459 research outputs found
Free-free and H42alpha emission from the dusty starburst within NGC 4945 as observed by ALMA
We present observations of the 85.69 GHz continuum emission and H42alpha line
emission from the central 30 arcsec within NGC 4945. Both sources of emission
originate from nearly identical structures that can be modelled as exponential
discs with a scale length of ~2.1 arcsec (or ~40 pc). An analysis of the
spectral energy distribution based on combining these data with archival data
imply that 84% +/- 10% of the 85.69 GHz continuum emission originates from
free-free emission. The electron temperature is 5400 +/- 600 K, which is
comparable to what has been measured near the centre of the Milky Way Galaxy.
The star formation rate (SFR) based on the H42alpha and 85.69 GHz free-free
emission (and using a distance of 3.8 Mpc) is 4.35 +/- 0.25 M/yr. This is
consistent with the SFR from the total infrared flux and with previous
measurements based on recombination line emission, and it is within a factor of
~2 of SFRs derived from radio data. The Spitzer Space Telescope 24 micron data
and Wide-field Infrared Survey Explorer 22 micron data yield SFRs ~10x lower
than the ALMA measurements, most likely because the mid-infrared data are
strongly affected by dust attenuation equivalent to A_V=150. These results
indicate that SFRs based on mid-infrared emission may be highly inaccurate for
dusty, compact circumnuclear starbursts.Comment: 19 pages, 9 figures, accepted for publication in MNRA
Kathryns Wheel: A spectacular galaxy collision discovered in the Galactic neighbourhood
We report the discovery of the closest collisional ring galaxy to the Milky
Way. Such rare systems occur due to "bulls-eye" encounters between two
reasonably matched galaxies. The recessional velocity of about 840 km/s is low
enough that it was detected in the AAO/UKST Survey for Galactic H
emission. The distance is only 10.0 Mpc and the main galaxy shows a full ring
of star forming knots, 6.1 kpc in diameter surrounding a quiescent disk. The
smaller assumed "bullet" galaxy also shows vigorous star formation. The
spectacular nature of the object had been overlooked because of its location in
the Galactic plane and proximity to a bright star and even though it is the
60 brightest galaxy in the HI Parkes All Sky Survey (HIPASS) HI
survey.
The overall system has a physical size of 15 kpc, a total mass of
M (stars + HI), a metallicity of
[O/H], and a star formation rate of 0.2-0.5 M\,yr,
making it a Magellanic-type system. Collisional ring galaxies therefore extend
to much lower galaxy masses than commonly assumed. We derive a space density
for such systems of , an order of magnitude
higher than previously estimated. This suggests Kathryn's Wheel is the nearest
such system. We present discovery images, CTIO 4-m telescope narrow-band
follow-up images and spectroscopy for selected emission components. Given its
proximity and modest extinction along the line of sight, this spectacular
system provides an ideal target for future high spatial resolution studies of
such systems and for direct detection of its stellar populations.Comment: 18 pages, 12 figures, accepted for publication in MNRA
ALMA observations of 99 GHz free-free and H40 line emission from star formation in the centre of NGC 253
We present Atacama Large Millimeter/submillimeter Array observations of 99.02
GHz free-free and H40 emission from the centre of the nearby starburst
galaxy NGC 253. We calculate electron temperatures of 3700-4500 K for the
photoionized gas, which agrees with previous measurements. We measure a
photoionizing photon production rate of s and
a star formation rate of M yr within the central
2010 arcsec, which fall within the broad range of measurements from
previous millimetre and radio observations but which are better constrained. We
also demonstrate that the dust opacities are ~3 dex higher than inferred from
previous near-infrared data, which illustrates the benefits of using millimetre
star formation tracers in very dusty sources.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
LABOCA and MAMBO-2 imaging of the dust ring of the Sombrero galaxy (NGC 4594)
The Sombrero galaxy (NGC 4594) is an Sa galaxy with a symmetric dust ring. We
have used the Large APEX BOlometer CAmera (LABOCA) at 870 micron and the
MAx-Planck Millimeter BOlometer (MAMBO-2) at 1.2 mm to detect the dust ring for
the first time at submillimetre and millimetre wavelengths. We have constructed
a model of the galaxy to separate the active galactic nucleus (AGN) and dust
ring components. The ring radius at both 870 micron and 1.2 mm agrees well with
the radius determined from optical absorption and atomic gas studies. The
spectral energy distribution of the ring is well fitted by a single grey-body
with dust emissivity index beta=2 and a dust temperature T_d=18.4 K. The dust
mass of the ring is found to be 1.6\pm0.2x10^7Msun which, for a Galactic
gas-to-dust ratio, implies a gas mass that is consistent with measurements from
the literature.Comment: 4 pages including 3 figures and 2 tables. Accepted for publication in
A&A Letters. Full resolution version at
http://www.strw.leidenuniv.nl/~vlahakis/vlahakis-highres.pd
Old stellar counter-rotating components in early-type galaxies from elliptical-spiral mergers
We investigate, by means of numerical simulations, the possibility of forming
counter-rotating old stellar components by major mergers between an elliptical
and a spiral galaxy. We show that counter-rotation can appear both in
dissipative and dissipationless retrograde mergers, and it is mostly associated
to the presence of a disk component, which preserves part of its initial spin.
In turn, the external regions of the two interacting galaxies acquire part of
the orbital angular momentum, due to the action of tidal forces exerted on each
galaxy by the companion.Comment: 6 pages, 15 figures. Accepted on Astronomy & Astrophysic
Tests of star formation metrics in the low metallicity galaxy NGC 5253 using ALMA observations of H30 line emission
We use Atacama Large Millimeter/submillimeter Array (ALMA) observations of
H30 (231.90 GHz) emission from the low metallicity dwarf galaxy NGC
5253 to measure the star formation rate (SFR) within the galaxy and to test the
reliability of SFRs derived from other commonly-used metrics. The H30
emission, which originates mainly from the central starburst, yields a
photoionizing photon production rate of (1.90.3)10 s
and an SFR of 0.0870.013 M yr based on conversions that
account for the low metallicity of the galaxy and for stellar rotation. Among
the other star formation metrics we examined, the SFR calculated from the total
infrared flux was statistically equivalent to the values from the H30
data. The SFR based on previously-published versions of the H flux that
were extinction corrected using Pa and Pa lines were lower than
but also statistically similar to the H30 value. The mid-infrared (22
m) flux density and the composite star formation tracer based on H
and mid-infrared emission give SFRs that were significantly higher because the
dust emission appears unusually hot compared to typical spiral galaxies.
Conversely, the 70 and 160 m flux densities yielded SFR lower than the
H30 value, although the SFRs from the 70 m and H30 data
were within 1-2 of each other. While further analysis on a broader
range of galaxies are needed, these results are instructive of the best and
worst methods to use when measuring SFR in low metallicity dwarf galaxies like
NGC 5253.Comment: 14 pages, 5 figures, accepted for publication in MNRA
Orbital Parameters of Merging Dark Matter Halos
In order to specify cosmologically motivated initial conditions for major
galaxy mergers (mass ratios 4:1) that are supposed to explain the
formation of elliptical galaxies we study the orbital parameters of major
mergers of cold dark matter halos using a high-resolution cosmological
simulation. Almost half of all encounters are nearly parabolic with
eccentricities and no correlations between the halo spin planes
or the orbital planes. The pericentric argument shows no correlation
with the other orbital parameters and is distributed randomly. In addition we
find that 50 % of typical pericenter distances are larger than half the halo's
virial radii which is much larger than typically assumed in numerical
simulations of galaxy mergers. In contrast to the usual assumption made in
semi-analytic models of galaxy formation the circularities of major mergers are
found to be not randomly distributed but to peak around a value of . Additionally all results are independent of the minimum
progenitor mass and major merger definitions (i.e. mass ratios 4:1; 3:1;
2:1).Comment: 11 pages, 20 figures, replaced by version accepted to A&A, figure 1
low re
Star formation in the centre of NGC 1808 as observed by ALMA
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of 85.69 and 99.02 GHz continuum emission and H42 and H40 lines
emission from the central 1~kpc of NGC 1808. These forms of emission are
tracers of photoionizing stars but unaffected by dust obscuration that we use
to test the applicability of other commonly star formation metrics. An analysis
of the spectral energy distributions shows that free-free emission contributes
about 60 to 90 per cent of the continuum emission in the 85-100 GHz frequency
range, dependent on the region. The star formation rate (SFR) derived from the
ALMA free-free emission is ~M~yr. This is comparable
to the SFRs measured from the infrared emission, mainly because most of the
bolometric energy from the heavily obscured region is emitted as infrared
emission. The radio 1.5~GHz emission yields a SFR 25 per cent lower than the
ALMA value, probably because of the diffusion of the electrons producing the
synchrotron emission beyond the star-forming regions. The SFRs measured from
the extinction-corrected H line emission are about 40 to 65 per cent of
the SFR derived from the ALMA data, likely because this metric was not
calibrated for high extinction regions. Some SFRs based on extinction-corrected
ultraviolet emission are similar to those from ALMA and infrared data, but
given that the ultraviolet terms in the extinction correction equations are
very small, these metrics seem inappropriate to apply to this dusty starburst.Comment: 18 pages, 7 figures, 10 tables, accepted for publication in MNRA
A Multi-Wavelength Infrared Study of NGC 891
We present a multi-wavlength infrared study of the nearby, edge-on, spiral
galaxy NGC 891. We have examined 20 independent, spatially resolved IR images
of this galaxy, 14 of which are newly reduced and/or previously unpublished
images. These images span a wavelength regime from 1.2 microns in which the
emission is dominated by cool stars, through the MIR, in which emission is
dominated by PAHs, to 850 microns, in which emission is dominated by cold dust
in thermal equilibrium with the radiation field. The changing morphology of the
galaxy with wavelength illustrates the changing dominant components. We detect
extra-planar dust emission in this galaxy, consistent with previously published
results, but now show that PAH emission is also in the halo, to a vertical
distance of z >= 2.5 kpc. We compare the vertical extents of various components
and find that the PAHs (from 7.7 and 8 micron data) and warm dust (24 microns)
extend to smaller z heights than the cool dust (450 microns). For six locations
in the galaxy for which the S/N was sufficient, we present SEDs of the IR
emission, including two in the halo - the first time a halo SED in an external
galaxy has been presented. We have modeled these SEDs and find that the PAH
fraction is similar to Galactic values (within a factor of two), with the
lowest value at the galaxy's center, consistent with independent results of
other galaxies. In the halo environment, the fraction of dust exposed to a
colder radiation field, is of order unity, consistent with an environment in
which there is no star formation. The source of excitation is likely from
photons escaping from the disk.Comment: 24 pages, 17 figures, 7 tables, accepted for publication in MNRA
The Herschel Virgo Cluster Survey: X.The relationship between cold dust and molecular gas content in Virgo spirals
Using the far-infrared emission, as observed by the Herschel Virgo Cluster
Survey (HeViCS), and the integrated HI and CO brightness, we infer the dust and
total gas mass for a magnitude limited sample of 35 metal rich spiral galaxies
in Virgo. The CO flux correlates tightly and linearly with far-infrared fluxes
observed by Herschel. Molecules in these galaxies are more closely related to
cold dust rather than to dust heated by star formation or to optical/NIR
brightness. We show that dust mass establishes a stronger correlation with the
total gas mass than with the atomic or molecular component alone. The
dust-to-gas ratio increases as the HI deficiency increases, but in highly HI
deficient galaxies it stays constant. Dust is in fact less affected than atomic
gas by weak cluster interactions, which remove most of the HI gas from outer
and high latitudes regions. Highly disturbed galaxies, in a dense cluster
environment, can instead loose a considerable fraction of gas and dust from the
inner regions of the disk keeping constant the dust-to-gas ratio. There is
evidence that the molecular phase is also quenched. This quencing becomes
evident by considering the molecular gas mass per unit stellar mass. Its
amplitude, if confirmed by future studies, highlights that molecules are
missing in Virgo HI deficient spirals, but to a somewhat lesser extent than
dust.Comment: 11 pages, 8 figures. Accepted for publication in A&
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