The Herschel perspective on nearby galaxies
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Abstract
This thesis presents an investigation of local galaxies using new data from the Herschel Space Observatory. Herschel observes the entire far-infrared peak of galaxies,
at higher sensitivities and angular resolution than previously possible, and can
observe large samples of galaxies or areas of sky. I developed data reduction routines
to optimise the data processing of SPIRE extragalactic fields, and found the best
methods of flux extraction for galaxies and for fitting of spectral energy distributions.
For all the objects I investigated, a single-temperature modified blackbody was a good
fit to the global fluxes between 100–500 μm. Within an individual galaxy (i.e., M31,
NGC4501 and NGC4567/8) the dust temperature varies between 15–30K. In M31
the dust emissivity index varies between 1.2–2.5 suggesting a change in the physical
properties of the grains. The dust and gas are highly correlated in M31, with
the gas-to-dust ratio varying from ∼20 in the centre to ∼200 at 18 kpc as expected
from the metallicity gradient of the galaxy. By averaging the radial profiles of the
late-type objects in the Herschel Reference Survey (HRS), I have shown that dust
emission can be traced to at least twice the optical radius (R25) of the galaxy. Within
the HRS, dust is detected in 24% of Ellipticals and 62% of S0s and has a mean temperature
of 23.9 ± 0.8K for early-type galaxies, warmer than that found for other
Herschel studies of late-type galaxies. The mean dust mass for the entire detected
early-type sample is logMd = 6.1 ± 0.1M⊙ with a mean dust-to-stellar-mass ratio of
log(Md/M∗) = −4.3 ± 0.1, a factor of ∼50 lower dust-to-stellar-mass ratio than for
the spiral galaxies in the HRS. The wide range in the dust-to-stellar-mass ratio for
ETGs and the lack of a correlation between dust mass and optical luminosity suggest
that much of the dust in the ETGs detected by Herschel has been acquired as the
result of interactions, although these are unlikely to have had a major effect on the
stellar masses of the ETGs