We present the type-1 active galactic nuclei (AGN) sample extracted from the VIMOS VLT Deep Survey's first observations of 21 000 spectra in 1.75 deg2. This sample, which is purely magnitude-limited ...and free of morphological or color-selection biases, contains 130 broad-line AGN (BLAGN) spectra with redshift up to 5. Our data are divided into a wide ($I_{\rm AB} \le 22.5$) and a deep ($I_{\rm AB} \le 24$) subsample containing 56 and 74 objects, respectively. Because of its depth and selection criteria, this sample is uniquely suited for studying the population of faint type-1 AGN. Our measured surface density (~$472 \pm 48$ BLAGN per square degree with $I_{\rm AB} \le 24$) is significantly higher than that of any other optically selected sample of BLAGN with spectroscopic confirmation. By applying a morphological and color analysis to our AGN sample, we find that (1) ~$23 \%$ of the AGN brighter than $I_{\rm AB}=22.5$ are classified as extended, and this percentage increases to ~42% for those with $z < 1.6$; (2) a non-negligible fraction of our BLAGN are lying close to the color-space area occupied by stars in the $u^*-g'$ versus $g'-r'$ color-color diagram. This leads us to the conclusion that the classical optical-ultraviolet preselection technique, if employed at such deep magnitudes ($I_{\rm AB}=22.5$) in conjuction with a preselection of point-like sources, can miss up to ~$35\%$ of the AGN population. Finally, we present a composite spectrum of our sample of objects. While the continuum shape is very similar to that of the SDSS composite at short wavelengths, it is much redder than that of the SDSS composite at $\lambda \ge 3000$ Å. We interpret this as due to significant contamination from emission of the host galaxies, as expected from the faint absolute magnitudes sampled by our survey.
We investigate the statistical properties of the polarized emission of extragalactic radio sources and estimate their contribution to the power spectrum of polarization fluctuations in the microwave ...region. The basic ingredients of our analysis are the NVSS polarization data, the multifrequency study of polarization properties of the B3-VLA sample (Mack et al. 2002) which has allowed us to quantify Faraday depolarization effects, and the 15 GHz survey by Taylor et al. (2001), which has provided strong constraints on the high-frequency spectral indices of sources. The polarization degree of both steep- and flat-spectrum sources at 1.4 GHz is found to be anti-correlated with the flux density. The median polarization degree at 1.4 GHz of both steep- and flat-spectrum sources brighter than $S(1.4\,\hbox{GHz})=80\,$mJy is $\simeq$ $2.2\%$. The data by Mack et al. (2002) indicate a substantial mean Faraday depolarization at 1.4 GHz for steep spectrum sources, while the depolarization is undetermined for most flat/inverted-spectrum sources. Exploiting this complex of information we have estimated the power spectrum of polarization fluctuations due to extragalactic radio sources at microwave frequencies. We confirm that extragalactic sources are expected to be the main contaminant of Cosmic Microwave Background (CMB) polarization maps on small angular scales. At frequencies <30 GHz the amplitude of their power spectrum is expected to be comparable to that of the E-mode of the CMB. At higher frequencies, however, the CMB dominates.
To understand the evolution of galaxies, we need to know as accurately as possible how many galaxies were present in the Universe at different epochs. Galaxies in the young Universe have hitherto ...mainly been identified using their expected optical colours, but this leaves open the possibility that a significant population remains undetected because their colours are the result of a complex mix of stars, gas, dust or active galactic nuclei. Here we report the results of a flux-limited I-band survey of galaxies at look-back times of 9 to 12 billion years. We find 970 galaxies with spectroscopic redshifts between 1.4 and 5. This population is 1.6 to 6.2 times larger than previous estimates, with the difference increasing towards brighter magnitudes. Strong ultraviolet continua (in the rest frame of the galaxies) indicate vigorous star formation rates of more than 10-100 solar masses per year. As a consequence, the cosmic star formation rate representing the volume-averaged production of stars is higher than previously measured at redshifts of 3 to 4.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The ATESP 5 GHz radio survey Prandoni, I.; de Ruiter, H. R.; Ricci, R. ...
Astronomy & astrophysics,
02/2010, Letnik:
510
Journal Article
Recenzirano
Odprti dostop
Aims. Physical and evolutionary properties of the sub-mJy radio population are not entirely known. The radio/optical analysis of the ATESP 5 GHz sample has revealed a significant class of compact ...flat/inverted radio-spectrum sources associated to early-type galaxies up to redshift 2. Such sources are most plausibly triggered by an AGN, but their observational properties are not entirely consistent with those of standard radio galaxy populations. In the present work we aim at a better understanding of the radio spectra of such sources and ultimately of the nature of AGNs at sub-mJy flux levels. In particular we are interested in assessing whether the AGN component of the sub-mJy population is more related to efficiently accreting systems – like radio-intermediate/quiet quasars – or to systems with low accretion rates – like e.g. FRI radio galaxies – or to low radiative efficiency accretion flows – like e.g. ADAF. Methods. We used the ATCA to get multi-frequency (4.8, 8.6 and 19 GHz) quasi-simultaneous observations for a representative sub-sample of ATESP radio sources associated with early-type galaxies (26 objects with S>0.6 mJy). This can give us insight into the accretion/radiative mechanism that is at work, since different regimes display different spectral signatures in the radio domain. Results. From the analysis of the radio spectra, we find that our sources are most probably jet-dominated systems. ADAF models are ruled out by the high frequency data, while ADAF+jet scenarios are still consistent with flat/moderately inverted-spectrum sources, but are not required to explain the data. We compared our sample with high (${\ga}20$ GHz) frequency selected surveys, finding spectral properties very similar to the ones of much brighter (S>500 mJy) radio galaxies extracted from the Massardi et al. (2008) sample. Linear sizes of ATESP 5 GHz sources associated with early type galaxies are also often consistent with the ones of brighter B2 and 3C radio galaxies, with possibly a very compact component that could be associated at least in part to (obscured) radio-quiet quasar-like objects and/or low power BL Lacs.
We discuss VLA polarization measurements at 8.5, 4.9 and 1.4 GHz for the B3-VLA sample of Compact Steep-spectrum Sources (CSS). The measurements at the two higher frequencies were presented in a ...previous paper (Fanti et al. 2001), while those at 1.4 GHz are from the NVSS. The study of the fractional polarization as a function of wavelength shows a variety of behaviours not always accounted for by the current simple models. In a large fraction of sources the integrated rotation angle appears to follow the lambda super(2) law down to 1.4 GHz, although this conclusion has to be treated with some care due to the small number of polarization measurements. For sources resolved at the two higher frequencies we find that a number exhibit asymmetries in the 8.5 GHz fractional polarization, in depolarization and rotation angle. We discuss depolarization effects and rotation of the polarization angle. We find that Faraday depth effects are very strong within 2-3 kpc of the nucleus, as found earlier by Cotton et al. (2003). A simple model for an external Faraday screen is able to account for the gross observed features. We also find that the Rotation Measure and the Faraday Dispersion appear to increase with the source red-shift.
Aims. We study the intra-cluster magnetic field in the poor galaxy cluster Abell 194 by complementing radio data, at different frequencies, with data in the optical and X-ray bands.Methods. We ...analyzed new total intensity and polarization observations of Abell 194 obtained with the Sardinia Radio Telescope (SRT). We used the SRT data in combination with archival Very Large Array observations to derive both the spectral aging and rotation measure (RM) images of the radio galaxies 3C 40A and 3C 40B embedded in Abell 194. To obtain new additional insights into the cluster structure, we investigated the redshifts of 1893 galaxies, resulting in a sample of 143 fiducial cluster members. We analyzed the available ROSAT and Chandra observations to measure the electron density profile of the galaxy cluster.Results. The optical analysis indicates that Abell 194 does not show a major and recent cluster merger, but rather agrees with a scenario of accretion of small groups, mainly along the NE−SW direction. Under the minimum energy assumption, the lifetimes of synchrotron electrons in 3C 40 B measured from the spectral break are found to be 157 ± 11 Myr. The break frequency image and the electron density profile inferred from the X-ray emission are used in combination with the RM data to constrain the intra-cluster magnetic field power spectrum. By assuming a Kolmogorov power-law power spectrum with a minimum scale of fluctuations of Λmin = 1 kpc, we find that the RM data in Abell 194 are well described by a magnetic field with a maximum scale of fluctuations of Λmax = (64 ± 24) kpc. We find a central magnetic field strength of ⟨ B0 ⟩ = (1.5 ± 0.2) μG, which is the lowest ever measured so far in galaxy clusters based on Faraday rotation analysis. Further out, the field decreases with the radius following the gas density to the power of η = 1.1 ± 0.2. Comparing Abell 194 with a small sample of galaxy clusters, there is a hint of a trend between central electron densities and magnetic field strengths.Key words: galaxies: clusters: general / galaxies: clusters: individual: Abell 194 / magnetic fields / large-scale structure of Universe
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 large numbers of radio sources and well defined control samples of radio-quiet objects. Methods: We use the 1.4 GHz VIMOS-VLA Deep Survey, 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 us to derive rest-frame colors and radio luminosity functions to a limit of a B rest-frame magnitude of M_B=-20. We derive spectrophotometric types, following the classification of Zucca et al. (2006, A&A, 455, 879), 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 than radio-quiet objects of the same class and this is related to the presence of more dust in stronger star forming galaxies. We estimate the 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 of a galaxy to be a radio emitter significantly increases at high values of these parameters. Radio-loud early type galaxies exhibit luminosity evolution in their bivariate radio-optical luminosity function, due to 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. In contrast, radio-loud late type objects exhibit a strong evolution, both in luminosity and density, of the radio luminosity function for z>0.7. This evolution is a direct effect of the strong optical evolution of this class and no significant change with redshift in 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.
We used the Vimos VLT Deep Survey in combination with other spectroscopic, photometric, and X-ray surveys from the literature to detect several galaxy structures in the Chandra Deep Field South ...(CDFS). Both a friend-of-friend-based algorithm applied to the spectroscopic redshift catalog and an adaptative kernel galaxy density and color maps correlated with photometric redshift estimates were used. We mainly detected a chain-like structure at $z=0.66$ and two massive groups at $z=0.735$ and 1.098 showing signs of ongoing collapse. We also detected two galaxy walls at $z=0.66$ and at $z=0.735$ (extremely compact in redshift space). The first one contains the chain-like structure and the last one contains one of the two massive groups in its center. Finally, other galaxy structures that are probably loose low mass groups were detected. We compared the group galaxy population with simulations in order to assess the richness of these structures and studied their galaxy morphological contents. The higher redshift structures appear to probably have lower velocity dispersion than the nearby ones. The number of moderately massive structures we detected is consistent with what is expected for an LCDM model, but a larger sample is required to put significant cosmological constraints.
We have reconstructed the three-dimensional density fluctuation maps to z\sim 1.5 using the distribution of galaxies observed in the VVDS-Deep survey. We use this overdensity field to measure the ...evolution of the probability distribution function and its lower-order moments over the redshift interval 0.7< z <1.5. We apply a self-consistent reconstruction scheme which includes a complete non-linear description of galaxy biasing and which has been thoroughly tested on realistic mock samples. We find that the variance and skewness of the galaxy distribution evolve over this redshift interval in a way that is remarkably consistent with predictions of first- and second-order perturbation theory. This finding confirms the standard gravitational instability paradigm over nearly 9 Gyr of cosmic time and demonstrates the importance of accounting for the non-linear component of galaxy biasing to consistently reproduce the higher-order moments of the galaxy distribution and their evolution.
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