77 research outputs found
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
Average Infrared Galaxy Spectra From Spitzer Flux Limited Samples
The mid-infrared spectroscopic analysis of a flux-limited sample of galaxies
with fv(24um) > 10 mJy is presented. Sources observed are taken from the
Spitzer First Look Survey (FLS) catalog and from the NOAO Deep Wide-Field
Survey region in Bootes (NDWFS). The spectroscopic sample includes 60 of the
100 sources in these combined catalogs having fv(24um) > 10 mJy. New spectra
from the Spitzer Infrared Spectrograph are presented for 25 FLS sources and for
11 Bootes AGN; these are combined with 24 Bootes starburst galaxies previously
published to determine the distribution of mid-infrared spectral
characteristics for the total 10 mJy sample. Sources have 0.01 < z < 2.4 and
41.8 < log vLv(15um) < 46.2 (ergs/s). Average spectra are determined as a
function of luminosity; lower luminosity sources (log vLv(15um) < 44.0) are
dominated by PAH features and higher luminosity sources (log vLv(15um) > 44.0)
are dominated by silicate absorption or emission. We find that a rest frame
equivalent width of 0.4um for the 6.2um PAH emission feature provides a well
defined division between lower luminosity, "pure" starbursts and higher
luminosity AGN or composite sources. Using the average spectra, fluxes fv(24um)
which would be observed with the Spitzer MIPS are predicted as a function of
redshift for sources with luminosities that correspond to the average spectra.
AGN identical to those in this 10 mJy sample could be seen to z = 3 with
fv(24um) > 1 mJy, but starbursts fall to fv(24um) < 1 mJy by z ~ 0.5. This
indicates that substantial luminosity evolution of starbursts is required to
explain the numerous starbursts found in other IRS results having fv(24um) > 1
mJy and z ~ 2.Comment: Accepted for publication in The Astrophysical Journa
Evolution of the Most Luminous Dusty Galaxies
A summary of mid-infrared continuum luminosities arising from dust is given
for very luminous galaxies, Lir > 10^12 solar luminosities, with 0.005 < z <
3.2 containing active galactic nuclei (AGN), including 115 obscured AGN and 60
unobscured (type 1) AGN. All sources have been observed with the Spitzer
Infrared Spectrograph. Obscured AGN are defined as having optical depth > 0.7
in the 9.7 um silicate absorption feature and unobscured AGN show silicate in
emission. Luminosity vLv(8 um) is found to scale as (1+z)^2.6 to z = 2.8, and
luminosities vLv(8 um) are approximately 3 times greater for the most luminous
unobscured AGN. Total infrared luminosities for the most luminous obscured AGN,
Lir(AGN_obscured) in solar luminosities, scale as log Lir(AGN_obscured) =
12.3+-0.25 + 2.6(+-0.3)log(1+z), and for the most luminous unobscured AGN,
scale as log Lir(AGN1) = 12.6+-0.15 + 2.6(+-0.3)log(1+z), indicating that the
most luminous AGN are about 10 times more luminous than the most luminous
starbursts. Results are consistent with obscured and unobscured AGN having the
same total luminosities with differences arising only from orientation, such
that the obscured AGN are observed through very dusty clouds which extinct
about 50% of the intrinsic luminosity at 8 um. Both obscured and unobscured AGN
should be detected to z ~ 6 by Spitzer surveys with fv(24 um) > 0.3 mJy, even
without luminosity evolution for z > 2.5. By contrast, the most luminous
starbursts cannot be detected for z > 3, even if luminosity evolution continues
beyond z = 2.5.Comment: Includes corrected Figure 3, as publishe
Millimetre observations of a sample of high-redshift obscured quasars
We present observations at 1.2 mm with MAMBO-II of a sample of z>~2
radio-intermediate obscured quasars, as well as CO observations of two sources
with the Plateau de Bure Interferometer. Five out of 21 sources (24%) are
detected at a significance of >=3sigma. Stacking all sources leads to a
statistical detection of = 0.96+-0.11 mJy and stacking only the
non-detections also yields a statistical detection, with = 0.51+-0.13
mJy. This corresponds to a typical far-infrared luminosity L_FIR~4x10^12 Lsol.
If the far-infrared luminosity is powered entirely by star-formation, and not
by AGN-heated dust, then the characteristic inferred star-formation rate is
~700 Msol yr-1. This far-infrared luminosity implies a dust mass of
M_dust~3x10^8 Msol. We estimate that such large dust masses on kpc scales can
plausibly cause the obscuration of the quasars. We present dust SEDs for our
sample and derive a mean SED for our sample. This mean SED is not well fitted
by clumpy torus models, unless additional extinction and far-infrared
re-emission due to cool dust are included. There is a hint that the host
galaxies of obscured quasars must have higher far-infrared luminosities and
cool-dust masses and are therefore often found at an earlier evolutionary phase
than those of unobscured quasars. For one source at z=2.767, we detect the
CO(3-2) transition, with S_CO Delta nu=630+-50 mJy km s-1, corresponding to
L_CO(3-2)= 3.2x10^7 Lsol, or L'_CO(3-2)=2.4x10^10 K km s-1 pc2. For another
source at z=4.17, the lack of detection of the CO(4-3) line yields a limit of
L'_CO(4-3)<1x10^10 K km s-1 pc2. Molecular gas masses, gas depletion timescales
and gas-to-dust ratios are estimated (Abridged).Comment: Accepted by ApJ, 25 pages, 11 figures, 4 table
Infrared Spectra and Spectral Energy Distributions for Dusty Starbursts and AGN
We present spectroscopic results for all galaxies observed with the Spitzer
Infrared Spectrograph (IRS) which also have total infrared fluxes f(ir)
measured with the Infrared Astronomical Satellite (IRAS), also using AKARI
photometry when available. Infrared luminosities and spectral energy
distributions (SEDs) from 8 um to 160 um are compared to polycyclic aromatic
hydrocarbon (PAH) emission from starburst galaxies or mid-infrared dust
continuum from AGN at rest frame wavelengths ~ 8 um. A total of 301 spectra are
analyzed for which IRS and IRAS include the same unresolved source, as measured
by the ratio fv(IRAS 25 um)/fv(IRS 25 um). Sources have 0.004 < z < 0.34 and
42.5 < log L(IR) < 46.8 (erg per s) and cover the full range of starburst
galaxy and AGN classifications. Individual spectra are provided electronically,
but averages and dispersions are presented. We find that log [L(IR)/vLv(7.7
um)] = 0.74 +- 0.18 in starbursts, that log [L(IR)/vLv(7.7 um)] = 0.96 +- 0.26
in composite sources (starburst plus AGN), that log [L(IR)/vLv(7.9 um)] = 0.80
+- 0.25 in AGN with silicate absorption, and log [L(IR)/vLv(7.9 um)] = 0.51 +-
0.21 in AGN with silicate emission. L(IR) for the most luminous absorption and
emission AGN are similar and 2.5 times larger than for the most luminous
starbursts. AGN have systematically flatter SEDs than starbursts or composites,
but their dispersion in SEDs overlaps starbursts. Sources with the strongest
far-infrared luminosity from cool dust components are composite sources,
indicating that these sources may contain the most obscured starbursts.Comment: Accepted for publication in The Astrophysical Journa
Interstellar Silicate Dust in the z=0.89 Absorber Towards PKS 1830-211: Crystalline Silicates at High Redshift?
We present evidence of a >10-sigma detection of the 10 micron silicate dust
absorption feature in the spectrum of the gravitationally lensed quasar PKS
1830-211, produced by a foreground absorption system at redshift 0.886. We have
examined more than 100 optical depth templates, derived from both observations
of Galactic and extragalactic sources and laboratory measurements, in order to
constrain the chemical structure of the silicate dust. We find that the best
fit to the observed absorption profile is produced by laboratory crystalline
olivine, with a corresponding peak optical depth of tau_10=0.27+/-0.05. The fit
is slightly improved upon by including small contributions from additional
materials such as silica, enstatite, or serpentine, which suggests that the
dust composition may consist of a blend of crystalline silicates. Combining
templates for amorphous and crystalline silicates, we find that the fraction of
crystalline silicates needs to be at least 95%. Given the rarity of
extragalactic sources with such a high degree of silicate crystallinity, we
also explore the possibility that the observed spectral features are produced
by amorphous silicates in combination with other molecular or atomic
transitions, or by foreground source contamination. While we cannot rule out
these latter possibilities, they lead to much poorer profile fits than for the
crystalline olivine templates. If the presence of crystalline interstellar
silicates in this distant galaxy is real, it would be highly unusual, given
that the Milky Way interstellar matter contains essentially only amorphous
silicates. It is possible that the z=0.886 absorber towards PKS 1830-211, well
known for its high molecular content, has a unique star-forming environment
that enables crystalline silicates to form and prevail.Comment: 67 pages, 21 figures, accepted for publication in the Astrophysical
Journa
Spitzer IRS 16 micron Observations of the GOODS Fields
We present Spitzer 16 micron imaging of the Great Observatories Origins Deep
Survey (GOODS) fields. We survey 150 square arcminutes in each of the two GOODS
fields (North and South), to an average 3 sigma depth of 40 and 65 micro-Jy
respectively. We detect about 1300 sources in both fields combined. We validate
the photometry using the 3-24 micron spectral energy distribution of stars in
the fields compared to Spitzer spectroscopic templates. Comparison with ISOCAM
and AKARI observations in the same fields show reasonable agreement, though the
uncertainties are large. We provide a catalog of photometry, with sources cross
correlated with available Spitzer, Chandra, and HST data. Galaxy number counts
show good agreement with previous results from ISOCAM and AKARI, with improved
uncertainties. We examine the 16 to 24 micron flux ratio and find that for most
sources it lies within the expected locus for starbursts and infrared luminous
galaxies. A color cut of S_{16}/S_{24}>1.4 selects mostly sources which lie at
1.1<z<1.6, where the 24 micron passband contains both the redshifted 9.7 micron
silicate absorption and the minimum between PAH emission peaks. We measure the
integrated galaxy light of 16 micron sources, and find a lower limit on the
galaxy contribution to the extragalactic background light at this wavelength to
be 2.2\pm 0.2$ nW m^{-2} sr^{-1}.Comment: Accepted for Publication in the AJ. 53 preprint pages, including 15
figures and 8 tables. Table 1-4 are truncated in the ms.tex but are included
in full in the tar file (and will be available in the online version of the
AJ
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
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