377 research outputs found
Blue colours of BL Lac host galaxies
Near-infrared and optical imaging of BL Lac host galaxies is used to
investigate their colour properties. We find that the R-H colour and colour
gradient distributions of the BL Lac hosts are much wider than those for normal
ellipticals, and many objects have very blue hosts and/or steep colour
gradients. The blue colours are most likely caused by recent star formation.
The lack of obvious signs of interaction may, however, require a significant
time delay between the interaction event with associated star formation
episodes and the onset of the nuclear activity.Comment: 2 pages, 1 figure, to appear in the proceedings of "The Interplay
among Black Holes, Stars and ISM in Galactic Nuclei", IAU 222, eds. T.
Storchi Bergmann, L.C. Ho, and H.R. Schmit
Low redshift quasars in the SDSS Stripe 82. The local environments
We study the environments of low redshift (z < 0.5) quasars based on a large
and homogeneous dataset from the Stripe 82 region of the Sloan Digital Sky
Survey (SDSS). We have compared the < 1 Mpc scale envi- ronments of 302 quasars
that were resolved in our recent study to those of 288 inactive galaxies with
closely matched redshifts. Crucially, the lu- minosities of the inactive
galaxies and the quasar host galaxies are also closely matched, unlike in most
previous studies. The environmental overdensities were studied by measuring the
num- ber density of galaxies within a projected distance of 200 kpc to 1 Mpc.
The galaxy number density of the quasar environments is comparable to that of
the inactive galaxies with similar luminosities, both classes of ob- jects
showing significant excess compared to the background galaxy density for
distances < 400 kpc. There is no significant dependence of the galaxy number
density on redshift, quasar or host galaxy luminosity, black hole mass or radio
loudness. This suggests that the fueling and triggering of the nuclear activity
is only weakly dependent on the local environment of quasars, and the quasar
phase may be a short-lived common phase in the life cycle of all massive
galaxies.Comment: Accepted for publication in MNRAS, 16 page
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
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