2,567 research outputs found
Early star formation traced by the highest redshift quasars
The iron abundance relative to alpha-elements in the circumnuclear region of
quasars is regarded as a clock of the star formation history and, more
specifically, of the enrichment by SNIa. We investigate the iron abundance in a
sample of 22 quasars in the redshift range 3.0<z<6.4 by measuring their rest
frame UV FeII bump, which is shifted into the near-IR, and by comparing it with
the MgII 2798 flux. The observations were performed with a device that can
obtain near-IR spectra in the range 0.8-2.4 um in one shot, thereby enabling an
optimal removal of the continuum underlying the FeII bump. We detect iron in
all quasars including the highest redshift (z=6.4) quasar currently known. The
uniform observational technique and the wide redshift range allows a reliable
study of the trend of the FeII/MgII ratio with redshift. We find the FeII/MgII
ratio is nearly constant at all redshifts, although there is marginal evidence
for a higher FeII/MgII ratio in the quasars at z~6. If the FeII/MgII ratio
reflects the Fe/alpha abundance, this result suggests that the z~6 quasars have
already undergone a major episode of iron enrichment. We discuss the possible
implications of this finding for the star formation history at z>6. We also
detect a population of weak iron emitters at z~4.5, which are possibly hosted
in systems that evolved more slowly. Alternatively, the trend of the FeII/MgII
ratio at high redshift may reflect significantly different physical conditions
of the circumnuclear gas in such high redshift quasars.Comment: Replaced to match the accepted version (ApJL in press), 5 page
Stellar and gaseous abundances in M82
The near infrared (IR) absorption spectra of starburst galaxies show several
atomic and molecular lines from red supergiants which can be used to infer
reliable stellar abundances. The metals locked in stars give a picture of the
galaxy metallicity prior to the last burst of star formation. The enrichment of
the new generation of stars born in the last burst can be traced by measuring
the hot gas in the X-rays. For the first time detailed stellar abundances in
the nuclear region of the starburst galaxy M82 have been obtained. They are
compared with those of the hot gas as derived from an accurate re-analysis of
the XMM and Chandra nuclear X-ray spectra. The cool stars and the hot gas
suggest [Fe/H]=-0.35+/-0.2 dex, and an overall [Si,Mg/Fe] enhancement by 0.4
and 0.5 dex, respectively. This is consistent with a major chemical enrichment
by SNe II explosions in recursive bursts on short timescales. Oxygen is more
puzzling to interpret since it is enhanced by 0.3 dex in stars and depleted by
0.2 dex in the hot gas. None of the standard enrichment scenarios can fully
explain such a behavior when compared with the other alpha-elements.Comment: APJ, in pres
The Nuclear Stellar Cluster in the Seyfert~1 Galaxy NGC 3227: High Angular Resolution NIR Imaging and Spectroscopy
NIR high angular resolution speckle imaging and imaging spectroscopy of the
nuclear region (10'' ~ 840pc) of the Seyfert1 galaxy NGC3227 are presented. A
nuclear stellar cluster is slightly resolved in the J and H band with
increasing contribution to the NIR continuum from the K to the J band. The
stellar absorption lines are extended compared to the neighboring continuum
suggesting a cluster size of ~ 70pc FWHM. Analysis of those lines suggests that
the stars are contributing about 65% (40%) of the total continuum emission in
the H (K) band in a 3.6'' aperture. Population synthesis in conjunction with
NIR spectral synthesis indicates an age of 25 to 50 Myr when red supergiants
contribute most to the NIR light. This is supported by published optical data
on the MgIb line and the CaII triplet. Although a higher age of ~ 0.5 Gyr where
AGB stars dominate the NIR light can not be excluded, the observed parameters
are at the limit of those expected for a cluster dominated by AGB stars.
However, in either case the resolved stellar cluster contributes only about ~
15 % of the total dynamical mass in the inner 300pc implying another much older
stellar population. Pure constant star formation over the last 10 Gyr can be
excluded. Therefore, at least two star formation/starburst events took place in
the nucleus of NGC3227. Since such sequences in the nuclear star formation
history are also observed in the nuclei of other galaxies a link between the
activity of the star formation and the AGN itself seems likely.Comment: accepted for publication in the Astrophysical Journal, 46 pages, 15
figure
High precision radial velocities with GIANO spectra
Radial velocities (RV) measured from near-infrared (NIR) spectra are a
potentially excellent tool to search for extrasolar planets around cool or
active stars. High resolution infrared (IR) spectrographs now available are
reaching the high precision of visible instruments, with a constant improvement
over time. GIANO is an infrared echelle spectrograph at the Telescopio
Nazionale Galileo (TNG) and it is a powerful tool to provide high resolution
spectra for accurate RV measurements of exoplanets and for chemical and
dynamical studies of stellar or extragalactic objects. No other high spectral
resolution IR instrument has GIANO's capability to cover the entire NIR
wavelength range (0.95-2.45 micron) in a single exposure. In this paper we
describe the ensemble of procedures that we have developed to measure high
precision RVs on GIANO spectra acquired during the Science Verification (SV)
run, using the telluric lines as wavelength reference. We used the Cross
Correlation Function (CCF) method to determine the velocity for both the star
and the telluric lines. For this purpose, we constructed two suitable digital
masks that include about 2000 stellar lines, and a similar number of telluric
lines. The method is applied to various targets with different spectral type,
from K2V to M8 stars. We reached different precisions mainly depending on the H
-magnitudes: for H ~ 5 we obtain an rms scatter of ~ 10 m s-1, while for H ~ 9
the standard deviation increases to ~ 50 - 80 m s-1. The corresponding
theoretical error expectations are ~4 m s-1 and 30 m s-1, respectively. Finally
we provide the RVs measured with our procedure for the targets observed during
GIANO Science Verification.Comment: 26 pages, 15 figures, 6 table
Stellar Dynamics and the implications on the merger evolution in NGC6240
We report near-infrared integral field spectroscopy of the luminous merging
galaxy NGC 6240. Stellar velocities show that the two K-band peaks separated by
1.6arcsec are the central parts of inclined, rotating disk galaxies with equal
mass bulges. The dynamical masses of the nuclei are much larger than the
stellar mass derived from the K-band light, implying that the progenitor
galaxies were galaxies with massive bulges. The K-band light is dominated by
red supergiants formed in the two nuclei in starbursts, triggered ~2x10^7 years
ago, possibly by the most recent perigalactic approach. Strong feedback effects
of a superwind and supernovae are responsible for a short duration burst
(~5x10^6 years) which is already decaying. The two galaxies form a
prograde-retrograde rotating system and from the stellar velocity field it
seems that one of the two interacting galaxies is subject to a prograde
encounter. Between the stellar nuclei is a prominent peak of molecular gas
(H_2, CO). The stellar velocity dispersion peaks there indicating that the gas
has formed a local, self-gravitating concentration decoupled from the stellar
gravitational potential. NGC 6240 has previously been reported to fit the
paradigm of an elliptical galaxy formed through the merger of two galaxies.
This was based on the near-infrared light distribution which follows a
r^1/4-law. Our data cast strong doubt on this conclusion: the system is by far
not relaxed, rotation plays an important role, as does self-gravitating gas,
and the near-infrared light is dominated by young stars.Comment: 34 pages, 11 figures, using AASTEX 5.0rc3.1, paper submitted to the
Astrophysical Journal, revised versio
Momentum Distribution for Bosons with Positive Scattering Length in a Trap
The coordinate-momentum double distribution function is calculated in the local density approximation for bosons with
positive scattering length in a trap. The calculation is valid to the first
order of . To clarify the meaning of the result, it is compared for a
special case with the double distribution function of
Wigner.Comment: Latex fil
A Close Look at Star Formation around Active Galactic Nuclei
We analyse star formation in the nuclei of 9 Seyfert galaxies at spatial
resolutions down to 0.085arcsec, corresponding to length scales of less than
10pc in some objects. Our data were taken mostly with the near infrared
adaptive optics integral field spectrograph SINFONI. The stellar light profiles
typically have size scales of a few tens of parsecs. In two cases there is
unambiguous kinematic evidence for stellar disks on these scales. In the
nuclear regions there appear to have been recent - but no longer active -
starbursts in the last 10-300Myr. The stellar luminosity is less than a few
percent of the AGN in the central 10pc, whereas on kiloparsec scales the
luminosities are comparable. The surface stellar luminosity density follows a
similar trend in all the objects, increasing steadily at smaller radii up to
10^{13}L_sun/kpc^2 in the central few parsecs, where the mass surface density
exceeds 10^4M_sun/pc^2. The intense starbursts were probably Eddington limited
and hence inevitably short-lived, implying that the starbursts occur in
multiple short bursts. The data hint at a delay of 50--100Myr between the onset
of star formation and subsequent fuelling of the black hole. We discuss whether
this may be a consequence of the role that stellar ejecta could play in
fuelling the black hole. While a significant mass is ejected by OB winds and
supernovae, their high velocity means that very little of it can be accreted.
On the other hand winds from AGB stars ultimately dominate the total mass loss,
and they can also be accreted very efficiently because of their slow speeds.Comment: 51 pages, including 27 figures; accepted by ApJ (paper reorganised,
but results & conclusions the same
Fluctuation theorem for constrained equilibrium systems
We discuss the fluctuation properties of equilibrium chaotic systems with
constraints such as iso-kinetic and Nos\'e-Hoover thermostats. Although the
dynamics of these systems does not typically preserve phase-space volumes, the
average phase-space contraction rate vanishes, so that the stationary states
are smooth. Nevertheless finite-time averages of the phase-space contraction
rate have non-trivial fluctuations which we show satisfy a simple version of
the Gallavotti-Cohen fluctuation theorem, complementary to the usual
fluctuation theorem for non-equilibrium stationary states, and appropriate to
constrained equilibrium states. Moreover we show these fluctuations are
distributed according to a Gaussian curve for long-enough times. Three
different systems are considered here, namely (i) a fluid composed of particles
interacting with Lennard-Jones potentials; (ii) a harmonic oscillator with
Nos\'e-Hoover thermostatting; (iii) a simple hyperbolic two-dimensional map.Comment: To appear in Phys. Rev.
The Nature of the Optical Light in Seyfert 2 Galaxies with Polarized Continuum
We investigate the nature of the optical continuum and stellar population in
the central kpc of the Seyfert 2s Mrk 348, Mrk 573, NGC 1358 and Mrk 1210 using
long-slit spectra obtained along the radio or extended emission axis. These
galaxies are known to have polarized continuum-including polarized broad lines
in Mrk 348 and Mrk 1210--and previous studies indicate featureless continuum
(FC) contributions in the 20-50% range at 5500 A. Nevertheless, our
measurements of the equivalent widths of absorption lines and continuum ratios
as a function of distance from the nuclei show no dilution of the lines nor
bluening of the spectrum, as expected if a blue FC was present at the nucleus
in the above proportions. We investigate one possibility to account for this
effect: that the stellar population at the nucleus is the same as that from the
surrounding bulge and dominates the nuclear light. A spectral analysis confirms
that this hypothesis works for Mrk 348, NGC 1358 and Mrk 1210, for which we
find stellar contributions at the nucleus larger than 90% at all wavelengths.
We find that a larger stellar population contribution to the nuclear spectra
can play the role of the ``second FC'' source inferred from previous studies.
Stellar population synthesis shows that the nuclear regions of Mrk 348 and Mrk
1210 have important contributions of young to intermediate age stars (0--100
Myr), not present in templates of elliptical galaxies. In the case of Mrk 1210,
this is further confirmed by the detection of a ``Wolf-Rayet feature'' in the
nuclear emission-line spectrum.Comment: ApJ, accepted. Uses aaspp4.sty. [22 pages
The complexity of parsec-scaled dusty tori in AGN
Warm gas and dust surround the innermost regions of active galactic nuclei
(AGN). They provide the material for accretion onto the super-massive black
hole and they are held responsible for the orientation-dependent obscuration of
the central engine. The AGN-heated dust distributions turn out to be very
compact with sizes on scales of about a parsec in the mid-infrared. Only
infrared interferometry currently provides the necessary angular resolution to
directly study the physical properties of this dust. Size estimates for the
dust distributions derived from interferometric observations can be used to
construct a size--luminosity relation for the dust distributions. The large
scatter about this relation suggests significant differences between the dust
tori in the individual galaxies, even for nuclei of the same class of objects
and with similar luminosities. This questions the simple picture of the same
dusty doughnut in all AGN. The Circinus galaxy is the closest Seyfert 2 galaxy.
Because its mid-infrared emission is well resolved interferometrically, it is a
prime target for detailed studies of its nuclear dust distribution. An
extensive new interferometric data set was obtained for this galaxy. It shows
that the dust emission comes from a very dense, disk-like structure which is
surrounded by a geometrically thick, similarly warm dust distribution as well
as significant amounts of warm dust within the ionisation cone.Comment: 8 pages, 3 figures, to appear in the proceedings of the conference
"The central kiloparsec in Galactic Nuclei: Astronomy at High Angular
Resolution 2011", open access Journal of Physics: Conference Series (JPCS),
published by IOP Publishin
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