that are not seen in the optically bright sample. Such sources are located at redshift z > 1. This study gives ∼1500 radio AGNs lying at the optically faint end and high-redshift regime not probed by previous searches.

A MERLIN pilot study of faint variable VLA FIRST radio sources

We present the results of a pilot study in which we obtained Multi-Element Radio Linked Interferometer Network (MERLIN) L-band snapshot images of the 20 strongest radio sources from a sample of 123 radio objects that exhibit significant flux density variations at 1.4 GHz on a seven year base-line. The sample was created using FIRST VLA B-array data from 1995 and 2002 on a strip around zero declination near the south Galactic cap. The primary purpose is to assess the presence of radio jets and provide direct evidence between long-term variability and jet structures, as several models suggest. The radio structure of all the sources in this subsample appears very compact and our high resolution MERLIN observations reveal for the first time the presence of small milli-arcsecond (mas) jets and/or jet-like extensions in 17 of the 20 variable sources (literature data show core-jet structures or hints of extended structure in the other three objects which are point sources in our MERLIN observations). In the future we will use the radio results from the complete study, in combination with their optical Sloan Digital Sky Survey (SDSS) data, in order to investigate trends and possible differences or similarities between the quasars and galaxies that host these radio sources, and so examine existing unification schemes or open up new aspects in AGN unification.

Radio and optical orientations of galaxies

We investigate the correlations between optical and radio isophotal position angles for 14302 SDSS galaxies with $r$ magnitudes brighter than 18 and which have been associated with extended FIRST radio sources. We identify two separate populations of galaxies using the colour, concentration and their principal components. Surprisingly strong statistical alignments are found: late-type galaxies are overwhelmingly biased towards a position angle differences of $0^{\circ}$ and early-type galaxies to $90^{\circ}$. The late-type alignment can be easily understood in terms of the standard picture in which the radio emission is intimately related to areas of recent star-formation. In early-type galaxies the radio emission is expected to be driven by accretion on to a nuclear black hole. We argue that the observed correlation of the radio axis with the minor axis of the large-scale stellar distribution gives a fundamental insight into the structure of elliptical galaxies, for example, whether or not the nuclear kinematics are decoupled form the rest of the galaxy. Our results imply that the galaxies are oblate spheroids with their radio emission aligned with the minor axis. Remarkably the strength of the correlation of the radio major axis with the optical minor axis depends on radio loudness. Those objects with a low ratio of FIRST radio flux density to total stellar light show a strong minor axis correlation while the stronger radio sources do not. This may reflect different formation histories for the different objects and we suggest we may be seeing the different behaviour of rationally supported and non-rotationally supported ellipticals.

The K‐z diagram of FIRST radio sources identified in the Boötes and Cetus fields

AbstractThis paper presents the Hubble diagram (K‐z relation) for FIRST (Faint Images of the Radio Sky at 20 cm) radio sources identified in the Boötes and Cetus fields. The correlation between the K magnitude of the FIRST‐NDWFS sample and the photometric redshifts found to be linear. The dispersion about the best fit line is given by 1.53 for the whole sample and 0.75 at z > 1. The paper also presents a composite K‐z diagram of FIRST radio sources and low‐frequency selected radio samples with progressively fainter flux‐density limits (3CRR, 6C, 7CRS and the EIS‐NVSS sample). The majority of FIRST radio sources lie fainter than the no evolution curve (3 L* galaxies) probably highlighting the fact that the galaxy luminosity is correlated with the radio power (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

THESPITZERDEEP, WIDE-FIELD SURVEY

The Spitzer Deep, Wide-Field Survey (SDWFS) is a four-epoch infrared survey of ten square degrees in the Bootes field of the NOAO Deep Wide-Field Survey using the IRAC instrument on the Spitzer Space Telescope. SDWFS, a Cycle four Spitzer Legacy project, occupies a unique position in the area-depth survey space defined by other Spitzer surveys. The four epochs that make up SDWFS permit -- for the first time -- the selection of infrared-variable and high proper motion objects over a wide field on timescales of years. Because of its large survey volume, SDWFS is sensitive to galaxies out to z~3 with relatively little impact from cosmic variance for all but the richest systems. The SDWFS datasets will thus be especially useful for characterizing galaxy evolution beyond z~1.5. This paper explains the SDWFS observing strategy and data processing, presents the SDWFS mosaics and source catalogs, and discusses some early scientific findings. The publicly-released, full-depth catalogs contain 6.78, 5.23, 1.20, and 0.96 x 10e5 distinct sources detected to the average 5-sigma, 4 diameter, aperture-corrected limits of 19.77, 18.83, 16.50, and 15.82 Vega mag at 3.6, 4.5, 5.8, and 8.0 micron, respectively. The SDWFS number counts and color-color distribution are consistent with other, earlier Spitzer surveys. At the 6 min integration time of the SDWFS IRAC imaging, more than 50% of isolated FIRST radio sources and more than 80% of on-axis XBootes sources are detected out to 8.0 micron. Finally, we present the four highest proper motion IRAC-selected sources identified from the multi-epoch imaging, two of which are likely field brown dwarfs of mid-T spectral class.

The FIRST radio survey: Panchromatic properties of FIRST radio sources identified in the Boötes and Cetus fields

AbstractIn this paper the availability of multi‐wavelength optical/infrared information of FIRST (Faint Images of the Radio Sky at 20 cm) radio sources counterparts over ∼9.2 deg2 in the Boötes field and ∼2.4 deg2 in the Cetus field is exploited to infer the physical properties of the faint radio population. The radio sources optically identified have been divided into resolved galaxies and stellar‐like objects finding that the faint radio population is mainly composed of early‐type galaxies with very red colour (Bw – R ∼ 4.6). A total number of 57 counterparts of FIRST radio sources have extremely red colour (R – K ≥ 5). Photometric redshift from Hyperz implies that the Extremely Red Objects (EROs) counterparts to FIRST radio sources are mostly located in the range z = 0.7–2, with the bulk of the population at z ∼ 1. Taking advantage of the near infrared imaging with FLAMEX (FLAMINGOS Extragalactic Infrared Survey), the EROs counterparts to FIRST radio sources are separated into passively‐evolving and dusty star‐forming galaxies using their RJK colours; the relatively blue J – K of these galaxies suggest that most are old elliptical galaxies (18/25) rather than dusty starburst galaxies (7/25). A total of 15 Distant Red Galaxy (DRGs) have been identified as counterparts to FIRST radio sources in the Cetus field and 3 DRGs in the Boötes field with J – K > 2.3 (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dependence of the Broad Absorption Line Quasar Fraction on Radio Luminosity

We find that the fraction of classical broad absorption line quasars (BALQSOs) among the FIRST radio sources in the Sloan Data Release 3, is -->20.5+ 7.3?5.9% at the faintest radio powers detected ( -->L1.4 GHz ~ 1032 erg s?1), and rapidly drops to 8% at -->L1.4 GHz ~ 3 ? 1033 erg s?1. Similarly, adopting the broader absorption index (AI) definition of Trump et al., we find the fraction of radio BALQSOs to be -->44+ 8.1?7.8%, reducing to -->23.1+ 7.3?6.1% at high luminosities. While the high fraction at low radio power is consistent with the recent near-IR estimates by Dai et al., the lower fraction at high radio powers is intriguing and confirms previous claims based on smaller samples. The trend is independent of the redshift range, the optical and radio flux selection limits, or the exact definition of a radio match. We also find that at fixed optical magnitude, the highest bins of radio luminosity are preferentially populated by non-BALQSOs, consistent with the overall trend. We do find, however, that those quasars identified as AI-BALQSOs but not under the classical definition do not show a significant drop in their fraction as a function of radio power, further supporting independent claims that these sources, characterized by lower equivalent width, may represent an independent class from the classical BALQSOs. We find the balnicity index, a measure of the absorption trough in BALQSOs, and the mean maximum wind velocity to be roughly constant at all radio powers. We discuss several plausible physical models which may explain the observed fast drop in the fraction of the classical BALQSOs with increasing radio power, although none is entirely satisfactory. A strictly evolutionary model for the BALQSO and radio emission phases requires a strong fine-tuning to work, while a simple geometric model, although still not capable of explaining polar BALQSOs and the paucity of FRII BALQSOs, is statistically successful in matching the data if part of the apparent radio luminosity function is due to beamed, non-BALQSOs.

Identifications of FIRST radio sources in the NOAO Deep Wide‐Field Survey

AbstractIn this paper we present the results of an optical and near infrared identification of 514 radio sources from the FIRST survey (Faint Images of the Radio Sky Survey at Twenty centimetres) with a flux‐density limit of 1 mJy in the NOAO Deep Wide‐Field Survey (NDWFS) Boötes field. Using optical (Bw, R, I) and K band data with approximate limits of Bw ∼ 25.5 mag, R ∼ 25.8 mag, I ∼ 25.5 mag and K ∼ 19.4 mag, optical counterparts have been identified for 378 of 514 FIRST radio sources. This corresponds to an identification rate of 34% in four bands (BwRIK), 60% in optical bands (BwRI) and 74% in the I band. Photometric redshifts for these sources have been computed using the hyperz code. The inclusion of quasar template spectra in hyperz is investigated. We note that the photometric data are, in many cases, best matched to templates with very short star‐formation timescales and the inferred ages of identified galaxies depend strongly on the assumptions about the star‐formation timescale. The redshifts obtained are fairly consistent with those expected from the K ‐z relation for brighter radio sources but there is more scatter in the K ‐z diagram at z < 1. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Science projects with the Armenian Virtual Observatory (ArVO)

The main goal of the Armenian Virtual Observatory is to develop efficient methods for science projects based on the digitized famous Markarian survey (Digitized First Byurakan Survey, DFBS) and other large astronomical databases, both Armenian and international. Two groups of projects are especially productive: search for new interesting objects of definite types by low-dispersion template spectra, and optical identifications of new gamma, X-ray, IR and radio sources. The first one is based on modeling of spectra for a number of types of objects: QSOs, Seyfert galaxies, white dwarfs, subdwarfs, cataclysmic variables, planetary nebulae, late-type stars (K-M, S, carbon), etc. Each kind of object appears in the DFBS with its typical SED and spectral lines (for objects having broad lines only, like white dwarfs and subdwarfs, quasars and Seyferts, etc.), however affected also by its brightness, so that each template works for definite range of magnitudes. The search criteria define how many objects will be found for further study, and may restrict these numbers leaving with the best candidates. Optical identifications have been proven to be rather efficient for IR sources from IRAS-psc and -fsc. Tests have been carried out for X-ray and radio sources as well. Special emphasis is put on search for bright QSOs missed by the Sloan Digital Sky Survey (SDSS), rather important for making up their complete sample, studies of the properties of the Local Universe, a comparison of their X-ray, IR, and radio properties and making up their multiwavelength SEDs, as well as for a refinement of the AGN classification. A project for search for new bright QSOs using the DFBS has been started in the region with δ>0° and |b|>20°. The Byurakan 2.6m telescope is being used for the spectral identification of the candidates. The first test resulted in 145 objects found, 81 being known QSOs/Sys, and 64 new candidates (including 23 NVSS and FIRST radio sources).

A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sources

Aims. We search for ultra luminous X-ray source (ULXs) radio counterparts located in nearby galaxies in order to constrain their physical nature. Methods. Our work is based on a systematic cross-identification of the most recent and extensive available ULX catalogues and archival radio data. Results. A catalogue of 70 positional coincidences is reported. Most of them are located within the galaxy nucleus. Among them, we find 11 new cases of non-nuclear ULX sources with possibly associated radio emission.

QSOs and Active Galactic Nuclei Associated with NGC 2639

NGC 2639 is a bright, active LINER galaxy that has previously been found to have a pair of bright optical, X-ray quasars paired across it (z = 0.305 and 0.323). In addition, PSPC X-ray observations reveal a line of seven X-ray sources lying along the northeast minor axis. Four of these have now been measured spectroscopically and are reported here to be QSO/active galactic nuclei with redshifts ranging between z = 0.337 and z = 2.63. FIRST radio sources are found to be aligned along this same minor axis, some of which are coincident with the optical objects. Redshifts of all of the known QSOs in the field lie close to the well-known periodicity peaks.

The 2dF galaxy redshift survey: clustering properties of radio galaxies

The clustering properties of local, S_{1.4 GHz} > 1 mJy, radio sources are investigated for a sample of 820 objects drawn from the joint use of the FIRST and 2dF Galaxy Redshift surveys. To this aim, we present 271 new bj < 19.45 spectroscopic counterparts of FIRST radio sources to be added to those already introduced in Magliocchetti et al. (2002). The two-point correlation function for the local radio population is found to be entirely consistent with estimates obtained for the whole sample of 2dFGRS galaxies. We estimate the parameters of the real-space correlation function xi(r)=(r/r_0)^{-\gamma}, r_0=6.7^{+0.9}_{-1.1} Mpc and \gamma=1.6\pm 0.1, where h=0.7 is assumed. Different results are instead obtained if we only consider sources that present signatures of AGN activity in their spectra. These objects are shown to be very strongly correlated, with r_0=10.9^{+1.0}_{-1.2} Mpc and \gamma=2\pm 0.1, a steeper slope than has been claimed in other recent works. No difference is found in the clustering properties of radio-AGNs of different radio luminosity. These results show that AGN-fuelled sources reside in dark matter halos more massive than \sim 10^{13.4} M_{\sun}},higher the corresponding figure for radio-quiet QSOs. This value can be converted into a minimum black hole mass associated with radio-loud, AGN-fuelled objects of M_{BH}^{min}\sim 10^9 M_{\sun}. The above results then suggest -at least for relatively faint radio objects -the existence of a threshold black hole mass associated with the onset of significant radio activity such as that of radio-loud AGNs; however, once the activity is triggered, there appears to be no evidence for a connection between black hole mass and level of radio output. (abridged)

A sample of radio-loud QSOs at redshift ∼ 4

We obtained spectra of 60 red, star-like objects (E < 18.8) identified with FIRST radio sources, S1.4 GHz > 1 mJy. Eight are quasi-stellar objects (QSOs) with redshift z > 3.6. Combined with our earlier pilot search, our sample of 121 candidates yields a total of 18 z > 3.6 QSOs (10 of these with z > 4.0). 8 per cent of candidates with S1.4 GHz< 10 mJy and 37 per cent of candidates with S1.4 GHz> 10 mJy are QSOs with z > 3.6. The surface density of E < 18.8, S1.4 GHz > 1 mJy, z > 4 QSOs is 0.003 deg−2. This is currently the only well-defined sample of radio-loud QSOs at z≈ 4 selected independently of radio spectral index. The QSOs are highly luminous in the optical (eight have MB < −28, q0= 0.5, H0= 50 km s−1 Mpc−1). The SEDs are as varied as those seen in optical searches for high-redshift QSOs, but the fraction of objects with weak (strongly self-absorbed) Lyα emission is marginally higher (3 out of 18) than for high-redshift QSOs from SDSS (5 out of 96).

Radio properties of FIRST radio sources at 1 mJy

ABSTRA C T This paper presents a detailed analysis of the radio properties for the sample of faint radio sources introduced by Magliocchetti et al. in 2000. The sample comprises mainly intrinsically low-power sources, the majority of which ( * 70 per cent) are FR I radio galaxies. These objects show some degree (at 1s confidence level) of luminosity evolution, which is also needed to reproduce correctly the total number and shape of the counts distribution at 1.4 GHz. Analysis of the de-evolved local radio luminosity function shows a good agreement between data and model predictions for this class of sources. Particular care has been devoted to the issue of ‘lined’ galaxies (i.e. objects presenting in their spectra a continuum typical of early-type galaxies plus emission lines of different nature), which appear as an intermediate class of sources between AGN-dominated and starburst galaxies. Different evolutionary behaviour is seen in the two subpopulations of lined and non-lined low-power radio galaxies, the first class indicating a tendency for the radio luminosity to decrease with look-back time, the second one showing positive evolution. We note that different evolutionary properties also seem to characterize BL Lacs selected in different bands, so that one might envisage an association between lined FR I and the subclass of BL Lacs selected in the X-ray band. Lastly, we find evidence for a negligible contribution of starburst galaxies at these low flux levels.

The redshift distribution of FIRST radio sources at 1 mJy

We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels. We targeted 365 sources and obtained 46 redshifts (13 per cent of the sample). We find that our sample is complete in redshift measurement to R∼18.6, corresponding to z∼0.2. Galaxies were assigned spectral types based on emission-line strengths. Early-type galaxies represent the largest subset (45 per cent) of the sample and have redshifts 0.15≲z≲0.5; late-type galaxies make up 15 per cent of the sample and have redshifts 0.05≲z≲0.2; starbursting galaxies are a small fraction (∼6 per cent), and are very nearby (z≲0.05). Some 9 per cent of the population have Seyfert 1/quasar-type spectra, all at z≳0.8, and 4 per cent are Seyfert 2 type galaxies at intermediate redshifts (z∼0.2). Using our measurements and data from the Phoenix survey (Hopkins et al.), we obtain an estimate for N(z) at S1.4 GHz≥1 mJy and compare this with model predictions. At variance with previous conclusions, we find that the population of starbursting objects makes up ≲5 per cent of the radio population at S∼1 mJy.

A Search for Sub-millisecond Pulsations in Unidentified FIRST and NVSS Radio Sources

AbstractWe have searched 92 unidentified sources from the FIRST and NVSS radio catalogs for pulsations at 610 MHz. The selected sources are bright, have no identifications, are unresolved and have significant linear polarization. Our search was sensitive to sub-millisecond pulsations from pulsars with a large range of dispersion measures. We have detected no radio pulsations from these sources and conclude that they are unlikely to be a population of previously undetected pulsars.

QSO Color Selection in the SDSS

The Sloan Digital Sky Survey (SDSS) will image 10,000 square degrees in the north galactic cap in five filters. We hope to identify and obtain spectra for about 100,000 quasars brighter than 20th magnitude in this area. The selection will be primarily on the basis of point spread function and colors, but we will also identify quasars from a catalog of FIRST radio sources. The selection areas in color space must be determined during the testing period prior to the official start of the survey. This task may determine the length of the test period. In anticipation of this becoming the critical path, we have written a body of software that will allow us to quickly analyze a set of multicolor data and make a first cut at the selection limits.