395 research outputs found
CO-Dark Star Formation and Black Hole Activity in 3C 368 at z = 1.131: Coeval Growth of Stellar and Supermassive Black Hole Masses
We present the detection of four far-infrared fine-structure oxygen lines, as
well as strong upper limits for the CO(2-1) and [N II] 205 um lines, in 3C 368,
a well-studied radio-loud galaxy at z = 1.131. These new oxygen lines, taken in
conjunction with previously observed neon and carbon fine-structure lines,
suggest a powerful active galactic nucleus (AGN), accompanied by vigorous and
extended star formation. A starburst dominated by O8 stars, with an age of ~6.5
Myr, provides a good fit to the fine-structure line data. This estimated age of
the starburst makes it nearly concurrent with the latest episode of AGN
activity, suggesting a link between the growth of the supermassive black hole
and stellar population in this source. We do not detect the CO(2-1) line, down
to a level twelve times lower than the expected value for star forming
galaxies. This lack of CO line emission is consistent with recent star
formation activity if the star-forming molecular gas has low metallicity, is
highly fractionated (such that CO is photodissociated through much of the
clouds), or is chemically very young (such that CO has not yet had time to
form). It is also possible, though we argue unlikely, that the ensemble of fine
structure lines are emitted from the region heated by the AGN.Comment: 10 pages, 4 figures, 2 tables, accepted for publication in the
Astrophysical Journa
The [Ne III]/[Ne II] line ratio in NGC 253
We present results of the mapping of the nucleus of the starburst galaxy NGC 253 and its immediate surroundings using the Infrared Spectrograph on board the Spitzer Space Telescope. The map is centered on the nucleus of the galaxy and spans the inner 800 × 688 pc^2. We perform a brief investigation of the implications of these measurement on the properties of the star formation in this region using theories developed to explain the deficiency of massive stars in starbursts
The Mid-IR Properties of Starburst Galaxies from Spitzer-IRS Spectroscopy
We present 5-38um mid-infrared spectra at a spectral resolution of R~65-130
of a large sample of 22 starburst nuclei taken with the Infrared Spectrograph
IRS on board the Spitzer Space Telescope. The spectra show a vast range in
starburst SEDs. The silicate absorption ranges from essentially no absorption
to heavily obscured systems with an optical depth of tau(9.8um)~5. The spectral
slopes can be used to discriminate between starburst and AGN powered sources.
The monochromatic continuum fluxes at 15um and 30um enable a remarkably
accurate estimate of the total infrared luminosity of the starburst. We find
that the PAH equivalent width is independent of the total starburst luminosity
L_IR as both continuum and PAH feature scale proportionally. However, the
luminosity of the 6.2um feature scales with L_IR and can be used to approximate
the total infrared luminosity of the starburst. Although our starburst sample
covers about a factor of ten difference in the [NeIII]/[NeII] ratio, we found
no systematic correlation between the radiation field hardness and the PAH
equivalent width or the 7.7um/11.3um PAH ratio. These results are based on
spatially integrated diagnostics over an entire starburst region, and local
variations may be ``averaged out''. It is presumably due to this effect that
unresolved starburst nuclei with significantly different global properties
appear spectrally as rather similar members of one class of objects.Comment: 22 pages, accepted for publication in ApJ, a high-resolution version
is available from http://www.strw.leidenuniv.nl/~brandl/IRS_starbursts.pd
Infrared 3-4 Micron Spectroscopic Investigations of a Large Sample of Nearby Ultraluminous Infrared Galaxies
We present infrared L-band (3-4 micron) nuclear spectra of a large sample of
nearby ultraluminous infrared galaxies (ULIRGs).ULIRGs classified optically as
non-Seyferts (LINERs, HII-regions, and unclassified) are our main targets.
Using the 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission and
absorption features at 3.1 micron due to ice-covered dust and at 3.4 micron
produced by bare carbonaceous dust, we search for signatures of powerful active
galactic nuclei (AGNs) deeply buried along virtually all lines-of-sight. The
3.3 micron PAH emission, the signatures of starbursts, is detected in all but
two non-Seyfert ULIRGs, but the estimated starburst magnitudes can account for
only a small fraction of the infrared luminosities. Three LINER ULIRGs show
spectra typical of almost pure buried AGNs, namely, strong absorption features
with very small equivalent-width PAH emission. Besides these three sources, 14
LINER and 3 HII ULIRGs' nuclei show strong absorption features whose absolute
optical depths suggest an energy source more centrally concentrated than the
surrounding dust, such as a buried AGN. In total, 17 out of 27 (63%) LINER and
3 out of 13 (23%) HII ULIRGs' nuclei show some degree of evidence for powerful
buried AGNs, suggesting that powerful buried AGNs may be more common in LINER
ULIRGs than in HII ULIRGs. The evidence of AGNs is found in non-Seyfert ULIRGs
with both warm and cool far-infrared colors. These spectra are compared with
those of 15 ULIRGs' nuclei with optical Seyfert signatures taken for
comparison.The overall spectral properties suggest that the total amount of
dust around buried AGNs in non-Seyfert ULIRGs is systematically larger than
that around AGNs in Seyfert 2 ULIRGs.Comment: 56 pages, 9 figures, accepted for publication in ApJ (20 January
2006, vol 637 issue
Quantifying the AGN-driven outflows in ULIRGs (QUADROS) – II. Evidencefor compact outflow regions from HST [O iii] imaging observations
The true importance of the warm, AGN-driven outflows for the evolution of galaxies remains
uncertain. Measurements of the radial extents of the outflows are key for quantifying their
masses and kinetic powers, and also establishing whether the AGN outflows are galaxywide.
Therefore, as part of a larger project to investigate the significance of warm, AGNdriven
outflows in the most rapidly evolving galaxies in the local universe, here we present
deep Hubble Space Telescope (HST) narrow-band [O III] λ5007 observations of a complete
sample of eight nearby ULIRGs with optical AGN nuclei. Combined with the complementary
information provided by our ground-based spectroscopy, the HST images show that the warm
gas outflows are relatively compact for most of the objects in the sample: in three objects, the
outflow regions are barely resolved at the resolution of HST (0.065 < R[O III] < 0.12 kpc); in a
further four cases, the outflows are spatially resolved but with flux-weighted mean radii in the
range 0.65 < R[O III] < 1.2 kpc; and in only one object (Mrk273) is there clear evidence for a
more extended outflow, with a maximum extent of R[O III] ∼ 5 kpc. Overall, our observations
show little evidence for the galaxy-wide outflows predicted by some models of AGN feedback
CASSIS: The Cornell Atlas of Spitzer/IRS Sources
We present the spectral atlas of sources observed in low resolution with the
Infrared Spectrograph on board the Spitzer Space Telescope. More than 11,000
distinct sources were extracted using a dedicated algorithm based on the SMART
software with an optimal extraction (AdOpt package). These correspond to all
13,000 low resolution observations of fixed objects (both single source and
cluster observations). The pipeline includes image cleaning, individual
exposure combination, and background subtraction. A particular attention is
given to bad pixel and outlier rejection at the image and spectra levels. Most
sources are spatially unresolved so that optimal extraction reaches the highest
possible signal-to-noise ratio. For all sources, an alternative extraction is
also provided that accounts for all of the source flux within the aperture.
CASSIS provides publishable quality spectra through an online database together
with several important diagnostics, such as the source spatial extent and a
quantitative measure of detection level. Ancillary data such as available
spectroscopic redshifts are also provided. The database interface will
eventually provide various ways to interact with the spectra, such as
on-the-fly measurements of spectral features or comparisons among spectra.Comment: Accepted for publication in ApJ Supplement Serie
Mid Infrared Properties of Low Metallicity Blue Compact Dwarf Galaxies From Spitzer/IRS
We present a {\em Spitzer}-based mid-infrared study of a large sample of Blue
Compact Dwarf galaxies (BCD) using the Infrared Spectrograph (IRS), including
the first mid-IR spectrum of IZw18, the archetype for the BCD class and among
the most metal poor galaxies known. We show the spectra of Polycyclic Aromatic
Hydrocarbon (PAH) emission in low-metallicity environment. We find that the
equivalent widths (EW) of PAHs at 6.2, 7.7, 8.6 and 11.2 m are generally
weaker in BCDs than in typical starburst galaxies and that the fine structure
line ratio, [NeIII]/[NeII], has a weak anti-correlation with the PAH EW. A much
stronger anti-correlation is shown between the PAH EW and the product of the
[NeIII]/[NeII] ratio and the UV luminosity density divided by the metallicity.
We conclude that PAH EW in metal-poor high-excitation environments is
determined by a combination of PAH formation and destruction effects.Comment: 41 pages, 14 figure
Mid-Infrared Spectroscopic Properties of Ultra-Luminous Infrared Quasars
We analyse mid-infrared (MIR) spectroscopic properties for 19 ultra-luminous
infrared quasars (IR QSOs) in the local universe based on the spectra from the
Infrared Spectrograph on board the Spitzer Space Telescope. The MIR properties
of IR QSOs are compared with those of optically-selected Palomar-Green QSOs (PG
QSOs) and ultra-luminous infrared galaxies (ULIRGs). The average MIR spectral
features from ~ 5 to 30um, including the spectral slopes, 6.2um PAH emission
strengths and [NeII] 12.81um luminosities of IR QSOs, differ from those of PG
QSOs. In contrast, IR QSOs and ULIRGs have comparable PAH and [NeII]
luminosities. These results are consistent with IR QSOs being at a transitional
stage from ULIRGs to classical QSOs. We also find that the colour index
alpha(30, 15) is a good indicator of the relative contribution of starbursts to
AGNs for all QSOs. Correlations between the [NeII] 12.81um and PAH 6.2um
luminosities and those between the [NeII], PAH with 60um luminosities for
ULIRGs and IR QSOs indicate that both [NeII] and PAH luminosities are
approximate star formation rate indicators for IR QSOs and starburst-dominated
galaxies; the scatters are, however, quite large (~ 0.7 to 0.8 dex). Finally
the correlation between the EW(PAH 6.2um) and outflow velocities suggests that
star formation activities are suppressed by feedback from AGNs and/or
supernovae.Comment: 19 pages, 8 figures, 6 tables, accepted for publication in MNRA
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