77 research outputs found

    The [Ne III]/[Ne II] line ratio in NGC 253

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    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

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    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

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    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

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    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

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    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?

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    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

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    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

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    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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