Surveys and Observational Cosmology

Involved IRA Scientists and Collaborators: M. Bondi, E. Carretti, I. M. Gioia, K-H. Mack, P. Parma, I. Prandoni, G. Vettolani, M. Vigotti, A. Zanichelli, L. Gregorini,  A. Mignano,   S. Poppi

The Institute is involved in many survey projects in the framework of large international collaborations. In particular the IRA has a long-standing tradition in radio surveys since the pioneering works with the Northern Cross in the early seventies which produced the B2 catalogs ( Colla et al. 1970; Colla et al. 1972; Colla et al. 1973; Fanti et al. 1974 ) with almost 10,000 radio sources detected at 408 MHz.
Nowadays the radio survey activity is mainly focused on the acquisition and analysis of deep radio fields, with the aim of studying the evolution of both star-forming galaxies and Active Galactic Nuclei (AGN), through the multi-wavelength characterization of the sub-mJy and microJy source populations, responsible for the flattening of the normalized source counts at 1.4 GHz.

Fig 1 - The normalized differential source counts at 1.4 GHz derived from the existing radio surveys - Courtesy of I. Prandoni.

Figure 1 shows the normalized differential source counts at 1.4 Ghz derived from the existing radio surveys. The relevance of radio-selected samples resides in their being free from dust-exctintion/gas-obscuration effects.  Here is a selection of the survey projects (in the radio, optical and X-ray bands) involving IRA staff.

RADIO, OPTICAL AND X-RAY SURVEYS:

ATESP 1.4 AND 5 GHz SURVEYS:

The ATESP 1.4 GHz survey, carried out with the Australia Telescope Compact Array (ATCA), is a collection of deep radio mosaics covering a strip of sky of 26x1 sq deg in the Southern Hemisphere. The survey has produced a sample of ~3000 radio sources complete down to a flux limit of ~0.4 mJy. A sub-region of 3 sq deg has been targeted for deep optical spectroscopy in order to study the composition of the sub-mJy population.

A sub-region of 2x0.5 sq deg of the sky region covered by the ATESP 1.4 GHz survey has been imaged at 5 GHz with the ATCA and followed-up with multi-color (UBVRIJK) deep optical imaging in the framework of the ESO Deep Public Survey (DPS). Such a multi-wavelength analysis is mainly aimed at studying the physical and evolutionary properties of sub-mJy radio sources, particularly with respect to the AGN component, which dominates at the flux densities probed by the ATESP surveys. For details and main results: Prandoni et al.  2000a, Prandoni et al. 2000b, Prandoni et al. 2001a, Prandoni et al. 2001b, Prandoni et al. 2006, Mignano et al. 2007

In order to better assess the nature and evolutionary properties of the faint radio population, with special regards to the AGN component, we are following up at several radio wavelengths and optical/NIR bands the region of 1 sq. deg. imaged at both 1.4 and 5 GHz as part of the ATESP. The radio/optical analysis of deep (R~25) multicolor (UBVRIJK) imaging of the 1 sq. deg. sample described above has revealed a class of flat/inverted radio spectrum sources associated to early-type galaxies, whose radio powers and sizes are not entirely consistent with those of standard radio galaxy populations (see Mignano et al. 2007b, A&A, in press, for details). Further clues will come from high radio frequency folllow-up (5, 8 and 20 GHz), undertaken with the Australia Telescope Compact Array (ATCA), which we are currently being reduced. To complement the obervational work,  evolutionary models are also developed that can best account for both the radio and the optical properties of the different components of the faint radio population. For preliminary results see Prandoni et al. 2004.
Such a work is also aimed at  providing reliable modeling for the sub-microJy sky which will be eventually probed by SKA.

VIRMOS-VLT DEEP SURVEY (VVDS):

The VIRMOS-VLT Deep Survey comprises: 1) an UBVRI photometric survey over four 2x2 sq deg fields and J,K imaging over selected areas; 2) a spectroscopic Wide Survey in the four fields to apparent magnitude I_{AB}=22.5, aiming at the study of Large Scale Structure up to z~1 by measuring ~100,000 redshifts; 3) a spectroscopic Deep Survey to I_{AB}=24 over a selected area, to study galaxy/AGN evolution up to z~5 by measuring ~50,000 redshifts; 4) an Ultra Deep survey of ~1,000 galaxy redshifts at I_{AB}=24.75. The spectroscopic sample currently lists ~35,000 redshifts in the Wide and ~12,000 redshifts in the Deep Survey. In the Deep survey region VLA radio data and XMM X-ray data have been acquired. Main results obtained so far include:
a) the presence of a significant population of distant (1.5<=5) galaxies 1.6 to 6.2 times larger than previous estimates (Le Fevre et al. 2005);
b) evolution of the rest-frame B-band luminosity function with the type local density (Ilbert et al. 2006);
c) results on the redshift and luminosity evolution of the galaxy colour-density relation (Cucciati et al. 2006);
e) derivation of the evolution of galaxy clustering with galaxy type and luminosity (Meneux et al. 2006; Pollo et al. 2006).

A further study deals with the radio-optical properties of the mJy population in the VVDS Wide survey. The large area and availability of spectroscopic and photometric data make the VVDS an ideal set for the study of the properties of large, faint radio galaxy samples.  The VVDS Wide and the NRAO FIRST Surveys have been used to build a sample of radio galaxies in the mJy regime. This sample is complementary both in radio and optical to the deeper VLA-VVDS sample (see below). These two datasets allow us to analyze and compare the nature and the properties of the mJy and sub-mJy populations in a consistent manner.

VLA-VVDS SURVEY:

The VVDS deep field has been observed at 1.4 GHz (VLA) and 0.61 GHz (GMRT) with matched resolution (6 arcsec).  Main results include: radio source counts at 1.4 GHz and 0.61 GHz, optical identification of the microJy radio sources, radio spectral index of the sub-mJy radio sources with evidence of different populations with flatter or steeper spectra at various flux density levels being present.

Fig 2 - A montage of various examples of radio and optical morphologies in the VLA-VVDS survey. Radio contours at 1.4 GHz are superimposed on the optical images - Courtesy of P. Ciliegi and M. Bondi.

Figure 2 shows a montage of various examples of radio and optical morphologies. Radio contours at 1.4 GHz are superimposed on the optical image. Spiral galaxies, ellpticals, interacting galaxies and peculiar objects are shown in this example.   Details can be found in Bondi et al. 2003; Ciliegi et al. 2005; Bondi et al. 2007.

VLA-COSMOS:

The VLA-COSMOS team (PI E. Schinnerer, MPIA-Heidelberg) is working on the analysis of the radio observations of the 2 sq deg COSMOS field. The analysis yielded a sample of about 3,500 radio sources down to 60 microJy/beam with a resolution of 1.5 arcsec. In particular, at the IRA we focussed on the extraction of the catalog of radio sources, the completeness issues and the radio source counts.

B3 VLA SURVEY:

The B3 VLA survey forms a sub-sample of the B3 catalog and is composed of 1049 sources with flux densities about 30 times fainter than the 3C catalog. Selected at a relatively low frequency (408 MHz), this catalog contains the progenitor sources of all classes of radio sources. A long-term project, initiated in 1994, has greatly increased the available information at several spectral bands, in particular in the radio domain where the frequency baseline ranges from 74 MHz to 250 GHz, the widest known frequency range for a survey of this size. The B3 VLA survey with its unique frequency coverage offers a new approach to characterise radio sources according to their spectral age and to find new, rare, objects. Also the polarization information obtained at at least four different frequencies (1.4, 2.7, 4.8, 10.5 GHz) qualifies this survey as one of the biggest and best-sampled databases for the study of rotation measures and depolarization. These measurements were used to estimate the contribution of polarized emission from extragalactic radio sources to the power spectra of cosmic background polarization fluctuations. See for details: Mack et al. 2003; Klein et al.  2003; Mack et al. 2005.

CMB FOREGROUND EMISSION SURVEYS:

The Cosmic Microwave Radiation (CMB) is the relic emission of the Big Bang and is a powerful tool to study the early Universe, how it formed and evolved. Unfortunately, the (CMB) signal is weak and contaminated by other types of astrophysical emission (foreground), like the Galactic diffuse emission and extragalactic point sources. The IRA leads three projects, aimed at studying such foreground emission, namely:

1) PGMS (Parkes Galactic Meridian Survey) and S-PASS (S-band Polarization All-Sky Survey). The PGMS survey is a 5x90 sq deg strip along a southern Galactic meridian from the Galactic plane to the south Galactic pole, aimed at exploring the behaviour of the polarized Galactic synchrotron emission with the Galactic latitude.  It is carried out with the Parkes radiotelescope (Australia) at 2.3 GHz and can detect the signal even at high Galactic latitude to characterize the Galactic signal even in the best regions for CMB polarization observations. S-PASS will extend to the whole southern sky and will enable pixel-based foreground separation procedures even more efficient than the ones allowed by the PGMS. Beside CMB, the other major goal of these surveys is to study the Galactic magnetism in both the halo and disc through synchrotron emission and Rotation Measure measurements.

2) MGMS (Medicina Galactic Meridian Survey). The MGMS surveys a 5x30 sq deg strip along a northern Galactic meridian from the Galactic plane up to the disc edge. This is carried out with the Medicina radiotelescope at 5 GHz.

3) Deep survey of the BOOMERanG region at 43 GHz. It is a deep 43 GHz polarization survey, carried out with the ATCA telescope, of extragalactic point sources in the 10x10 sq deg area observed with the BOOMERanG 2003 experiments. Its main aim is to study the extragalactic sources as foreground to the CMB polarization in one of the candidate fields of CMB experiments. The observing frequency is already in the CMB window, in order to provide information with minimal frequency extrapolation.

THE NORTH ECLIPTIC POLE SURVEY

The North Ecliptic Pole survey is an International project carried out at X-ray wavelengths using data from the Rosat All-Sky Survey (RASS). The whole team involves both European and American scientists (PI: J. P. Henry, IfA, University of Hawai'i). The survey has been completed in the early 2000's but several research projects that use sub-samples of sources extracted from the survey are still ongoing. Here we present a brief description of the survey and of the main results obtained.

The sky around the North Ecliptic Pole (NEP) at R.A. = 18h 00m 00s and DEC = +66deg 33' 39", has the deepest exposure of the entire RASS and is an undistinguished region at moderate Galactic latitude of (b = 29.8 deg) (Fig. 2a).

Figure 2a - ROSAT X-ray color image of the NEP region. The colors encode the X-ray energy with red and yellow for 0.11-0.40 keV, green 0.40-1.0 keV, and blue and purple for 1.0-2.40 energy photons. White implies bright sources in the 0.40-1.0 and/or 1.5-2.4 keV bands. Note that only the brighter two-thirds of the NEP sources are apparent in this particular image (from Voges et al. 2001)

It is hence suitable for compiling statistcal samples of both galactic and extragalactic objects. Such a compilation was made in the 80.6 sq. deg surrounding the NEP, and yields the deepest large solid angle contiguous sample of X-ray sources to date. 442 unique sources are found above a flux limit ~ 2 x 10^14 ergs/cm^2 s in the 0.5-2.0 keV band. The optical identification procedure is described with some detail in Gioia et al. 2003.

Ninety-nine per cent of the sources have been identified and redshifts for the extragalactic objects have been measured using mostly the optical telescopes on top Mauna Kea in Hawaii. One of the results of the NEP project is the confirmation of the “negative” evolution of X-ray selected galaxy clusters (Gioia et al. 2001) first seen in the EMSS survey and more recently by other independent complete samples of galaxy clusters (Mullis et al.  2004).

The X-ray data including positions, fluxes, spectral information in the form of hardness ratios, and angular sizes are described in Henry et al. 2006.