1,721,609 research outputs found
Radio properties of FIRST radio sources at 1 mJy
No full textThis paper presents a detailed analysis of the radio properties for the sample of faint radio sources introduced by Magliocchetti et al. in 2000. The sample comprises mainly intrinsically low-power sources, the majority of which (greater than or similar to 70 per cent) are FR I radio galaxies. These objects show some degree (at 1sigma-confidence level) of luminosity evolution, which is also needed to reproduce correctly the total number and shape of the counts distribution at 1.4 GHz. Analysis of the de-evolved local radio luminosity function shows a good agreement between data and model predictions for this class of sources. Particular care has been devoted to the issue of 'lined' galaxies (i.e. objects presenting in their spectra a continuum typical of early-type galaxies plus emission lines of different nature), which appear as an intermediate class of sources between AGN-dominated and starburst galaxies. Different evolutionary behaviour is seen in the two subpopulations of lined and non-lined low-power radio galaxies, the first class indicating a tendency for the radio luminosity to decrease with look-back time, the second one showing positive evolution., We note that different evolutionary properties also seem to characterize BL Lacs selected in different bands, so that one might envisage an association between lined FR I and the subclass of BL Lacs selected in the X-ray band. Lastly, we find evidence for a negligible contribution of starburst galaxies at these low flux levels
Faint radio-loud quasars: clues on their evolution
No full textThe quasar sample selected by cross-correlating the FIRST and the 2dF Quasar Redshift Surveys allows us to explore, for the first time, the faint end of the radio and optical luminosity functions up to z = 2.2. We find indications (~3 \sigma) of a negative evolution for these faint sources at z > 1.8, both in radio and optical bands. This corresponds to a decrement in the space density of faint quasars of about a factor 2 at z=2.2 and confirms the presence of a differential evolution for the population of radio-active quasars. The faint end of both luminosity functions flattens and the comparison with the (optical) number density of the whole quasar population supports a dependence of the fraction of radio detected quasars on the optical luminosity. A progressive decrease in the fraction of quasars in the whole radio source population can be consistently accounted for within the `receding torus' scenario. The population of low luminosity quasars, which the FIRST-2dF detects, appears to depart from the `classical' scheme for radio-loud quasars
Evidence for anisotropy in the distribution of short-lived gamma-ray bursts
No full textMeasurements of the two-point angular correlation function w(\theta) for 407 short gamma-ray bursts collected in the Current BATSE Catalogue reveal a ~2 \sigma deviation from isotropy on angular scales \theta ~ 2-4 degrees. Such an anisotropy is not observed in the distribution of long gamma-ray bursts and hints to the presence of repeated bursts for up to ~13% of the sources under exam. However, the available data cannot exclude the signal as due to the presence of large-scale structure. Under this assumption, the amplitude of the observed w(\theta) is compatible with those derived for different populations of galaxies up to redshifts ~0.5, result that suggests short gamma-ray bursts to be relatively local sources
The role of black hole mass in quasar radio activity
No full textWe use a homogeneous sample of ∼300, 0.3 ≲z≲ 3, radio-loud quasars (QSOs) drawn from the FIRST and 2dF QSO surveys to investigate a possible dependence of radio activity on black hole mass. By analysing composite spectra for the populations of radio-quiet and radio-loud QSOs - chosen to have the same redshift and luminosity distributions - we find with high statistical significance that radio-loud quasars are on average associated with black holes of masses ∼108.6 M⊙, about twice as large as those measured for radio-quiet quasars (∼108.3 M ⊙). We also find a clear dependence of black hole mass on optical luminosity of the form log(MBH/M⊙) RL= 8.57(±0.06) - 0.27(±0.06) (MB+ 24.5) and log(MBH/M⊙) RQ= 8.43(±0.05) - 0.32(±0.06) (MB+ 24.5), respectively, for the cases of radio-loud and radio-quiet quasars. It is intriguing to note that these two trends run roughly parallel to each other, implying that radio-loud quasars are associated to black holes more massive than those producing the radio-quiet case at all sampled luminosities. On the other hand, in the case of radio-loud quasars, we find evidence for only a weak (if any) dependence of the black hole mass on radio power. The above findings seem to support the belief that there exists - at a given optical luminosity - a threshold black hole mass associated with the onset of significant radio activity such as that of radio-loud QSOs; however, once the activity is triggered, there appears to be very little connection between black hole mass and level of radio output. © 2005 RAS
Is there a dichotomy in the radio loudness distribution of quasars?
Full text linkWe present a new approach to tackling the issue of radio loudness in quasars. We constrain a (simple) prescription for the intrinsic distribution of radio-to-optical ratios by comparing properties of Monte Carlo simulated samples with those of observed optically selected quasars. We find strong evidence for a dependence of the radio luminosity on the optical one, even though with a large scatter. The dependence of the fraction of radio-loud quasars on apparent and absolute optical magnitudes results in a selection effect related to the radio and optical limits of current surveys. The intrinsic distribution of the radio-to-optical ratios shows a peak at R 1.4* ∼ 0.3, with only ≲5 per cent of objects being included in a high-R1.4* tail, which identifies the radio-loud regime. No lack or deficit of sources - but only a steep transition region - is present between the radio-loud and radio-quiet populations at any R 1.4*. Briefly, we discuss possible origins for this behaviour (e.g. an absence of jets in radio-quiet sources, a large range of radiative radio efficiency, different lifetimes for the accretion and jet ejection phenomena, etc.)
The connection between spheroidal galaxies and QSOs
No full textIn view of the extensive evidence of tight inter-relationships between spheroidal galaxies (and galactic bulges) with massive black holes hosted at their centers, a consistent model must deal jointly with the evolution of the two components. We describe one such model, which successfully accounts for the local luminosity function of spheroidal galaxies, for their photometric and chemical properties, for deep galaxy counts in different wavebands, including those in the (sub)-mm region which proved to be critical for current semi-analytic models stemming from the standard hierarchical clustering picture, for clustering properties of SCUBA galaxies, of EROs, and of LBGs, as well as for the local mass function of massive black holes and for quasar evolution. Predictions that can be tested by surveys carried out by SIRTF are presented
The environmental properties of radio-emitting AGN
No full textWe study the environmental properties of z less than or similar to 1.2
radio-selected active galactic nuclei (AGNs) belonging to the similar to
2 deg(2) of the COSMOS field, finding that about 20 per cent of them
appear within overdense structures. AGNs with P-1.4 GHz > 10(23.5) W
Hz(-1) sr(-1) are twice more likely to be found in clusters with respect
to fainter sources (similar to 38 per cent vs similar to 15 per cent),
just as radio-selected AGNs with stellar masses M-* > 10(11) M-circle
dot are twice more likely to be found in overdense environments with
respect to objects of lower mass (similar to 24 per cent vs similar to
11 per cent). Comparisons with galaxy samples further suggest that
radio-selected AGN of large stellar mass tend to avoid underdense
environments more than normal galaxies with the same stellar content.
Stellar masses also seem to determine the location of radio-active AGN
within clusters: similar to 100 per cent of the sources found as
satellite galaxies have M* < 10(11.3) M-circle dot, while similar to
100 per cent of the AGNs coinciding with a cluster central galaxy have
M* > 10(11) M-circle dot. No different location within the cluster is
instead observed for AGN of various radio luminosities. Radio AGN, which
also emit in the Mid-Infrared show a marked preference to be found as
isolated galaxies (similar to 70 per cent) at variance with those also
active in the X-ray that all seem to reside within overdensities. What
emerges from our work is a scenario whereby physical processes on sub-pc
and kpc scales (e.g. emission, respectively, related to the AGN and to
star formation) are strongly interconnected with the large-scale
environment of the AGN itself
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
First stars contribution to the near-infrared background fluctuations
Full text linkWe show that the emission from the first, metal-free stars inside Population III objects (Pop IIIs) is needed to explain the level of fluctuations in the near-infrared background (NIRB) recently discovered by Kashlinsky et al., at least at the shortest wavelengths. Clustering of (unresolved) Pop IIIs can in fact account for the entire signal at almost all the ∼1–30 arcsec scales probed by observations in the J band. Their contribution fades away at shorter frequencies and becomes negligible in the K band. ‘Normal’, highly clustered, 〈z〉≃ 3 galaxies undergoing intense star formation such as those found in the Hubble Deep Fields can ‘fill in’ this gap and provide for the missing signal. It is in fact found that their contribution to the intensity fluctuations is the dominant one at λ= 2.17 μm, while it gradually loses importance in the H andJ bands. The joint contribution from these two populations of cosmic objects is able, within the errors, to reproduce the observed power spectrum in the whole near-infrared range on small angular scales (θ≲ 200 arcsec for Pop III protogalaxies). Signals on larger scales detected by other experiments instead require the presence of more local sources
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