53 research outputs found
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
On quasar host galaxies as tests of non-cosmological redshifts
Despite a general consensus in the astronomical community that all quasars
are located at the distances implied by their redshifts, a number of
observations still challenge this interpretation, possibly indicating that some
subpopulation of quasars may harbour significant redshift components not
related to the expansion of the universe. It has been suggested that these
objects may have been ejected from local galaxies and are likely to evolve into
new galaxies themselves. Here, a test of such exotic scenarios is proposed,
based on the spectral energy distribution of the galaxies hosting quasars with
suspected ejection origin. Provided that the time scales over which the ejected
objects manifest themselves as quasars is short, one would in the framework of
ejection scenarios expect to find either no quasar host galaxy, a pseudo-host
consisting of gas ionized by the quasar, or a host galaxy consisting of young
stars only. It is argued that the spectral energy distributions corresponding
to the latter two options should differ significantly from those of most quasar
host galaxies detected at low redshift so far, thus providing a potential test
of the claimed existence of ejected quasars. A minimal implementation of this
test, involving optical and near-IR broadband photometry, is suggested.Comment: 9 pages, 4 figure
The stellar content of low redshift radio galaxies from near-infrared spectroscopy
We present medium spectral resolution near-infrared (NIR) HK-band spectra for
8 low redshift (z<0.06) radio galaxies to study the NIR stellar properties of
their host galaxies. As a homogeneous comparison sample, we used 9 inactive
elliptical galaxies that were observed with similar resolution and wavelength
range. The aim of the study is to compare the NIR spectral properties of radio
galaxies to those of inactive early-type galaxies and, furthermore, produce the
first NIR HK-band spectra for low redshift radio galaxies. For both samples
spectral indices of several diagnostic absorption features, SiI(1.589microns),
CO(1.619microns), NaI(2.207microns), CaI(2.263microns), CO(>2.29microns), were
measured. To characterize the age of the populations, the measured EWs of the
absorption features were fitted with the corresponding theoretical evolutionary
curves of the EWs calculated by the stellar synthesis model. On average, EW(CO
2.29) of radio galaxies is somewhat greater than that of inactive ellipticals.
Most likely, EW(CO 2.29) is not significantly affected by dilution, and thus
indicating that elliptical galaxies containing AGN are in a different stage in
their evolution than inactive ellipticals. This is also supported by comparing
other NIR features, such as CaI and NaI, with each other. Absorption features
are consistent with the intermediate age stellar population, suggesting that
host galaxies contain both an old and intermediate age components. It is
consistent with previous optical spectroscopy studies which have shown evidence
on the intermediate age (~2 Gyr) stellar population of radio galaxies, and also
in some of the early-type galaxies. The existence of intermediate age
population is a link between the star formation episode, possibly induced by
interaction or merging event, and the triggering of the nuclear activity.Comment: 10 pages, 7 figure
High-redshift quasar host galaxies with adaptive optics
We present K band adaptive optics observations of three high-redshift (z ~
2.2) high-luminosity quasars, all of which were studied for the first time. We
also bserved several point spread function (PSF) calibrators,
non-simultaneously because of the small field of view. The significant temporal
PSF variations on timescales of minutes inhibited a straightforward scaled PSF
removal from the quasar images. Characterising the degree of PSF concentration
by the radii encircling 20% and 80% of the total flux, respectively, we found
that even under very different observing conditions the r_20 vs. r_80 relation
varied coherently between individual short exposure images, delineating a
well-defined relation for point sources. Placing the quasar images on this
relation, we see indications that all three objects were resolved. We designed
a procedure to estimate the significance of this result, and to estimate host
galaxy parameters, by reproducing the statistical distribution of the
individual short exposure images. We find in all three cases evidence for a
luminous host galaxy, with a mean absolute magnitude of M_R = -27.0 and scale
lengths around ~ 4-12 kpc. Together with a rough estimate of the central black
hole masses obtained from C_iv line widths, the location of the objects on the
bulge luminosity vs. black hole mass relation is not significantly dfferent
from the low-redshift regime, assuming only passive evolution of the host
galaxy. Corresponding Eddington luminosities are L_nuc/L_Edd ~ 0.1-0.6.Comment: 17 pages, 16 figures; submitted to Astronomy&Astrophysics. v2:
Absolute magnitudes corrected and as a consequence updated two figures, black
hole masses and stellar population age estimates in Discussion, but main
conclusions remain the same; accepted by A&
Shape Abnormalities of the Caudate Nucleus Correlate with Poorer Gait and Balance: Results from a Subset of the LADIS Study
Functional deficits seen in several neurodegenerative disorders have been linked with dysfunction in fronto-striatal circuits and with associated shape alterations in striatal structures. The severity of visible white matter changes (WMC) on MRI has been found to correlate with poorer performance on measures of gait and balance. This study aimed to determine whether striatal volume and shape changes were correlated with gait dysfunction
Quasar host galaxies at intermediate and high redshifts
Quasars form one of the most energetic phenomena in the universe, and can be traced out to very large redshifts. By studying the galaxies which host the active nuclei, important insights can be gained into the processes that trigger and maintain the quasar powerhouse. The evolution rate of the quasar population is furthermore similar to that of ordinary galaxies, which implies a connection between black hole accretion and star formation in the host galaxies. While the properties of quasar host galaxies at low redshift have become better constrained in recent years, less is known about hosts at earlier cosmic epochs. In addition, though radio-quiet quasars are by far more common than their radio-loud counterparts their host galaxies have not been studied to the same extent, in particular not at higher redshifts. An imaging campaign of a large sample of quasars at intermediate redshift (0.4 < z < 0.8) was carried out at optical wavelengths using the Nordic Optical Telescope, and is studied in this thesis together with two smaller samples. The joint material forms more than half of the total number of observed sources in this redshift interval and increases the number of resolved radio-quiet hosts at z>0.4 considerably. The morphology and mean magnitudes are found to be similar for radio-loud and radio-quiet host galaxies. Both types of host are shown to have optical colours as blue as those of present-day late-type spirals and starburst galaxies, which is likely the result of ongoing star formation. With increasing redshift, observations of host galaxies become more difficult. High spatial resolution can be achieved with adaptive optics, but the variation of the point spread function in the near-infrared wavelength band which is most suited for detection is large and rapid. A statistical approach to the problem of characterizing the point spread function has been developed, making use of simulated objects which are matched to the different atmospheric conditions. Bright, compact host galaxies showing signs of merging and interaction were detected in this way for three quasars at z~2.2, which were observed with the ESO 3.6 m telescope. The method is not restricted to host galaxy analysis but can be utilized in other applications as well, provided that the underlying extended source can be described by an analytical model
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