29 research outputs found
The Infrared Database of Extragalactic Observables from Spitzer I: the redshift catalog
This is the first of a series of papers on the Infrared Database of
Extragalactic Observables from Spitzer (IDEOS). In this work we describe the
identification of optical counterparts of the infrared sources detected in
Spitzer Infrared Spectrograph (IRS) observations, and the acquisition and
validation of redshifts. The IDEOS sample includes all the spectra from the
Cornell Atlas of Spitzer/IRS Sources (CASSIS) of galaxies beyond the Local
Group. Optical counterparts were identified from correlation of the extraction
coordinates with the NASA Extragalactic Database (NED). To confirm the optical
association and validate NED redshifts, we measure redshifts with unprecedented
accuracy on the IRS spectra ({\sigma}(dz/(1+z))=0.0011) by using an improved
version of the maximum combined pseudo-likelihood method (MCPL). We perform a
multi-stage verification of redshifts that considers alternate NED redshifts,
the MCPL redshift, and visual inspection of the IRS spectrum. The statistics is
as follows: the IDEOS sample contains 3361 galaxies at redshift 0<z<6.42 (mean:
0.48, median: 0.14). We confirm the default NED redshift for 2429 sources and
identify 124 with incorrect NED redshifts. We obtain IRS-based redshifts for
568 IDEOS sources without optical spectroscopic redshifts, including 228 with
no previous redshift measurements. We provide the entire IDEOS redshift catalog
in machine-readable formats. The catalog condenses our compilation and
verification effort, and includes our final evaluation on the most likely
redshift for each source, its origin, and reliability estimates.Comment: 11 pages, 6 figures, 1 table. Accepted for publication in MNRAS. Full
redshift table in machine-readable format available at
http://ideos.astro.cornell.edu/redshifts.htm
Resolving the AGN and host emission in the mid-infrared using a model-independent spectral decomposition
We present results on the spectral decomposition of 118 Spitzer Infrared
Spectrograph (IRS) spectra from local active galactic nuclei (AGN) using a
large set of Spitzer/IRS spectra as templates. The templates are themselves IRS
spectra from extreme cases where a single physical component (stellar,
interstellar, or AGN) completely dominates the integrated mid-infrared
emission. We show that a linear combination of one template for each physical
component reproduces the observed IRS spectra of AGN hosts with unprecedented
fidelity for a template fitting method, with no need to model extinction
separately. We use full probability distribution functions to estimate
expectation values and uncertainties for observables, and find that the
decomposition results are robust against degeneracies. Furthermore, we compare
the AGN spectra derived from the spectral decomposition with sub-arcsecond
resolution nuclear photometry and spectroscopy from ground-based observations.
We find that the AGN component derived from the decomposition closely matches
the nuclear spectrum, with a 1-sigma dispersion of 0.12 dex in luminosity and
typical uncertainties of ~0.19 in the spectral index and ~0.1 in the silicate
strength. We conclude that the emission from the host galaxy can be reliably
removed from the IRS spectra of AGN. This allows for unbiased studies of the
AGN emission in intermediate and high redshift galaxies -currently inaccesible
to ground-based observations- with archival Spitzer/IRS data and in the future
with the Mid-InfraRed Instrument of the James Webb Space Telescope. The
decomposition code and templates are available at
http://www.denebola.org/ahc/deblendIRS.Comment: 16 pages, 15 figures, 2 tables, accepted for publication in Ap
Extended Warm Gas in the ULIRG Mrk273: Galactic Outflows and Tidal Debris
We present new HST ACS medium- and narrow-band images and long-slit, optical
(4000 - 7200A) spectra obtained using the Isaac Newton Telescope (INT) on La
Palma, of the merging system Mrk273. The HST observations sample the
[OIII]4959,5007 emission from the galaxy and the nearby continuum. The images
show that the morphologies of the extended continuum and the ionised gas
emission from the galaxy are decoupled, extending almost perpendicular to each
other. In particular, we detect for the first time a spectacular structure of
ionised gas in the form of filaments extending ~23 kpc to the east of the
nuclear region. The quiescent ionised gas kinematics at these locations
suggests that these filaments are tidal debris left over from a secondary
merger event that are illuminated by an AGN in the nuclear regions. The images
also reveal a complex morphology in the nuclear region of the galaxy for both
the continuum and the [OIII] emission. Kinematic disturbance, in the form of
broad (FWHM > 500 km s-1) and/or strongly shifted (abs(\DeltaV) >150 km s-1)
emission line components, is found at almost all locations within a radius of
~4 kpc to the east and west of the northern nucleus. We fit the profiles of all
the emission lines of different ionisation with a kinematic model using up to 3
Gaussian components. From these fits we derive diagnostic line ratios that are
used to investigate the ionisation mechanisms at the different locations in the
galaxy. We show that, in general, the line ratios are consistent with
photoionization by an AGN as the main ionisation mechanism. Finally, the
highest surface brightness [OIII] emission is found in a compact region that is
coincident with the so-called SE nuclear component. The compactness, kinematics
and emission line ratios of this component suggest that it is a separate
nucleus with its own AGN.Comment: Accepted for Publication in A&
Mid-infrared properties of OH megamaser host galaxies. I: Spitzer IRS low- and high-resolution spectroscopy
We present mid-infrared spectra and photometry from the Infrared Spectrograph
on the Spitzer Space Telescope for 51 OH megamasers (OHMs), along with 15
galaxies confirmed to have no megamaser emission above L_OH = 10^2.3 L_sun. The
majority of galaxies display moderate-to-deep 9.7 um amorphous silicate
absorption, with OHM galaxies showing stronger average absorption and steeper
20-30 um continuum emission than non-masing galaxies. Emission from multiple
polycyclic aromatic hydrocarbons (PAHs), especially at 6.2, 7.7, and 11.3 um,
is detected in almost all systems. Fine-structure atomic emission (including
[Ne II], [Ne III], [S III], and [S IV]) and multiple H2 rotational transitions
are observed in more than 90% of the sample. A subset of galaxies show emission
from rarer atomic lines, such as [Ne V], [O IV], and [Fe II]. 50% of the OHMs
show absorption from water ice and hydrogenated amorphous carbon grains (HACs),
while absorption features from CO2, HCN, C2H2, and crystalline silicates are
also seen in several OHMs. Column densities of OH derived from 34.6 um OH
absorption are similar to those derived from 1667 MHz OH absorption in
non-masing galaxies, indicating that the abundance of masing molecules is
similar for both samples. This data paper presents full mid-infrared spectra
for each galaxy, along with measurements of line fluxes and equivalent widths,
absorption feature depths, and spectral indices.Comment: 28 pages, 10 figures; accepted to ApJS. Ancillary data includes full
IRS spectra of the complete sampl
Mid-infrared properties of OH megamaser host galaxies. II: Analysis and modeling of the maser environment
We present a comparison of Spitzer IRS data for 51 OH megamaser (OHM) hosts
and 15 non-masing ULIRGs. 10-25% of OHMs show evidence for the presence of an
AGN, significantly lower than the estimated AGN fraction from previous optical
and radio studies. Non-masing ULIRGs have a higher AGN fraction (50-95%) than
OHMs, although some galaxies in both samples show evidence of co-existing
starbursts and AGN. Radiative transfer models of the dust environment reveal
that non-masing galaxies tend to have clumpy dust geometries commonly
associated with AGN, while OHMs have deeper absorption consistent with a
smooth, thick dust shell. Statistical analyses show that the major differences
between masing and non-masing ULIRGs in the mid-IR relate to the optical depth
and dust temperature, which we measure using the 9.7 um silicate depth and
30-20 um spectral slope from the IRS data. Dust temperatures of 40-80 K derived
from the IRS data are consistent with predictions of OH pumping models and with
a minimum T_dust required for maser production. The best-fit dust opacities
({\tau}_V ~ 100 - 400), however, are nearly an order of magnitude larger than
those predicted for OH inversion, and suggest that modifications to the model
may be required. These diagnostics offer the first detailed test of an OHM
pumping model based only on the properties of its host galaxy and provide
important restrictions on the physical conditions relevant to OHM production.Comment: 15 pages, 12 figures; accepted to Ap
Mid-Infrared Properties of Luminous Infrared Galaxies II: Probing the Dust and Gas Physics of the GOALS Sample
The Great Observatories All-Sky LIRG Survey (GOALS) is a comprehensive,
multiwavelength study of luminous infrared galaxies (LIRGs) in the local
universe. Here we present the results of a multi-component, spectral
decomposition analysis of the low resolution mid-IR Spitzer IRS spectra from
5-38um of 244 LIRG nuclei. The detailed fits and high quality spectra allow for
characterization of the individual PAH features, warm molecular hydrogen
emission, and optical depths for silicate dust grains and water ices. We find
that starbursting LIRGs, which make up the majority of GOALS, are very
consistent in their MIR properties (i.e. tau_9.7um, tau_ice, neon line and PAH
feature ratios). However, as their PAH EQW decreases, usually an indicator of
an increasingly dominant AGN, LIRGs cover a larger spread in these MIR
parameters. The contribution from PAHs to the total L(IR) in LIRGs varies from
2-29% and LIRGs prior to their first encounter show higher L(PAH)/L(IR) ratios
on average. We observe a correlation between the strength of the starburst
(IR8) and the PAH fraction at 8um but not with the 7.7 to 11.3 PAH ratio,
suggesting the fractional PDR emission, and not the overall grain properties,
is associated with the rise in IR8 for galaxies off the starburst main
sequence. We detect crystalline silicate features in 6% of the sample but only
in the most obscured sources (s_9.7um < -1.24). Ice absorption features are
observed in 11% (56%) of GOALS LIRGs (ULIRGs). Most GOALS LIRGs have
L(H2)/L(PAH) ratios elevated above those observed for normal star-forming
galaxies and exhibit a trend for increasing L(H2)/L(PAH) ratio with increasing
L(H2). While star formation appears to be the dominant process responsible for
exciting the H2 in most of the GOALS galaxies, a subset of LIRGs (10%) show
excess H2 emission that is inconsistent with PDR models and may be excited by
shocks or AGN-induced outflows.Comment: 21 pages with 20 figures plus 2 table
Extinction in the 11.2 micron PAH band and the low L_11.2/L_IR in ULIRGs
We present a method for recovering the intrinsic (extinction-corrected)
luminosity of the 11.2 micron PAH band in galaxy spectra. Using 105 high S/N
Spitzer/IRS spectra of star-forming galaxies, we show that the equivalent width
ratio of the 12.7 and 11.2 micron PAH bands is independent on the optical
depth, with small dispersion of ~5% indicative of a nearly constant intrinsic
flux ratio R_int = (f_12.7/f_11.2)_int = 0.377 +/- 0.020. Conversely, the
observed flux ratio, R_obs = (f_12.7/f_11.2)_obs strongly correlates with the
silicate strength (S_sil) confirming that differences in R_obs reflect
variation in the optical depth. The relation between R_obs and S_sil reproduces
predictions for the Galactic Centre extinction law but disagrees with other
laws. We calibrate the total extinction affecting the 11.2 micron PAH from
R_obs, which we apply to another sample of 215 galaxies with accurate
measurements of the total infrared luminosity (L_IR) to investigate the impact
of extinction on L_11.2/L_IR. Correlation between L_11.2/L_IR and R_obs
independently on L_IR suggests that increased extinction explains the well
known decrease in the average L_11.2/L_IR at high L_IR. The
extinction-corrected L_11.2 is proportional to L_IR in the range L_IR/L_sun =
10^9--10^13. These results consolidate L_11.2 as a robust tracer of star
formation in galaxies.Comment: 14 pages, 9 figures, 2 tables. Accepted for publication in MNRA
Ultra-deep Spitzer Mid-Infrared Spectroscopy of LIRGs and ULIRGs at z ~ 1-2
We present ultra-deep mid-IR spectra of 48 infrared-luminous galaxies in the
GOODS-South field obtained with the InfraRed Spectrograph (IRS) on the Spitzer
Space Telescope. These galaxies are selected among faint infrared sources (0.14
- 0.5 mJy at 24 um) in two redshift bins (0.76-1.05 and 1.75-2.4) to sample the
major contributors to the cosmic infrared background at the most active epochs.
We estimate redshifts for 92% of the sample using PAH and Si absorption
features. Only few of these galaxies (5% at z~1 and 12% at z~2) have their
total infrared luminosity dominated by emission from AGN. The averaged mid-IR
spectra of the z~1 LIRGs and of the z~2 ULIRGs are very similar to the averaged
spectrum of local starbursts and HII-like ULIRGs, respectively. We find that
6.2um PAH equivalent widths reach a plateau of ~1 um for L(24 mu) < 1E11
L(sun). At higher luminosities, EW (6.2 mu) anti-correlates with L(24 um).
Intriguingly, high-z ULIRGs and SMG lie above the local EW (6.2 um) - L(24 um)
relationship suggesting that, at a given luminosity, high-z ULIRGs have AGN
contributions to their dust emission lower than those of local counterparts. A
quantitative analysis of their morphology shows that most of the luminous IR
galaxies have morphologies similar to those of IR-quiet galaxies at the same
redshift. All z~2 ULIRGs of our sample are IR-excess BzK galaxies and most of
them have L(FIR)/L(1600A) ratios higher than those of starburst galaxies at a
given UV slope. The ``IR excess'' (Daddi et al. 2007) is mostly due to strong
7.7 um PAH emission and under-estimation of UV dust extinction. On the basis of
the AGN-powered L (6 um) continuum measured directly from the mid-IR spectra,
we estimate an average intrinsic X-ray AGN luminosity of L(2-10 keV) = (0.1 +/-
0.6) 1E43 erg/s, a value substantially lower than the prediction by Daddi et
al. (2007).Comment: 45 pages, 36 figures, 2 tables, accepted for publication on Ap
Detection of the Buried Active Galactic Nucleus in NGC 6240 with the Infrared Spectrograph on the Spitzer Space Telescope
International audienceWe present mid-infrared spectra of the nearby ultraluminous infrared galaxy NGC 6240 taken with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The spectrum of NGC 6240 is dominated by strong fine-structure lines, rotational H2 lines, and polycyclic aromatic hydrocarbon (PAH) emission features. The H2 line fluxes suggest molecular gas at a variety of temperatures. A simple two-temperature fit to the S(0) through S(7) lines implies a mass of ~6.7Ă—106 Msolar at T~957 K and ~1.6Ă—109 Msolar at T~164 K, or about 15% of the total molecular gas mass in this system. Notably, we have detected the [Ne V] 14.3 mum emission line, with a flux of 5Ă—10-14 ergs cm-2 s-1, providing the first direct detection of the buried active galactic nucleus (AGN) in the mid-infrared. Modeling of the total spectral energy distribution (SED) from near- to far-infrared wavelengths requires the presence of a hot dust (T~700 K) component, which we also associate with the buried AGN. The small [Ne V]/[Ne II] and [Ne V]/IR flux ratios, the relative fraction of hot dust emission, and the large 6.2 mum PAH equivalent width (EQW), are all consistent with an apparent AGN contribution of only 3%-5% to the bolometric luminosity. However, correcting the measured [Ne V] flux by the extinction implied by the silicate optical depth and our SED fitting suggests an intrinsic fractional AGN contribution to the bolometric luminosity of ~20%-24% in NGC 6240, which lies within the range implied by fits to the hard X-ray spectrum