1,855 research outputs found
The Distribution and Cosmic Density of Relativistic Iron Lines in Active Galactic Nuclei
X-ray observations of several active galactic nuclei show prominent iron
K-shell fluorescence lines that are sculpted due to special and general
relativistic effects. These observations are important because they probe the
space-time geometry close to distant black holes. However, the intrinsic
distribution of Fe line strengths in the cosmos has never been determined. This
uncertainty has contributed to the controversy surrounding the relativistic
interpretation of the emission feature. Now, by making use of the latest
multi-wavelength data, we show theoretical predictions of the cosmic density of
relativistic Fe lines as a function of their equivalent width and line flux. We
are able to show unequivocally that the most common relativistic iron lines in
the universe will be produced by neutral iron fluorescence in Seyfert galaxies
and have equivalent widths < 100 eV. Thus, the handful of very intense lines
that have been discovered are just the bright end of a distribution of line
strengths. In addition to validating the current observations, the predicted
distributions can be used for planning future surveys of relativistic Fe lines.
Finally, the predicted sky density of equivalent widths indicate that the X-ray
source in AGNs can not, on average, lie on the axis of the black hole.Comment: 12 pages, 3 figures, accepted by ApJ Letter
Growing black holes and galaxies: black hole accretion versus star formation rate
We present a new suite of hydrodynamical simulations and use it to study, in
detail, black hole and galaxy properties. The high time, spatial and mass
resolution, and realistic orbits and mass ratios, down to 1:6 and 1:10, enable
us to meaningfully compare star formation rate (SFR) and BH accretion rate
(BHAR) timescales, temporal behaviour and relative magnitude. We find that (i)
BHAR and galaxy-wide SFR are typically temporally uncorrelated, and have
different variability timescales, except during the merger proper, lasting
~0.2-0.3 Gyr. BHAR and nuclear (<100 pc) SFR are better correlated, and their
variability are similar. Averaging over time, the merger phase leads typically
to an increase by a factor of a few in the BHAR/SFR ratio. (ii) BHAR and
nuclear SFR are intrinsically proportional, but the correlation lessens if the
long-term SFR is measured. (iii) Galaxies in the remnant phase are the ones
most likely to be selected as systems dominated by an active galactic nucleus
(AGN), because of the long time spent in this phase. (iv) The timescale over
which a given diagnostic probes the SFR has a profound impact on the recovered
correlations with BHAR, and on the interpretation of observational data.Comment: Accepted for publication in MNRA
Black hole accretion versus star formation rate: theory confronts observations
We use a suite of hydrodynamical simulations of galaxy mergers to compare
star formation rate (SFR) and black hole accretion rate (BHAR) for galaxies
before the interaction ('stochastic' phase), during the `merger' proper,
lasting ~0.2-0.3 Gyr, and in the `remnant' phase. We calculate the bi-variate
distribution of SFR and BHAR and define the regions in the SFR-BHAR plane that
the three phases occupy. No strong correlation between BHAR and galaxy-wide SFR
is found. A possible exception are galaxies with the highest SFR and the
highest BHAR. We also bin the data in the same way used in several
observational studies, by either measuring the mean SFR for AGN in different
luminosity bins, or the mean BHAR for galaxies in bins of SFR. We find that the
apparent contradiction or SFR versus BHAR for observed samples of AGN and star
forming galaxies is actually caused by binning effects. The two types of
samples use different projections of the full bi-variate distribution, and the
full information would lead to unambiguous interpretation. We also find that a
galaxy can be classified as AGN-dominated up to 1.5 Gyr after the merger-driven
starburst took place. Our study is consistent with the suggestion that most
low-luminosity AGN hosts do not show morphological disturbances.Comment: MNRAS Letters, in pres
A Thermal Wind Model for GRO J1655-40
Recent Chandra observations of an outflowing gas in GRO J1655-40 resulted in
a suggestion by Miller et al. (2006) that the wind in this system must be
powered by a magnetic process that can also drive accretion through the disk
around the black hole. The alternative explanations, of radiation pressure or
thermally driven flows, were considered unsatisfactory because of the highly
ionized level of the gas and because of the derived small distance from the
black hole, well inside the minimum distance required for an efficient X-ray
heated wind. The present paper shows that there is a simple photoionized wind
solution for this system where the gas is much further out than assumed by
Miller et al., at r/r_g = 10^(4.7-5.7). The expected wind velocity, as well as
the computed equivalent widths of more than 50 absorption lines in this
single-component 1D model, are all in good agreement with the Chandra
observations.Comment: 4 pages, 2 figures, accepted for publication in ApJ
Near Infrared Spectroscopy of High Redshift Active Galactic Nuclei. II. Disappearing Narrow Line Regions and the Role of Accretion
We present new near infrared spectroscopic measurements for 29 luminous
high-z quasars and use the data to discuss the size and other properties of the
NLRs in those sources. The high resolution spectra have been used to carefully
model the Fe II blends and to provide reliable [O III], Fe II and Hb
measurements. We find that about 2/3 of all high luminosity sources show strong
[O III] lines while the remaining objects show no or very weak such line. While
weak [O III] emitters are also found among lower luminosity AGN, we argue that
the implications for very high luminosity objects are different. In particular,
we suggest that the averaging of these two populations in other works gave rise
to claims of a Baldwin relationship in [O III] which is not confirmed by our
data. We also argue that earlier proposed relations of the type R_NLR \propto
L_[O III]^{1/2}, where R_NLR is the NLR radius, are theoretically sound yet
they must break down for R_NLR exceeding a few kpc. This suggests that the NLR
properties in luminous sources are different from those observed in nearby AGN.
In particular, we suggest that some sources lost their very large, dynamically
unbound NLR while others are in a phase of violent star-forming events that
produce a large quantity of high density gas in the central kpc. This gas is
ionized and excited by the central radiation source and its spectroscopic
properties may be different from those observed in nearby, lower luminosity
NLRs. We also discuss the dependence of EW(Hb) and Fe II/Hb on L, M_BH, and
accretion rate for a large sample of AGNs. The strongest dependence of the two
quantities is on the accretion rate and the Fe II/Hb correlation is probably
due to the EW(Hb) dependence on accretion rate. We show the most extreme values
measured so far of Fe II/Hb and address its correlation with EW([O III]).Comment: 10 pages (emulateapj), 9 figures. Accepted by Ap
Black-Hole Mass and Growth Rate at High Redshift
We present new H and K bands spectroscopy of 15 high luminosity active
galactic nuclei (AGNs) at redshifts 2.3-3.4 obtained on Gemini South. We
combined the data with spectra of additional 29 high-luminosity sources to
obtain a sample with 10^{45.2}<\lambda L_{\lambda}(5100A)<10^{47.3} ergs/sec
and black hole (BH) mass range, using reverberation mapping relationships based
on the H_beta method, of 10^{8.8}-10^{10.7} M_sun. We do not find a correlation
of L/L_Edd with M_BH but find a correlation with \lambda L_{\lambda}(5100A)
which might be due to selection effects. The L/L_Edd distribution is broad and
covers the range ~0.07-1.6, similar to what is observed in lower redshift,
lower luminosity AGNs. We suggest that this consistently measured and
calibrated sample gives the best representation of L/L_Edd at those redshifts
and note potential discrepancies with recent theoretical and observational
studies. The lower accretion rates are not in accord with growth scenarios for
BHs at such redshifts and the growth times of many of the sources are longer
than the age of the universe at the corresponding epochs. This suggests earlier
episodes of faster growth at z>~3 for those sources. The use of the C IV method
gives considerably different results and a larger scatter; this method seems to
be a poor M_BH and L/L_Edd estimator at very high luminosity.Comment: 8 pages (emulateapj), 4 figures. Accepted for publication in Ap
Locating Star-Forming Regions in Quasar Host Galaxies
We present a study of the morphology and intensity of star formation in the
host galaxies of eight Palomar-Green quasars using observations with the Hubble
Space Telescope. Our observations are motivated by recent evidence for a close
relationship between black hole growth and the stellar mass evolution in its
host galaxy. We use narrow-band [O II] 3727, H, [O III]
5007 and Pa images, taken with the WFPC2 and NICMOS
instruments, to map the morphology of line-emitting regions, and, after
extinction corrections, diagnose the excitation mechanism and infer
star-formation rates. Significant challenges in this type of work are the
separation of the quasar light from the stellar continuum and the
quasar-excited gas from the star-forming regions. To this end, we present a
novel technique for image decomposition and subtraction of quasar light. Our
primary result is the detection of extended line-emitting regions with sizes
ranging from 0.5 to 5 kpc and distributed symmetrically around the nucleus,
powered primarily by star formation. We determine star-formation rates of order
a few tens of M/yr. The host galaxies of our target quasars have
stellar masses of order M and specific star formation rates
on a par with those of M82 and luminous infrared galaxies. As such they fall at
the upper envelope or just above the star-formation mass sequence in the
specific star formation vs stellar mass diagram. We see a clear trend of
increasing star formation rate with quasar luminosity, reinforcing the link
between the growth of the stellar mass of the host and the black hole mass
found by other authors.Comment: Accepted for publication in M.N.R.A.
Mid-Infrared line diagnostics of Active Galaxies -- A spectroscopic AGN survey with ISO-SWS
We present medium resolution (R approx. 1500) ISO-SWS 2.4--45 micron spectra
of a sample of 29 galaxies with active nuclei. This data set is rich in fine
structure emission lines tracing the narrow line regions and (circum-)nuclear
star formation regions, and it provides a coherent spectroscopic reference for
future extragalactic studies in the mid-infrared. We use the data set to
briefly discuss the physical conditions in the narrow line regions (density,
temperature, excitation, line profiles) and to test for possible differences
between AGN sub-types. Our main focus is on new tools for determining the
propertibes of dusty galaxies and on the AGN-starburst connection. We present
mid-IR line ratio diagrams which can be used to identify composite (starburst +
AGN) sources and to distinguish between emission excited by active nuclei and
emission from (circum-nuclear) star forming regions. For instance, line ratios
of high to low excitation lines like [O IV]25.9um/[Ne II]12.8um, that have been
used to probe for AGNs in dusty objects, can be examined in more detail and
with better statistics now. In addition, we present two-dimensional diagnostic
diagrams that are fully analogous to classical optical diagnostic diagrams, but
better suited for objects with high extinction. Finally, we discuss
correlations of mid-infrared line fluxes to the mid- and far-infrared
continuum. We compare these relations to similar relations in starburst
galaxies in order to examine the contribution of AGNs to the bolometric
luminosities of their host galaxies. The spectra are available in electronic
form from the authors.Comment: 24 pages, 23 figures, 5 tables. Accepted for A&
Structural and vibrational properties of two-dimensional nanolayers on Pd(100)
Using different experimental techniques combined with density functional
based theoretical methods we have explored the formation of
interface-stabilized manganese oxide structures grown on Pd(100) at
(sub)monolayer coverage. Amongst the multitude of phases experimentally
observed we focus our attention on four structures which can be classified into
two distinct regimes, characterized by different building blocks. Two
oxygen-rich phases are described in terms of MnO(111)-like O-Mn-O trilayers,
whereas the other two have a lower oxygen content and are based on a
MnO(100)-like monolayer structure. The excellent agreement between calculated
and experimental scanning tunneling microscopy images and vibrational electron
energy loss spectra allows for a detailed atomic description of the explored
models.Comment: 14 pages, 11 figure
The outflow in Mrk 509: A method to calibrate XMM-Newton EPIC-pn and RGS
We have analyzed three XMM-Newton observations of the Seyfert 1 galaxy Mrk
509, with the goal to detect small variations in the ionized outflow
properties. Such measurements are limited by the quality of the
cross-calibration between RGS, the best instrument to characterize the
spectrum, and EPIC-pn, the best instrument to characterize the variability. For
all three observations we are able to improve the relative calibration of RGS
and pn consistently to 4 %. In all observations we detect three different
outflow components and, thanks to our accurate cross-calibration we are able to
detect small differences in the ionization parameter and column density in the
highest ionized component of the outflow. This constrains the location of this
component of the outflow to within 0.5 pc of the central source. Our method for
modeling the relative effective area is not restricted to just this source and
can in principle be extended to other types of sources as well.Comment: 11 pages, 9 figure
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