520 research outputs found
Detections of water ice, hydrocarbons, and 3.3um PAH in z~2 ULIRGs
We present the first detections of the 3um water ice and 3.4um amorphous
hydrocarbon (HAC) absorption features in z~2 ULIRGs. These are based on deep
rest-frame 2-8um Spitzer IRS spectra of 11 sources selected for their
appreciable silicate absorption. The HAC-to-silicate ratio for our z~2 sources
is typically higher by a factor of 2-5 than that observed in the Milky Way.
This HAC `excess' suggests compact nuclei with steep temperature gradients as
opposed to predominantly host obscuration. Beside the above molecular
absorption features, we detect the 3.3um PAH emission feature in one of our
sources with three more individual spectra showing evidence for it. Stacking
analysis suggests that water ice, hydrocarbons, and PAH are likely present in
the bulk of this sample even when not individually detected. The most
unexpected result of our study is the lack of clear detections of the 4.67um CO
gas absorption feature. Only three of the sources show tentative signs of this
feature and at significantly lower levels than has been observed in local
ULIRGs. Overall, we find that the closest local analogs to our sources, in
terms of 3-4um color, HAC-to-silicate and ice-to-silicate ratios, as well as
low PAH equivalent widths are sources dominated by deeply obscured nuclei. Such
sources form only a small fraction of ULIRGs locally and are commonly believed
to be dominated by buried AGN. Our sample suggests that, in absolute number,
such buried AGN are at least an order of magnitude more common at z~2 than
today. The presence of PAH suggests that significant levels of star-formation
are present even if the obscured AGN typically dominate the power budget.Comment: 39 pages, 14 figures, accepted for publication in Ap
The nature and evolution of Ultraluminous Infrared Galaxies: A mid-infrared spectroscopic survey
We report the first results of a low resolution mid-infrared spectroscopic
survey of an unbiased, far-infrared selected sample of 60 ultraluminous
infrared galaxies, using ISOPHOT-S on board ISO. We use the ratio of the 7.7um
`PAH' emission feature to the local continuum as a discriminator between
starburst and AGN activity. About 80% of all the ULIRGs are found to be
predominantly powered by star formation but the fraction of AGN powered objects
increases with luminosity.
Observed ratios of the PAH features in ULIRGs differ slightly from those in
lower luminosity starbursts. This can be plausibly explained by the higher
extinction and/or different physical conditions in the interstellar medium of
ULIRGs. The PAH feature-to-continuum ratio is anticorrelated with the ratio of
feature-free 5.9um continuum to the IRAS 60um continuum, confirming suggestions
that strong mid-IR continuum is a prime AGN signature. The location of
starburst-dominated ULIRGs in such a diagram is consistent with previous
ISO-SWS spectroscopy which implies significant extinction even in the
mid-infrared.
We have searched for indications that ULIRGs which are advanced mergers might
be more AGN-like, as postulated by the classical evolutionary scenario. No such
trend has been found amongst those objects for which near infrared images are
available to assess their likely merger status.Comment: aastex, 4 eps figures. Revised version, accepted by ApJ (Letters
Mid-infrared spectral evidence for a luminous dust enshrouded source in Arp220
We have re-analyzed the 6-12 micron ISO spectrum of the ultra-luminous
infrared galaxy Arp220 with the conclusion that it is not consistent with that
of a scaled up version of a typical starburst. Instead, both template fitting
with spectra of the galaxies NGC4418 and M83 and with dust models suggest that
it is best represented by combinations of a typical starburst component,
exhibiting PAH emission features, and a heavily absorbed dust continuum which
contributes ~40% of the 6-12 micron flux and likely dominates the luminosity.
Of particular significance relative to previous studies of Arp220 is the fact
that the emission feature at 7.7 micron comprises both PAH emission and a
broader component resulting from ice and silicate absorption against a heavily
absorbed continuum. Extinction to the PAH emitting source, however, appears to
be relatively low. We tentatively associate the PAH emitting and heavily
dust/ice absorbed components with the diffuse emission region and the two
compact nuclei respectively identified by Soifer et al. (2002) in their higher
spatial resolution 10 micron study. Both the similarity of the absorbed
continuum with that of the embedded Galactic protostars and results of the dust
models imply that the embedded source(s) in Arp220 could be powered by, albeit
extremely dense, starburst activity. Due to the high extinction, it is not
possible with the available data to exclude that AGN(s) also contribute some or
all of the observed luminosity. In this case, however, the upper limit measured
for its hard X-ray emission would require Arp220 to be the most highly obscured
AGN known.Comment: 11 pages, 9 figures. Accepted for publication in A&A. Also available
at http://www.astro.rug.nl/~spoon/publications.htm
Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2 II: Diagnostics
We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected
ULIRGs spanning z~1-3 and likely L_IR~10^12-10^13Lsun. Our study aims at
quantifying the star-formation and AGN processes in these sources which recent
results suggest have evolved strongly between the observed epoch and today. To
do this, we study the mid-IR contribution of PAH emission, continuum, and
extinction. About 3/4 of our sample are continuum- (i.e. AGN) dominated
sources, but ~60% of these show PAH emission, suggesting the presence of
star-formation activity. These sources have redder mid-IR colors than typical
optically-selected quasars. About 25% of our sample have strong PAH emission,
but none are likely to be pure starbursts as reflected in their relatively high
5um hot dust continua. However, their steep 30um-to-14um slopes suggest that
star-formation might dominate the total infrared luminosity. Six of our z~2
sources have EW6.2>~0.3um and L_14um>~10^12Lsun (implying L_IR>~10^13Lsun). At
these luminosities, such high EW6.2 ULIRGs do not exist in the local Universe.
We find a median optical depth at 9.7um of =1.4. This is consistent
with local IRAS-selected ULIRGs, but differs from early results on
SCUBA-selected z~2 ULIRGs. Similar to local ULIRGs about 25% of our sample show
extreme obscuration (tau_9.7>~3) suggesting buried nuclei. In general, we find
that our sources are similar to local ULIRGs, but are an order of magnitude
more luminous. It is not clear whether our z~2 ULIRGs are simply scaled-up
versions of local ULIRGs, or subject to fundamentally different physical
processes.Comment: 60 pages, 15 figures, accepted for publication in Ap
The extraordinary mid-infrared spectral properties of FeLoBAL Quasars
We present mid-infrared spectra of six FeLoBAL QSOs at 1<z<1.8, taken with
the Spitzer space telescope. The spectra span a range of shapes, from hot dust
dominated AGN with silicate emission at 9.7 microns, to moderately obscured
starbursts with strong Polycyclic Aromatic Hydrocarbon (PAH) emission. The
spectrum of one object, SDSS 1214-0001, shows the most prominent PAHs yet seen
in any QSO at any redshift, implying that the starburst dominates the mid-IR
emission with an associated star formation rate of order 2700 solar masses per
year. With the caveats that our sample is small and not robustly selected, we
combine our mid-IR spectral diagnostics with previous observations to propose
that FeLoBAL QSOs are at least largely comprised of systems in which (a) a
merger driven starburst is ending, (b) a luminous AGN is in the last stages of
burning through its surrounding dust, and (c) which we may be viewing over a
restricted line of sight range.Comment: ApJ, accepte
Deep Mid-Infrared Silicate Absorption as a Diagnostic of Obscuring Geometry Toward Galactic Nuclei
The silicate cross section peak near 10um produces emission and absorption
features in the spectra of dusty galactic nuclei observed with the Spitzer
Space Telescope. Especially in ultraluminous infrared galaxies, the observed
absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A
foreground screen of obscuration cannot reproduce this observed feature, even
at large optical depth. Instead, the deep absorption requires a nuclear source
to be deeply embedded in a smooth distribution of material that is both
geometrically and optically thick. In contrast, a clumpy medium can produce
only shallow absorption or emission, which are characteristic of
optically-identified active galactic nuclei. In general, the geometry of the
dusty region and the total optical depth, rather than the grain composition or
heating spectrum, determine the silicate feature's observable properties. The
apparent optical depth calculated from the ratio of line to continuum emission
generally fails to accurately measure the true optical depth. The obscuring
geometry, not the nature of the embedded source, also determines the far-IR
spectral shape.Comment: To appear in ApJ
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