267 research outputs found
Initial design and evaluation of automatic restructurable flight control system concepts
Results of efforts to develop automatic control design procedures for restructurable aircraft control systems is presented. The restructurable aircraft control problem involves designing a fault tolerance control system which can accommodate a wide variety of unanticipated aircraft failure. Under NASA sponsorship, many of the technologies which make such a system possible were developed and tested. Future work will focus on developing a methodology for integrating these technologies and demonstration of a complete system
The physical characteristics of the gas in the disk of Centaurus A using the Herschel Space Observatory
We search for variations in the disk of Centaurus A of the emission from
atomic fine structure lines using Herschel PACS and SPIRE spectroscopy. In
particular we observe the [C II](158 m), [N II](122 and 205 m), [O
I](63 and 145 m) and [O III](88 m) lines, which all play an important
role in cooling the gas in photo-ionized and photodissociation regions. We
determine that the ([C II]+[O I])/ line ratio, a proxy for the
heating efficiency of the gas, shows no significant radial trend across the
observed region, in contrast to observations of other nearby galaxies. We
determine that 10 - 20% of the observed [C II] emission originates in ionized
gas. Comparison between our observations and a PDR model shows that the
strength of the far-ultraviolet radiation field, , varies between
and and the hydrogen nucleus density varies between
and cm, with no significant radial trend in
either property. In the context of the emission line properties of the
grand-design spiral galaxy M51 and the elliptical galaxy NGC 4125, the gas in
Cen A appears more characteristic of that in typical disk galaxies rather than
elliptical galaxies.Comment: Accepted for publication in the Astrophysical Journal. 22 pages, 10
figures, 5 table
The Ionized Gas in Nearby Galaxies as Traced by the [NII] 122 and 205 \mu m Transitions
The [NII] 122 and 205 \mu m transitions are powerful tracers of the ionized
gas in the interstellar medium. By combining data from 21 galaxies selected
from the Herschel KINGFISH and Beyond the Peak surveys, we have compiled 141
spatially resolved regions with a typical size of ~1 kiloparsec, with
observations of both [NII] far-infrared lines. We measure [NII] 122/205 line
ratios in the ~0.6-6 range, which corresponds to electron gas densities
~1-300 cm, with a median value of =30 cm. Variations in
the electron density within individual galaxies can be as a high as a factor of
~50, frequently with strong radial gradients. We find that increases as a
function of infrared color, dust-weighted mean starlight intensity, and star
formation rate surface density (). As the intensity of the [NII]
transitions is related to the ionizing photon flux, we investigate their
reliability as tracers of the star formation rate (SFR). We derive relations
between the [NII] emission and SFR in the low-density limit and in the case of
a log-normal distribution of densities. The scatter in the correlation between
[NII] surface brightness and can be understood as a property of
the distribution. For regions with close to or higher than the
[NII] line critical densities, the low-density limit [NII]-based SFR
calibration systematically underestimates the SFR since [NII] emission is
collisionally quenched. Finally, we investigate the relation between [NII]
emission, SFR, and by comparing our observations to predictions from the
MAPPINGS-III code.Comment: 18 pages, 9 figures, accepted for publication in The Astrophysical
Journa
The far-infrared view of M87 as seen by the Herschel Space Observatory
The origin of the far-infrared emission from the nearby radio galaxy M87
remains a matter of debate. Some studies find evidence of a far-infrared excess
due to thermal dust emission, whereas others propose that the far-infrared
emission can be explained by synchrotron emission without the need for an
additional dust emission component. We observed M87 with PACS and SPIRE as part
of the Herschel Virgo Cluster Survey (HeViCS). We compare the new Herschel data
with a synchrotron model based on infrared, submm and radio data to investigate
the origin of the far-infrared emission. We find that both the integrated SED
and the Herschel surface brightness maps are adequately explained by
synchrotron emission. At odds with previous claims, we find no evidence of a
diffuse dust component in M87.Comment: 4 pages, 2 figures, proceedings IAU Symposium 275 (Jets at all
scales
Insights into gas heating and cooling in the disc of NGC 891 from Herschel far-infrared spectroscopy
We present Herschel PACS and SPIRE spectroscopy of the most important
far-infrared cooling lines in the nearby edge-on spiral galaxy, NGC 891: [CII]
158 m, [NII] 122, 205 m, [OI] 63, 145 m, and [OIII] 88 m.
We find that the photoelectric heating efficiency of the gas, traced via the
([CII]+[OII]63)/ ratio, varies from a mean of
3.510 in the centre up to 810 at increasing
radial and vertical distances in the disc. A decrease in
([CII]+[OII]63)/ but constant
([CII]+[OI]63)/ with increasing FIR colour suggests that
polycyclic aromatic hydrocarbons (PAHs) may become important for gas heating in
the central regions. We compare the observed flux of the FIR cooling lines and
total IR emission with the predicted flux from a PDR model to determine the gas
density, surface temperature and the strength of the incident far-ultraviolet
(FUV) radiation field, . Resolving details on physical scales of ~0.6
kpc, a pixel-by-pixel analysis reveals that the majority of the PDRs in NGC
891's disc have hydrogen densities of 1 < log (/cm) < 3.5
experiencing an incident FUV radiation field with strengths of 1.7 < log
< 3. Although these values we derive for most of the disc are consistent with
the gas properties found in PDRs in the spiral arms and inter-arm regions of
M51, observed radial trends in and are shown to be sensitive to
varying optical thickness in the lines, demonstrating the importance of
accurately accounting for optical depth effects when interpreting observations
of high inclination systems. With an empirical relationship between the MIPS 24
m and [NII] 205 m emission, we estimate an enhancement of the FUV
radiation field strength in the far north-eastern side of the disc.Comment: Accepted for publication in A&A. 25 pages, including 17 figures and 3
tables, abstract abridged for arXi
Linking dust emission to fundamental properties in galaxies: The low-metallicity picture
In this work, we aim at providing a consistent analysis of the dust
properties from metal-poor to metal-rich environments by linking them to
fundamental galactic parameters. We consider two samples of galaxies: the Dwarf
Galaxy Survey (DGS) and KINGFISH, totalling 109 galaxies, spanning almost 2 dex
in metallicity. We collect infrared (IR) to submillimetre (submm) data for both
samples and present the complete data set for the DGS sample. We model the
observed spectral energy distributions (SED) with a physically-motivated dust
model to access the dust properties. Using a different SED model (modified
blackbody), dust composition (amorphous carbon), or wavelength coverage at
submm wavelengths results in differences in the dust mass estimate of a factor
two to three, showing that this parameter is subject to non-negligible
systematic modelling uncertainties. For eight galaxies in our sample, we find a
rather small excess at 500 microns (< 1.5 sigma). We find that the dust SED of
low-metallicity galaxies is broader and peaks at shorter wavelengths compared
to more metal-rich systems, a sign of a clumpier medium in dwarf galaxies. The
PAH mass fraction and the dust temperature distribution are found to be driven
mostly by the specific star-formation rate, SSFR, with secondary effects from
metallicity. The correlations between metallicity and dust mass or total-IR
luminosity are direct consequences of the stellar mass-metallicity relation.
The dust-to-stellar mass ratios of metal-rich sources follow the well-studied
trend of decreasing ratio for decreasing SSFR. The relation is more complex for
highly star-forming low-metallicity galaxies and depends on the chemical
evolutionary stage of the source (i.e., gas-to-dust mass ratio). Dust growth
processes in the ISM play a key role in the dust mass build-up with respect to
the stellar content at high SSFR and low metallicity. (abridged)Comment: 44 pages (20 pages main body plus 5 Appendices), 11 figures, 9
tables, accepted for publication in A&
Herschel SPIRE-FTS Observations of Excited CO and [CI] in the Antennae (NGC 4038/39): Warm and Cold Molecular Gas
We present Herschel SPIRE-FTS observations of the Antennae (NGC 4038/39), a
well studied, nearby ( Mpc) ongoing merger between two gas rich spiral
galaxies. We detect 5 CO transitions ( to ), both [CI]
transitions and the [NII] transition across the entire system, which
we supplement with ground based observations of the CO , and
transitions, and Herschel PACS observations of [CII] and [OI].
Using the CO and [CI] transitions, we perform both a LTE analysis of [CI], and
a non-LTE radiative transfer analysis of CO and [CI] using the radiative
transfer code RADEX along with a Bayesian likelihood analysis. We find that
there are two components to the molecular gas: a cold ( K)
and a warm ( K) component. By comparing the warm gas mass
to previously observed values, we determine a CO abundance in the warm gas of
. If the CO abundance is the same in the warm and
cold gas phases, this abundance corresponds to a CO luminosity-to-mass
conversion factor of $\alpha_{CO} \sim 7 \ M_{\odot}{pc^{-2} \ (K \ km \
s^{-1})^{-1}}_263\mu m\sim 0.01 L_{\odot}/M_{\odot}G_0\sim 1000$. Finally, we find
that a combination of turbulent heating, due to the ongoing merger, and
supernova and stellar winds are sufficient to heat the molecular gas.Comment: 50 pages, 15 figures, 8 tables, Accepted for publication in The
Astrophysical Journa
The Herschel Virgo Cluster Survey: I. Luminosity functions
We describe the Herschel Virgo Cluster Survey (HeViCS) and the first data
obtained as part of the Science Demonstration Phase (SDP). The data cover a
central 4x4 sq deg region of the cluster. We use SPIRE and PACS photometry data
to produce 100, 160, 250, 350 and 500 micron luminosity functions (LFs) for
optically bright galaxies that are selected at 500 micron and detected in all
bands. We compare these LFs with those previously derived using IRAS, BLAST and
Herschel-ATLAS data. The Virgo Cluster LFs do not have the large numbers of
faint galaxies or examples of very luminous galaxies seen previously in surveys
covering less dense environments.Comment: Letter accepted for publication in A&A (Herschel special issue
The Herschel Virgo Cluster Survey: IV. Resolved dust analysis of spiral galaxies
We present a resolved dust analysis of three of the largest angular size
spiral galaxies, NGC 4501 and NGC 4567/8, in the Herschel Virgo Cluster Survey
(HeViCS) Science Demonstration field. Herschel has unprecedented spatial
resolution at far-infrared wavelengths and with the PACS and SPIRE instruments
samples both sides of the peak in the far infrared spectral energy distribution
(SED).We present maps of dust temperature, dust mass, and gas-to-dust ratio,
produced by fitting modified black bodies to the SED for each pixel. We find
that the distribution of dust temperature in both systems is in the range ~19 -
22 K and peaks away from the centres of the galaxies. The distribution of dust
mass in both systems is symmetrical and exhibits a single peak coincident with
the galaxy centres. This Letter provides a first insight into the future
analysis possible with a large sample of resolved galaxies to be observed by
Herschel.Comment: Letter accepted for publication in A&A (Herschel special issue
The spatially resolved correlation between [NII] 205μm line emission and the 24μm continuum in nearby galaxies
A correlation between the 24 μm continuum and the [Nii] 205 μm line emission may arise if both quantities trace the star formation activity on spatially-resolved scales within a galaxy, yet has so far only been observed in the nearby edge-on spiral galaxy NGC 891. We therefore assess whether the [Nii] 205−24 μm emission correlation has some physical origin or is merely an artefact of line-of-sight projection effects in an edge-on disc. We search for the presence of a correlation in Herschel and Spitzer observations of two nearby face-on galaxies, M 51 and M 83, and the interacting Antennae galaxies NGC 4038 and 4039. We show that not only is this empirical relationship also observed in face-on galaxies, but also that the correlation appears to be governed by the star formation rate (SFR). Both the nuclear starburst in M 83 and the merger-induced star formation in NGC 4038/9 exhibit less [Nii] emission per unit SFR surface density than the normal star-forming discs. These regions of intense star formation exhibit stronger ionization parameters, as traced by the 70/160 μm far-infrared (FIR) colour. These observations suggest the presence of higher ionization lines that may become more important for gas cooling, thereby reducing the observed [Nii] 205 μm line emission in regions with higher star formation rates. Finally, we present a general relation between the [Nii] 205 μm line flux density and SFR density for normal star-forming galaxies, yet note that future studies should extend this analysis by including observations with wider spatial coverage for a larger sample of galaxies
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