32 research outputs found
Millimetre and Submillimetre continuum imaging studies of nearby active galaxies
The thesis presents millimetre (mm) and submm continuum imaging observations of the
four, nearby active galaxies Cygnus A, NCC 4374, Centaurus A and M 82. These were
undertaken using the Submillimetre Common-User Bolometer Array (SCUBA) on the
James Clerk Maxwell Telescope in Hawaii. The study focused on the properties of the
cool to cold dust (~ 80 to 10K) and its associations with star formation and nuclear
activity in the galaxies.
The observations of Cygnus A constrained the galaxy's cold dust temperature between
37 and 85K, corresponding to dust masses between 1.0 x 10 8M® and 1.4 x 106M®
respectively. Based on the dust spatial and mass limits, it is possible the heating of the
cold dust in Cygnus A is dominated by high-energy nuclear activity or star formation.
Detailed photometric observations of the core and two major hotspots (A and D) in
Cygnus A showed that their mm to submm spectral indices extend smoothly to about
450μm (677GHz), with no evidence of spectral steepening. The results allow tight determinations
for model parameters of synchrotron electron lifetimes in these components.
In pilot survey observations of the elliptical galaxy NGC 4374, a mass upper-limit
Of ~ 2 x 107M® (i.e. comparable to the dust contents of luminous spiral galaxies) was
placed on the diffuse dust that, if it exists, would affect colour and metallicity studies of elliptical galaxies. Following the pilot observations, a strategy was devised to select a good sample of ellipticals in which to search for this low-level, very cold dust.
For the first time ever, the observations of Centaurus A delineated the galaxy's submm
unresolved core, an inner-jet interacting with gas in the dust lane, and extended emission
from multi-temperature cold dust at ~ 40, 30 and 12K. Also for the first time ever, a
submm high surface brightness ringed, spiral structure of radii ~ 90", that coincides with
the regions of intense star formation and mid-infrared continuum, was revealed in the
optical dust-lane, confirming predictions by a geometric warped disc model consisting of
tilted rings. From the spatial associations in the galaxy, star formation seems to dominate
at lease 50% of the heating of the cold dust, while the rest appears to be heated by diluted
stellar radiation.
In M82, the 850μm continuum has been detected out to 1.5kpc, at least 10% farther
in radius than previous detections in this starburst galaxy. The overall submm morphology
and nuclear spectral index distribution have a general north-south asymmetry similar
to that of H. and X-ray winds, supporting the association of the extended continuum
with outflows of dust grains from the nucleus into the halo. The work of M 82 raises many
interesting points about the origin and structure of the submm emission, however the
results are displayed here still under investigation and therefore are rather preliminary.
In short, the submm continuum observations of Cygnus A, NGC 4374, CentaurusA
and M 82 are presented in this thesis that constrain the physical properties of cold dust
in these active galaxies and elucidate the associations of the dust with star formation,
nuclear activity and stellar components in these galaxies in particular, and other similar
galaxies in general
Observations of cold dust in nearby elliptical galaxies
Spectral energy distribution (SED) analyses that include new millimeter to far-infrared (FIR) observations obtained with continuum instruments on the Nobeyama and James Clerk Maxwell Telescopes and the Infrared Space Observatory are presented for seven nearby (<45 Mpc) FIR-bright elliptical galaxies. These are analyzed together with archival FIR and shortwave radio data obtained from the NASA/IPAC Extragalactic Database (NED). The radio to infrared SEDs are best-fitted by power law plus graybody models of dust residing in the central galactic regions within a 2.4 kpc diameter and with temperatures between ~21 and 28 K, emissivity index simeq2, and masses from ~1.6 to 19 × 105 M☉. The emissivity index is consistent with dust constituting amorphous silicate and carbonaceous grains previously modeled for stellar-heated dust observed in the Galaxy and other nearby extragalactic sources. Using updated dust absorption coefficients for this type of dust, dust masses are estimated that are similar to those determined from earlier FIR data alone, even though the latter results implied hotter dust temperatures. Fluxes and masses that are consistent with the new FIR and submillimeter data are estimated for dust cooler than 20 K within the central galactic regions. Tighter physical constraints for such cold, diffuse dust (if it exists) with low surface brightness will need sensitive FIR to submillimeter observations with the Spitzer Space Telescope, SCUBA2, or ALMA
Submillimeter Continuum Properties of Cold Dust in the Inner Disk and Outflows of M82
Deep submillimeter (submm) continuum imaging observations of the starburst
galaxy M82 are presented at 350, 450, 750 and 850 micron wavelengths, that were
undertaken with the Submillimetre Common-User Bolometer Array (SCUBA) on the
James Clerk Maxwell Telescope in Hawaii. The presented maps include a
co-addition of submm data mined from the SCUBA Data Archive. The co-added data
produce the deepest submm continuum maps yet of M82, in which low-level 850
micron continuum has been detected out to 1.5kpc, at least 10% farther in
radius than any previously published submm detections of this galaxy. The
overall submm morphology and spatial spectral energy distribution of M82 have a
general north-south asymmetry consistent with H-alpha and X-ray winds,
supporting the association of the extended continuum with outflows of dust
grains from the disk into the halo. The new data raise interesting points about
the origin and structure of the submm emission in the inner disk of M82. In
particular, SCUBA short wavelength evidence of submm continuum peaks that are
asymmetrically distributed along the galactic disk suggests the inner-disk
emission is re-radiation from dust concentrations along a bar (or perhaps a
spiral) rather than edges of a dust torus, as is commonly assumed. Higher
resolution submm interferometery data from the Smithsonian Submillimeter Array
and later Atacama Large Millimeter Array should spatially resolve and further
constrain the reported dust emission structures in M82.Comment: Accepted by the Astronomical Journal -- 28 pages and 14 figure
The atomic-to-molecular hydrogen transition in the TNG50 simulation:Using realistic UV fields to create spatially resolved H i maps
Cold gas in galaxies provides a crucial test to evaluate the realism of cosmological hydrodynamical simulations. To extract the atomic and molecular hydrogen properties of the simulated galaxy population, post-processing methods taking the local UV field into account are required. We improve upon previous studies by calculating realistic UV fields with the dust radiative transfer code SKIRT to model the atomic-to-molecular transition in TNG50, the highest-resolution run of the IllustrisTNG suite. Comparing integrated quantities such as the H i mass function, we study to what detail the UV field needs to be modelled in order to calculate realistic cold gas properties. We then evaluate new, spatially resolved comparisons for cold gas in galaxies by exploring synthetic maps of atomic hydrogen at redshift zero and compare them to 21-cm observations of local galaxies from the WHISP survey. In terms of non-parametric morphologies, we find that TNG50 H i maps are less concentrated than their WHISP counterparts (median ΔC ≈ 0.3), due in part to central H i deficits related to the ejective character of supermassive black hole feedback in TNG. In terms of the H i column density distribution function, we find discrepancies between WHISP and IllustrisTNG that depend on the total H i abundance in these data sets as well as the post-processing method. To fully exploit the synergy between cosmological simulations and upcoming deep H i/H2 data, we advocate the use of accurate methods to estimate the UV radiation field and to generate mock maps.</p
MeerKLASS: MeerKAT Large Area Synoptic Survey
We discuss the ground-breaking science that will be possible with a wide area
survey, using the MeerKAT telescope, known as MeerKLASS (MeerKAT Large Area
Synoptic Survey). The current specifications of MeerKAT make it a great fit for
science applications that require large survey speeds but not necessarily high
angular resolutions. In particular, for cosmology, a large survey over for hours will potentially provide the first
ever measurements of the baryon acoustic oscillations using the 21cm intensity
mapping technique, with enough accuracy to impose constraints on the nature of
dark energy. The combination with multi-wavelength data will give unique
additional information, such as exquisite constraints on primordial
non-Gaussianity using the multi-tracer technique, as well as a better handle on
foregrounds and systematics. Such a wide survey with MeerKAT is also a great
match for HI galaxy studies, providing unrivalled statistics in the pre-SKA era
for galaxies resolved in the HI emission line beyond local structures at z >
0.01. It will also produce a large continuum galaxy sample down to a depth of
about 5\,Jy in L-band, which is quite unique over such large areas and
will allow studies of the large-scale structure of the Universe out to high
redshifts, complementing the galaxy HI survey to form a transformational
multi-wavelength approach to study galaxy dynamics and evolution. Finally, the
same survey will supply unique information for a range of other science
applications, including a large statistical investigation of galaxy clusters as
well as produce a rotation measure map across a huge swathe of the sky. The
MeerKLASS survey will be a crucial step on the road to using SKA1-MID for
cosmological applications and other commensal surveys, as described in the top
priority SKA key science projects (abridged).Comment: Larger version of the paper submitted to the Proceedings of Science,
"MeerKAT Science: On the Pathway to the SKA", Stellenbosch, 25-27 May 201
A Large Sky Survey with MeerKAT
We discuss the ground-breaking science that will be possible with a wide area survey, using
the MeerKAT telescope, known as MeerKLASS (MeerKAT Large Area Synoptic Survey). The
current specifications of MeerKAT make it a great fit for cosmological applications, which require
large volumes. In particular, a large survey over ~4,000 deg^2 for ~4,000 hours will potentially
provide the first ever measurements of the baryon acoustic oscillations using the 21cm intensity
mapping technique, with enough accuracy to impose constraints on the nature of dark energy. The
combination with multi-wavelength data will give unique additional information, such as the first
constraints on primordial non-Gaussianity using the multi-tracer technique, as well as a better
handle on foregrounds and systematics. The survey will also produce a large continuum galaxy
sample down to a depth of 5 µJy in L-band, unmatched by any other concurrent telescope, which
will allow to study the large-scale structure of the Universe out to high redshifts. Finally, the same
survey will supply unique information for a range of other science applications, including a large
statistical investigation of galaxy clusters, and the discovery of rare high-redshift AGN that can be
used to probe the epoch of reionization as well as produce a rotation measure map across a huge
swathe of the sky. The MeerKLASS survey will be a crucial step on the road to using SKA1-MID
for cosmological applications, as described in the top priority SKA key science projects
The causes of the red sequence, the blue cloud, the green valley, and the green mountain
The galaxies found in optical surveys fall in two distinct regions of a diagram of optical colour versus absolute magnitude: the red sequence and the blue cloud with the green valley in between. We show that the galaxies found in a submillimetre survey have almost the opposite distribution in this diagram, forming a `green mountain'. We show that these distinctive distributions follow naturally from a single, continuous, curved Galaxy Sequence in a diagram of specific star-formation rate versus stellar mass without there being the need for a separate star-forming galaxy Main Sequence and region of passive galaxies. The cause of the red sequence and the blue cloud is the geometric mapping between stellar mass/specific star-formation rate and absolute magnitude/colour, which distorts a continuous Galaxy Sequence in the diagram of intrinsic properties into a bimodal distribution in the diagram of observed properties. The cause of the green mountain is Malmquist bias in the submillimetre waveband, with submillimetre surveys tending to select galaxies on the curve of the Galaxy Sequence, which have the highest ratios of submillimetre-to-optical luminosity. This effect, working in reverse, causes galaxies on the curve of the Galaxy Sequence to be underrepresented in optical samples, deepening the green valley. The green valley is therefore not evidence (1) for there being two distinct populations of galaxies, (2) for galaxies in this region evolving more quickly than galaxies in the blue cloud and the red sequence, (c) for rapid quenching processes in the galaxy population