60 research outputs found
HST NICMOS imaging of z~2, 24 micron-selected Ultraluminous Infrared Galaxies
We present Hubble Space Telescope NICMOS H-band imaging of 33 Ultraluminous
Infrared Galaxies (ULIRGs) at z~2 that were selected from the 24 micron catalog
of the Spitzer Extragalactic First Look Survey. The images reveal that at least
17 of the 33 objects are associated with interactions. Up to one fifth of the
sources in our sample could be minor mergers whereas only 2 systems are merging
binaries with luminosity ratio <=3:1, which is characteristic of local ULIRGs.
The rest-frame optical luminosities of the sources are of the order 10^10-10^11
L_sun and their effective radii range from 1.4 to 4.9 kpc. The most compact
sources are either those with a strong active nucleus continuum or those with a
heavy obscuration in the mid-infrared regime, as determined from Spitzer
Infra-Red Spectrograph data. The luminosity of the 7.7 micron feature produced
by polycyclic aromatic hydrocarbon molecules varies significantly among compact
systems whereas it is typically large for extended systems. A bulge-to-disk
decomposition performed for the 6 brightest (m_H<20) sources in our sample
indicates that they are best fit by disk-like profiles with small or negligible
bulges, unlike the bulge-dominated remnants of local ULIRGs. Our results
provide evidence that the interactions associated with ultraluminous infrared
activity at z~2 can differ from those at z~0.Comment: ApJ, in press. Document revised to match the journal versio
Mid-infrared spectroscopy of high-redshift obscured quasars
We present mid-infrared observations of 18 sources from a sample of 21 z ~ 2 radio-intermediate obscured (type 2) quasars. The mid-infrared spectra of the sources are continuum dominated, and 12 sources show deep silicate absorption with τ_9.7 ~ 1–2. Combining mid-infrared and optical spectra, we achieve 86% spectroscopic completeness which allows us to confirm that most (63^(+14)_(−22%)) z ~ 2 radio-intermediate quasars are obscured. The new spectra also prove that many high-redshift type 2 quasars do not show any rest-frame ultraviolet emission lines. From the 18 individual mid-infrared spectra, we classify most of the sources into three subsamples: those with hints of the 7.7 and 6.2 μm polyaromatic hydrocarbons (3/18 sources show PAHs; subsample A), those with an excess of emission around 8 μm but no hint of the 6.2 μm PAH (7/18 cases; subsample B), and pure-continuum sources with no visible excess (4/18 sources; subsample C). The remaining 4/18 sources have spectra that are featureless or too noisy for any features to be visible. In subsample A, averaging the spectra leads to a statistical detection of both 6.2 and 7.7 μm PAHs over the continuum, with the strength of the 7.7 μm PAH comparable to that of submillimeter-selected galaxies (SMGs) at similar redshifts. These sources are in a phase of coeval growth of a supermassive black hole and a host galaxy
The stellar, molecular gas and dust content of the host galaxies of two z~2.8 dust obscured quasars
We present optical through radio observations of the host galaxies of two
dust obscured, luminous quasars selected in the mid-infrared, at z=2.62 and
z=2.99, including a search for CO emission. Our limits on the CO luminosities
are consistent with these objects having masses of molecular gas <~10^10 solar
masses, several times less than those of luminous submillimeter-detected
galaxies (SMGs) at comparable redshifts. Their near-infrared spectral energy
distributions, however, imply that these galaxies have high stellar masses
(~10^11-12 solar masses). The relatively small reservoirs of molecular gas and
low dust masses are consistent with them being relatively mature systems at
high-z.Comment: AJ, in pres
Spitzer- and Herschel-based Spectral Energy Distributions of 24 μm Bright z ~ 0.3-3.0 Starbursts and Obscured Quasars
In this paper, we characterize the infrared spectral energy distributions (SEDs) of mid-IR-selected z ∼ 0.3–3.0 and
LIR ∼ 10^(11)–10^(13) L_☉ galaxies, and study how their SEDs differ from those of local and high-z analogs. Infrared
SEDs depend both on the power source (AGN or star formation) and the dust distribution. Therefore, differences
in the SEDs of high-z and local galaxies provide clues as to differences in their physical conditions. Our mid-IR
flux-limited sample of 191 sources is unique in size, and spectral coverage, including Spitzer mid-IR spectroscopy.
Here, we add Herschel photometry at 250μm, 350μm, and 500μm, which allows us, through fitting an empirical
SED model, to obtain accurate total IR luminosities, as well as constrain the relative contributions of AGNs and
starbursts to those luminosities. Our sample includes three broad categories of SEDs: ∼23% of the sources are
AGNs (i.e., where the AGN contributes >50% of L_(IR)), ∼30% are starbursts where an AGN contributes <20%
of LIR, and the mid-IR spectra are starburst-like (i.e., strong polycyclic aromatic hydrocarbon features); and the
largest group (∼47%) are composites which show both significant AGN and starburst activity. The AGN-dominated
sources divide into ones that show a strong silicate 9.7μm absorption feature, implying highly obscured systems,
and ones that do not. The high-τ_(9.7) sources are half of our z > 1.2 AGNs, but show SEDs that are extremely
rare among local AGNs. The 30% of the sample that are starbursts, even the z ∼ 2, L_(IR) ∼ 10^(13) L_☉ ones, have
lower far-IR to mid-IR continuum ratios than local Ultra Luminous Infrared Galaxies (ULIRGs) or the z ∼ 2 submm
galaxies—effectively the SEDs of our z ∼ 2 starburst-dominated ULIRGs are much closer to those of local
Luminous Infrared Galaxies than ULIRGs. This is consistent with our earlier finding that, unlike local ULIRGs,
our high-z starbursts are typically only in the early stages of a merger. The SEDs of the composite sources are most similar to the local archetypal warm ULIRG, Mrk231, which supports the interpretation of their consisting of both AGN and starburst components. In summary, our results show that there is strong evolution in the SEDs between
local and z ∼ 2 IR-luminous galaxies, as well as that there is a wide range of SEDs among high redshift IR-luminous sources. The publicly available SED templates we derive from our sample will be particularly useful for infrared population synthesis models, as well as in the interpretation of other mid-IR high-z galaxies, in particular those detected by the recent all sky Wide-field Infrared Survey Explorer
An Application of Multi-band Forced Photometry to One Square Degree of SERVS: Accurate Photometric Redshifts and Implications for Future Science
We apply The Tractor image modeling code to improve upon existing multi-band
photometry for the Spitzer Extragalactic Representative Volume Survey (SERVS).
SERVS consists of post-cryogenic Spitzer observations at 3.6 and 4.5 micron
over five well-studied deep fields spanning 18 square degrees. In concert with
data from ground-based near-infrared (NIR) and optical surveys, SERVS aims to
provide a census of the properties of massive galaxies out to z ~ 5. To
accomplish this, we are using The Tractor to perform "forced photometry." This
technique employs prior measurements of source positions and surface brightness
profiles from a high-resolution fiducial band from the VISTA Deep Extragalactic
Observations (VIDEO) survey to model and fit the fluxes at lower-resolution
bands. We discuss our implementation of The Tractor over a square degree test
region within the XMM-LSS field with deep imaging in 12 NIR/optical bands. Our
new multi-band source catalogs offer a number of advantages over traditional
position-matched catalogs, including 1) consistent source cross-identification
between bands, 2) de-blending of sources that are clearly resolved in the
fiducial band but blended in the lower-resolution SERVS data, 3) a higher
source detection fraction in each band, 4) a larger number of candidate
galaxies in the redshift range 5 < z < 6, and 5) a statistically significant
improvement in the photometric redshift accuracy as evidenced by the
significant decrease in the fraction of outliers compared to spectroscopic
redshifts. Thus, forced photometry using The Tractor offers a means of
improving the accuracy of multi-band extragalactic surveys designed for galaxy
evolution studies. We will extend our application of this technique to the full
SERVS footprint in the future.Comment: accepted to ApJ, 22 pages, 12 figure
Detections of Water Ice, Hydrocarbons, and 3.3 \u3cem\u3eμ\u3c/em\u3em PAH in \u3cem\u3ez\u3c/em\u3e ~ 2 ULIRGs
We present the first detections of the 3 μm water ice and 3.4 μm amorphous hydrocarbon (HAC) absorption features in z ~ 2 ULIRGs. These are based on deep rest-frame 2-8 μm Spitzer Infrared Spectrograph spectra of 11 sources selected for their appreciable silicate absorption. The HAC-to-silicate ratio for our z ~ 2 sources is typically higher by a factor of 2-5 than that observed in the Milky Way. This HAC “excess” suggests compact nuclei with steep temperature gradients as opposed to predominantly host obscuration. Beside the above molecular absorption features, we detect the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature in one of our sources with three more individual spectra showing evidence for it. Stacking analysis suggests that water ice, hydrocarbons, and PAH are likely present in the bulk of this sample even when not individually detected. The most unexpected result of our study is the lack of clear detections of the 4.67 μm CO gas absorption feature. Only three of the sources show tentative signs of this feature at significantly lower levels than has been observed in local ULIRGs. Overall we find that the closest local analogs to our sources, in terms of 3-4 μm color, HAC-to-silicate and ice-to-silicate ratios, as well as low PAH equivalent widths, are sources dominated by deeply obscured nuclei. Such sources form only a small fraction of ULIRGs locally and are commonly believed to be dominated by buried active galactic nuclei (AGNs). Our sample suggests that, in an absolute number, such buried AGNs are at least an order of magnitude more common at z ~ 2 than today. The presence of PAH suggests that significant levels of star formation are present even if the obscured AGNs typically dominate the power budget
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