2,351 research outputs found
Thick gas discs in faint dwarf galaxies
We determine the intrinsic axial ratio distribution of the 'gas' disks of
extremely faint M_B > -14.5 dwarf irregular galaxies. We start with the
measured (beam corrected) distribution of apparent axial ratios in the HI 21cm
images of dwarf irregular galaxies observed as part of the Faint Irregular
Galaxy GMRT Survey (FIGGS). Assuming that the disks can be approximated as
oblate spheroids, the intrinsic axial ratio distribution can be obtained from
the observed apparent axial ratio distribution. We use a couple of methods to
do this, and our final results are based on using Lucy's deconvolution
algorithm. This method is constrained to produce physically plausible
distributions, and also has the added advantage of allowing for observational
errors to be accounted for. While one might a priori expect that gas disks
would be thin (because collisions between gas clouds would cause them to
quickly settle down to a thin disk), we find that the HI disks of faint dwarf
irregulars are quite thick, with mean axial ratio ~ 0.6. While this is
substantially larger than the typical value of ~ 0.2 for the 'stellar' disks of
large spiral galaxies, it is consistent with the much larger ratio of velocity
dispersion to rotational velocity (sigma/v_c) in dwarf galaxy HI disks as
compared to that in spiral galaxies. Our findings have implications for studies
of the mass distribution in and the Tully - Fisher relation for faint dwarf
irregular galaxies, where it is often assumed that the gas is in a thin disk.Comment: 5 pages, 4 figures. Minor changes in revised version. The definitive
version is available at http://www.blackwell-synergy.com
A search for HI in some peculiar faint dwarf galaxies
We present a deep Giant Metrewave Radio Telescope (GMRT) search for HI 21 cm
emission from three dwarf galaxies, viz. POX 186, SC 24 and KKR 25. Based, in
part, on previous single dish HI observations, these galaxies have been
classified as a BCD, a dwarf irregular and a transition galaxy respectively.
However, in conflict with previous single dish detections, we do not detect HI
in SC 24 or KKR 25. We suggest that the previous single dish measurements were
probably confused with the local galactic emission. In the case of POX 186, we
confirm the previous non detection of HI but with substantially improved limits
on its HI mass. Our derived upper limits on the HI mass of SC 24 and KKR 25 are
similar to the typical HI mass limit for dwarf spheroidal galaxies, whereas in
the case of POX 186, we find that its gas content is somewhat smaller than is
typical of BCD galaxies.Comment: Accepted for publication in MNRA
Demarcation and definition: explicating the meaning and scope of ‘decolonisation’ in the social and political sciences
Decolonisation of the curriculum is a burgeoning yet controversial project of radical change, gaining slow but steady traction in higher education politics departments across the country. At its heart is the acknowledgement and systematic unravelling of colonial and imperial practices in the UK university system. This article pins down what decolonisation is and is not, highlighting the barriers and tentative opportunities to effective decolonisation work. This is discussed in the context of the structural constraints that critical scholars of race — particularly those at the intersection of marginalised racial and gender identities — work against in the academy
Decolonising the curriculum
Social science courses are increasingly coming under fire for the over-representation of white male authors and theorists. Campaigns such as ‘Why Is My Curriculum White?’ call into question the ‘Dead White Men’ approach to teaching political theory, where few female and theorists of colour are included on reading lists. The ways in which knowledge is produced, propagated and perpetuated through White, Western perspectives also spawned the related campaign ‘Why Is My Professor White?’ These campaigns are taking place against a backdrop of immense changes in the higher education sector, which earlier this year saw thousands of university academic staff go on strike over pensions, and a spate of anti-casualisation campaigns crop up at universities across the country. Changes such as these disproportionately affect women and ethnic minorities because of the extent to which we are subject to structural inequalities. Ethnic and gender penalties are present at every academic pay grade. Women are more likely to be on casual, part-time contracts. And ethnic minorities still constitute a minor proportion of senior academic and management staff in most universities. As women of colour (WOC) in the academy – emerging scholars of race who have yet to begin permanent academic roles – the decolonisation campaigns hold personal as well as professional resonance for us. They fuel our desire to impart real change in the way politics is taught in the United Kingdom and to help make a space for scholars like us. However, this desire must sit alongside the realities of our future in the academy. We both started out PhDs in the mid-2010s with the hope of becoming critical and radical but essentially fully fledged and secure academic employees. The structural changes the academy is undergoing not only undermines the work we do to represent the work of subaltern scholars in the field of politics but makes us question our ability as well as our desire to survive and thrive as academics
Turbulence in the Harassed Galaxy NGC 4254
Galaxy harassment is an important mechanism for the morphological evolution
of galaxies in clusters. The spiral galaxy NGC 4254 in the Virgo cluster is
believed to be a harassed galaxy. We have analyzed the power spectrum of HI
emission fluctuations from NGC 4254 to investigate whether it carries any
imprint of galaxy harassment. The power spectrum, as determined using the 16
central channels which contain most of the HI emission, is found to be well
fitted by a power law with at
length-scales to . This is similar to
other normal spiral galaxies which have a slope of and is
interpreted as arising from two dimensional turbulence at length-scales larger
than the galaxy's scale-height. NGC 4254 is hence yet another example of a
spiral galaxy that exhibits scale-invariant density fluctuations out to
length-scales comparable to the diameter of the HI disk. While a large variety
of possible energy sources like proto-stellar winds, supernovae, shocks, etc.
have been proposed to produce turbulence, it is still to be seen whether these
are effective on length-scales comparable to that of the entire HI disk. On
separately analyzing the HI power spectrum in different parts of NGC 4254, we
find that the outer parts have a different slope ()
compared to the central part of the galaxy (). Such a
change in slope is not seen in other, undisturbed galaxies. We suggest that, in
addition to changing the overall morphology, galaxy harassment also effects the
fine scale structure of the ISM, causing the power spectrum to have a steeper
slope in the outer parts.Comment: 6 figures, 1 table, accepted for publication in MNRAS letter
The Scale Height of NGC 1058 Measured from its HI Power Spectrum
We have measured the HI power spectrum of the nearly face-on spiral galaxy
NGC 1058 from radio-interferometric observations using a visibility based
estimator. The power spectrum is well fitted by two different power laws
, one with at small length-scales
and another with
at large length-scales . We interpret this change in the slope of the power spectrum as a
transition from 3D turbulence at small length-scales to 2D turbulence in the
plane of the galaxy's disk at large length-scales. We use the observed break in
the power spectrum to estimate the galaxy's scale-height, which we find to be pc.Comment: Accepted for publication in Mon. Not. of Royal Astron. Soc. Letter
Baryonic Tully-Fisher Relation for Extremely Low Mass Galaxies
We study Tully-Fisher relations for a sample that combines extremely faint
(M_B > -14.0) galaxies along with bright (i.e. L_*) galaxies. Accurate (~ 10%)
distances, I band photometry, and B-V colors are known for the majority of the
galaxies in our sample. The faint galaxies are drawn from the Faint Irregular
Galaxy GMRT survey (FIGGS), and we have HI rotation velocities derived from
aperture synthesis observations for all of them. For the faint galaxies, we
find that even though the median HI and stellar masses are comparable, the HI
mass correlates significantly better with the circular velocity indicators than
the stellar mass. We also find that W correlates better with mass than
the rotation velocity, although the difference is not statistically
significant. The faint galaxies lie systematically below the I band TF relation
defined by bright galaxies, and also show significantly more intrinsic scatter.
This implies that the integrated star formation in these galaxies has been both
less efficient and also less regulated than in large galaxies. We find that
while the faint end deviation is greatly reduced in Baryonic Tully-Fisher (BTF)
relations, the existence of a break at the faint end of the BTF is subject to
systematics such as the assumed stellar mass to light ratio. If we assume that
there is an intrinsic BTF and try to determine the baryonic mass by searching
for prescriptions that lead to the tightest BTF, we find that scaling the HI
mass leads to a much more significant tightening than scaling the stellar mass
to light ratio. The most significant tightening that we find however, is if we
scale the entire baryonic mass of the faint (but not the bright) galaxies. Such
a scenario would be consistent with models where dwarf (but not large) galaxies
have a large fraction of dark or ``missing'' baryons (Slightly abridged)Comment: 7 pages, 7 figures. Accepted for publication in MNRA
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