3,649 research outputs found
The Relationship of Hard X-ray and Optical Line Emission in Low Redshift Active Galactic Nuclei
In this paper we assess the relationship of the population of Active Galactic
Nuclei (AGN) selected by hard X-rays to the traditional population of AGN with
strong optical emission lines. First, we study the emission-line properties of
a new hard X-ray selected sample of 47 local AGN (classified optically as both
Type 1 and Type 2 AGN). We find that the hard X- ray (3-20 keV) and
[OIII]5007 optical emission-line luminosities are well-correlated over
a range of about four orders-of-magnitude in luminosity (mean luminosity ratio
2.15 dex with a standard deviation of = 0.51 dex). Second, we study
the hard X-ray properties of a sample of 55 local AGN selected from the
literature on the basis of the flux in the [OIII] line. The correlation between
the hard X-ray (2-10 keV) and [OIII] luminosity for the Type 1 AGN is
consistent with what is seen in the hard X-ray selected sample. However, the
Type 2 AGN have a much larger range in the luminosity ratio, and many are very
weak in hard X-rays (as expected for heavily absorbed AGN). We then compare the
hard X-ray (3-20 keV) and [OIII] luminosity functions of AGN in the local
universe. These have similar faint-end slopes with a luminosity ratio of 1.60
dex (0.55 dex smaller than the mean value for individual hard X-ray selected
AGN). We conclude that at low redshift, selection by narrow optical emission-
lines will recover most AGN selected by hard X-rays (with the exception of BL
Lac objects). However, selection by hard X-rays misses a significant fraction
of the local AGN population with strong emission lines
The Excess Far-Infrared Emission of AGN in the Local Universe
We have cross-correlated the Sloan Digital Sky Survey (SDSS) second data
release spectroscopic galaxy sample with the IRAS faint-source catalogue (FSC).
Optical emission line ratios are used to classify the galaxies with reliable
IRAS 60 and 100 microns detections into AGN and normal star-forming galaxies.
We then create subsamples of normal galaxies and AGN that are very closely
matched in terms of key physical properties such as stellar mass, redshift,
size, concentration and mean stellar age (as measured by absorption line
indicators in the SDSS spectra). We then quantify whether there are systematic
differences between the IR luminosities of the galaxies and the AGN in the
matched subsamples. We find that the AGN exhibit a significant excess in far-IR
emission relative to the star-forming galaxies in our sample. The excesses at
60 and 100 microns are 0.21 +/- 0.03 dex and 0.12 +/- 0.035 dex in
log[L(60)/M*] and log[L(100)/M*], respectively. We then discuss whether the
far-IR excess is produced by radiation from the active nucleus that is absorbed
by dust or alternatively, by an extra population of young stars that is not
detectable at optical wavelengths.Comment: 12 pages, 14 figures, accepted by MNRA
Detectability of High Redshift Ellipticals in the Hubble Deep Field
Relatively few intensively star-forming galaxies at redshifts z>2.5 have been
found in the Hubble Deep Field (HDF). This has been interpreted to imply a low
space density of elliptical galaxies at high z, possibly due to a late (z<2.5)
epoch of formation, or to dust obscuration of the ellipticals that are forming
at z~3. I use HST UV (2300 Ang) images of 25 local early-type galaxies to
investigate a third option, that ellipticals formed at z>4.5, and were fading
passively by 2<z<4.5. Present-day early-types are faint and centrally
concentrated in the UV. If ellipticals formed their stars in a short burst at
z>4.5, and have faded passively to their present brightnesses at UV
wavelengths, they would generally be below the HDF detection limits in any of
its bands at z>2.5. Quiescent z ~ 3 ellipticals, if they exist, should turn up
in sufficiently deep IR images.Comment: AAS LaTex, 11 pages, 1 table, 1 figure, some corrections and
clarifications, accepted for publication in ApJ
The Ages of Elliptical Galaxies in a Merger Model
The tightness of the observed colour-magnitude and Mg- velocity
dispersion relations for elliptical galaxies has often been cited as an
argument against a picture in which ellipticals form by the merging of spiral
disks. A common view is that merging would mix together stars of disparate ages
and produce a large scatter in these relations. Here I use semi-analytic models
of galaxy formation to derive the distribution of the mean ages, colours and
metallicities of the stars in elliptical galaxies formed by mergers in a flat
CDM universe. It is seen that most of the stars in ellipticals form at
relatively high redshift (z > 1.9) and that the predicted scatter in the
colour-magnitude and Mg_2 - sigma relations falls within observational bounds.
I conclude that the apparent homogeneity in the properties of the stellar
populations of ellipticals is not inconsistent with a merger scenario for the
origin of these systems.Comment: latex file, figures available upon reques
The Effect of Noise on the Dust Temperature - Spectral Index Correlation
We investigate how uncertainties in flux measurements affect the results from
modified blackbody SED fits. We show that an inverse correlation between the
dust temperature T and spectral index (beta) naturally arises from least
squares fits due to the uncertainties, even for sources with a single T and
beta. Fitting SEDs to noisy fluxes solely in the Rayleigh-Jeans regime produces
unreliable T and beta estimates. Thus, for long wavelength observations (lambda
>~ 200 micron), or for warm sources (T >~ 60 K), it becomes difficult to
distinguish sources with different temperatures. We assess the role of noise in
recent observational results that indicate an inverse and continuously varying
T - beta relation. Though an inverse and continuous T - beta correlation may be
a physical property of dust in the ISM, we find that the observed inverse
correlation may be primarily due to noise.Comment: 14 pages, including 5 Figures; Accepted for publication in Ap
Spectral and morphological properties of quasar hosts in SPH simulations of AGN feeding by mergers
We present a method for generating virtual observations from
smoothed-particle-hydrodynamics (SPH) simulations. This method includes stellar
population synthesis models and the reprocessing of starlight by dust to
produce realistic galaxy images. We apply this method and simulate the merging
of two identical giant Sa galaxies. The merger remnant is an elliptical galaxy.
The merger concentrates the gas content of the two galaxies into the nuclear
region. The gas that flows into the nuclear region refuels the central black
holes of the merging galaxies. We follow the refuelling of the black holes
during the merger semi-analytically.
In the simulation presented in this article, the black holes grow from 3 x
10^7 to 1.8X 10^8 Solar masses, with a peak AGN luminosity of M_B ~ -23.7. We
study how the morphological and spectral properties of the system evolve during
the merger and work out the predictions of this scenario for the properties of
host galaxies during the active phase. The peak of AGN activity coincides with
the merging of the two galactic nuclei and occurs at a stage when the remnant
looks like a lenticular galaxy. The simulation predicts the formation of a
circumnuclear starburst ring/dusty torus with an opening angle of 30-40 degrees
and made of clouds with n_H=10^24 cm^-2. The average optical depth of the torus
is quite high, but the obscuring medium is patchy, so that there still exist
lines of sight where the AGN is visible in a nearly edge-on view. For the same
reason, there are lines of sight where the AGN is completely obscured in the
face-on view.Comment: 14 pages, 11 figure
Clustering of Galaxies in a Hierarchical Universe: I. Methods and Results at z=0
We introduce a new technique for following the formation and evolution of
galaxies in cosmological N-body simulations. Dissipationless simulations are
used to track the formation and merging of dark matter halos as a function of
redshift. Simple prescriptions, taken directly from semi-analytic models of
galaxy formation, are adopted for cooling, star formation, supernova feedback
and the merging of galaxies within the halos. This scheme enables us to study
the clustering properties of galaxies and to investigate how selection by type,
colour or luminosity influences the results. In this paper, we study properties
of the galaxy distribution at z=0. These include luminosity functions, colours,
correlation functions, pairwise peculiar velocities, cluster M/L ratios and
star formation rates. We focus on two variants of a CDM cosmology: a high-
density model with Gamma=0.21 (TCDM) and a low-density model with Omega=0.3 and
Lambda=0.7 (LCDM). Both are normalized to reproduce the I-band Tully-Fisher
relation near a circular velocity of 220 km/s. Our results depend strongly both
on this normalization and on the prescriptions for star formation and feedback.
Very different assumptions are required to obtain an acceptable model in the
two cases. For TCDM, efficient feedback is required to suppress the growth of
galaxies low-mass field halos. Without it, there are too many galaxies and the
correlation function turns over below 1 Mpc. For LCDM, feedback must be weak,
otherwise too few L* galaxies are produced and the correlation function is too
steep. Given the uncertainties in modelling some of the key physical processes,
we conclude that it is not yet possible to draw conclusions about the values of
cosmological parameters from studies of this kind. Further work on global star
formation and feedback effects is required to narrow the range of possibilitiesComment: 43 pages, Latex, 16 figures included, 2 additional GIF format
figures, submitted to MNRA
Sterile neutrinos as subdominant warm dark matter
In light of recent findings which seem to disfavor a scenario with (warm)
dark matter entirely constituted of sterile neutrinos produced via the
Dodelson-Widrow (DW) mechanism, we investigate the constraints attainable for
this mechanism by relaxing the usual hypothesis that the relic neutrino
abundance must necessarily account for all of the dark matter. We first study
how to reinterpret the limits attainable from X-ray non-detection and
Lyman-alpha forest measurements in the case that sterile neutrinos constitute
only a fraction fs of the total amount of dark matter. Then, assuming that
sterile neutrinos are generated in the early universe solely through the DW
mechanism, we show how the X-ray and Lyman-alpha results jointly constrain the
mass-mixing parameters governing their production. Furthermore, we show how the
same data allow us to set a robust upper limit fs < 0.7 at the 2 sigma level,
rejecting the case of dominant dark matter (fs = 1) at the ~ 3 sigma level.Comment: Minor changes; added references; version accepted for publication in
Phys. Rev.
Perspectives for Future Research on European Urban Development: Workshop im Rahmen eines deutsch-russischen Kooperationsprojekts
Das im September 2012 auf Initiative der damaligen Abteilung Stadtökonomik gestartete Projekt zur Anbahnung gemeinsamer Forschungsarbeiten des Leontief Centre St. Petersburg (LC) und des IWH zu Fra-gen der wirtschaftlichen Neupositionierung post-sozialistischer StĂ€dte fand am 25. und 26. Februar 2015 mit einem Workshop in Halle (Saale) seinen vorlĂ€ufigen Abschluss. Neben den beteiligten Mitarbeiterinnen und Mitarbeitern des LC und des IWH nahmen auch Wissenschaftlerinnen und Wissenschaftler anderer Forschungseinrichtungen teil, deren Forschung BezĂŒge zur Fragestellung des vom Bundesministerium fĂŒr Bildung und Forschung (BMBF) geförderten Projekts aufweist. In den vorangehenden zweieinhalb Jahren gab es einen inten-siven Austausch zwischen beiden Instituten u. a. in Form von Gastaufenthalten, gegenseitiger Hilfe bei der Datenbeschaffung und Âaufbereitung und gemeinsamen Auftritten bei wissenschaftlichen Veranstaltungen. Im Rahmen des Abschlussworkshops wurden Ergebnisse der gemeinsamen Arbeit wie auch thematisch verwandter Untersuchungen vorgetragen und diskutiert
A possible observational bias in the estimation of the virial parameter in virialized clumps
The dynamics of massive clumps, the environment where massive stars
originate, is still unclear. Many theories predict that these regions are in a
state of near-virial equilibrium, or near energy equi-partition, while others
predict that clumps are in a sub-virial state. Observationally, the majority of
the massive clumps are in a sub-virial state with a clear anti-correlation
between the virial parameter and the mass of the clumps ,
which suggests that the more massive objects are also the more gravitationally
bound. Although this trend is observed at all scales, from massive clouds down
to star-forming cores, theories do not predict it. In this work we show how,
starting from virialized clumps, an observational bias is introduced in the
specific case where the kinetic and the gravitational energies are estimated in
different volumes within clumps and how it can contribute to the spurious
anti-correlation in these data. As a result, the observed
effective virial parameter , and in some
circumstances it might not be representative of the virial state of the
observed clumps.Comment: A&A letter, accepte
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