arXiv subjects

J. I. Gonzalez-Serrano

Publications and source records attributed to J. I. Gonzalez-Serrano.

35 records · Page 2Linked to original sources

arXiv2004

Selection of quasar candidates from combined radio and optical surveys using neural networks

The application of supervised artificial neural networks (ANNs) for quasar selection from combined radio and optical surveys with photometric and morphological data is investigated, using the list of candidates and their classification from White et al. (2000). Seven input parameters and one output, evaluated to 1 for quasars and 0 for nonquasars, were used, with architectures 7:1 and 7:2:1. Both models were trained on samples of ~800 sources and yielded similar performance on independent test samples, with reliability as large as 87% at 80% completeness. For comparison the quasar fraction from the original candidate list was 56%. The accuracy is similar to that found by White et al. using supervised learning with oblique decision trees. This performance probably approaches the maximum value achievable with this database. Probabilities for the 98 candidates without spectroscopic classification in White et al. are presented and compared with the results from their work, showing a good agreement. This is the first analysis of the performance of ANNs for the selection of quasars. Our work shows that ANNs provide a promising technique for the selection of specific object types in astronomical databases.

astro-ph↗

arXiv2003

A sample of radio-loud QSOs at redshift ~ 4

We obtained spectra of 60 red, starlike objects (E< 18.8) identified with FIRST radio sources, S_{1.4GHz} > 1 mJy. Eight are QSOs with redshift z> 3.6.Combined with our pilot search (Benn et al 2002), our sample of 121 candidates yields a total of 18 z > 3.6 QSOs (10 of these with z > 4.0). 8% of candidates with S_{1.4GHz}< 10 mJy, and 37% of candidates with S_{1.4GHz}> 10 mJy are QSOs with z > 3.6. The surface density of E < 18.8, S_{1.4GHz} > 1mJy, 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 (8 have M_B < -28, q_0 = 0.5, H_0 = 50 kms^{-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) Ly alpha emission is marginally higher (3 out of 18) than for high-redshift QSOs from SDSS (5 out of 96).

astro-ph↗

arXiv2003

The Near-Infrared properties of the host galaxies of radio quasars

We present K-band images of 31 radio quasars selected from the B3-VLA quasar sample. A new method has been developed to detect and restore the host galaxies of these quasars. We have detected the host galaxies of 16 of them. Using these data together with previous data from the B3-VLA quasars (Carballo et al. 1998, hereafter Paper I) and literature data from other samples, we have analyzed a sample of 69 hosts of radio quasars, covering a redshift range between 0<z<3. The host galaxies are large (r_e}~15kpc) and luminous elliptical galaxies (~75% of them brighter than L_*), with an evolution similar to that of radio galaxies. A significant fraction (~40%) of them shows evidence of a possible collision/merging process. They follow a mu_e-r_e relation similar to that of normal elliptical galaxies. The morphological and photometric similarities between these galaxies and radio galaxies in this wide range of redshifts is a good test of the reliability of unification schemes. All of them show little evolution from z=3 to the present epoch. Their K-z distribution is consistent with a no-evolution model, with a fraction of the dispersion due to differences in radio power. We have found a correlation between the quasar radio power and the host luminosity. We have also found a correlation between the host and nuclear source luminosities. These correlations could be induced by a physical relation between the central black hole and the bulge mass (Magorrian et al. 1998).

astro-ph↗

arXiv2003

On the origin of the X-ray emission from a narrow-line radioquasar at z>1

We present new XMM-Newton X-ray observations of the z=1.246 narrow-line radioquasar RX J1011.2+5545 serendipitously discovered by ROSAT. The flat X-ray spectrum previously measured by ROSAT and ASCA is shown to be the result of a steep Gamma~1.8 power law spectrum seen through a moderate intrinsic absorbing column NH~4E21 cm^-2. The position of the X-ray source is entirely coincident with the nucleus of the radio source that we have resolved in new sensitive VLA observations at 3.6 and 6 cm, implying that scattering in the radio lobes is not responsible for the bulk of X-ray emission. In the EPIC pn image, a faint patch of X-ray emission is apparent 14'' to the NE of the main X-ray source. The former is positionally coincident with an apparently extended optical object with R~21.9, but there is no associated radio emission, thus ruling out the possibility that this represents a hotspot in a jet emanating from the primary X-ray source. No reflection features are detected in the X-ray spectrum of the narrow-line radioquasar, although an Fe line with equivalent width of up to 600 eV cannot be ruled out.

astro-ph↗

arXiv2003

Tomography of high-redshift clusters with OSIRIS

High-redshift clusters of galaxies are amongst the largest cosmic structures. Their properties and evolution are key ingredients to our understanding of cosmology: to study the growth of structure from the inhomogeneities of the cosmic microwave background; the processes of galaxy formation, evolution, and differentiation; and to measure the cosmological parameters (through their interaction with the geometry of the universe, the age estimates of their component galaxies, or the measurement of the amount of matter locked in their potential wells). However, not much is yet known about the properties of clusters at redshifts of cosmological interest. We propose here a radically new method to study large samples of cluster galaxies using microslits to perform spectroscopy of huge numbers of objects in single fields in a narrow spectral range-chosen to fit an emission line at the cluster redshift. Our objective is to obtain spectroscopy in a very restricted wavelength range (~100 A in width) of several thousands of objects for each single 8x8 square arcmin field. Approximately 100 of them will be identified as cluster emission-line objects and will yield basic measurements of the dynamics and the star formation in the cluster (that figure applies to a cluster at z~0.50, and becomes ~40 and ~20 for clusters at z~0.75 and z~1.00 respectively). This is a pioneering approach that, once proven, will be followed in combination with photometric redshift techniques and applied to other astrophysical problems.

astro-ph↗

arXiv2002

Cluster of galaxies around seven radio-loud QSOs at 1<z<1.6: K-band images

We have conducted a NIR study of the environments of seven radio-loud quasars at redshifts 1<z<1.6. In present paper we describe deep $K$ band images obtained for the fields of ~6X6 arcmin around the quasars with 3$σ$ limiting magnitudes of K~20.5. These fields were previously studied using deep B and R band images (Sanchez & Gonzalez-Serrano 1999). Using together optical and NIR data, it has been found a significant excess of galaxies which optical-NIR colours, luminosity, spatial scale, and number of galaxies are compatible with clusters at the redshift of the quasar. We have selected a sample of cluster candidates analyzing the R-K vs. K diagram. A ~25% of the candidates present red optical-NIR colours and an ultraviolet excess. This population has been also found in clusters around quasars at the same redshifts (Tanaka et al. 2000; Haines et al. 2001). These galaxies seem to follow a mixed evolution: a main passive evolution plus late starformation processes. The quasars do not inhabit the core of the clusters, being found in the outer regions. This result agrees with the hypothesis that the origin/feeding mechanism of the nuclear activity were merging processes. The quasars inhabit the region were a collision is most probably to produce a merger.

astro-ph↗

arXiv2002

Observations of the Hubble Deep Field South with the Infrared Space Observatory - I. Observations, data reduction and mid-infrared source counts

We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 7 and 15 micron with the CAM instrument on board the Infrared Space Observatory (ISO). The final map in each band was constructed by the coaddition of four independent rasters, registered using bright sources securely detected in all rasters, with the absolute astrometry being defined by a radio source detected at both 7 and 15 micron. We sought detections of bright sources in a circular region of radius 2.5 arcmin at the centre of each map, in a manner that simulations indicated would produce highly reliable and complete source catalogues using simple selection criteria. Merging source lists in the two bands yielded a catalogue of 35 distinct sources, which we calibrated photometrically using photospheric models of late-type stars detected in our data. We present extragalactic source count results in both bands, and discuss the constraints they impose on models of galaxy evolution models, given the volume of space sampled by this galaxy population.

astro-ph↗

arXiv2002

Observations of the Hubble Deep Field South with the Infrared Space Observatory - II. Associations and star formation rates

We present results from a deep mid-IR survey of the Hubble Deep Field South (HDF-S) region performed at 7 and 15um with the CAM instrument on board ISO. We found reliable optical/near-IR associations for 32 of the 35 sources detected in this field by Oliver et al. (2002, Paper I): eight of them were identified as stars, one is definitely an AGN, a second seems likely to be an AGN, too, while the remaining 22 appear to be normal spiral or starburst galaxies. Using model spectral energy distributions (SEDs) of similar galaxies, we compare methods for estimating the star formation rates (SFRs) in these objects, finding that an estimator based on integrated (3-1000um) IR luminosity reproduces the model SFRs best. Applying this estimator to model fits to the SEDs of our 22 spiral and starburst galaxies, we find that they are forming stars at rates of ~1-100 M_sol/yr, with a median value of ~40M_sol/yr, assuming an Einstein - de Sitter universe with a Hubble constant of 50 km/s/Mpc, and star formation taking place according to a Salpeter (1955) IMF across the mass range 0.1-100M_sol. We split the redshift range 0.0<z<0.6 into two equal-volume bins to compute raw estimates of the star formation rate density contributed by these sources, assuming the same cosmology and IMF as above and computing errors based on estimated uncertainties in the SFRs of individual galaxies. We compare these results with other estimates of the SFR density made with the same assumptions, showing them to be consistent with the results of Flores et al. (1999) from their ISO survey of the CFRS 1415+52 field. However, the relatively small volume of our survey means that our SFR density estimates suffer from a large sampling variance, implying that our results, by themselves, do not place tight constraints on the global mean SFR density.

astro-ph↗

arXiv2001

The European Large Area ISO Survey VI - Discovery of a new hyperluminous infrared galaxy

We report the discovery of the first hyperluminous infrared galaxy (HyLIG) in the course of the European Large Area ISO Survey (ELAIS). This object has been detected by ISO at 6.7, 15, and 90 microns, and is found to be a broad-line, radio-quiet quasar at a redshift: z = 1.099. From a detailed multi-component model fit of the spectral energy distribution, we derive a total infrared luminosity: L_IR (1-1000 microns) ~ 1.0 x 10E13 h_65^-2 L_sun (q_0 = 0.5), and discuss the possible existence of a starburst contributing to the far-IR output. Observations to date present no evidence for lens magnification. This galaxy is one of the very few HyLIGs with an X-ray detection. On the basis of its soft X-ray properties, we suggest that this broad-line object may be the face-on analogue of narrow-line, Seyfert-like HyLIGs.

astro-ph↗

arXiv1999

{Excesses of faint galaxies around seven radio QSOs at 1.0<z<1.6

We have conducted an optical study of the environments of seven radio-loud quasars at redshifts 1 22.5 and R>22.0) galaxies on scales of r<170'' and r<35'' around the quasars. The number of excess galaxies, their magnitudes and the angular extension of the excesses are compatible with clusters of galaxies at the redshift of the QSOs. We also found that the quasars of our sample are in general {\it not} located at the peak of the density distribution, lying in a projected distance of about $\sim$70'' from it. This result agrees with the scenario of a link between overdensities of galaxies around radio sources and the origin of their radio emission.

astro-ph↗

arXiv1999

X-ray Spectra of the RIXOS source sample

We present results of an extensive study of the X-ray spectral properties of sources detected in the RIXOS survey, that is nearly complete down to a flux limit of 3e-14 cgs (0.5-2 keV). We show that for X-ray surveys containing sources with low count rate spectral slopes estimated using simple hardness ratios in the ROSAT band can be biased. Instead we analyse three-colour X-ray data using statistical techniques appropriate to the Poisson regime which removes the effects of this bias. We have then applied this technique to the RIXOS survey to study the spectral properties of the sample. For the AGN we find an average energy index of 1.05+-0.05 with no evidence for spectral evolution with redshift. Individual AGN are shown to have a range of properties including soft X-ray excesses and intrinsic absorption. Narrow Emission Line Galaxies also seem to fit to a power-law spectrum, which may indicate a non-thermal origin for their X-ray emission. We infer that most of the clusters in the sample have a bremsstrahlung temperature >3 keV, although some show evidence for a cooling flow. The stars deviate strongly from a power-law model but fit to a thermal model. Finally, we have analysed the whole RIXOS sample containing 1762 sources. We find that the mean spectral slope of the sources hardens at lower fluxes in agreement with results from other samples. However, a study of the individual sources demonstrates that the hardening of the mean is caused by the appearance of a population of very hard sources at the lowest fluxes. This has implications for the nature of the soft X-ray background.

astro-ph↗

arXiv1998

Clustering around QSOs at 1<z<1.6

Deep $B$ and $R$ band images (R<24 and B<25) have been obtained of a field of $\sim 6 \times 6$ arcmin around seven radio-loud quasars with $1 3σ_{N{exp}}$). The galaxies have magnitudes compatible with being at the redshift of the QSOs.

astro-ph↗

arXiv1998

Discovery of an X-ray selected radio-loud obscured AGN at z=1.246

We have discovered an obscured active galaxy at redshift z = 1.246 identified with the ROSAT X-ray source RX J1011.2+5545. We report on multiwavelength observations of this source and discuss its X-ray, optical and radio properties. This is the first X-ray selected, obscured active galaxy at high redshift to be shown to be radio-loud, with a radio counterpart exhibiting a classical double-lobe morphology.

astro-ph↗

arXiv1997

K-band imaging of 52 B3-VLA quasars: Nucleus and host properties

We present K-band imaging and photometry of a sample of 52 radio loud quasars (RQs) selected from the B3 survey with flux densities above 0.5 Jy at 408 MHz. The optical completeness of the sample is 90% and the quasars cover the redshift range 0.4 - 2.3. For ~57% of the sources for which the quality of the images allowed a detailed morphological study (16/28) resolved extended emission was detected around the QSO, and its K flux was measured. Interpreting this ``fuzz'' as starlight emission from the host galaxy, its location on the K-z plane at z<1 is consistent with radio quasars being hosted by galaxies similar to radio galaxies (RGs) or giant ellipticals (gEs). At higher redshifts the detected host galaxies of RQs are more luminous than typical RGs and gEs, although some weak detections or upper limits are consistent with a similar fraction of RQs being hosted by galaxies with the expected luminosities for RGs or gEs. We found a significant correlation between radio power and nuclear infrared luminosity indicating a direct link between the radio synchrotron emission and the nuclear emission in K. This correlation is more tight for the steep-spectrum sources (99.97% significance). In addition, a trend is found between radio power and infrared luminosity of the host galaxy (or mass), in the sense that the most powerful quasars inhabit the most luminous galaxies.

astro-ph↗

arXiv1997

Red quasars not so dusty

Webster et al (1995) claimed that up to 80% of QSOs may be obscured by dust. They inferred the presence of this dust from the remarkably broad range of B-K optical-infrared colours of a sample of flat-spectrum PKS radio QSOs. If such dust is typical of QSOs, it will have rendered invisible most of those which would otherwise been have detected by optical surveys. We used the William Herschel Telescope on La Palma to obtain K infrared images of 54 B3 radio quasars selected at low frequency (mainly steep-spectrum), and we find that although several have very red optical-infrared colours, most of these can be attributed to an excess of light in K rather than a dust-induced deficit in B. We present evidence that some of the infrared excess comes from the light of stars in the host galaxy (some, as previously suggested, comes from synchrotron radiation associated with flat-spectrum radio sources). The B-K colours of the B3 QSOs provide no evidence for a large reddened population. Either the Webster et al QSOs are atypical in having such large extinctions, or their reddening is not due to dust; either way, the broad range of their B-K colours does not provide evidence that a large fraction of QSOs has been missed from optical surveys.

astro-ph↗

arXiv1995

INTERMEDIATE RESOLUTION SPECTROSCOPY OF THE RADIO GALAXY B2 0902+34 AT $Z\approx 3.4$

We have carried out spectroscopic observations of the high redshift ($z\approx 3.4$) radio galaxy 0902+34 at intermediate resolution with the William Herschel Telescope. The dynamical spectral ranges covered are 4600-5480 Åand 5920-7680 Åwith resolutions of 5.4 Åand 9.5 Å, respectively. We detect a continuum that is almost flat and resolve three emission lines: Ly$α$, C IV $λ$1549 and He II $λ$1640, the last one previously undetected. The line ratios are similar to the typical values found for narrow-line high redshift radio galaxies. Line ratios observed in different regions of the galaxy seem to indicate the presence of strong ionization and/or dust density gradients. We have not detected any Ly$α$ absorption at z=3.3968 (red wing of the Ly$α$ emission line) as might be expected from the absorption found at 21 cm by other authors using the VLA and Arecibo antennas. We discuss possible models for the H I absorbing cloud.

astro-ph↗

arXiv1994

R-Band Imaging of Fields Around 1<z<2 Radiogalaxies

We have taken deep $R$-band images of fields around five radiogalaxies: 0956+47, 1217+36, 3C256, 3C324 and 3C294 with $1<z<2$ . 0956+47 is found to show a double nucleus. Our data on 1217+36 suggest the revision of its classification as a radiogalaxy. We found a statistically significant excess of bright ($19.5<R<22$) galaxies on scales of 2 arcmin around the radiogalaxies (which have $R \approx 21.4$) in our sample. The excess has been determined empirically to be at $\gtrsim 99.5\%$ level. It is remarkable that this excess is not present for $22<R<23.75$ galaxies within the same area, suggesting that the excess is not physically associated to the galaxies but due to intervening groups and then related to gravitational lensing.

astro-ph↗