839 research outputs found
Accretion-Inhibited Star Formation in the Warm Molecular Disk of the Green-valley Elliptical Galaxy NGC 3226
We present archival Spitzer photometry and spectroscopy, and Herschel
photometry, of the peculiar "Green Valley" elliptical galaxy NGC~3226. The
galaxy, which contains a low-luminosity AGN, forms a pair with NGC~3227, and is
shown to lie in a complex web of stellar and HI filaments. Imaging at 8 and
16m reveals a curved plume structure 3 kpc in extent, embedded within the
core of the galaxy, and coincident with the termination of a 30 kpc-long HI
tail. In-situ star formation associated with the IR plume is identified from
narrow-band HST imaging. The end of the IR-plume coincides with a warm
molecular hydrogen disk and dusty ring, containing 0.7-1.1 10
M detected within the central kpc. Sensitive upper limits to the
detection of cold molecular gas may indicate that a large fraction of the H
is in a warm state. Photometry, derived from the UV to the far-IR, shows
evidence for a low star formation rate of 0.04 M yr
averaged over the last 100 Myrs. A mid-IR component to the Spectral Energy
Distribution (SED) contributes 20 of the IR luminosity of the galaxy,
and is consistent with emission associated with the AGN. The current measured
star formation rate is insufficient to explain NGC3226's global UV-optical
"green" colors via the resurgence of star formation in a "red and dead" galaxy.
This form of "cold accretion" from a tidal stream would appear to be an
inefficient way to rejuvenate early-type galaxies, and may actually inhibit
star formation.Comment: Accepted for Publication ApJ Oct 201
Testing the Seyfert Unification Theory: Chandra HETGS Observations of NGC 1068
We present spatially resolved Chandra HETGS observations of the Seyfert 2 galaxy NGC 1068. X-ray imaging and high resolution spectroscopy are used to test the Seyfert unification theory. Fe K-alpha emission is concentrated in the nuclear region, as are neutral and ionized continuum reflection. This is consistent with reprocessing of emission from a luminous, hidden X-ray source by the obscuring molecular torus and X-ray narrow-line region (NLR). We detect extended hard X-ray emission surrounding the X-ray peak in the nuclear region, which may come from the outer portion of the torus. Detailed modeling of the spectrum of the X-ray NLR confirms that it is excited by photoionization and photoexcitation from the hidden X-ray source. K-shell emission lines from a large range of ionization states of H-like and He-like N, O, Ne, Mg, Al, Si, S, and Fe XVII-XXIV L-shell emission lines are modeled. The emission measure distribution indicates roughly equal masses at all observed ionization levels in the range log xi=1-3. We separately analyze the spectrum of an off-nuclear cloud. We find that it has a lower column density than the nuclear region, and is also photoionized. The nuclear X-ray NLR column density, optical depth, outflow velocity, and electron temperature are all consistent with values predicted by optical spectropolarimetry for the region which provides a scattered view of the hidden Seyfert 1 nucleus
A Spitzer Unbiased Ultradeep Spectroscopic Survey
We carried out an unbiased, spectroscopic survey using the low-resolution
module of the infrared spectrograph (IRS) on board Spitzer targeting two 2.6
square arcminute regions in the GOODS-North field. IRS was used in spectral
mapping mode with 5 hours of effective integration time per pixel. One region
was covered between 14 and 21 microns and the other between 20 and 35 microns.
We extracted spectra for 45 sources. About 84% of the sources have reported
detections by GOODS at 24 microns, with a median F_nu(24um) ~ 100 uJy. All but
one source are detected in all four IRAC bands, 3.6 to 8 microns. We use a new
cross-correlation technique to measure redshifts and estimate IRS spectral
types; this was successful for ~60% of the spectra. Fourteen sources show
significant PAH emission, four mostly SiO absorption, eight present mixed
spectral signatures (low PAH and/or SiO) and two show a single line in
emission. For the remaining 17, no spectral features were detected. Redshifts
range from z ~ 0.2 to z ~ 2.2, with a median of 1. IR Luminosities are roughly
estimated from 24 microns flux densities, and have median values of 2.2 x
10^{11} L_{\odot} and 7.5 x 10^{11} L_{\odot} at z ~ 1 and z ~ 2 respectively.
This sample has fewer AGN than previous faint samples observed with IRS, which
we attribute to the fainter luminosities reached here.Comment: Published in Ap
Growth of Captive Juvenile Tripletail Lobotes surinamensis
Early-juvenile tripletail Lobotes surinamensis (n = 27; range 45–115 mm TL, 0 = 73.0 mm; range 3.2–34.7 g TW, 0 = 12.9 g) captured in pelagic Sargassum algae off coastal Mississippi in mid-July 1999 were reared in a recirculating seawater system for 210 days. Fish were maintained on a natural light-dark cycle and fed to satiation 3 times per day. Water temperature ranged from 25.2° to 29.0° C and salinity was 28.0‰. All fish were measured for length and weight on days 1, 60, 135 and 210 of the study. Between these dates, mean daily TL growth rates were 2.2 mm/day, 1.2 mm/day, and 1.0 mm/day, respectively, where as 0 daily TW growth rates were 2.9 g/day, 4.3 g/day, and 7.1 g/day. Over the entire study, 0 TL and TW growth rates were 1.4 mm/day and 4.9 g/day, respectively. There was a significant correlation between length and weight vs. date of measurement. At the end of the study, specimens ranged from 272–431 mm TL (0 = 359 mm) and from 443.9–2,380.0 g TW (0 = 1,012.5 g)
Strong Far-IR Cooling Lines, Peculiar CO Kinematics and Possible Star Formation Suppression in Hickson Compact Group 57
We present [C II] and [O I] observations from Herschel and CO(1-0) maps from
the Combined Array for{\dag} Research in Millimeter Astronomy (CARMA) of the
Hickson Compact Group HCG 57, focusing on the galaxies HCG 57a and HCG 57d. HCG
57a has been previously shown to contain enhanced quantities of warm molecular
hydrogen consistent with shock and/or turbulent heating. Our observations show
that HCG 57d has strong [C II] emission compared to L and weak
CO(1-0), while in HCG 57a, both the [C II] and CO(1-0) are strong. HCG 57a lies
at the upper end of the normal distribution of [C II]/CO and [C II]/FIR ratios,
and its far-IR cooling supports a low density warm diffuse gas that falls close
to the boundary of acceptable PDR models. However, the power radiated in the [C
II] and warm H emission have similar magnitudes, as seen in other
shock-dominated systems and predicted by recent models. We suggest that
shock-heating of the [C II] is a viable alternative to photoelectric heating in
violently disturbed diffuse gas. The existence of shocks is also consistent
with peculiar CO kinematics in the galaxy, indicating highly non-circular
motions are present. These kinematically disturbed CO regions also show
evidence of suppressed star formation, falling a factor of 10-30 below normal
galaxies on the Kennicutt-Schmidt relation. We suggest that the peculiar
properties of both galaxies are consistent with a highly dissipative off-center
collisional encounter between HCG 57d and 57a, creating ring-like morphologies
in both systems. Highly dissipative gas-on-gas collisions may be more common in
dense groups because of the likelihood of repeated multiple encounters. The
possibility of shock-induced SF suppression may explain why a subset of these
HCG galaxies have been found previously to fall in the mid-infrared green
valley.Comment: ApJ accepted, 16 pages, 12 figures, 3 table
Thrifty Phenotype vs Cold Adaptation: Trade-offs in Upper Limb Proportions of Himalayan Populations of Nepal
The multi-stress environment of high altitude has been associated with growth deficits in humans, particularly in zeugopod elements (forearm, lower leg). This is consistent with the thrifty phenotype hypothesis, which has been observed in Andeans, but has yet to be tested in other high altitude populations. In Himalayan populations, other factors, such as cold stress, may shape limb proportions. The current study investigated whether relative upper limb proportions of Himalayan adults (n=254) differ between highland and lowland populations, and whether cold adaptation or a thrifty phenotype mechanism may be acting here. Height, weight, humerus length, ulna length, hand length, and hand width were measured using standard methods. Relative to height, total upper limb and ulna lengths were significantly shorter in highlanders compared to lowlanders in both sexes, whilst hand and humerus length were not. Hand width did not significantly differ between populations. These results support the thrifty phenotype hypothesis, as hand and humerus proportions are conserved at the expense of the ulna. The reduction in relative ulna length could be attributed to cold adaptation, but the lack of difference between populations in both hand length and width indicate that cold adaptation is not shaping hands proportions in this case
Warm Molecular Hydrogen Emission in Normal Edge-On Galaxies NGC 4565 and NGC 5907
We have observed warm molecular hydrogen in two nearby edge-on disk galaxies,
NGC 4565 and NGC 5907, using the Spitzer high-resolution infrared spectrograph.
The 0-0 S(0) 28.2 micron and 0-0 S(1) 17.0 micron pure rotational lines were
detected out to 10 kpc from the center of each galaxy on both sides of the
major axis, and in NGC 4565 the S(0) line was detected at r = 15 kpc on one
side. This location lies beyond a steep drop in the radio continuum emission
from cosmic rays in the disk. Despite indications that star formation activity
decreases with radius, the H2 excitation temperature and the ratio of the H2
line and the far-IR luminosity surface densities, Sigma_L(H2}/Sigma_L(TIR},
change very little as a function of radius, even into the diffuse outer region
of the disk of NGC 4565. This suggests that the source of excitation of the H2
operates over a large range of radii, and is broadly independent of the
strength and relative location of UV emission from young stars. Although
excitation in photodissociation regions is the most common explanation for the
widespread H2 emission, cosmic ray heating or shocks cannot be ruled out. The
inferred mass surface densities of warm molecular hydrogen in both edge-on
galaxies differ substantially, being 4(-60) M_solar/pc^2 and 3(-50)
M_solar/pc^2 at r = 10 kpc for NGC 4565 and NGC 5907, respectively. The higher
values represent very unlikely point-source upper limits. The point source case
is not supported by the observed emission distribution in the spectral slits.
These mass surface densities cannot support the observed rotation velocities in
excess of 200 km/s. Therefore, warm molecular hydrogen cannot account for dark
matter in these disk galaxies, contrary to what was implied by a previous ISO
study of the nearby edge-on galaxy NGC 891.Comment: Accepted for publication in the Astronomical Journal (20 pages, 17
figures, 7 tables
Advancing national greenhouse gas inventories for agriculture in developing countries : improving activity data, emission factors and software technology
Peer reviewedPublisher PD
PAH Emission from Ultraluminous Infrared Galaxies
We explore the relationships between the Polycyclic Aromatic Hydrocarbon
(PAH) feature strengths, mid-infrared continuum luminosities, far-infrared
spectral slopes, optical spectroscopic classifications, and silicate optical
depths within a sample of 107 ULIRGs observed with the Infrared Spectrograph on
the Spitzer Space Telescope. The detected 6.2 micron PAH equivalent widths
(EQWs) in the sample span more than two orders of magnitude (0.006-0.8 micron),
and ULIRGs with HII-like optical spectra or steep far-infrared spectral slopes
(S_{25} / S_{60} < 0.2) typically have 6.2 micron PAH EQWs that are half that
of lower-luminosity starbursts. A significant fraction (~40-60%) of HII-like,
LINER-like, and cold ULIRGs have very weak PAH EQWs. Many of these ULIRGs also
have large (tau_{9.7} > 2.3) silicate optical depths. The far-infrared spectral
slope is strongly correlated with PAH EQW, but not with silicate optical depth.
In addition, the PAH EQW decreases with increasing rest-frame 24 micron
luminosity. We argue that this trend results primarily from dilution of the PAH
EQW by continuum emission from dust heated by a compact central source,
probably an AGN. High luminosity, high-redshift sources studied with Spitzer
appear to have a much larger range in PAH EQW than seen in local ULIRGs, which
is consistent with extremely luminous starburst systems being absent at low
redshift, but present at early epochs.Comment: 15 pages, 9 Figures; Accepted for publication in Ap
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