483 research outputs found
The cosmic evolution of quasar host galaxies
We present near-infrared imaging of the host galaxies of 17 quasars in the
redshift range 1 < z < 2, carried out at the ESO VLT UT1 8m telescope under
excellent seeing conditions (~0.4 arcsec). The sample includes radio-loud (RLQ)
and radio-quiet (RQQ) quasars with similar distribution of redshift and optical
luminosity. For all the observed objects but one we have derived the global
properties of the surrounding nebulosity. The host galaxies of both types of
quasars follow the expected trend in luminosity of massive ellipticals
undergoing simple passive evolution, but there is a systematic difference by a
factor ~2 in the host luminosity between RLQs and RQQs (M_K(RLQ) = -27.55 +-
0.12 and M_K(RQQ) = -26.83 +- 0.25). Comparison with quasar hosts at similar
and lower redshift indicates that the difference in the host luminosity between
RLQs and RQQs remains the same from z = 2 to the present epoch. No significant
correlation is found between the nuclear and the host luminosities. Assuming
that the host luminosity is proportional to the black hole mass, as observed in
nearby massive spheroids, these quasars emit at very different levels (spread
\~1.5dex) with respect to their Eddington luminosity and with the same
distribution for RLQs and RQQs. Apart from a factor of ~2 difference in
luminosity, the hosts of RLQs and RQQs appear to follow the same cosmic
evolution as massive inactive spheroids. Our results support a view where
nuclear activity can occur in all luminous ellipticals without producing a
significant change in their global properties and evolution. Quasar hosts
appear to be already well formed at z ~2, in disagreement with models for the
joint formation and evolution of galaxies and active nuclei based on the
hierarchical structure formation scenario.Comment: Astrophysical Journal, accepted; 34 page
On the cool gaseous haloes of quasars
We present optical spectroscopy of projected QSO pairs to investigate the
MgII and the CIV absorption features imprinted on the spectrum of the
background object by the gaseous halo surrounding the foreground QSO. We
observed 13 projected pairs in the redshift range 0.7<z<2.2 spanning projected
separations between 60 kpc and 120 kpc. In the spectra of the background QSOs,
we identify MgII intervening absorption systems associated to the foreground
QSOs in 7 out of 10 pairs, and 1 absorption system out of 3 is found for CIV.
The distribution of the equivalent width as a function of the impact parameter
shows that, unlike the case of normal galaxies, some strong absorption systems
(EWr > 1 Ang) are present also beyond a projected radius of ~70 kpc. If we take
into account the mass of the galaxies as an additional parameter that influence
the extent of the gaseous haloes, the distribution of the absorptions connected
to the QSOs is consistent to that of galaxies. In the spectra of the foreground
QSOs we do not detect any MgII absorption lines originated by the gas
surrounding the QSO itself, but in 2 cases these features are present for CIV.
The comparison between the absorption features observed in the transverse
direction and those along the line of sight allows us to comment on the
distribution of the absorbing gas and on the emission properties of the QSOs.
Based on observations undertaken at the European Southern Observatory (ESO)
Very Large Telescope (VLT) under Programmes 085.B-0210(A) and 086.B-0028(A).Comment: 15 pages, 3 tables, 9 figures. Accepted to be published on MNRA
Reclassification of the nearest quasar pair candidate: SDSS J15244+3032 - RXS J15244+3032
We present optical spectroscopy of the nearest quasar pair listed in the 13th
edition of the Veron-Cetty & Veron catalogue, i.e. the two quasars SDSS
J15244+3032 and RXS J15244+3032 (redshift z~0.27, angular separation ~7 arcsec,
and line-of-sight velocity difference ~1900 km/s). This system would be an
optimal candidate to investigate the mutual interaction of the host galaxies
with ground based optical imaging and spectroscopy. However, new optical data
demonstrate that RXS J15244+3032 is indeed a star of spectral type G.
This paper includes data gathered with the Asiago 1.82m telescope (Cima Ekar
Observatory, Asiago, Italy).Comment: 5 pages, 2 figures, 1 table. Accepted for publication in APS
Examining the Seyfert - Starburst Connection with Arcsecond Resolution Radio Continuum Observations
We compare the arcsecond-scale circumnuclear radio continuum properties
between five Seyfert and five starburst galaxies, concentrating on the search
for any structures that could imply a spatial or causal connection between the
nuclear activity and a circumnuclear starburst ring. No evidence is found in
the radio emission for a link between the triggering or feeding of nuclear
activity and the properties of circumnuclear star formation. Conversely, there
is no clear evidence of nuclear outflows or jets triggering activity in the
circumnuclear rings of star formation. Interestingly, the difference in the
angle between the apparent orientation of the most elongated radio emission and
the orientation of the major axis of the galaxy is on average larger in
Seyferts than in starburst galaxies, and Seyferts appear to have a larger
physical size scale of the circumnuclear radio continuum emission. The
concentration, asymmetry, and clumpiness parameters of radio continuum emission
in Seyferts and starbursts are comparable, as are the radial profiles of radio
continuum and near-infrared line emission. The circumnuclear star formation and
supernova rates do not depend on the level of nuclear activity. The radio
emission usually traces the near-infrared Br-gamma and H2 1-0 S(1) line
emission on large spatial scales, but locally their distributions are
different, most likely because of the effects of varying local magnetic fields
and dust absorption and scattering.Comment: 21 pages, 10 figures. Accepted for publication in the Astronomical
Journa
The nuclear to host galaxy relation of high redshift quasars
We present near-infrared imaging with ESO VLT+ISAAC of the host galaxies of
low luminosity quasars in the redshift range 1 < z < 2, aimed at investigating
the relationship between the nuclear and host galaxy luminosities at high
redshift. This work complements our previous study to trace the cosmological
evolution of the host galaxies of high luminosity quasars (Falomo et al. 2004).
The new sample includes 15 low luminosity quasars, nine radio-loud (RLQ) and
six radio-quiet (RQQ). They have similar distribution of redshift and optical
luminosity, and together with the high luminosity quasars they cover a large
range (~4 mag) of the quasar luminosity function. The host galaxies of both
types of quasars are in the range of massive inactive ellipticals between L*
and 10 L*. RLQ hosts are systematically more luminous than RQQ hosts by a
factor of ~2. This difference is similar to that found for the high luminosity
quasars. This luminosity gap appears to be independent of the rest-frame U-band
luminosity but clearly correlated with the rest-frame R-band luminosity. The
color difference between the RQQs and the RLQs is likely a combination of an
intrinsic difference in the strength of the thermal and nonthermal components
in the SEDs of RLQs and RQQs, and a selection effect due to internal dust
extinction. For the combined set of quasars, we find a reasonable correlation
between the nuclear and the host luminosities. This correlation is less
apparent for RQQs than for RLQs. If the R-band luminosity is representative of
the bolometric luminosity, and assuming that the host luminosity is
proportional to the black hole mass, as observed in nearby massive spheroids,
quasars emit with a relatively narrow range of power with respect to their
Eddington luminosity and with the same distribution for RLQs and RQQs.Comment: Accepted for publication in ApJ, 24 pages, 4 figure
Chernobyl still with us : (137)Caesium activity contents in seabed sediments from the Gulf of Bothnia, northern Baltic Sea
Anthropogenic radionuclides are among those human impacts, which can be seen widely in the marine and terrestrial ecosystems. Fallout from the 1986 Chernobyl nuclear power plant accident has rendered the Baltic Sea as the most polluted marine body in the world with respect to Cs-137. This research investigated sediment cores from 56 sites around the Gulf of Bothnia, Baltic Sea. Radioactivity from Cs-137 in sediments has generally declined due to natural/radioactive decay of Cs-137 over the last decades. However, Cs-137 contents in subsurface sediments remain at elevated levels compared to pre-Chernobyl levels. The highest Cs-137 activity contents in subsurface sediments (>4000 Bg kg(-1)) occur in coastal areas including estuaries. These areas often experience severe anthropogenic pressure. The southern Bothnian Sea, Kvarken archipelago, and southern Bothnian Bay all show elevated Cs-137 values in subsurface sediments. Sedimentary Cs-137 can also help constrain recent rates of sedimentation. Post-Chernobyl sedimentation rates in the Gulf of Bothnia varied from 0.1 to 4.8 cm/year with an average sedimentation rate of 0.54 cm/year.Peer reviewe
Circum-galactic medium in the halo of quasars
The properties of circum-galactic gas in the halo of quasar host galaxies are
investigated analyzing Mg II 2800 and C IV 1540 absorption-line systems along
the line of sight close to quasars. We used optical spectroscopy of closely
aligned pairs of quasars (projected distance 200 kpc, but at very
different redshift) obtained at the VLT and Gran Telescopio Canarias to
investigate the distribution of the absorbing gas for a sample of quasars at
z1. Absorption systems of EW 0.3 associated with the
foreground quasars are revealed up to 200 kpc from the centre of the host
galaxy, showing that the structure of the absorbing gas is patchy with a
covering fraction quickly decreasing beyond 100 kpc. In this contribution we
use optical and near-IR images obtained at VLT to investigate the relations
between the properties of the circum-galactic medium of the host galaxies and
of the large scale galaxy environments of the foreground quasars.Comment: 6 pages, 3 figures, proceedings of the conference "QUASARS at all
cosmic epochs", accepted for publication on Frontiers in Astronomy and Space
Scienc
Host Galaxies of low z Radio-loud Quasars: A search of HST archives
We searched the HST archives for unpublished WFPC2 images of low redshift
(z<0.5) radio loud quasars (RLQ). This led to the identification of 11 objects.
We present here the results of the analysis of these images from which we
derive the properties of their host galaxies. All objects are clearly resolved
and their surrounding nebulosity is consistent with an elliptical galaxy model.
These new data, together with previous published HST observations, form a
sample of 34 sources which significantly expands all previous studies of low
redshift RLQ based on HST data. For this full sample we derive the average
absolute magnitude of the host galaxies =-24.01+/-0.48, and the effective
radius =10.5+/-3.7kpc. No significant correlation is found between the
nucleus and the host galaxy luminosity. Using the relationship between black
hole mass (M_BH) and bulge luminosity we investigate the relation between M_BH
and total radio power for RLQ and compare with other classes of radio sources.
The overall distribution of AGN in the plane M_BH-P(radio) exhibits a trend for
increasing M_BH with increasing P(radio) but with a substantial spread. RLQ
occupy the region of most powerful sources and most massive BH. The quasars
appear to emit over a wide range of power with respect to their Eddington
luminosity as deduced by the estimated M_BH.Comment: 23 pages, 8 figures, ApJ in pres
XMM-Newton View of PKS 2155-304: Characterizing the X-ray Variability Properties with EPIC-PN
Starting from XMM-Newton EPIC-PN data, we present the X-ray variability
characteristics of PKS 2155-304 using a simple analysis of the excess variance,
\xs, and of the fractional rms variability amplitude, fvar. The scatter in \xs\
and \fvar, calculated using 500 s long segments of the light curves, is smaller
than the scatter expected for red noise variability. This alone does not imply
that the underlying process responsible for the variability of the source is
stationary, since the real changes of the individual variance estimates are
possibly smaller than the large scatters expected for a red noise process. In
fact the averaged \xs and \fvar, reducing the fluctuations of the individual
variances, chang e with time, indicating non-stationary variability. Moreover,
both the averaged \sqxs (absolute rms variability amplitude) and \fvar show
linear correlation with source flux but in an opposite sense: \sqxs correlates
with flux, but \fvar anti-correlates with flux. These correlations suggest that
the variability process of the source is strongly non-stationary as random
scatters of variances should not yield any correlation. \fvar spectra were
constructed to compare variability amplitudes in different energy bands. We
found that the fractional rms variability amplitude of the source, when
significant variability is observed, increases logarithmically with the photon
energy, indicating significant spectral variability. The point-to-point
variability amplitude may also track this trend, suggesting that the slopes of
the power spectral density of the source are energy-independent. Using the
normalized excess variance the black hole mass of \pks was estimated to be
about . This is compared and contrasted with the
estimates derived from measurements of the host galaxies.Comment: Accepted for publication in The Astrophysical Journa
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