476 research outputs found
Disentangling AGN and Star Formation in Soft X-rays
We have explored the interplay of star formation and AGN activity in soft
X-rays (0.5-2 keV) in two samples of Seyfert 2 galaxies (Sy2s). Using a
combination of low resolution CCD spectra from Chandra and XMM-Newton, we
modeled the soft emission of 34 Sy2s using power law and thermal models. For
the 11 sources with high signal-to-noise Chandra imaging of the diffuse host
galaxy emission, we estimate the luminosity due to star formation by removing
the AGN, fitting the residual emission. The AGN and star formation
contributions to the soft X-ray luminosity (i.e. L and L)
for the remaining 24 Sy2s were estimated from the power law and thermal
luminosities derived from spectral fitting. These luminosities were scaled
based on a template derived from XSINGS analysis of normal star forming
galaxies. To account for errors in the luminosities derived from spectral
fitting and the spread in the scaling factor, we estimated L and
L from Monte Carlo simulations. These simulated luminosities agree
with L and L derived from Chandra imaging analysis within a
3\sigma\ confidence level. Using the infrared [NeII]12.8\mu m and [OIV]26\mu m
lines as a proxy of star formation and AGN activity, respectively, we
independently disentangle the contributions of these two processes to the total
soft X-ray emission. This decomposition generally agrees with L and
L at the 3\sigma\ level. In the absence of resolvable nuclear
emission, our decomposition method provides a reasonable estimate of emission
due to star formation in galaxies hosting type 2 AGN.Comment: accepted for publication in ApJ; 34 pages, 9 tables, 4 figure
Evidence for Black Hole Growth in Local Analogs to Lyman Break Galaxies
We have used XMM-Newton to observe six Lyman Break Analogs (LBAs): members of
the rare population of local galaxies that have properties that are very
similar to distant Lyman Break Galaxies. Our six targets were specifically
selected because they have optical emission-line properties that are
intermediate between starbursts and Type 2 (obscured) AGN. Our new X-ray data
provide an important diagnostic of the presence of an AGN. We find X-ray
luminosities of order 10^{42} erg/s and ratios of X-ray to far-IR luminosities
that are higher than values in pure starburst galaxies by factors ranging from
~ 3 to 30. This strongly suggests the presence of an AGN in at least some of
the galaxies. The ratios of the luminosities of the hard (2-10 keV) X-ray to [O
III]\lambda 5007 emission-line are low by about an order-of-magnitude compared
to Type 1 AGN, but are consistent with the broad range seen in Type 2 AGN.
Either the AGN hard X-rays are significantly obscured or the [O III] emission
is dominated by the starburst. We searched for an iron emission line at ~ 6.4
keV, which is a key feature of obscured AGN, but only detected emission at the
~ 2\sigma level. Finally, we find that the ratios of the mid-infrared (24\mu m)
continuum to [O III]\lambda 5007 luminosities in these LBAs are higher than the
values for Type 2 AGN by an average of 0.8 dex. Combining all these clues, we
conclude that an AGN is likely to be present, but that the bolometric
luminosity is produced primarily by an intense starburst. If these black holes
are radiating at the Eddington limit, their masses would lie in the range of
10^5 to 10^6 M_{sun}. These objects may offer ideal local laboratories to
investigate the processes by which black holes grew in the early universe.Comment: Accepted for publication in Ap
Using R-W1 to Uncover Obscured AGN in X-ray Surveys
X-ray surveys provide perhaps the most efficient method for discovering AGN across the Universe, though a major challenge is the time investment involved in following-up X-ray sources to identify them. To make the best use of limited resources, we explored the efficacy of m_r (optical) - m_3.4-micron (mid-infrared), hereafter R-W1, as a color diagnostic to hone in on signatures of obscured black hole growth in wide-area X-ray surveys which would otherwise be missed in optical and infrared surveys. We calibrated this color on the 16.5 deg^2 release of the Stripe 82X survey, finding that most X-ray emitting stars lie along a well-defined locus in R-W1 v. R-K color space and most AGN hosted in optically normal galaxies beyond z>0.5 have red R-W1 colors (R-W1 > 4 Vega). Our results will be useful for crafting target lists for current and future wide-area X-ray surveys to follow-up the most promising obscured AGN candidates
Chandra reveals heavy obscuration and circumnuclear star formation in Seyfert 2 Galaxy NGC 4968
The Ubiquity and Magnitude of Large FeK Equivalent Widths in AGN Extended Regions
Narrow Fe K fluorescent emission lines arising at kpc-scale
separations from the nucleus have only been detected in a few AGN. The
detections require that the extended line emission be spatially resolved and
sufficiently bright. Compared to narrow Fe K lines arising closer to
the nucleus, they have much lower fluxes but show substantially larger
equivalent widths, EW. We show that, in the optically-thin
limit, a purely analytical argument naturally predicts large, EW1 keV, values for such lines, regardless of the details of
equivalent hydrogen column density, , or reprocessor geometry. Monte Carlo
simulations corroborate this result and show that the simple analytic EW prescription holds up to higher approaching the Compton-thick
regime. We compare to observations from the literature and discuss
that our results are consistent with the large EW values
reported for local AGN, for which the line is detected in extended, up to
kpc-scale, regions. We argue that large EW from
kpc-scale regions in AGN should be ubiquitous, because they do not depend on
the absolute luminosity of the central X-ray source, and are measured only
against the scattered continuum. We predict values to be of the order of
1 keV or larger, even for covering factors 1, and for arbitrarily
small column densities. We propose that the large-scale molecular material that
is now routinely being detected with the Atacama Large Millimeter/Submillimeter
Array (ALMA) may act as an extended X-ray scattering reprocessor giving rise to
kpc-scale Fe K emission.Comment: Accepted for publication in Physical Review D. 9 pages, 6 figure
NGC 4388: A Test Case for Relativistic Disk Reflection and Fe K Fluorescence Features
We present a new analysis of the Suzaku X-ray spectrum of the Compton-thin
Seyfert 2 galaxy NGC 4388. The spectrum above 2 keV can be described by a
remarkably simple and rather mundane model, consisting of a uniform, neutral
spherical distribution of matter, with a radial column density of cm, and an Fe abundance of
relative to solar. The model does not require any
phenomenological adjustments to self-consistently account for the low-energy
extinction, the Fe K and Fe K fluorescent emission lines, the Fe
K edge, and the Compton-scattered continuum from the obscuring material. The
spherical geometry is not a unique description, however, and the
self-consistent, solar abundance MYTORUS model, applied with toroidal and
non-toroidal geometries, gives equally good descriptions of the data. In all
cases, the key features of the spectrum are so tightly locked together that for
a wide range of parameters, a relativistic disk-reflection component
contributes no more than 2% to the net spectrum in the 2-20 keV band. We
show that the commonly invoked explanations for weak X-ray reflection features,
namely a truncated and/or very highly ionized disk, do not work for NGC 4388.
If relativistically-broadened Fe K lines and reflection are ubiquitous
in Seyfert 1 galaxies, they should also be ubiquitous in Compton-thin Seyfert 2
galaxies. The case of NGC 4388 shows the need for similar studies of more
Compton-thin AGN to ascertain whether this is true.Comment: MNRAS accepted. 21 pages, 7 figures, 4 tables; Appendix with
historical notes and 1 table. This version: Corrected minor typo and
affiliatio
Suprathermal electron isotropy in high-beta solar wind and its role in heat flux dropouts
[1] Time variations in plasma beta and a parameter which measures isotropy in suprathermal electron pitch angle distributions show a remarkably close correspondence throughout the solar wind. The finding implies that high-beta plasma, with its multiple magnetic holes and sharp field and plasma gradients, is conducive to electron pitch-angle scattering, which reduces heat flux from the Sun without field-line disconnection. Thus the finding impacts our understanding of signatures we use to determine magnetic topology in the heliosphere
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