30 research outputs found
Dense Molecular Gas and the Role of Star Formation in the Host Galaxies of Quasi-Stellar Objects
New millimeter-wave CO and HCN observations of the host galaxies of
infrared-excess Palomar Green quasi-stellar objects (PG QSOs) previously
detected in CO are presented. These observations are designed to assess the
validity of using the infrared luminosity to estimate star formation rates of
luminous AGN by determining the relative significance of dust-heating by young,
massive stars and active galactic nuclei (AGN) in QSO hosts and IRAS galaxies
with warm, AGN-like infrared colors. The HCN data show the PG QSO host IZw1 and
most of the warm IRAS galaxies to have high L_IR / L'_HCN (>1600) relative to
the cool IRAS galaxy population for which the median L_IR / L'_HCN ~
890(+440,-470). If the assumption is made that the infrared emission from cool
IRAS galaxies is reprocessed light from embedded star-forming regions, then
high values of L_IR / L'_HCN are likely the result of dust heating by the AGN.
Further, if the median ratio of L'_HCN / L'_CO ~ 0.06 observed for Seyfert
galaxies and IZw1 is applied to the PG QSOs not detected in HCN, then the
derived L_IR / L'_HCN correspond to a stellar contribution to the production of
L_IR of ~ 7-39%, and star formation rates ~ 2-37 M_sun/yr are derived for the
QSO hosts. Alternatively, if the far-infrared is adopted as the star formation
component of the total infrared in cool galaxies, the stellar contributions in
QSO hosts to their L_FIR are up to 35% higher than the percentages derived for
L_IR. This raises the possibility that the L_FIR in several of the PG QSO
hosts, including IZw1, could be due entirely to dust heated by young, massive
stars. Finally, there is no evidence that the global HCN emission is enhanced
relative to CO in galaxies hosting luminous AGN.Comment: LaTex, 31 pages, including 9 postscript figures, AJ, in press
(December 2006
Massive Star Cluster Formation and Destruction in Luminous Infrared Galaxies in GOALS
We present the results of a {\it Hubble Space Telescope} ACS/HRC FUV, ACS/WFC
optical study into the cluster populations of a sample of 22 Luminous Infrared
Galaxies in the Great Observatories All-Sky LIRG Survey. Through integrated
broadband photometry we have derived ages and masses for a total of 484 star
clusters contained within these systems. This allows us to examine the
properties of star clusters found in the extreme environments of LIRGs relative
to lower luminosity star-forming galaxies in the local Universe. We find that
by adopting a Bruzual \& Charlot simple stellar population (SSP) model and
Salpeter initial mass function, the age distribution of clusters declines as
, consistent with the age distribution derived
for the Antennae Galaxies, and interpreted as evidence for rapid cluster
disruption occuring in the strong tidal fields of merging galaxies. The large
number of young clusters identified in the sample also
suggests that LIRGs are capable of producing more high-mass clusters than what
is observed to date in any lower luminosity star-forming galaxy in the local
Universe. The observed cluster mass distribution of is consistent with the canonical -2 power law used to describe the
underlying initial cluster mass function (ICMF) for a wide range of galactic
environments. We interpret this as evidence against mass-dependent cluster
disruption, which would flatten the observed CMF relative to the underlying
ICMF distribution.Comment: 63 pages, 58 Figures, 56 Tables, Accepted for publication in Ap
Molecular Gas and the Host-Galaxy System of the z ~ 0.3 QSO PG 1700+518
The detection of CO(1→0) emission in the massive (i.e., MH ~ –26.13 mag), z ~ 0.3 host-galaxy system of the broad absorption line quasi-stellar object (QSO) PG1700+518 is reported. The host system has a CO luminosity of L'_CO ~ 1.4 × 10^(10) K km s^(–1) pc^2, and thus a star-forming molecular gas mass of M(H_2) ~ 6 × 10^(10) M_☉ (adopting an α = 4 M_☉[K km s^(–1) pc^2]^(–1)), making it one of the most molecular gas-rich Palomar-Green QSO hosts observed to date. New Hubble Space Telescope WFPC2 direct and NICMOS coronagraphic images show the highest resolution view yet of the host and companion. The new NICMOS image reveals the underlying, apparently tidally disrupted structure seen previously from high-resolution ground-based optical imaging. Light from the host galaxy is overwhelmed by the central point source in the WFPC2 images. The companion galaxy is well resolved in both data sets, and the WFPC2 provides for the first time a clear picture of the optically visible ring structure. The CO redshift is within the range of redshifts derived from optical QSO emission lines, thus the observed CO is associated with the QSO host. However, it cannot be ruled out that the companion has at least ~10^(10) M_☉ of molecular gas. Finally, if the far-infrared luminosity, which is 1/5 of the bolometric luminosity, is the luminosity of the starburst population, the star formation rate is estimated to be ~210 M_☉ yr^(–1). There is thus sufficient molecular gas in the QSO host galaxy to fuel both star formation and QSO activity for another ~10^8 yr. We speculate that we may be witnessing the fueling event in progress that resulted from a collision between the QSO host and the companion galaxy, and that there is an accompanying expulsion of material along our line of sight in the form of broad absorption line gas
Mid-Infrared Spectral Diagnostics of Luminous Infrared Galaxies
We present a statistical analysis of the mid-infrared (MIR) spectra of 248
luminous infrared (IR) galaxies (LIRGs) which comprise the Great Observatories
All-sky LIRG Survey (GOALS) observed with the Infrared Spectrograph (IRS)
on-board the Spitzer Space Telescope. The GOALS sample enables a direct
measurement of the relative contributions of star-formation and active galactic
nuclei (AGN) to the total IR emission from a large sample of local LIRGs. The
AGN contribution to the MIR emission (f-AGN) is estimated by employing several
diagnostics based on the properties of the [NeV], [OIV] and [NeII] fine
structure gas emission lines, the 6.2 microns PAH and the shape of the MIR
continuum. We find that 18% of all LIRGs contain an AGN and that in 10% of all
sources the AGN contributes more than 50% of the total IR luminosity. Summing
up the total IR luminosity contributed by AGN in all our sources suggests that
AGN supply ~12% of the total energy emitted by LIRGs. The average spectrum of
sources with an AGN looks similar to the average spectrum of sources without an
AGN, but it has lower PAH emission and a flatter MIR continuum. AGN dominated
LIRGs have higher IR luminosities, warmer MIR colors and are found in
interacting systems more often than pure starbursts LIRGs. However we find no
linear correlations between these properties and f-AGN. We used the IRAC colors
of LIRGs to confirm that finding AGN on the basis of their MIR colors may miss
~40% of AGN dominated (U)LIRGsComment: accepted for publication in ApJ, 34 pages, 12 figure
Spectral Energy Distributions of Local Luminous And Ultraluminous Infrared Galaxies
Luminous and ultraluminous infrared galaxies ((U)LIRGs) are the most extreme
star forming galaxies in the universe. The local (U)LIRGs provide a unique
opportunity to study their multi-wavelength properties in detail for comparison
to their more numerous counterparts at high redshifts. We present common large
aperture photometry at radio through X-ray wavelengths, and spectral energy
distributions (SEDs) for a sample of 53 nearby LIRGs and 11 ULIRGs spanning log
(LIR/Lsun) = 11.14-12.57 from the flux-limited Great Observatories All-sky LIRG
Survey (GOALS). The SEDs for all objects are similar in that they show a broad,
thermal stellar peak and a dominant FIR thermal dust peak, where nuLnu(60um) /
nuLnu(V) increases from ~2-30 with increasing LIR. When normalized at
IRAS-60um, the largest range in the luminosity ratio,
R(lambda)=log[nuLnu(lambda)/nuLnu(60um)] observed over the full sample is seen
in the Hard X-rays (HX=2-10 keV). A small range is found in the Radio (1.4GHz),
where the mean ratio is largest. Total infrared luminosities, LIR(8-1000um),
dust temperatures, and dust masses were computed from fitting thermal dust
emission modified blackbodies to the mid-infrared (MIR) through submillimeter
SEDs. The new results reflect an overall ~0.02 dex lower luminosity than the
original IRAS values. Total stellar masses were computed by fitting stellar
population synthesis models to the observed near-infrared (NIR) through
ultraviolet (UV) SEDs. Mean stellar masses are found to be log(M/Msun) =
10.79+/-0.40. Star formation rates have been determined from the infrared
(SFR_IR~45Msun/yr) and from the monochromatic UV luminosities
(SFR_UV~1.3Msun/yr), respectively. Multiwavelength AGN indicators have be used
to select putative AGN: about 60% of the ULIRGs would have been classified as
an AGN by at least one of the selection criteria.Comment: 39 pages, including 12 figures and 11 tables; accepted for
publication in ApJ
The Great Observatories All-Sky LIRG Survey: Comparison of Ultraviolet and Far-Infrared Properties
The Great Observatories All-sky LIRG Survey (GOALS) consists of a complete
sample of 202 Luminous Infrared Galaxies (LIRGs) selected from the IRAS Revised
Bright Galaxy Sample (RBGS). The galaxies span the full range of interaction
stages, from isolated galaxies to interacting pairs to late stage mergers. We
present a comparison of the UV and infrared properties of 135 galaxies in GOALS
observed by GALEX and Spitzer. For interacting galaxies with separations
greater than the resolution of GALEX and Spitzer (2-6"), we assess the UV and
IR properties of each galaxy individually. The contribution of the FUV to the
measured SFR ranges from 0.2% to 17.9%, with a median of 2.8% and a mean of 4.0
+/- 0.4%. The specific star formation rate of the GOALS sample is extremely
high, with a median value (3.9*10^{-10} yr^{-1}) that is comparable to the
highest specific star formation rates seen in the Spitzer Infrared Nearby
Galaxies Survey sample. We examine the position of each galaxy on the IR
excess-UV slope (IRX-beta) diagram as a function of galaxy properties,
including IR luminosity and interaction stage. The LIRGs on average have
greater IR excesses than would be expected based on their UV colors if they
obeyed the same relations as starbursts with L_IR < 10^{11}L_0 or normal
late-type galaxies. The ratio of L_IR to the value one would estimate from the
IRXg-beta relation published for lower luminosity starburst galaxies ranges
from 0.2 to 68, with a median value of 2.7. A minimum of 19% of the total IR
luminosity in the RBGS is produced in LIRGs and ULIRGs with red UV colors (beta
> 0). Among resolved interacting systems, 32% contain one galaxy which
dominates the IR emission while the companion dominates the UV emission. Only
21% of the resolved systems contain a single galaxy which dominates both
wavelengths.Comment: 37 pages, 10 figures, accepted for publication in Ap
Investigation of Dual Active Nuclei, Outflows, Shock-Heated Gas, and Young Star Clusters in Markarian 266
Results of observations with the Spitzer, Hubble, GALEX, Chandra, and
XMM-Newton space telescopes are presented for the Luminous Infrared Galaxy
(LIRG) merger Mrk 266. The SW (Seyfert 2) and NE (LINER) nuclei reside in
galaxies with Hubble types SBb (pec) and S0/a (pec), respectively. Both
galaxies have L > L*, and they are inferred to each contain a ~2.5x10^8 M_sun
black hole. Mrk 266 SW is likely the primary source of a bright Fe K-alpha line
detected from the system, consistent with the reflection-dominated X-ray
spectrum of a heavily obscured AGN. Optical knots embedded in an arc with
aligned radio continuum radiation, combined with luminous H_2 line emission,
provide evidence for a radiative bow shock in an AGN-driven outflow surrounding
the NE nucleus. Soft X-ray emission modeled as shock-heated plasma is
co-spatial with radio continuum emission between the galaxies. Mid-IR
diagnostics indicate roughly equal contributions of AGN and starburst radiation
powering the bolometric luminosity. Approximately 120 star clusters have been
detected, with most having estimated ages < 50 Myr. Detection of 24 micron
emission extending ~34 arcsec (20 kpc) north of the galaxies is interpreted as
~2x10^7 M_sun of dust entrained in an outflowing superwind; at optical
wavelengths this region is resolved into a fragmented morphology indicative of
Rayleigh-Taylor instabilities in an expanding shell of ionized gas. Mrk 266
demonstrates that the dust "blow-out" phase can begin in a LIRG well before the
galaxies fully coalesce during a subsequent ULIRG phase, and rapid gas
consumption in luminous dual AGNs (kpc scale separations) early in the merger
process may explain the paucity of detected binary QSOs (sub-pc scale orbits)
in large surveys. An evolutionary sequence is proposed representing a
progression from dual to binary AGNs, accompanied by an increase in observed
L_x/L_ir ratios by 10^4 or more.Comment: Accepted for publication in the Astronomical Journal, 40 pages, 31
figures, 15 table
The Buried Starburst in the Interacting Galaxy II Zw 096 as Revealed by the Spitzer Space Telescope
An analysis of data from the Spitzer Space Telescope, Hubble Space Telescope, Chandra X-ray Observatory, and AKARI Infrared Astronomy Satellite is presented for the z = 0.036 merging galaxy system II Zw 096 (CGCG 448-020). Because II Zw 096 has an infrared luminosity of log(L_(IR)/L_☉) = 11.94, it is classified as a Luminous Infrared Galaxy (LIRG), and was observed as part of the Great Observatories All-sky LIRG Survey (GOALS). The Spitzer data suggest that 80% of the total infrared luminosity comes from an extremely compact, red source not associated with the nuclei of the merging galaxies. The Spitzer mid-infrared spectra indicate no high-ionization lines from a buried active galactic nucleus in this source. The strong detection of the 3.3 μm and 6.2 μm polycyclic aromatic hydrocarbon emission features in the AKARI and Spitzer spectra also implies that the energy source of II Zw 096 is a starburst. Based on Spitzer infrared imaging and AKARI near-infrared spectroscopy, the star formation rate is estimated to be 120 M_☉ yr^(-1) and >45 M_☉ yr^(-1), respectively. Finally, the high-resolution B-, I-, and H-band images show many star clusters in the interacting system. The colors of these clusters suggest at least two populations—one with an age of 1-5 Myr and one with an age of 20-500 Myr, reddened by 0-2 mag of visual extinction. The masses of these clusters span a range between 10^6 and 10^8 M_☉. This starburst source is reminiscent of the extranuclear starburst seen in NGC 4038/9 (the Antennae Galaxies) and Arp 299 but approximately an order of magnitude more luminous than the Antennae. The source is remarkable in that the off-nuclear infrared luminosity dominates the entire system
Mid-Infrared Properties of Nearby Luminous Infrared Galaxies I: Spitzer IRS Spectra for the GOALS Sample
The Great Observatories All-Sky LIRG Survey (GOALS) is a multiwavelength
study of luminous infrared galaxies (LIRGs) in the local universe. Here we
present low resolution Spitzer spectra covering 5-38um and provide a basic
analysis of the mid-IR spectral properties for nearby LIRGs. In a companion
paper, we discuss detailed fits to the spectra. The GOALS sample of 244 nuclei
in 180 luminous and 22 ultraluminous IR galaxies represents a complete subset
of the IRAS RBGS and covers a range of merger stages, morphologies and spectral
types. The majority (>60%) of GOALS LIRGs have high 6.2um PAH equivalent widths
(EQW > 0.4um) and low levels of silicate absorption (s_9.7um >-1.0). There is a
general trend among the U/LIRGs for silicate depth and MIR slope to increase
with LIR. U/LIRGs in the late stages of a merger also have on average steeper
MIR slopes and higher levels of dust obscuration. Together these trends suggest
that as gas & dust is funneled towards the center of a coalescing merger, the
nuclei become more compact and obscured. The sources that depart from these
correlations have very low PAH EQW (EQW < 0.1um) consistent with their MIR
emission being dominated by an AGN. The most heavily dust obscured sources are
the most compact in their MIR emission, suggesting that the obscuring (cool)
dust is associated with the outer regions of the starburst. As the merger
progresses a marked decline is seen for the fraction of high EQW (star
formation dominated) sources while the fraction of composite sources increases
but the fraction of AGN-dominated sources remains low. When compared to the MIR
spectra of submillimeter galaxies (SMGs) at z~2, the average GOALS LIRG is more
absorbed at 9.7um and has more PAH emission. However, when the AGN
contributions to both the local LIRGs and the high-z SMGs are removed, the
average local starbursting LIRG closely resembles the starbursting SMGs.Comment: Accepted for publication in ApJ
Tracing PAHs and Warm Dust Emission in the Seyfert Galaxy NGC 1068
We present a study of the nearby Seyfert galaxy NGC 1068 using mid- and far-
infrared data acquired with the IRAC, IRS, and MIPS instruments aboard the
Spitzer Space Telescope. The images show extensive 8 um and 24 um emission
coinciding with star formation in the inner spiral approximately 15" (1 kpc)
from the nucleus, and a bright complex of star formation 47" (3 kpc) SW of the
nucleus. The brightest 8 um PAH emission regions coincide remarkably well with
knots observed in an Halpha image. Strong PAH features at 6.2, 7.7, 8.6, and
11.3 um are detected in IRS spectra measured at numerous locations inside,
within, and outside the inner spiral. The IRAC colors and IRS spectra of these
regions rule out dust heated by the AGN as the primary emission source; the
SEDs are dominated by starlight and PAH emission. The equivalent widths and
flux ratios of the PAH features in the inner spiral are generally consistent
with conditions in a typical spiral galaxy ISM. Interior to the inner spiral,
the influence of the AGN on the ISM is evident via PAH flux ratios indicative
of a higher ionization parameter and a significantly smaller mean equivalent
width than observed in the inner spiral. The brightest 8 and 24 um emission
peaks in the disk of the galaxy, even at distances beyond the inner spiral, are
located within the ionization cones traced by [O III]/Hbeta, and they are also
remarkably well aligned with the axis of the radio jets. Although it is
possible that radiation from the AGN may directly enhance PAH excitation or
trigger the formation of OB stars that subsequently excite PAH emission at
these locations in the inner spiral, the orientation of collimated radiation
from the AGN and star formation knots in the inner spiral could be
coincidental. (abridged)Comment: 20 pages, 11 figures; AJ, accepted; full resolution version available
at http://spider.ipac.caltech.edu/staff/jhhowell/astro/howelln1068.pd