394 research outputs found
The host galaxies of radio-quiet quasars at 0.5<z<1.0
We present near-infrared H-band imaging of 15 intermediate redshift (0.5<z<1)
radio quiet quasars (RQQ) in order to characterize the properties of their host
galaxies. We are able to clearly detect the surrounding nebulosity in 12
objects, whereas the object remains unresolved in three cases. For all the
resolved objects, we find that the host galaxy is well represented by a de
Vaucouleurs r^{1/4} surface brightness law. This is the first reasonably sized
sample of intermediate redshift RQQs studied in the near-infrared.
The RQQ host galaxies are luminous (average M_H=-26.3+-0.6) and large giant
elliptical galaxies (average bulge scale length R_e = 11.3\pm5.8 kpc). RQQ
hosts are about 1 mag brighter than the typical low redshift galaxy luminosity
L^*, and their sizes are similar to those of galaxies hosting lower redshift
RQQs, indicating that there is no significant evolution at least up to z=1 of
the host galaxy structure. We also find that RQQ hosts are about 0.5-1 mag
fainter than radio-loud quasars (RLQ) hosts at the similar redshift range. The
comparison of the host luminosity of intermediate redshift RQQ hosts with that
for lower z sources shows a trend that is consistent with that expected from
the passive evolution of the stars in the host galaxies. The nuclear luminosity
and the nucleus/host galaxy luminosity ratio of the objects in our sample are
intermediate between those of lower redshift RQQs and those of higher redshift
(z>1) RQQs.Comment: A&A, accepted, 15 pages, 6 figures. Fig. 2 will only be published in
the electronic version of A&
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
The stellar content of low redshift BL Lac host galaxies from multicolour imaging
We present B-band imaging of 18 low redshift (z<0.3) BL Lac objects for which
their host galaxies were previously resolved in the R-band and the
near-infrared H-band. For a subset of the objects, also U- and V-band imaging
is presented. These data are used to investigate the blue-red-near-infrared
colours and the colour gradients of the host galaxies of BL Lacs in comparison
with other elliptical galaxies with and without nuclear activity. In all cases
galaxies are well represented by an elliptical model, with average absolute
magnitude M_B=-21.6+-0.7 and average scale length R_e=7.6+-3.2 kpc. The
best-fit B-band Kormendy relation is in reasonable agreement with that obtained
for normal ellipticals and radio galaxies. This structural and dynamical
similarity indicates that all massive elliptical galaxies can experience
nuclear activity without significant perturbation of their global structure.
The distributions of the integrated blue/near-infrared colour (with average
B-H=3.5+-0.5) and colour gradient (with average Delta(B-R)/Delta(log
r)=-0.14+-0.75) of the BL Lac hosts are much wider than those for normal
ellipticals, and most BL Lac objects have bluer hosts and/or steeper colour
gradients than those in normal ellipticals. The blue colours are likely caused
by a young stellar population component, and indicates a link between star
formation caused by an interaction/merging event and the onset of the nuclear
activity. This result is corroborated by stellar population modelling,
indicating a presence of young/intermediate age populations in the majority of
the sample, in agreement with low redshift quasar hosts. The lack of strong
signs of interaction may require a significant time delay between the event
with associated star formation episodes and the start of the nuclear activity.Comment: 16 pages, 10 figure
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
An infrared study of the double nucleus in NGC3256
We present new resolved near and mid-IR imaging and N-band spectroscopy of
the two nuclei in the merger system NGCA3256, the most IR luminous galaxy in
the nearby universe. The results from the SED fit to the data are consistent
with previous estimates of the amount of obscuration towards the nuclei and the
nuclear star formation rates. However, we also find substantial differences in
the infrared emission from the two nuclei which cannot be explained by
obscuration alone. We conclude that the northern nucleus requires an additional
component of warm dust in order to explain its properties. This suggests that
local starforming conditions can vary significantly within the environment of a
single system.Comment: Accepted for publication (MNRAS
Using Hubble Space Telescope Imaging of Nuclear Dust Morphology to Rule Out Bars Fueling Seyfert Nuclei
If AGN are powered by the accretion of matter onto massive black holes, how
does the gas in the host galaxy lose the required angular momentum to approach
the black hole? Gas easily transfers angular momentum to stars in strong bars,
making them likely candidates. Although ground-based searches for bars in
active galaxies using both optical and near infrared surface brightness have
not found any excess of bars relative to quiescent galaxies, the searches have
not been able to rule out small-scale nuclear bars. To look for these nuclear
bars we use HST WFPC2-NICMOS color maps to search for the straight dust lane
signature of strong bars. Of the twelve Seyfert galaxies in our sample, only
three have dust lanes consistent with a strong nuclear bar. Therefore, strong
nuclear bars cannot be the primary fueling mechanism for Seyfert nuclei. We do
find that a majority of the galaxies show an spiral morphology in their dust
lanes. These spiral arms may be a possible fueling mechanism.Comment: To be published in the Astronomical Journal, June 1999. 25 pages and
14 figures. Full resolution figures are available at
ftp://www.ciw.edu/pub/mregan/fullfigs.tar.g
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