188 research outputs found
The Herschel Space Observatory view of dust in M81
We use Herschel Space Observatory data to place observational constraints on the peak and Rayleigh-Jeans slope of dust emission observed at 70â500 ÎŒm in the nearby spiral galaxy M81. We find that the ratios of wave bands between 160 and 500 ÎŒm are primarily dependent on radius but that the ratio of 70 to 160 ÎŒm emission shows no clear dependence on surface brightness or radius. These
results along with analyses of the spectral energy distributions imply that the 160â500 ÎŒm emission traces 15â30 K dust heated by evolved stars in the bulge and disc whereas the 70 ÎŒm emission includes dust heated by the active galactic nucleus and young stars in star forming regions
The identification of dust heating mechanisms in nearby galaxies using Herschel 160/250 and 250/350 micron surface brightness ratios
We examined variations in the 160/250 and 250/350 micron surface brightness
ratios within 24 nearby (<30 Mpc) face-on spiral galaxies observed with the
Herschel Space Observatory to identify the heating mechanisms for dust emitting
at these wavelengths. The analysis consisted of both qualitative and
quantitative comparisons of the 160/250 and 250/350 micron ratios to H alpha
and 24 micron surface brightnesses, which trace the light from star forming
regions, and 3.6 micron emission, which traces the light from the older stellar
populations of the galaxies. We find broad variations in the heating mechanisms
for the dust. In one subset of galaxies, we found evidence that emission at
<=160 microns (and in rare cases potentially at <=350 microns) originates from
dust heated by star forming regions. In another subset, we found that the
emission at >=250 microns (and sometimes at >=160 microns) originates from dust
heated by the older stellar population. In the rest of the sample, either the
results are indeterminate or both of these stellar populations may contribute
equally to the global dust heating. The observed variations in dust heating
mechanisms does not necessarily match what has been predicted by dust emission
and radiative transfer models, which could lead to overestimated dust
temperatures, underestimated dust masses, false detections of variability in
dust emissivity, and inaccurate star formation rate measurements.Comment: Accepted for publication in MNRA
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&
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
Correcting the extended-source calibration for the <i>Herschel</i>-SPIRE Fourier-transform spectrometer
We describe an update to the Herschel-Spectral and Photometric Imaging Receiver (SPIRE) Fourier-transform spectrometer (FTS) calibration for extended sources, which incorporates a correction for the frequency-dependent far-field feedhorn efficiency, ηff. This significant correction affects all FTS extended-source calibrated spectra in sparse or mapping mode, regardless of the spectral resolution. Line fluxes and continuum levels are underestimated by factors of 1.3â2 in thespectrometer long wavelength band (447â1018 GHz; 671â294 ÎŒm) and 1.4â1.5 in the spectrometer short wavelength band (944â1568 GHz; 318â191 ÎŒm). The correction was implemented in the FTS pipeline version 14.1 and has also been described in the SPIRE Handbook since 2017 February. Studies based on extended-source calibrated spectra produced prior to this pipeline version should be critically reconsidered using the current products available in the Herschel Science Archive. Once the extended-source calibrated spectra are corrected for ηff, the synthetic photometry and the broad-band intensities from SPIRE photometer maps agree within 2â4âperâcent â similar levels to the comparison of point-source calibrated spectra and photometry from point-source calibrated maps. The two calibration schemes for the FTS are now self-consistent: the conversion between the corrected extended-source and point-source calibrated spectra can be achieved with the beam solid angle and a gain correction that accounts for the diffraction loss
Calibration of the Herschel SPIRE Fourier Transform Spectrometer
The Herschel SPIRE instrument consists of an imaging photometric camera and
an imaging Fourier Transform Spectrometer (FTS), both operating over a
frequency range of 450-1550 GHz. In this paper, we briefly review the FTS
design, operation, and data reduction, and describe in detail the approach
taken to relative calibration (removal of instrument signatures) and absolute
calibration against standard astronomical sources. The calibration scheme
assumes a spatially extended source and uses the Herschel telescope as primary
calibrator. Conversion from extended to point-source calibration is carried out
using observations of the planet Uranus. The model of the telescope emission is
shown to be accurate to within 6% and repeatable to better than 0.06% and, by
comparison with models of Mars and Neptune, the Uranus model is shown to be
accurate to within 3%. Multiple observations of a number of point-like sources
show that the repeatability of the calibration is better than 1%, if the
effects of the satellite absolute pointing error (APE) are corrected. The
satellite APE leads to a decrement in the derived flux, which can be up to ~10%
(1 sigma) at the high-frequency end of the SPIRE range in the first part of the
mission, and ~4% after Herschel operational day 1011. The lower frequency range
of the SPIRE band is unaffected by this pointing error due to the larger beam
size. Overall, for well-pointed, point-like sources, the absolute flux
calibration is better than 6%, and for extended sources where mapping is
required it is better than 7%.Comment: 20 pages, 18 figures, accepted for publication in MNRA
The Vega Debris Disk -- A Surprise from Spitzer
We present high spatial resolution mid- and far-infrared images of the Vega
debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS).
The disk is well resolved and its angular size is much larger than found
previously. The radius of the disk is at least 43" (330 AU), 70"(543 AU), and
105" (815 AU) in extent at 24, 70 and 160 um, respectively. The disk images are
circular, smooth and without clumpiness at all three wavelengths. The radial
surface brightness profiles imply an inner boundary at a radius of 11"+/-2" (86
AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the
disk can be modeled as an axially symmetric and geometrically thin disk, viewed
face-on, with the surface particle number density following an r^-1 power law.
The disk radiometric properties are consistent with a range of models using
grains of sizes ~1 to ~50 um. We find that a ring, containing grains larger
than 180 um and at radii of 86-200 AU from the star, can reproduce the observed
850 um flux, while its emission does not violate the observed MIPS profiles.
This ring could be associated with a population of larger asteroidal bodies
analogous to our own Kuiper Belt. Cascades of collisions starting with
encounters amongthese large bodies in the ring produce the small debris that is
blown outward by radiation pressure to much larger distances where we detect
its thermal emission. The dust production rate is >~10^15 g/s based on the MIPS
results. This rate would require a very massive asteroidal reservoir for the
dust to be produced in a steady state throughout Vega's life. Instead, we
suggest that the disk we imaged is ephemeral and that we are witnessing the
aftermath of a large and relatively recent collisional event, and subsequent
collisional cascade.Comment: 13 pages, 17 figures, accepted for publication in ApJ. (Figures 2,
3a, 3b and 4 have been degraded to lower resolutions.
AGN are cooler than you think: the intrinsic far-IR emission from QSOs
We present an intrinsic AGN spectral energy distribution (SED) extending from the optical to the submm, derived with a sample of unobscured, optically luminous (ÎœLÎœ,5100 >1043.5 erg sâ1) QSOs at z < 0.18 from the Palomar Green survey. The intrinsic AGN SED was computed by removing the contribution from stars using the 11.3 ÎŒm polycyclic aromatic hydrocarbon (PAH) feature in the QSOsâ mid-IR spectra; the 1Ï uncertainty on the SED ranges between 12 and 45 per cent as a function of wavelength and is a combination of PAH flux measurement errors and the uncertainties related to the conversion between PAH luminosity and star-forming luminosity. Longwards of 20 ÎŒm, the shape of the intrinsic AGN SED is independent of the AGN power indicating that our template should be applicable to all systems hosting luminous AGN (ÎœLÎœ, 5100 or LX(2â10 keV) _ 1043.5 erg sâ1). We note that for our sample of luminous QSOs, the average AGN emission is at least as high as, and mostly higher than, the total stellar powered emission at all wavelengths from the optical to the submm. This implies that in many galaxies hosting powerful AGN, there is no âsafeâ broad-band photometric observation (at λ < 1000 ÎŒm) which can be used in calculating star formation rates without subtracting the AGN contribution. Roughly, the AGN contribution may be ignored only if the intrinsic AGN luminosity at 5100 AA is at least a factor of 4 smaller than the total infrared luminosity (LIR, 8â1000 ÎŒm) of the galaxy. Finally, we examine the implication of our work in statistical studies of star formation in AGN host galaxies
- âŠ