The sub-mJy radio sky in the Extended Chandra Deep Field South: source population

The sub-mJy radio population is a mixture of active systems, that is star forming galaxies (SFGs) and active galactic nuclei (AGNs). We study a sample of 883 radio sources detected at 1.4 GHz in a deep Very Large Array survey of the Extended Chandra Deep Field South (E-CDFS) that reaches a best rms sensitivity of 6 microJy. We have used a simple scheme to disentangle SFGs, radio-quiet (RQ), and radio-loud (RL) AGNs based on the combination of radio data with Chandra X-ray data and mid-infrared observations from Spitzer. We find that at flux densities between about 30 and 100 microJy the radio population is dominated by SFGs (~60%) and that RQ AGNs become increasingly important over RL ones below 100 microJy. We also compare the host galaxy properties of the three classes in terms of morphology, optical colours and stellar masses. Our results show that both SFG and RQ AGN host galaxies have blue colours and late type morphology while RL AGNs tend to be hosted in massive red galaxies with early type morphology. This supports the hypothesis that radio emission in SFGs and RQ AGNs mainly comes from the same physical process: star formation in the host galaxy.Comment: 13 pages, 11 figures, 1 table, accepted for publication in MNRA

Star formation properties of sub-mJy radio sources

We investigate the star formation properties of ~800 sources detected in one of the deepest radio surveys at 1.4 GHz. Our sample spans a wide redshift range (~0.1 - 4) and about four orders of magnitude in star formation rate (SFR). It includes both star forming galaxies (SFGs) and active galactic nuclei (AGNs), further divided into radio-quiet and radio-loud objects. We compare the SFR derived from the far infrared luminosity, as traced by Herschel, with the SFR computed from their radio emission. We find that the radio power is a good SFR tracer not only for pure SFGs but also in the host galaxies of RQ AGNs, with no significant deviation with redshift or specific SFR. Moreover, we quantify the contribution of the starburst activity in the SFGs population and the occurrence of AGNs in sources with different level of star formation. Finally we discuss the possibility of using deep radio survey as a tool to study the cosmic star formation history.Comment: 18 pages, 14 figures, 1 table (available in its entirety as ancillary data

The VLA 1.4GHz Survey of the Extended Chandra Deep Field South: Second Data Release

Deep radio observations at 1.4GHz for the Extended Chandra Deep Field South were performed in June through September of 2007 and presented in a first data release (Miller et al. 2008). The survey was made using six separate pointings of the Very Large Array (VLA) with over 40 hours of observation per pointing. In the current paper, we improve on the data reduction to produce a second data release (DR2) mosaic image. This DR2 image covers an area of about a third of a square degree and reaches a best rms sensitivity of 6 uJy and has a typical sensitivity of 7.4 uJy per 2.8" by 1.6" beam. We also present a more comprehensive catalog, including sources down to peak flux densities of five or more times the local rms noise along with information on source sizes and relevant pointing data. We discuss in some detail the consideration of whether sources are resolved under the complication of a radio image created as a mosaic of separate pointings each suffering some degree of bandwidth smearing, and the accurate evaluation of the flux densities of such sources. Finally, the radio morphologies and optical/near-IR counterpart identifications (Bonzini et al. 2012) are used to identify 17 likely multiple-component sources and arrive at a catalog of 883 radio sources, which is roughly double the number of sources contained in the first data release.Comment: to appear in the Astrophysical Journal Supplement Series; 41 page

Accretion and nuclear activity in Virgo early-type galaxies

We use Chandra observations to estimate the accretion rate of hot gas onto the central supermassive black hole in four giant (of stellar mass 10E11 - 10E12 solar masses) early-type galaxies located in the Virgo cluster. They are characterized by an extremely low radio luminosity, in the range L < 3E25 - 10E27 erg/s/Hz. We find that, accordingly, accretion in these objects occurs at an extremely low rate, 0.2 - 3.7 10E-3 solar masses per year, and that they smoothly extend the relation accretion - jet power found for more powerful radio-galaxies. This confirms the dominant role of hot gas and of the galactic coronae in powering radio-loud active galactic nuclei across ~ 4 orders of magnitude in luminosity. A suggestive trend between jet power and location within the cluster also emerges.Comment: Accepted for publication in A&

VizieR Online Data Catalog: ECDFS sources optical/IR counterparts (Bonzini+, 2013)

We consider a sample of 883 radio sources detected at 1.4GHz in a deep Very Large Array (VLA) survey of the Extended Chandra Deep Field-South (E-CDFS) that reaches a best rms sensitivity of 6uJy. We used deep Spitzer InfraRed Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) data. The IRAC data were obtained as part of the Spitzer IRAC/MUSYC Public Legacy Survey in the Extended CDF-South (SIMPLE) survey (Damen et al., 2011ApJ...727....1D, Cat. J/ApJ/727/1). It covers an area of about 1600 arcmin2 centred on the E-CDFS. The typical 5σ flux density limits are 1.1, 1.3, 6.3 and 7.6uJy at 3.6, 4.5, 5.8 and 8.0um, respectively. We also use MIPS 24um data from the Far-Infrared Deep Extragalactic Legacy (FIDEL) survey (Dickinson & FIDEL Team, 2007AAS...211.5216D). The E-CDFS has been mapped in the X-ray band by Chandra. A total of 129 radio sources have a counterpart in the 4 Ms observations of the CDFS presented in Xue et al. (2011ApJS..195...10X, Cat. J/ApJS/195/10) and another 99 in the main E-CDFS catalogue by Lehmer et al. (2005ApJS..161...21L, Cat. J/ApJS/161/21) obtained with shallower (250ks) observations in each of four pointings. The list of the X-ray counterparts of the radio sources is given in Bonzini et al. (2012ApJS..203...15B, Cat. J/ApJS/203/15)

VizieR Online Data Catalog: Sub-mJy radio sources SF properties (Bonzini+, 2015)

In this work, we investigated the SF properties of the faint radio population as detected by one of the deepest 1.4 GHz survey up-to-date conducted with the VLA in the E-CDFS. This study builds upon the results presented in Bonzini et al. (2012, Cat. J/ApJS/203/15 and 2013, Cat. J/MNRAS/436/3759) where we have exploited the wealth of multiwavelength data available in this field to identify the AGNs, further divide them into RL and RQ, and characterize the properties of the radio selected galaxies (e.g. redshift, stellar mass)

The Radio - X-ray relation as a star formation indicator: Results from the VLA--E-CDFS Survey

In order to trace the instantaneous star formation rate at high redshift, and hence help understanding the relation between the different emission mechanisms related to star formation, we combine the recent 4 Ms Chandra X-ray data and the deep VLA radio data in the Extended Chandra Deep Field South region. We find 268 sources detected both in the X-ray and radio band. The availability of redshifts for $\sim 95$ of the sources in our sample allows us to derive reliable luminosity estimates and the intrinsic properties from X-ray analysis for the majority of the objects. With the aim of selecting sources powered by star formation in both bands, we adopt classification criteria based on X-ray and radio data, exploiting the X-ray spectral features and time variability, taking advantage of observations scattered across more than ten years. We identify 43 objects consistent with being powered by star formation. We also add another 111 and 70 star forming candidates detected only in the radio or X-ray band, respectively. We find a clear linear correlation between radio and X-ray luminosity in star forming galaxies over three orders of magnitude and up to $z \sim 1.5$. We also measure a significant scatter of the order of 0.4 dex, higher than that observed at low redshift, implying an intrinsic scatter component. The correlation is consistent with that measured locally, and no evolution with redshift is observed. Using a locally calibrated relation between the SFR and the radio luminosity, we investigate the L_X(2-10keV)-SFR relation at high redshift. The comparison of the star formation rate measured in our sample with some theoretical models for the Milky Way and M31, two typical spiral galaxies, indicates that, with current data, we can trace typical spirals only at z<0.2, and strong starburst galaxies with star-formation rates as high as $\sim 100 M_\odot yr^{-1}$, up to $z\sim 1.5$.Comment: 21 pages, 10 figures, 5 table

Chemical evolution of classical and ultra-faint dwarf spheroidal galaxies

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2014 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reservedPeer reviewedFinal Published versio

Spectrum of the unresolved cosmic X-ray background: what is unresolved 50 years after its discovery

Aims. We study the spectral properties of the unresolved cosmic X-ray background (CXRB) in the 1.5–7.0 keV energy band with the aim of providing an observational constraint on the statistical properties of those sources that are too faint to be individually probed. Methods. We made use of the Swift X-ray observation of the Chandra deep field South complemented by the Chandra data. Exploiting the lowest instrument background (Swift) together with the deepest observation ever performed (Chandra) we measured the unresolved emission at the deepest level and with the best accuracy available today. Results. We find that the unresolved CXRB emission can be modeled by a single power law with a very hard photon index Γ = 0.1 ± 0.7 and a flux of 5-2.6+3.2 erg s-1 cm-2 deg-2 in the 2.0–10 keV energy band (1σ error). Thanks to the low instrument background of the Swift-XRT, we significantly improved the accuracy with respect to previous measurements. Conclusions. These results point towards a novel ingredient in AGN population synthesis models, namely a positive evolution of the Compton-thick AGN population from local Universe to high redshift