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A Two-Parameter Model for the Infrared/Submillimeter/Radio Spectral Energy Distributions of Galaxies and AGN

TL;DR: In this paper, a two-parameter semi-empirical model is presented for the spectral energy distributions of galaxies with contributions to their infrared-submillimeter-radio emission from both star formation and accretion disk-powered activity.
Abstract: A two-parameter semi-empirical model is presented for the spectral energy distributions of galaxies with contributions to their infrared-submillimeter-radio emission from both star formation and accretion disk-powered activity. This model builds upon a previous one-parameter family of models for star-forming galaxies, and includes an update to the mid-infrared emission using an average template obtained from Spitzer Space Telescope observations of normal galaxies. Star-forming/AGN diagnostics based on PAH equivalent widths and broadband infrared colors are presented, and example mid-infrared AGN fractional contributions are estimated from model fits to the GOALS sample of nearby U/LIRGS and the 5MUSES sample of 24um-selected sources at redshifts 0 < z < 2.

Summary (3 min read)

1. INTRODUCTION

  • There have been many recent developments in modeling galaxy infrared spectral energy distributions (SEDs).
  • Similarly, Dale & Helou (2002) use a “single-parameter family” (denoted by their αSF) to coherently govern changes across their templates in polycyclic aromatic hydrocarbon (PAH) emission, the peak wavelength of the broad far-infrared bump, and the far-infrared/submillimeter dust emissivity.
  • In deciding which set(s) of models to adopt, the end users ultimately must balance their need for sophisticated interpretation with ease of use.
  • Thus, ideally each set of infrared galaxy SED models would have a convenient methodology for consideration of AGN contributions.

2. SAMPLES FOR TESTING THE MODEL

  • Two galaxy surveys are used to check the utility of the SED models described below .
  • Overall, secure SPIRE photometry for all three passbands exists for 74 5MUSES targets.
  • This survey includes deep Spitzer IRS spectroscopy for 202 nearby LIRGs and ULIRGs covering a redshift range of 0 z 0.09.

3. THE UPDATED SPECTRAL ENERGY DISTRIBUTION MODELS

  • In the original construction of these templates (Dale et al. 2001; Dale & Helou 2002), a series of “local” SEDs were created to represent the emission from dust exposed to a wide range of heating intensities 0.3 U 105 where U = 1 corresponds to the local interstellar radiation field in the solar neighborhood.
  • A power-law combination of these local curves can effectively mimic the spatially integrated (“global”) dust emission, i.e., dMd ∝ U−αSFdU, (1) where Md is the dust mass heated by a radiation field at intensity U and the exponent αSF represents the relative contributions of the different local SEDs.
  • Various modifications based on observations of star-forming galaxies were made by Dale and collaborators to the Désert et al. (1990) framework, including the insertion of an empirical PAH spectrum, the incorporation of a wavelength-dependent far-infrared/submillimeter dust emissivity, and the extension of their modeling to radiation fields U > 103.
  • Though this ISO spectrum represented a step forward in infrared SED modeling at the time, a description here of some of its features and limitations is warranted.
  • Finally, the 11.5 μm cut-off to the red end of the ISOPHOT spectrum unfortunately resulted in a truncated tracing of the 11.3 μm PAH feature and the omission of the 12.7 μm PAH emission feature and those at any longer wavelengths.

3.1. Modifications to the Star-forming Templates

  • For the updated mid-infrared spectrum the authors adopt the 5–34 μm “pure” star-forming curve from (Spoon et al. 2007, their spectrum “1C”), who utilized the Spitzer archives to analyze a sequence of mid-infrared spectral shapes among AGNs, ULIRGs, and star-forming galaxies.
  • Important benefits to updating the mid-infrared with Spitzer data are the inclusion of prominent fine-structure lines (e.g., [Ne iii] 15.6 μm, [S iii] 18.7 μm, and [S iii] 33.5 μm) and the 17 μm PAH complex, the latter of which accounts for up to 10% of the total PAH emission in normal star-forming galaxies .
  • As was done in Dale et al. (2001), the authors scale the empirical mid-infrared spectrum to the amplitude of the Désert et al. (1990) PAH templates via integrating over the 12 μm IRAS filter.
  • Besides these modifications to the mid-infrared spectrum, the star-forming templates are otherwise unchanged.
  • Moreover, it should be noted that the authors continue to assume optically thin infrared emission, and thus do not include in their model any absorption features such as the 9.7 μm silicate trough found in many ULIRGs (e.g., Armus et al. 2007).

3.2. Addition of an AGN Template

  • While the star-forming templates themselves from Dale & Helou (2002) are only slightly modified, the authors introduce here a fundamental addition to the templates by incorporating a second parameter, one that accounts for accretion disk-powered infrared luminosity.
  • The data involved in their analysis include X-ray, far- and near-ultraviolet, optical, near-, mid-, and farinfrared, and radio spectroscopy and/or photometry.
  • For this work the authors have developed mixed combinations for 5–20 μm AGN fractions running from 0% to 100%, spaced at 5% intervals.
  • Figure 4 shows how the resulting infrared–radio SEDs appear for a variety of combinations of AGN and star-forming emission.

4.1. Model Color Distributions

  • To date most efforts to disentangle infrared emission from AGNs and star formation have focused on utilizing mid-infrared continuum data sets (e.g., Laurent et al.
  • A common complementary technique for identifying AGN contributions, especially useful when midinfrared spectral data are unavailable, involves combinations of flux ratios that utilize data from three or four broadband filters (e.g., Lacy et al.
  • This restriction is by design: the Spoon et al. (2007) star-forming template begins at 5 μm, a feature which conveniently minimizes complications arising from stellar emission.

4.2. PAH Equivalent Width Distributions

  • The strengths of various PAH features have been widely used to diagnose the main power source of a galaxy (e.g., Genzel et al.
  • The different curves show the trends for a variety of AGN fractions; the 6.2 μm equivalent width for the Spoon et al. (2007) pure star-forming curve used here is ∼0.5 μm.
  • In addition, even “pure” star-forming galaxies exhibit a significant dispersion in the equivalent width of PAH features (e.g., 0.2 dex at 6.2 μm; Wu et al. 2010), a dispersion that these simple models do not incorporate.
  • In agreement with the references listed above, this plot shows that EW (PAH 6.2 μm) ≈ 0.2–0.3 μm could roughly be used as a demarcation between sources powered by star formation and AGN activity.

4.3. Spectral Energy Distribution Fits

  • Figure 7 shows the best fits of the AGN/star-forming curves to the subset of 5MUSES sources that have Herschel SPIRE data available; Figure 8 provides similar displays for fits to GOALS sources.
  • The values of αSF and the mid-infrared AGN percentage found in each subpanel of Figures 7 and 8 correspond to the median value obtained after carrying out 1000 Monte Carlo simulations of each fit.
  • For each Monte Carlo simulation, a random (Gaussian deviate) flux offset, scaled according to the measured uncertainty, was added to each flux.

4.4. Comparison with Other AGN Fractional Estimates

  • The mid-infrared AGN fractions for galaxies from the 5MUSES and GOALS surveys are provided within the subpanels of Figures 7 and 8.
  • The 5MUSES mid-infrared AGN fractions are compared to those from Wu et al. (2011), who utilize the 5–35 μm continuum data and various templates of AGN and star-forming systems.
  • Thus, the difference is more fundamental than merely the difference in wavelength ranges utilized in their fits and those of Wu et al. (2011).
  • For the comparison involving GOALS, a Spearman rank correlation test yields a correlation coefficient of 0.43 for the 58 targets for which reliable mid-infrared AGN fractions are available from both Petric et al. (2011) and this work.
  • Thus, for both samples there is less than a 1% probability that the correlation occurred purely through chance.

4.5. Total Infrared Estimators and AGN Fraction

  • For the pure star-forming sequence (i.e., the top row of 0% AGN), the coefficients are similar to those already published in Dale & Helou (2002).
  • Spitzer fluxes better captures the full range of model variations than using just the three Spitzer/MIPS fluxes.
  • Figure 10 shows how the 5MUSES sample is biased toward AGNs at higher infrared luminosities.

5. SUMMARY

  • The first parameter governs the variety of long-wavelength spectral shapes observed for star-forming galaxies, whereas the second parameter quantifies the fractional contribution of AGN mid-infrared emission.
  • Because only two parameters are utilized, the fine-scale interpretive power of these models is necessarily limited, and they do not capture the full range of observed spectra.
  • The authors appreciate helpful discussions with Adam Myers, Mike Brotherton, Zhaohui Shang, Allison Kirkpatrick, Emeric Le Floc’h, and James Mullaney.
  • Funding for the Sloan Digital Sky Survey (SDSS) and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the NSF, the U.S. Department of Energy, NASA, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England.

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The Astrophysical Journal, 784:83 (11pp), 2014 March 20 doi:10.1088/0004-637X/784/1/83
C
2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
A TWO-PARAMETER MODEL FOR THE INFRARED/SUBMILLIMETER/RADIO SPECTRAL ENERGY
DISTRIBUTIONS OF GALAXIES AND ACTIVE GALACTIC NUCLEI
Daniel A. Dale
1
, George Helou
2
, Georgios E. Magdis
3
, Lee Armus
4
,TanioD
´
ıaz-Santos
4
, and Yong Shi
5
1
Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA; ddale@uwyo.edu
2
Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
3
Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
4
Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA
5
School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China
Received 2013 November 25; accepted 2014 February 6; published 2014 March 6
ABSTRACT
A two-parameter semi-empirical model is presented for the spectral energy distributions of galaxies with
contributions to their infrared–submillimeter–radio emission from both star formation and accretion disk-powered
activity. This model builds upon a previous one-parameter family of models for star-forming galaxies, and
includes an update to the mid-infrared emission using an average template obtained from Spitzer Space Telescope
observations of normal galaxies. Star-forming/active galactic nucleus (AGN) diagnostics based on polycyclic
aromatic hydrocarbon equivalent widths and broadband infrared colors are presented, and example mid-infrared
AGN fractional contributions are estimated from model fits to the Great Observatories All-Sky LIRG Survey sample
of nearby U/LIRGS and the Five mJy Unbiased Spitzer Extragalactic Survey sample of 24 μm selected sources at
redshifts 0 z 4.
Key words: dust, extinction galaxies: active galaxies: star formation infrared: ISM
Online-only material: color figures, extended figures
1. INTRODUCTION
There have been many recent developments in modeling
galaxy infrared spectral energy distributions (SEDs). Some
of these models are quite sophisticated, and when fitted to a
galaxy’s observed spectrum, their various parameters can yield
insight into the physical characteristics of the system (e.g.,
Silva et al. 1998; Popescu et al. 2000; Gordon et al. 2001;
Siebenmorgen & Kr
¨
ugel 2007;Draine&Li2007; da Cunha
et al. 2008; Galliano et al. 2008; Groves et al. 2008;Hermelo
et al. 2013). Such models are often referred to as “grids” to reflect
their multi-dimensional nature. At the other extreme of infrared
galaxy spectral models are one-dimensional “templates, typi-
cally a suite of synthetic or empirical spectra that essentially rely
on a single parameter to characterize a galaxy’s infrared spec-
tral shape. For example, Chary & Elbaz (2001) and Rieke et al.
(2009) provide template spectra sequenced according to their
bolometric infrared luminosity L
TIR
. Similarly, Dale & Helou
(2002) use a “single-parameter family” (denoted by their α
SF
)to
coherently govern changes across their templates in polycyclic
aromatic hydrocarbon (PAH) emission, the peak wavelength of
the broad far-infrared bump, and the far-infrared/submillimeter
dust emissivity. The work presented in Spoon et al. (2007) rep-
resents an example of a spectral set that is intermediate in com-
plexity between grids and templates, whereby the strength of
the 9.7 μm silicate absorption and the 6.2 μm PAH equiva-
lent width form the basis of their two-dimensional system for
describing mid-infrared spectra. In deciding which set(s) of
models to adopt, the end users ultimately must balance their
need for sophisticated interpretation with ease of use. This
choice depends on a project’s science goals and the richness
of the observational data set.
Complicating this choice is the additional issue of infrared
emission from active galactic nuclei (AGNs); for many galax-
ies, a full accounting of their infrared energy budget must
include dust for which the heating can be traced to accretion
disk-powered luminosity around supermassive black holes, es-
pecially for more luminous systems (e.g., Del Moro et al. 2013;
Shi et al. 2013; Kirkpatrick et al. 2013). This concern is espe-
cially true for interpreting galaxies at higher redshifts (z 2–3),
where the fraction of quasars and strong AGN galaxies is higher
than at the present epoch (Fan et al. 2001). For example, work
by Goto et al. (2010) and Fu et al. (2010) suggest that a signifi-
cant portion of the evolution with redshift in the cosmic infrared
luminosity function can be attributed to the increased fraction
of AGNs in the overall galaxy population at higher redshifts.
Thus, ideally each set of infrared galaxy SED models would
have a convenient methodology for consideration of AGN con-
tributions. Some models do incorporate dust emission from both
AGNs and star formation (Siebenmorgen & Kr
¨
ugel 2007;Rieke
et al. 2009; Berta et al. 2013), but most do not. However, the clear
challenge in this arena is the paucity of robust multi-wavelength
AGN databases, particularly databases where it is clear that the
majority of the mid-infrared emission does indeed come from
the AGN and not from star-forming regions. Fortunately, recent
progress in robust panchromatic AGN data sets present oppor-
tunities to remedy this limitation in galaxy spectral modeling
(e.g., Richards et al. 2006; Shang et al. 2011; Mullaney et al.
2011; Shi et al. 2013).
We report here efforts to update the infrared/submillimeter/
radio SED models from Dale & Helou (2002) in two important
ways. First, we update the mid-infrared portion of these star-
forming models, which was originally based on ISOPHOT data
from the Infrared Space Observatory (ISO), using results from
the Spitzer Space Telescope. The main improvement resulting
from this modification is the inclusion of the prominent 17 μm
PAH complex, which can produce up to 10% of the total
PAH emission. Second, we add another spectral component
that represents emission from AGNs; the models in Dale
& Helou (2002) were purely for star-forming systems. For
1

The Astrophysical Journal, 784:83 (11pp), 2014 March 20 Dale et al.
Figure 1. (Subsets of the) two comparison samples used in this work. The subset of the 5MUSES sample (Wu et al. 2010) is the 74 systems with available Spitzer and
Herschel/SPIRE photometry (Magdis et al. 2013) and the subset of the GOALS sample (Armus et al. 2009) is the 64 targets with Spitzer photometry (U et al. 2012).
The luminosities in the right-hand panel come from U et al. (2012; GOALS) and this work (5MUSES).
(A color version of this figure is available in the online journal.)
this AGN component, a panchromatic database of unobscured
Type 1 quasars is employed. We test this new model using
data from the Spitzer Space Telescope and Herschel Space
Observatory and the 5MUSES (Wu et al. 2010) and GOALS
(Armus et al. 2009) surveys, surveys for which AGN percentages
have been independently estimated from infrared data (Petric
et al. 2011;Wuetal.2011). In the process, we show these
models are applicable to the luminous infrared galaxy (LIRG)
and ultraluminous infrared galaxy (ULIRG) regimes (L
TIR
>
10
11
L
and L
TIR
> 10
12
L
, respectively); the models were
originally developed using only “normal” star-forming galaxies
(L
TIR
10
10
L
).
In Section 2 we review the two galaxy samples against
which the updated models are tested. Section 3 describes how
the templates are updated using recent AGN and star-forming
galaxy databases, and Section 4 presents the results from this
work. The final section summarizes our findings.
2. SAMPLES FOR TESTING THE MODEL
Two galaxy surveys are used to check the utility of the SED
models described below (see Figure 1). The first survey is the
Five mJy Unbiased Spitzer Extragalactic Survey (5MUSES; Wu
et al. 2010), a 24 μm selected sample of 330 galaxies spanning
redshifts 0 z 4forwhichSpitzer Infrared Spectrograph
(IRS) low-resolution spectra (5–35 μm) were obtained (high-
resolution IRS spectroscopy was also obtained for a subset of
the sample). In addition to the extant Spitzer IRAC and MIPS
photometry for 5MUSES that is available from the SWIRE
(Lonsdale et al. 2003) and First Look Surveys (Fadda et al.
2006; Frayer et al. 2006), we also have new Herschel 250, 350,
and 500 μm SPIRE fluxes (Magdis et al. 2013). These Herschel
data were taken as part of the HerMES project (Oliver et al.
2010), and probe to a 50% source recovery limit of 12–30 mJy.
Overall, secure SPIRE photometry for all three passbands exists
for 74 5MUSES targets.
The second comparative sample stems from the Great Obser-
vatories All-Sky LIRG Survey (GOALS; Armus et al. 2009).
This survey includes deep Spitzer IRS spectroscopy for 202
nearby LIRGs and ULIRGs covering a redshift range of 0
z 0.09. Broadband infrared data from Spitzer IRAC 3.6/4.5/
5.8/8.0 μm and Spitzer MIPS 24/70/160 μm also exist for
GOALS, and we utilize the published photometry for a subset
of 64 sources (U et al. 2012). Using the mid-infrared contin-
uum spectral diagnostics developed in Laurent et al. (2000), the
typical AGN fractional contribution to the mid-infrared energy
budget in GOALS sources is 15% (Petric et al. 2011).
3. THE UPDATED SPECTRAL ENERGY
DISTRIBUTION MODELS
In the original construction of these templates (Dale et al.
2001;Dale&Helou2002), a series of “local” SEDs were created
to represent the emission from dust exposed to a wide range of
heating intensities 0.3 U 10
5
where U = 1 corresponds
to the local interstellar radiation field in the solar neighborhood.
A power-law combination of these local curves can effectively
mimic the spatially integrated (“global”) dust emission, i.e.,
dM
d
U
α
SF
dU, (1)
where M
d
is the dust mass heated by a radiation field at intensity
U and the exponent α
SF
represents the relative contributions of
the different local SEDs.
These templates were built on the framework of D
´
esert
et al. (1990) and are comprised of emission from stochastically
heated PAHs, emission from semi-stochastically heated very
small grains, and thermal emission from large dust grains.
Various modifications based on observations of star-forming
galaxies were made by Dale and collaborators to the D
´
esert
et al. (1990) framework, including the insertion of an empirical
PAH spectrum, the incorporation of a wavelength-dependent
far-infrared/submillimeter dust emissivity, and the extension of
their modeling to radiation fields U>10
3
.
The average 2.5–11.5 μm mid-infrared spectrum of Lu
et al. (2003) was used to replace the PAH spectrum of D
´
esert
et al. (1990). This SED was derived from the average of 40
normal star-forming disk galaxies from the ISO Key Project on
Normal Galaxies (Dale et al. 2000). Though this ISO spectrum
represented a step forward in infrared SED modeling at the
time, a description here of some of its features and limitations is
warranted. Those data were taken with the ISOPHOT instrument
aboard ISO, an instrument which had a 24

× 24

field of
view, resulting in relatively large 4 kpc sizescales over which
the galaxies were sampled; the average ISOPHOT spectra
2

The Astrophysical Journal, 784:83 (11pp), 2014 March 20 Dale et al.
Figure 2. Collection of seven pure star-forming model spectral energy distributions along with that for a pure AGN. The star-forming spectra are essentially the suite
of curves presented in Dale & Helou (2002), but with the ISOPHOT mid-infrared template replaced by the star-forming template of Spoon et al. (2007, their “1C”
curve). The different star-forming curves portrayed here represent different α
SF
values. The AGN spectrum derives from the median quasar spectral energy distribution
of Shi et al. (2013); see Section 3.2.
(A color version of this figure is available in the online journal.)
undoubtedly contained contributions from a wide range of
environments (e.g., H ii regions; photo-dissociation regions,
molecular clouds, etc.), a feature that may not be desirable for
certain modeling applications. A significant limitation to the
Lu et al. (2003) spectrum is that it is restricted to wavelengths
2.5 μm λ 11.5 μm with a gap in coverage between 4.8 and
5.8 μm. The wavelength gap was bridged with a simple linear
interpolation of the average spectrum. Finally, the 11.5 μm
cut-off to the red end of the ISOPHOT spectrum unfortunately
resulted in a truncated tracing of the 11.3 μm PAH feature and
the omission of the 12.7 μm PAH emission feature and those
at any longer wavelengths. This latter limitation was partly
remedied in Dale et al. (2001) by a schematic extension to
15 μm that was guided by ISOCAM circular variable filter
observations.
3.1. Modifications to the Star-forming Templates
For the updated mid-infrared spectrum we adopt the 5–34 μm
“pure” star-forming curve from (Spoon et al. 2007, their spec-
trum “1C”), who utilized the Spitzer archives to analyze a se-
quence of mid-infrared spectral shapes among AGNs, ULIRGs,
and star-forming galaxies. Important benefits to updating the
mid-infrared with Spitzer data are the inclusion of prominent
fine-structure lines (e.g., [Ne iii] 15.6 μm, [S iii] 18.7 μm, and
[S iii] 33.5 μm) and the 17 μm PAH complex, the latter of
which accounts for up to 10% of the total PAH emission in nor-
mal star-forming galaxies (see Figure 2 of this work and Table 7
of Smith et al. 2007). As was done in Dale et al. (2001), we
scale the empirical mid-infrared spectrum to the amplitude of
the D
´
esert et al. (1990) PAH templates via integrating over the
12 μm IRAS filter. The shape of the mid-infrared continuum
beyond 15 μm was also fixed to that of the D
´
esert et al. (1990)
PAH templates. Besides these modifications to the mid-infrared
spectrum, the star-forming templates are otherwise unchanged.
We continue to utilize a single-parameter family (i.e., α
SF
)to
describe the full range of PAH/very small grain/large grain and
overall spectral shapes for normal star-forming galaxies. More-
over, it should be noted that we continue to assume optically
thin infrared emission, and thus do not include in our model
any absorption features such as the 9.7 μm silicate trough found
in many ULIRGs (e.g., Armus et al. 2007). While this simpli-
fication will fail to appropriately characterize all the nuances
in mid-infrared spectra for samples specifically selected to be
infrared-luminous (e.g., GOALS; Stierwalt et al. 2013), there
are very few deeply obscured systems in infrared flux-limited
surveys like 5MUSES (Wu et al. 2010).
3.2. Addition of an AGN Template
While the star-forming templates themselves from Dale &
Helou (2002) are only slightly modified, we introduce here a
fundamental addition to the templates by incorporating a second
parameter, one that accounts for accretion disk-powered infrared
luminosity.
Until recently, the state-of-the-art in panchromatic AGN
SEDs was still the pioneering work of Elvis et al. (1994), who
studied 47 non-blazar quasars from the radio through 10 keV
3

The Astrophysical Journal, 784:83 (11pp), 2014 March 20 Dale et al.
Figure 3. Compilation of several infrared AGN/quasar templates/models from
the literature (Richards et al. 2006; Schartmann et al. 2008; Mullaney et al.
2011; Shang et al. 2011; Kirkpatrick et al. 2012; Shi et al. 2013).
(A color version of this figure is available in the online journal.)
X-rays. However, the recent influx of large multi-wavelength
databases has allowed for more complete reconstructions of
AGN SEDs. For example, in 2011 Shang and collaborators
updated the Elvis et al. (1994) work using data from 85 non-
blazar quasars. The data involved in their analysis include
X-ray, far- and near-ultraviolet, optical, near-, mid-, and far-
infrared, and radio spectroscopy and/or photometry. Shi et al.
(2013) utilize Spitzer infrared spectral (5–30 μm) and imaging
(24, 70, and 160 μm) data from all Palomar–Green quasars
to generate a median mid-infrared spectrum similar to that of
Shang et al. (2011). These quasars are UV selected and thus
are minimally obscured Type 1 AGNs. Figure 3 provides a
comparison of several AGN/quasar infrared templates available
from the literature as well as the maximum and minimum curves
based on the clumpy torus models of Schartmann et al. (2008).
We adopt here the median spectrum of Shi et al. (2013) since
they have carefully attempted to remove any star formation-
related contributions from the host galaxies, including forcing
the template beyond 70 μm to drop like a blackbody (νf
ν
λ
4
;
see their Figure 3). Several features are evident in this median
quasar spectrum of Shi et al. (2013), including the broad silicate
emission features near 10 and 18 μm and the [O iv] 25.9 μm
fine-structure lines that are seen in many AGNs (Hao et al. 2005;
Armus et al. 2007).
Figure 2 displays this median quasar SED of Shi et al. (2013)
in addition to a suite of normal star-forming galaxy curves
spanning a range in α
SF
. To simulate the spectral appearance
of a source for which the emission has contributions from
both an AGN and normal star formation, we employ linear
mixing over the 5–20 μm wavelength range. For this work
we have developed mixed combinations for 5–20 μmAGN
fractions running from 0% to 100%, spaced at 5% intervals.
6
Figure 4 shows how the resulting infrared–radio SEDs appear
for a variety of combinations of AGN and star-forming emission.
6
Available at physics.uwyo.edu/ddale/research/seds/seds.html
4. RESULTS
4.1. Model Color Distributions
To date most efforts to disentangle infrared emission from
AGNs and star formation have focused on utilizing mid-infrared
continuum data sets (e.g., Laurent et al. 2000; Murphy et al.
2009; Mullaney et al. 2011;Wuetal.2011) or a combination
of the mid-infrared continuum plus mid-infrared fine-structure
lines (e.g., Genzel et al. 1998; Peeters et al. 2004; Armus et al.
2007;Daleetal.2009). A common complementary technique
for identifying AGN contributions, especially useful when mid-
infrared spectral data are unavailable, involves combinations of
flux ratios that utilize data from three or four broadband filters
(e.g., Lacy et al. 2004; Stern et al. 2005; Yan et al. 2013; Kirk-
patrick et al. 2013; Mendez et al. 2013). Figure 5 shows how the
various combinations of the star-forming and AGN templates
appear in two different infrared color–color diagrams (assum-
ing rest wavelengths). Using such continuum diagnostics, one
can estimate both the AGN fractional contribution as well as
the characteristics of the star-forming portion of the galaxy,
e.g., dust temperature. The colors for local actively star-forming
galaxies (filled squares) shown in Figure 5 come from Sieben-
morgen & Kr
¨
ugel (2007), the colors for normal star-forming
galaxies (open circles) are from Dale et al. (2012), and the col-
ors for the local AGN M87 (filled triangle) are from NED; the
f
ν
(70 μm)/f
ν
(500 μm) color for M87 falls below the displayed
range since the 500 μm flux is overwhelmed by synchrotron
radiation (Baes et al. 2010). While some (SINGS/KINGFISH)
galaxies from Dale et al. (2012) have nuclei that are distin-
guished by Seyfert or LINER characteristics, very few have
their global luminosity dominated by an active nucleus (Mous-
takas et al. 2010). Note that our color–color analysis does not
to extend to wavelengths shorter than 8 μm and thus cannot be
directly compared to Spitzer IRAC color–color analyses (e.g.,
Lacy et al. 2004; Stern et al. 2005). This restriction is by design:
the Spoon et al. (2007) star-forming template begins at 5 μm,
a feature which conveniently minimizes complications arising
from stellar emission.
4.2. PAH Equivalent Width Distributions
The strengths of various PAH features have been widely used
to diagnose the main power source of a galaxy (e.g., Genzel et al.
1998; Laurent et al. 2000; Armus et al. 2007; Smith et al. 2007;
Spoon et al. 2007;Daleetal.2009;Hern
´
an-Caballero et al. 2009;
Diamond-Stanic & Rieke 2010;Wuetal.2010; Shang et al.
2011). These studies suggest that EW (PAH 6.2 μm) 0.2 μm
is an approximate delineation between sources predominantly
powered by AGNs and those mostly powered by star formation.
Figure 6 shows how the PAH (6.2 μm) equivalent widths for our
models depend on far-infrared color. The different curves show
the trends for a variety of AGN fractions; the 6.2 μm equivalent
width for the Spoon et al. (2007) pure star-forming curve used
here is 0.5 μm. As can be seen from the figure, larger AGN
fractions correspond to lower equivalent width, attributable to
the fact that the adopted AGN template is essentially devoid
of PAH emission features. Moreover, each trend of connected
points in Figure 6 dips to lower equivalent widths at warmer
far-infrared colors, a feature that complicates using the 6.2 μ
m
equivalent width as a pure AGN/star-forming diagnostic. This
effect of diminished PAH strength (equivalent width) as a
function of star formation activity level is weakly built in to
the star-forming models (see Figure 6 of Dale et al. 2001),
echoing the results of diminished PAH emission for regions
4

The Astrophysical Journal, 784:83 (11pp), 2014 March 20 Dale et al.
Figure 4. Left: the model curves that result from combining a fixed star-forming template (α
SF
=−0.2) with a variable contribution to the 5–20 μm mid-infrared
emission from the radio-quiet quasar curve in Figure 2. Right: the model curves that result from equally combining star-forming templates with the quasar curve.
“Equal” implies a 50% contribution to the emission over 5–20 μm.
(A color version of this figure is available in the online journal.)
Figure 5. Rest-frame color–color diagrams for the joint AGN–star-forming spectral energy distribution models. The colors for actively star-forming galaxies (filled
squares) come from Siebenmorgen & Kr
¨
ugel (2007), the colors for normal star-forming galaxies (open circles) are from Dale et al. (2012), and the colors for the
local AGN M87 (filled triangle) are from the NASA/IPAC Extragalactic Database. The range of model colors displayed for a given mid-infrared AGN percentage
represents the diversity of colors in the star-forming templates; the color spread for a single AGN percentage indicates the impact of varying α
SF
. Note that the terms
“actively star-forming” and “quiescent” can be directly tied to the average dust temperature, which in turn is parameterized by α
SF
(Chapman et al. 2003).
(A color version of this figure is available in the online journal.)
5

Citations
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Journal ArticleDOI
TL;DR: CIGALE as mentioned in this paper is a tool to estimate the spectral energy distribution (SED) of galaxies from the far-ultraviolet (FUV) to radio spectrum of galaxies.
Abstract: Context . Measuring how the physical properties of galaxies change across cosmic times is essential to understand galaxy formation and evolution. With the advent of numerous ground-based and space-borne instruments launched over the past few decades we now have exquisite multi-wavelength observations of galaxies from the far-ultraviolet (FUV) to the radio domain. To tap into this mine of data and obtain new insight into the formation and evolution of galaxies, it is essential that we are able to extract information from their spectral energy distribution (SED).Aims . We present a completely new implementation of Code Investigating GALaxy Emission (CIGALE). Written in python, its main aims are to easily and efficiently model the FUV to radio spectrum of galaxies and estimate their physical properties such as star formation rate, attenuation, dust luminosity, stellar mass, and many other physical quantities.Methods . To compute the spectral models, CIGALE builds composite stellar populations from simple stellar populations combined with highly flexible star formation histories, calculates the emission from gas ionised by massive stars, and attenuates both the stars and the ionised gas with a highly flexible attenuation curve. Based on an energy balance principle, the absorbed energy is then re-emitted by the dust in the mid- and far-infrared domains while thermal and non-thermal components are also included, extending the spectrum far into the radio range. A large grid of models is then fitted to the data and the physical properties are estimated through the analysis of the likelihood distribution.Results . CIGALE is a versatile and easy-to-use tool that makes full use of the architecture of multi-core computers, building grids of millions of models and analysing samples of thousands of galaxies, both at high speed. Beyond fitting the SEDs of galaxies and parameter estimations, it can also be used as a model-generation tool or serve as a library to build new applications.

548 citations


Cites background or methods from "A Two-Parameter Model for the Infra..."

  • ...However, this flexibility comes at the cost of a much larger parameter space to explore compared to the Dale et al. (2014) templates and is therefore more expensive in terms of processing power and memory....

    [...]

  • ...While less physically motivated than the Draine & Li (2007) models and not based on observations as the Dale et al. (2014) templates, the Casey (2012) models are very flexible and can be easily used for local and high-redshift galaxies....

    [...]

  • ...For CIGALE, we consider three different sets of models: the Dale et al. (2014) empirical templates, the Draine & Li (2007) models (including the updates of Draine et al. 2014), and the Casey (2012) analytic model....

    [...]

  • ...3.5.1. dale2014 module The dust templates of Dale et al. (2014) are based on a sample of nearby star-forming galaxies originally presented in Dale & Helou (2002)....

    [...]

  • ...…page 16 of 33 A103, page 17 of 33 gas (Inoue 2011), attenuation by dust (Calzetti et al. 2000; Charlot & Fall 2000) and re-emission of the energy at longer wavelengths (Draine & Li 2007; Casey 2012; Dale et al. 2014), active nuclei (Fritz et al. 2006; Dale et al. 2014), and the IGM (Meiksin 2006)....

    [...]

01 May 2001
TL;DR: The SWIRE project as mentioned in this paper is the largest of the SIRTF Legacy programs, which surveys 65 sq. deg. in seven high latitude fields selected to be the best wide low-extinction windows into the extragalactic sky.
Abstract: The largest of the SIRTF Legacy programs, SWIRE will survey 65 sq. deg. in seven high latitude fields selected to be the best wide low-extinction windows into the extragalactic sky. SWIRE will detect millions of spheroids, disks and starburst galaxies to z>3 and will map L* and brighter systems on scales up to 150 Mpc at z∼0.5–1. It will also detect ∼104 low extinction AGN and large numbers of obscured AGN. An extensive program of complementary observations is underway. The data are non-proprietary and will be made available beginning in Spring 2004.

484 citations

Journal ArticleDOI
TL;DR: CIGALE as discussed by the authors is a tool for modeling the FUV to radio spectrum of galaxies and estimating their physical properties such as star formation rate, attenuation, dust luminosity, stellar mass, and many other physical quantities.
Abstract: Context. Measuring how the physical properties of galaxies change across cosmic times is essential to understand galaxy formation and evolution. With the advent of numerous ground-based and space-borne instruments launched over the past few decades we now have exquisite multi-wavelength observations of galaxies from the FUV to the radio domain. To tap into this mine of data and obtain new insight into the formation and evolution of galaxies, it is essential that we are able to extract information from their SED. Aims. We present a completely new implementation of CIGALE. Written in python, its main aims are to easily and efficiently model the FUV to radio spectrum of galaxies and estimate their physical properties such as star formation rate, attenuation, dust luminosity, stellar mass, and many other physical quantities. Methods. To compute the spectral models, CIGALE builds composite stellar populations from simple stellar populations combined with highly flexible star formation histories, calculates the emission from gas ionised by massive stars, and attenuates both the stars and the ionised gas with a highly flexible attenuation curve. Based on an energy balance principle, the absorbed energy is then re-emitted by the dust in the mid- and far-infrared domains while thermal and non-thermal components are also included, extending the spectrum far into the radio range. A large grid of models is then fitted to the data and the physical properties are estimated through the analysis of the likelihood distribution. Results. CIGALE is a versatile and easy-to-use tool that makes full use of the architecture of multi-core computers, building grids of millions of models and analysing samples of thousands of galaxies, both at high speed. Beyond fitting the SEDs of galaxies and parameter estimations, it can also be used as a model-generation tool or serve as a library to build new applications.

376 citations

Journal ArticleDOI
09 Aug 2019-Science
TL;DR: In this paper, the authors reported the interferometric localization of the single-pulse fast radio burst (FRB 180924) to a position 4 kiloparsecs from the center of a luminous galaxy at redshift 0.3214.
Abstract: Fast radio bursts (FRBs) are brief radio emissions from distant astronomical sources. Some are known to repeat, but most are single bursts. Nonrepeating FRB observations have had insufficient positional accuracy to localize them to an individual host galaxy. We report the interferometric localization of the single-pulse FRB 180924 to a position 4 kiloparsecs from the center of a luminous galaxy at redshift 0.3214. The burst has not been observed to repeat. The properties of the burst and its host are markedly different from those of the only other accurately localized FRB source. The integrated electron column density along the line of sight closely matches models of the intergalactic medium, indicating that some FRBs are clean probes of the baryonic component of the cosmic web.

357 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used the latest version of CIGALE, a fast state-of-the-art galaxy SED-fitting model relying on energy balance, to study the influence of an AGN in a self consistent manner in estimating both the star formation rate and stellar mass in galaxies, as well as to calculate the contribution of the AGN to the power output of the host.
Abstract: Detailed studies of the spectral energy distribution (SED) of normal galaxies have increasingly been used to understand the physical mechanism dominating their integrated emission, mainly owing to the availability of high quality multi-wavelength data from the UV to the far-infrared (FIR). However, systems hosting dust-enshrouded nuclear starbursts and/or an accreting supermassive black hole (an active galactic nucleus or AGN) are especially challenging to study. This is due to the complex interplay between the heating by massive stars and the AGN, the absorption and emission of radiation from dust, as well as the presence of the underlying old stellar population. We used the latest release of CIGALE, a fast state-of-the-art galaxy SED-fitting model relying on energy balance, to study the influence of an AGN in a self consistent manner in estimating both the star formation rate (SFR) and stellar mass in galaxies, as well as to calculate the contribution of the AGN to the power output of the host. Using the semi-analytical galaxy formation model galform, we created a suite of mock galaxy SEDs using realistic star formation histories (SFH). We also added an AGN of Type-1, Type-2, or intermediate-type whose contribution to the bolometric luminosity can be variable. We performed an SED-fitting of these catalogues with CIGALE, assuming three different SFHs: a single-exponentially-decreasing (1τ-dec), a double-exponentially-decreasing (2τ-dec), and a delayed SFH. Constraining the overall contribution of an AGN to the total infrared luminosity (fracAGN) is very challenging for fracAGN< 20%, with uncertainties of ~5–30% for higher fractions depending on the AGN type, while FIR and sub-mm are essential. The AGN power has an impact on the estimation of M∗ in Type-1 and intermediate-type AGNs but has no effect on galaxies hosting Type-2 AGNs. We find that in the absence of AGN emission, the best estimates of M∗ are obtained using the 2τ-dec model but at the expense of realistic ages of the stellar population. The delayed SFH model provides good estimates of M∗ and SFR, with a maximum offset of 10% as well as better estimates of the age. Our analysis shows that the under-estimation of the SFR increases with fracAGN for Type-1 systems, as well as for low contributions of an intermediate AGN type, but it is quite insensitive to the emission of Type-2 AGNs up to fracAGN ~ 45%. A lack of sampling the FIR, or sub-mm domain systematically over-estimates the SFR (<20%), independent of the contribution of the AGN. Similarly, the UV emission is critical in accurately retrieving both the M∗ for Type-1 and intermediate- type AGN and the SFR of all three AGN types. We show that the presence of AGN emission introduces a scatter to the SFR-M∗ main sequence relation derived from SED-fitting, which is driven by the uncertainties on M∗. Finally, we used our mock catalogues to test the popular IR SED-fitting code DecompIR and show that fracAGN is under-estimated but that the SFR is recovered well for Type-1 and intermediate-types of AGN. The fracAGN, SFR, and LIR estimates of Type-2 AGNs are more problematic owing to a FIR emission disagreement between predicted and observed models.

206 citations


Cites background or methods from "A Two-Parameter Model for the Infra..."

  • ...The energy absorbed by the dust is reemitted in the IR using a choice of different dust templates (Dale & Helou 2002; Dale et al. 2014; Draine & Li 2007; Casey 2012)....

    [...]

  • ...Although the mock galaxies dust emission is modeled with the Draine & Li (2007) library, we use the Dale et al. (2014) templates in the SED fitting....

    [...]

  • ...The UV/optical stellar emission absorbed by dust is remitted in the IR assuming the Dale et al. (2014) templates....

    [...]

  • ...This disagreement is attributed to compatibility problems between Dale et al. (2014) and Draine & Li (2007) dust emission libraries in this range (Ciesla et al. 2014) that have no impact on the results of this study....

    [...]

References
More filters
Journal ArticleDOI
TL;DR: In this paper, the authors calculated IR emission spectra for dust heated by starlight, for mixtures of amorphous silicate and graphitic grains, including varying amounts of PAH particles.
Abstract: IR emission spectra are calculated for dust heated by starlight, for mixtures of amorphous silicate and graphitic grains, including varying amounts of PAH particles. The models are constrained to reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating. Updated IR absorption properties for the PAHs are presented that are consistent with observed emission spectra, including those newly obtained by Spitzer. We find a size distribution for the PAHs giving emission band ratios consistent with the observed spectra of the Milky Way and other galaxies. Emission spectra are presented for a wide range of starlight intensities. We calculate how the efficiency of emission into different IR bands depends on PAH size; the strong 7.7 μm emission feature is produced mainly by PAH particles containing Umin. We present graphical procedures using Spitzer IRAC and MIPS photometry to estimate the parameters qPAH, Umin, and γ, the fraction fPDR of the dust luminosity coming from photodissociation regions with U > 100, and the total dust mass Mdust.

2,102 citations


"A Two-Parameter Model for the Infra..." refers background in this paper

  • ...…their various parameters can yield insight into the physical characteristics of the system (e.g., Silva et al. 1998; Popescu et al. 2000; Gordon et al. 2001; Siebenmorgen & Krügel 2007; Draine & Li 2007; da Cunha et al. 2008; Galliano et al. 2008; Groves et al. 2008; Hermelo et al. 2013)....

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Journal ArticleDOI
TL;DR: In this article, the spectral energy distributions (SEDs) of normal, nonblazar, quasars over the whole available range (radio to 10 keV X-rays) of the electromagnetic spectrum are presented.
Abstract: We present an atlas of the spectral energy distributions (SEDs) of normal, nonblazar, quasars over the whole available range (radio to 10 keV X-rays) of the electromagnetic spectrum. The primary (UVSX) sample includes 47 quasars for which the spectral energy distributions include X-ray spectral indices and UV data. Of these, 29 are radio quiet, and 18 are radio loud. The SEDs are presented both in figures and in tabular form, with additional tabular material published on CD-ROM. Previously unpublished observational data for a second set of quasars excluded from the primary sample are also tabulated. The effects of host galaxy starlight contamination and foreground extinction on the UVSX sample are considered and the sample is used to investigate the range of SED properties. Of course, the properties we derive are influenced strongly by the selection effects induced by quasar discovery techniques. We derive the mean energy distribution (MED) for radio-loud and radio-quiet objects and present the bolometric corrections derived from it. We note, however, that the dispersion about this mean is large (approximately one decade for both the infrared and ultraviolet components when the MED is normalized at the near-infrared inflection). At least part of the dispersion in the ultraviolet may be due to time variability, but this is unlikely to be important in the infrared. The existence of such a large dispersion indicates that the MED reflects only some of the properties of quasars and so should be used only with caution.

1,923 citations


"A Two-Parameter Model for the Infra..." refers background or methods in this paper

  • ...For example, in 2011 Shang and collaborators updated the Elvis et al. (1994) work using data from 85 nonblazar quasars....

    [...]

  • ...Until recently, the state-of-the-art in panchromatic AGN SEDs was still the pioneering work of Elvis et al. (1994), who studied 47 non-blazar quasars from the radio through 10 keV (A color version of this figure is available in the online journal.)...

    [...]

Journal ArticleDOI
TL;DR: In this article, the evolution of the mid-infrared local luminosity function with redshift to the spectrum of the cosmic infrared background (CIRB) at j[ 5 km and the galaxy counts from various surveys at midinfrared, far infrared, and submillimeter wavelengths was investigated.
Abstract: The mid-infrared local luminosity function is evolved with redshift to —t the spectrum of the cosmic infrared background (CIRB) at j[ 5 km and the galaxy counts from various surveys at mid-infrared, far-infrared, and submillimeter wavelengths. A variety of evolutionary models provide satisfactory —ts to the CIRB and the number counts. The degeneracy in the range of models cannot be broken by current observations. However, the diUerent evolutionary models yield approximately the same comoving number density of infrared luminous galaxies as a function of redshift. Since the spectrum of the cosmic background at j[ 200 km is quite sensitive to the evolution at high redshift, i.e., z [ 1, all models that —t the counts require a —attening at z D 0.8 to avoid overproducing the CIRB. About 80% of the 140 km CIRB is produced at 0 \ z \ 1.5, while only about 30% of the 850 km background is produced within the same redshift range. The nature of the evolution is then translated into a measure of the dustenshrouded star formation rate (SFR) density as a function of redshift and compared with estimates from rest-frame optical/ultraviolet surveys. The dust-enshrouded SFR density appears to peak at z \ 0.8 ^ 0.1, much sooner than previously thought, with a value of yr~1 Mpc~3, and remains almost 0.25 ~0.10.12 M _ constant up to z D 2. At least 70% of this star formation takes place in infrared luminous galaxies with The long-wavelength observations that constrain our evolutionary models do not strongL IR [ 1011 L _ . ly trace the evolution at z [ 2 and a drop-oU in the dust-enshrouded SFR density is consistent with both the CIRB spectrum and the number counts. However, a comparison with the infrared luminosity function derived from extinction-corrected rest-frame optical/ultraviolet observations of the Lyman break galaxy population at z D 3 suggests that the almost —at comoving SFR density seen between redshifts of 0.8 and 2 extends up to a redshift of z D 4. (%)

1,292 citations


"A Two-Parameter Model for the Infra..." refers methods in this paper

  • ...For example, Chary & Elbaz (2001) and Rieke et al. (2009) provide template spectra sequenced according to their bolometric infrared luminosity LTIR....

    [...]

Journal ArticleDOI
TL;DR: In this paper, the authors examined the infrared (IR) 3-500μm spectral energy distributions (SEDs) of galaxies at 0 < z < 2.5, supplemented by a local reference sample from IRAS, ISO, Spitzer, and AKARI data.
Abstract: We present the deepest 100 to 500 μm far-infrared observations obtained with the Herschel Space Observatory as part of the GOODS-Herschel key program, and examine the infrared (IR) 3–500 μm spectral energy distributions (SEDs) of galaxies at 0 < z < 2.5, supplemented by a local reference sample from IRAS, ISO, Spitzer, and AKARI data. We determine the projected star formation densities of local galaxies from their radio and mid-IR continuum sizes. We find that the ratio of total IR luminosity to rest-frame 8 μm luminosity, IR8 (≡ L_(IR)^(tot)/L_8), follows a Gaussian distribution centered on IR8 = 4 (σ = 1.6) and defines an IR main sequence for star-forming galaxies independent of redshift and luminosity. Outliers from this main sequence produce a tail skewed toward higher values of IR8. This minority population ( 3 × 10^(10) L_⊙ kpc^(-2)) and a high specific star formation rate (i.e., starbursts). The rest-frame, UV-2700 A size of these distant starbursts is typically half that of main sequence galaxies, supporting the correlation between star formation density and starburst activity that is measured for the local sample. Locally, luminous and ultraluminous IR galaxies, (U)LIRGs (L_(IR)^(tot)≥ 10^(11) L_☉), are systematically in the starburst mode, whereas most distant (U)LIRGs form stars in the “normal” main sequence mode. This confusion between two modes of star formation is the cause of the so-called “mid-IR excess” population of galaxies found at z > 1.5 by previous studies. Main sequence galaxies have strong polycyclic aromatic hydrocarbon (PAH) emission line features, a broad far-IR bump resulting from a combination of dust temperatures (T_(dust) ~ 15–50 K), and an effective T_(dust) ~ 31 K, as derived from the peak wavelength of their infrared SED. Galaxies in the starburst regime instead exhibit weak PAH equivalent widths and a sharper far-IR bump with an effective T_(dust)~ 40 K. Finally, we present evidence that the mid-to-far IR emission of X-ray active galactic nuclei (AGN) is predominantly produced by star formation and that candidate dusty AGNs with a power-law emission in the mid-IR systematically occur in compact, dusty starbursts. After correcting for the effect of starbursts on IR8, we identify new candidates for extremely obscured AGNs.

1,235 citations


Additional excerpts

  • ...…= ξ0νLν(8 μm) + ξ1νLν(24 μm) + ξ2νLν(70 μm) + ξ3νLν(160 μm) (Spitzer-based), (5) where the coefficients are functions of the AGN fractional contribution to the 5–20 μm mid-infrared luminosity (for other recent formulations see also Boquien et al. 2010; Elbaz et al. 2011; Galametz et al. 2013)....

    [...]

Journal ArticleDOI
TL;DR: In this paper, the authors present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultraluminous IRAS galaxies (LIR ≥ 1012 L ).
Abstract: We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultraluminous IRAS galaxies (LIR ≥ 1012 L☉). We combine the survey results with a detailed case study, based on arcsecond resolution, near-IR, and millimeter imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these ultraluminous galaxies to ISO and literature data of 30 starburst and active galactic nuclei (AGN) template galaxies. We find the following: 1. Of the ultraluminous IRAS galaxies in our sample, 70%-80% are predominantly powered by recently formed massive stars, and 20%-30% are powered by a central AGN. These conclusions are based on a new infrared diagnostic diagram involving the ratio of high- to low-excitation mid-IR emission lines on the one hand, and the strength of the 7.7 μm PAH feature on the other hand. 2. At least half of the sources probably have simultaneously an active nucleus and starburst activity in a 1-2 kpc diameter circumnuclear disk/ring. 3. The mid-IR emitting regions are highly obscured [Av(screen) ~ 5-50 or Av(mixed) ~ 50-1000]. In a model where star-forming regions and dense molecular clouds are fully mixed, the ISO-derived, V-band dust extinctions approach the dust column densities inferred from CO millimeter measurements. After correction for these extinctions, we estimate that the star-forming regions in ultraluminous infrared galaxies have ages between 107 and 108 yr, similar to but somewhat larger than those found in lower luminosity starburst galaxies. 4. In the sample we have studied there is no obvious trend for the AGN component to dominate in the most compact, and thus most advanced mergers. Instead, at any given time during the merger evolution, the time-dependent compression of the circumnuclear interstellar gas, the accretion rate onto the central black hole, and the associated radiation efficiency may determine whether star formation or AGN activity dominates the luminosity of the system.

1,227 citations

Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "C: " ?

A two-parameter semi-empirical model is presented for the spectral energy distributions of galaxies with contributions to their infrared–submillimeter–radio emission from both star formation and accretion disk-powered activity.