1,137 research outputs found
Multicolour imaging of z= 2 QSO hosts
We present multicolour images of the hosts of three z=2 QSOs previously
detected in R-band by our group. The luminosities, colours and sizes of the
hosts overlap with those of actively star-forming galaxies in the nearby
Universe. Radial profiles over the outer resolved areas roughly follow de
Vaucouleur or exponential disk laws. These properties give support to the host
galaxy interpretation of the extended light around QSOs at high-redshift. The
rest-frame UV colours and upper limits derived for the rest-frame UV-optical
colours are inconsistent with the hypothesis of a scattered halo of light from
the active nucleus by a simple optically-thin scattering process produced by
dust or hot electrons. If the UV light is indeed stellar, star formation rates
of hundreds of solar masses per year are implied, an order of magnitude larger
than field galaxies at similar redshifts and above. This might indicate that
the QSO phenomenon (at least the high-luminosity one) is preferentially
acompanied by enhanced galactic activity at high-redshifts.Comment: Accepted to be published in MNRAS. 11 pages, Latex, uses mn macros,
also available at http://www.mpa-garching.mpg.de/~itzia
Wide and deep near-UV (360nm) galaxy counts and the extragalactic background light with the Large Binocular Camera
Deep multicolour surveys are the main tool to explore the formation and
evolution of the faint galaxies which are beyond the spectroscopic limit with
the present technology. The photometric properties of these faint galaxies are
usually compared with current renditions of semianalytical models to provide
constraints on the fundamental physical processes involved in galaxy formation
and evolution, namely the mass assembly and the star formation. Galaxy counts
over large sky areas in the near-UV band are important because they are
difficult to obtain given the low efficiency of near-UV instrumentation, even
at 8m class telescopes. A large instrumental field of view helps in minimizing
the biases due to the cosmic variance. We have obtained deep images in the
360nm U band provided by the blue channel of the Large Binocular Camera at the
prime focus of the Large Binocular Telescope. We have derived over an area of
~0.4 sq. deg. the galaxy number counts down to U=27 in the Vega system
(corresponding to U=27.86 in the AB system) at a completeness level of 30%
reaching the faintest current limit for this wavelength and sky area. The shape
of the galaxy counts in the U band can be described by a double power-law, the
bright side being consistent with the shape of shallower surveys of comparable
or greater areas. The slope bends over significantly at U>23.5 ensuring the
convergence of the contribution by star forming galaxies to the EBL in the
near-UV band to a value which is more than 70% of the most recent upper limits
derived for this band. We have jointly compared our near-UV and K band counts
collected from the literature with few selected hierarchical CDM models
emphasizing critical issues in the physical description of the galaxy formation
and evolution.Comment: Accepted for publication in A&A. Uses aa.cls, 9 pages, 4 figures.
Citations update
Fundamental Parameters of He-Weak and He-Strong Stars
We carried out low resolution spectroscopic observations in the wavelength
range 3400-4700 A of 20 He-weak and 8 He-strong stars to determine their
fundamental parameters by means of the Divan-Chalonge-Barbier (BCD)
spectrophotometric system. For a few He-weak stars we also estimate the
effective temperatures and the angular diameters by integrating absolute fluxes
observed over a wide spectral range. Non-LTE model calculations are carried out
to study the influence of the He/H abundance ratio on the emergent radiation of
He-strong stars and on their Teff determination. We find that the effective
temperatures, surface gravities and bolometric absolute magnitudes of He-weak
stars estimated with the BCD system and the integrated flux method are in good
agreement between each other, and they also agree with previous determinations
based on several different methods. The mean discrepancy between the visual
absolute magnitudes derived using the Hipparcos parallaxes and the BCD values
is on average 0.3 mag for He-weak stars, while it is 0.5 mag for He-strong
stars. For He-strong stars, we note that the BCD calibration, based on stars in
the solar environment, leads to overestimated values of Teff. By means of model
atmosphere calculations with enhanced He/H abundance ratios we show that larger
He/H ratios produce smaller BD which naturally explains the Teff
overestimation. We take advantage of these calculations to introduce a method
to estimate the He/H abundance ratio in He-strong stars. The BD of HD 37479
suggests that the Teff of this star remains fairly constant as the star
spectrum undergoes changes in the intensity of H and He absorption lines. Data
for the He-strong star HD 66765 are reported for the first time.Comment: Accepted for publication in A&
The effect of stellar limb darkening values on the accuracy of the planet radii derived from photometric transit observations
We study how the precision of the exoplanet radius determination is affected
by our present knowledge of limb darkening in two cases: when we fix the limb
darkening coefficients and when we adjust them. We also investigate the effects
of spots in one-colour photometry. We study the effect of limb darkening on the
planetary radius determination both via analytical expressions and by numerical
experiments. We also compare some of the existing limb darkening tables. When
stellar spots affect the fit, we replace the limb darkening coefficients,
calculated for the unspotted cases, with effective limb darkening coefficients
to describe the effect of the spots. There are two important cases. (1) When
one fixes the limb darkening values according to some theoretical predictions,
the inconsistencies of the tables do not allow us to reach accuracy in the
planetary radius of better than 1-10% (depending on the impact parameter) if
the host star's surface effective temperature is higher than 5000 K. Below 5000
K the radius ratio determination may contain even 20% error. (2) When one
allows adjustment of the limb darkening coefficients, the a/Rs ratio, the
planet-to-stellar radius ratio, and the impact parameter can be determined with
sufficient accuracy (<1%), if the signal-to-noise ratio is high enough.
However, the presence of stellar spots and faculae can destroy the agreement
between the limb darkening tables and the fitted limb darkening coefficients,
but this does not affect the precision of the planet radius determination. We
also find that it is necessary to fit the contamination factor, too. We
conclude that the present inconsistencies of theoretical stellar limb darkening
tables suggests one should not fix the limb darkening coefficients. When one
allows them to be adjusted, then the planet radius, impact parameter, and the
a/Rs can be obtained with the required precision.Comment: Astronomy & Astrophysics Vol. 549, A9 (2013) - 11 page
XMM-Newton and INTEGRAL observations of the very faint X-ray transient IGRJ17285-2922/XTEJ1728-295 during the 2010 outburst
We report the first broad-band (0.5-150 keV) simultaneous X-ray observations
of the very faint X-ray transient IGRJ17285-2922/XTEJ1728-295 performed with
XMM-Newton and INTEGRAL satellites during its last outburst, started on 2010,
August 28. XMM-Newton observed the source on 2010 September 9-10, for 22ks.
INTEGRAL observations were part of the publicly available Galactic Bulge
program, and overlapped with the times covered by XMM-Newton. The broad-band
spectroscopy resulted in a best-fit with an absorbed power law displaying a
photon index of 1.61+/-0.01, an absorbing column density of (5.10+/-0.05)E21
cm-2, and a flux of 2.4E-10 erg/cm2/s (1-100 keV), corrected for the
absorption. The data did not require either a spectral cut-off (E>50 keV) or an
additional soft component. The slopes of the XMM-Newton and INTEGRAL separate
spectra were compatible, within the uncertainties. The timing analysis does not
show evidence either for X-ray pulsations or for type I X-ray bursts. The broad
band X-ray spectrum as well as the power density spectrum are indicative of a
low hard state in a low mass X-ray binary, although nothing conclusive can be
said about the nature of the compact object (neutron star or black hole). The
results we are reporting here allow us to conclude that IGRJ17285-2922 is a low
mass X-ray binary, located at a distance greater than 4 kpc.Comment: Accepted for publication in MNRAS; 7 pages, 6 figure, 1 table.
Accepted 2011 April 5. Received 2011 April 5; in original form 2011 February
2
A study on the multicolour evolution of Red Sequence galaxy populations: insights from hydrodynamical simulations and semi-analytical models
By means of our own cosmological-hydrodynamical simulation and
semi-analytical model we studied galaxy population properties in clusters and
groups, spanning over 10 different bands from UV to NIR, and their evolution
since redshift z=2. We compare our results in terms of galaxy red/blue
fractions and luminous-to-faint ratio (LFR) on the Red Sequence (RS) with
recent observational data reaching beyond z=1.5. Different selection criteria
were tested in order to retrieve galaxies belonging to the RS: either by their
quiescence degree measured from their specific SFR ("Dead Sequence"), or by
their position in a colour-colour plane which is also a function of sSFR. In
both cases, the colour cut and the limiting magnitude threshold were let
evolving with redshift, in order to follow the natural shift of the
characteristic luminosity in the LF.
We find that the Butcher-Oemler effect is wavelength-dependent, with the
fraction of blue galaxies increasing steeper in optical colours than in NIR.
Besides, only when applying a lower limit in terms of fixed absolute magnitude,
a steep BO effect can be reproduced, while the blue fraction results less
evolving when selecting samples by stellar mass or an evolving magnitude limit.
We then find that also the RS-LFR behaviour, highly debated in the literature,
is strongly dependent on the galaxy selection function: in particular its very
mild evolution recovered when measured in terms of stellar mass, is in
agreement with values reported for some of the highest redshift confirmed
(proto)clusters. As to differences through environments, we find that normal
groups and (to a lesser extent) cluster outskirts present the highest values of
both star forming fraction and LFR at low z, while fossil groups and cluster
cores the lowest: this separation among groups begins after z~0.5, while
earlier all group star forming properties are undistinguishable.Comment: revised version, A&A accepted (11 pages, 6 figures
Secular changes in the quiescence of WZ Sge: the development of a cavity in the inner disk
We find a dimming during optical quiescence of the cataclysmic variable WZ
Sge by about half a magnitude between superoutbursts. We connect the dimming
with the development of a cavity in the inner part of the accretion disk. We
suggest that, when the cavity is big enough, accretion of material is governed
by the magnetic field of the white dwarf and pulsations from the weakly
magnetic white dwarf appear. The time scale of forming the cavity is about a
decade, and it persists throughout the whole quiescent phase. Such a cavity can
be accommodated well by the proposed magnetic propeller model for WZ Sge, where
during quiescence mass is being expelled by the magnetic white dwarf from the
inner regions of the accretion disk to larger radii.Comment: 10 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics; following referee report, many textual changes, figures
improved, more historic data added, conclusions unchange
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