A New Method to Separate Star‐forming from AGN Galaxies at Intermediate Redshift: The Submillijansky Radio Population in the VLA‐COSMOS Survey (original) (raw)
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Properties and environment of radio-emitting galaxies in the VLA-zCOSMOS survey
Astronomy and Astrophysics, 2010
Aims. We investigate the properties and the environment of radio sources with optical counterpart from the combined VLA-COSMOS and zCOSMOS samples. The advantage of this sample is the availability of optical spectroscopic information, high quality redshifts, and accurate density determination. Methods. By comparing the star formation rates estimated from the optical spectral energy distribution with those based on the radio luminosity, we divide the radio sources in three families, passive AGN, non-passive AGN and star forming galaxies. These families occupy specific regions of the 8.0-4.5 µm infrared color-specific star formation plane, from which we extract the corresponding control samples. Results. Only the passive AGN have a significantly different environment distribution from their control sample. The fraction of radio-loud passive AGN increases from ∼ 2% in underdense regions to ∼ 15% for overdensities (1+δ) greater than 10. This trend is also present as a function of richness of the groups hosting the radio sources. Passive AGN in overdensities tend to have higher radio luminosities than those in lower density environments. Since the black hole mass distribution is similar in both environments, we speculate that, for low radio luminosities, the radio emission is controlled (through fuel disponibility or confinement of radio jet by local gas pressure) by the interstellar medium of the host galaxy, while in other cases it is determined by the structure (group or cluster) in which the galaxy resides.
Monthly Notices of the Royal Astronomical Society, 2002
We have cross-matched the 1.4 GHz NRAO VLA Sky Survey (NVSS) with the first 210 fields observed in the 2dF Galaxy Redshift Survey (2dFGRS), covering an effective area of 325 square degrees (about 20% of the final 2dFGRS area). This yields a set of optical spectra of 912 candidate NVSS counterparts, of which we identify 757 as genuine radio IDs -the largest and most homogeneous set of radio-source spectra ever obtained. The 2dFGRS radio sources span the redshift range z = 0.005 to 0.438, and are a mixture of active galaxies (60%) and star-forming galaxies (40%). About 25% of the 2dFGRS radio sources are spatially resolved by NVSS, and the sample includes three giant radio galaxies with projected linear size greater than 1 Mpc. The high quality of the 2dF spectra means we can usually distinguish unambiguously between AGN and star-forming galaxies. We have made a new determination of the local radio luminosity function at 1.4 GHz for both active and star-forming galaxies, and derive a local star-formation density of 0.022 ± 0.004 M ⊙ yr −1 Mpc −3 (H 0 =50 km s −1 Mpc −1 ).
The faint radio sky: VLBA observations of the COSMOS field
Astronomy & Astrophysics
Context. Quantifying the fraction of active galactic nuclei (AGN) in the faint radio population and understanding their relation with star-forming activity are fundamental to studies of galaxy evolution. Very long baseline interferometry (VLBI) observations are able to identify AGN above relatively low redshifts (z > 0.1) since they provide milli-arcsecond resolution. Aims. We have created an AGN catalogue from 2865 known radio sources observed in the Cosmic Evolution Survey (COSMOS) field, which has exceptional multi-wavelength coverage. With this catalogue we intend to study the faint radio sky with statistically relevant numbers and to analyse the AGN-host galaxy co-evolution, making use of the large amount of ancillary data available in the field. Methods. Wide-field VLBI observations were made of all known radio sources in the COSMOS field at 1.4 GHz to measure the AGN fraction, in particular in the faint radio population. We describe in detail the observations, data calibration, source detection and flux density measurements, parts of which we have developed for this survey. The combination of number of sources, sensitivity, and area covered with this project are unprecedented. Results. We have detected 468 radio sources, expected to be AGN, with the Very Long Baseline Array (VLBA). This is, to date, the largest sample assembled of VLBI detected sources in the sub-mJy regime. The input sample was taken from previous observations with the Very Large Array (VLA). We present the catalogue with additional optical, infrared and X-ray information. Conclusions. We find a detection fraction of 20 ± 1%, considering only those sources from the input catalogue which were in principle detectable with the VLBA (2361). As a function of the VLA flux density, the detection fraction is higher for higher flux densities, since at high flux densities a source could be detected even if the VLBI core accounts for a small percentage of the total flux density. As a function of redshift, we see no evolution of the detection fraction over the redshift range 0.5 < z < 3. In addition, we find that faint radio sources typically have a greater fraction of their radio luminosity in a compact core-∼70% of the sub-mJy sources detected with the VLBA have more than half of their total radio luminosity in a VLBI-scale component, whereas this is true for only ∼30% of the sources that are brighter than 10 mJy. This suggests that fainter radio sources differ intrinsically from brighter ones. Across our entire sample, we find the predominant morphological classification of the host galaxies of the VLBA detected sources to be early type (57%), although this varies with redshift and at z > 1.5 we find that spiral galaxies become the most prevalent (48%). The number of detections is high enough to study the faint radio population with statistically significant numbers. We demonstrate that wide-field VLBI observations, together with new calibration methods such as multi-source self-calibration and mosaicing, result in information which is difficult or impossible to obtain otherwise.
Radio galaxies of the local universe
Astronomy & Astrophysics, 2012
Context. To understand the feedback of black holes on their environment or the acceleration of ultra-high energy cosmic rays in the present cosmic epoch, a comprehensive inventory of radio galaxies in the local universe is needed. This requires an all-sky catalog of radio-emitting galaxies, that hitherto has not been available. Aims. We present such an all-sky sample. Our catalog allows one to build volume-limited subsamples containing all low-power radio galaxies, similar to the prototypical low-power radio galaxies Cen A or M87, within some hundred Mpc. Methods. We match radio emission from the NVSS and SUMSS surveys to galaxies of the 2MASS Redshift Survey (2MRS) using an image-level algorithm that properly treats the extended structure of radio sources. Results. The bright master sample we present contains 575 radio-emitting galaxies with a flux greater than 213 mJy at 1.4 GHz. Over 30% of the galaxies in our catalog are not contained in existing large-area extra-galactic radio samples. We compute the optical and radio luminosity functions and the fraction of radio galaxies as a function of galaxy luminosity. 94% of the radio galaxies within z = 0.03 are of Hubble type E/S0. The local galaxy density in a sphere of 2 Mpc centered on the radio galaxies is 1.7 times higher than around non-radio galaxies of the same luminosity and morphology, which is a statistically significant enhancement (> 3σ). Conclusions. Our sample presents the deepest all-sky catalog of low-power radio galaxies. The observed enhancement of the galaxy density around radio galaxies suggests a causal relation between external galaxy properties, such as environment or merger history, and the formation of powerful jets in the present universe. Since the enhancement is observed with respect to galaxies of the same luminosity and Hubble type, it is not primarily driven by black hole mass. Our automated matching procedure is found to select radio-emitting galaxies with high efficiency (99%) and purity (91%), which is key for future processing of deeper, larger samples.
The 2dF Galaxy Redshift Survey: the population of nearby radio galaxies at the 1-mJy level
Monthly Notices of the Royal Astronomical Society, 2002
We use redshift determinations and spectral analysis of galaxies in the 2dF Galaxy Redshift Survey to study the properties of local radio sources with S ≥ 1 mJy. 557 objects (hereafter called the spectroscopic sample) drawn from the FIRST survey, corresponding to 2.3 per cent of the total radio sample, are found in the 2dFGRS catalogue within the area 9 h 48 m ∼ < RA(2000) ∼ < 14 h 32 m and −2.77 • ∼ < dec(2000) ∼ < 2.25 • , down to a magnitude limit b J = 19.45. The excellent quality of 2dF spectra allows us to divide these sources into classes, according to their optical spectra.
The AMIGA sample of isolated galaxies VI. Radio
Context. This paper is part of a series that describes the results of the AMIGA (Analysis of the interstellar Medium of Isolated GAlaxies) project, studying the largest sample of very isolated galaxies in the local Universe. Aims. The study of the radio properties of the AMIGA sample is intended to characterize the radio continuum emission for a sample least affected by the local environment, thus providing a reference against which less isolated and interacting samples can be compared. Methods. Radio continuum data at 325, 1420, and 4850 MHz were extracted from the WENSS, NVSS/FIRST, and GB6 surveys, respectively. The source extractions have been obtained from reprocessing the data and new detections added to the cross-matched detections with the respective survey catalogs. We focus on the complete AMIGA subsample composed of 719 galaxies. Results. A catalog of radio fluxes was obtained from the above four surveys. Comparison between the NVSS and FIRST detections indicates that the radio continuum is coming from disk-dominated emission in spiral galaxies, in contrast to the results found in high-density environments where nuclear activity is more frequent. The comparison of the radio continuum power with a comparable sample, which is however not selected with respect to its environment, the Condon et al. UGC-SF sample of starforming field galaxies, shows a lower mean value for the AMIGA sample. We have obtained radio-to-optical flux ratios (R) using the NVSS radio continuum flux. The distribution of R for the AMIGA galaxies is consistent with a sample dominated by radio emission from star formation (SF) and a small number of active galactic nuclei (AGN), with less than 3% of the sample with R > 100. We derived the radio luminosity function (RLF) and total power density of the radio continuum emission for the AMIGA sample at 1.4 GHz, and compared them with results from other low-redshift studies. The Schechter fit of the RLF indicates a major weight of the low-luminosity galaxies. Conclusions. The results indicate the very low level of radio continuum emission in our sample of isolated galaxies, which is dominated by mild disk SF. It confirms thus the AMIGA sample as a suitable template to effectively quantify the role of interactions in samples extracted from denser environments.
AGN and Starburst Galaxies Seen through Radio Surveys
The New Era of Wide Field Astronomy, 2001
The emergence of a new population of radio galaxies at mJy and sub-mJy levels is responsible for the change in the slope of the radio source counts. This population seems to include both star forming galaxies and classical (AGN-powered) radio sources, but the relative importance of the two classes is still debated. We present results from the ATESP radio survey and its optical follow-up and show that the fraction of starburst galaxies changes from ∼ 15% at fluxes ≥ 1 mJy to ≥ 50% at lower fluxes.
Astronomy & Astrophysics, 2007
Aims. The availability of wide angle and deep surveys, both in the optical and the radio band, allows us to explore the evolution of radio sources with optical counterparts up to redshift z ∼ 1.1 in an unbiased way, using not only large numbers of radio sources but also well defined control samples of radio quiet objects. Methods. We use the 1.4 GHz VIMOS-VLA Deep Survey and the optical VIMOS-VLT Deep Survey and the CFHT Legacy Survey to compare the properties of radio loud galaxies with respect to the whole population of optical galaxies. The availability of multiband photometry and high quality photometric redshifts allows to derive rest frame colors and radio luminosity functions down to a limit of a B rest-frame magnitude of M B = −20. Moreover, we derive spectrophotometric types, following the classification of , in order to have a priori knowledge of the optical evolution of different galaxy classes. Results. Galaxy properties and luminosity functions are estimated up to z ∼ 1 for radio loud and radio quiet early and late type galaxies. Radio loud late type galaxies show significantly redder colors with respect to radio quiet objects of the same class and this is an effect related to the presence of more dust in stronger star forming galaxies. Moreover, we estimate optical luminosity functions, stellar masses and star formation rate distributions for radio sources and compare them with those derived for a well defined control sample, finding that the probability for a galaxy to be a radio emitter significantly increases at high values of these parameters. Radio loud early type galaxies show luminosity evolution of their bivariate radio-optical luminosity function, due to an evolution in the radio-optical ratio. The lack of evolution of the mass function of radio loud early type galaxies means that no new AGN are formed at redshift z < 1. On the contrary, radio loud late type objects show a strong evolution, both in luminosity and in density, of the radio luminosity function for z > 0.7. This evolution is the direct effect of the strong optical evolution of this class and no significant change with redshift of the radio-optical ratio is required. With the knowledge of the radio-optical ratio and the optical and radio luminosity functions for late type galaxies, we show that it is possible to estimate the star formation history of the Universe up to redshift z ∼ 1.5, using optical galaxies as tracers of the global radio emission.
The Spitzer High-redshift Radio Galaxy Survey
The Astrophysical …, 2010
We present results from a comprehensive imaging survey of 70 radio galaxies at redshifts 1 < z < 5.2 using all three cameras onboard the Spitzer Space Telescope. The resulting spectral energy distributions unambiguously show a stellar population in 46 sources and hot dust emission associated with the active nucleus in 59. Using a new restframe S 3 µm /S 1.6 µm versus S 5 µm /S 3 µm criterion, we identify 42 sources where the restframe 1.6 µm emission from the stellar population can be measured. For these radio galaxies, the median stellar mass is high, 2 × 10 11 M ⊙ , and remarkably constant within the range 1 < z < 3. At z > 3, there is tentative evidence for a factor of two decrease in stellar mass. This suggests that radio galaxies have assembled the bulk of their stellar mass by z ∼ 3, but confirmation by more detailed decomposition of stellar and AGN emission is needed.
The VLA-COSMOS 3 GHz Large Project: Cosmic star formation history sincez~ 5
Astronomy & Astrophysics
We make use of the deep Karl G. Jansky Very Large Array (VLA) COSMOS radio observations at 3 GHz to infer radio luminosity functions of star-forming galaxies up to redshifts of z ∼ 5 based on approximately 6000 detections with reliable optical counterparts. This is currently the largest radio-selected sample available out to z ∼ 5 across an area of 2 square degrees with a sensitivity of rms ≈2.3 µJy beam −1. By fixing the faint and bright end shape of the radio luminosity function to the local values, we find a strong redshift trend that can be fitted with a pure luminosity evolution L 1.4 GHz ∝ (1 + z) (3.16±0.2)−(0.32±0.07)z. We estimate star formation rates (SFRs) from our radio luminosities using an infrared (IR)-radio correlation that is redshift dependent. By integrating the parametric fits of the evolved luminosity function we calculate the cosmic SFR density (SFRD) history since z ∼ 5. Our data suggest that the SFRD history peaks between 2 < z < 3 and that the ultraluminous infrared galaxies (100 M yr −1 < SFR < 1000 M yr −1) contribute up to ∼25% to the total SFRD in the same redshift range. Hyperluminous infrared galaxies (SFR > 1000 M yr −1) contribute an additional 2% in the entire observed redshift range. We find evidence of a potential underestimation of SFRD based on ultraviolet (UV) rest-frame observations of Lyman break galaxies at high redshifts (z 4) on the order of 15-20%, owing to appreciable star formation in highly dust-obscured galaxies, which might remain undetected in such UV observations.