35 research outputs found
Extended Schmidt law holds for faint dwarf irregular galaxies
The extended Schmidt law (ESL) is a variant of the Schmidt law which relates
the surface densities of gas and star formation, with the surface density of
stellar mass added as an extra parameter. We empirically investigate for the
first time whether low metallicity faint dwarf irregular galaxies (dIrrs)
follow the ESL. Here we consider the `global' law where surface densities are
averaged over the galactic discs. dIrrs are unique not only because they are at
the lowest end of mass and star formation scales for galaxies, but also because
they are metal-poor compared to the general population of galaxies. Our sample
is drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS) which is the
largest survey of atomic hydrogen in such galaxies. The gas surface densities
are determined using their atomic hydrogen content. The star formation rates
are calculated using GALEX far ultraviolet fluxes after correcting for dust
extinction, whereas the stellar surface densities are calculated using Spitzer
3.6 m fluxes. All surface densities are calculated over stellar discs
defined by the 3.6 m images. We find dIrrs indeed follow the extended
Schmidt law. The mean deviation of the FIGGS galaxies from the relation is 0.01
dex, with a scatter around the relation of less than half that seen in the
original relation. In comparison, we also show that the FIGGS galaxies are much
more deviant when compared to the `canonical' Kennicutt-Schmidt relation. Our
results help strengthen the universality of the extended Schmidt law,
especially for galaxies with low metallicities. We suggest that models of star
formation in which feedback from previous generations of stars set the pressure
in the ISM, are promising candidates for explaining the ESL. We also confirm
that ESL is an independent relation and not a form of a relation between star
formation efficiency and metallicity.Comment: Accepted for publication in Astronomy & Astrophysics. Figure 2 on
Page 5 shows the main resul
First measurement of HI 21cm emission from a GRB host galaxy indicates a post-merger system
We report the detection and mapping of atomic hydrogen in HI 21cm emission
from ESO 184-G82, the host galaxy of the gamma ray burst 980425. This is the
first instance where HI in emission has been detected from a galaxy hosting a
gamma ray burst. ESO 184-G82 is an isolated galaxy and contains a Wolf-Rayet
region close to the location of the gamma ray burst and the associated
supernova, SN 1998bw. This is one of the most luminous HII regions identified
in the local Universe, with a very high inferred density of star formation. The
HI 21cm observations reveal a high HI mass for the galaxy, twice as large as
the stellar mass. The spatial and velocity distribution of the HI 21cm emission
reveals a disturbed rotating gas disk, which suggests that the galaxy has
undergone a recent minor merger that disrupted its rotation. We find that the
Wolf-Rayet region and the gamma ray burst are both located in the highest HI
column density region of the galaxy. We speculate that the merger event has
resulted in shock compression of the gas, triggering extreme star formation
activity, and resulting in the formation of both the Wolf-Rayet region and the
gamma ray burst. The high HI column density environment of the GRB is
consistent with the high HI column densities seen in absorption in the host
galaxies of high redshift gamma ray bursts.Comment: Accepted for publication in MNRAS Letters. 5 pages, 5 figures, 2
tables. For the definitive version visit http://mnrasl.oxfordjournals.org
The spatially resolved Kennicutt-Schmidt relation in the HI dominated regions of spiral and dwarf irregular galaxies
We study the Kennicutt-Schmidt relation between average star formation rate
and average cold gas surface density in the Hi dominated ISM of nearby spiral
and dwarf irregular galaxies. We divide the galaxies into grid cells varying
from sub-kpc to tens of kpc in size. Grid-cell measurements of low SFRs using
H-alpha emission can be biased and scatter may be introduced because of
non-uniform sampling of the IMF or because of stochastically varying star
formation. In order to alleviate these issues, we use far-ultraviolet emission
to trace SFR, and we sum up the fluxes from different bins with the same gas
surface density to calculate the average at a given value of
. We study the resulting Kennicutt-Schmidt relation in 400 pc, 1
kpc and 10 kpc scale grids in nearby massive spirals and in 400 pc scale grids
in nearby faint dwarf irregulars. We find a relation with a power law slope of
1.5 in the HI-dominated regions for both kinds of galaxies. The relation is
offset towards longer gas consumption timescales compared to the molecular
hydrogen dominated centres of spirals, but the offset is an order-of-magnitude
less than that quoted by earlier studies. Our results lead to the surprising
conclusion that conversion of gas to stars is independent of metallicity in the
HI dominated regions of star-forming galaxies. Our observed relations are
better fit by a model of star formation based on thermal and hydrostatic
equilibrium in the ISM, in which feedback driven turbulence sets the thermal
pressure.Comment: 11 pages, 7 figures, 5 tables. Accepted for publication in MNRAS Main
Journal. For the definitive version visit http://mnras.oxfordjournals.org
New insights into the interstellar medium of the dwarf galaxy IC 10 : connection between magnetic fields, the radio--infrared correlation and star formation
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. Available at doi: https://doi.org/10.1093/mnras/stx1567.We present the highest sensitivity and angular resolution study at 0.32 GHz of the dwarf irregular galaxy IC\,10, observed using the Giant Metrewave Radio Telescope, probing pc spatial scales. We find the galaxy-averaged radio continuum spectrum to be relatively flat, with a spectral index (), mainly due to a high contribution from free--free emission. At 0.32 GHz, some of the H{\sc ii} regions show evidence of free--free absorption as they become optically thick below GHz with corresponding free electron densities of . After removing the free--free emission, we studied the radio--infrared relations on 55, 110 and 165 pc spatial scales. We find that on all scales the non-thermal emission at 0.32 and 6.2 GHz correlates better with far-infrared (FIR) emission at m than mid-infrared emission at m. The dispersion of the radio--FIR relation arises due to variations in both magnetic field and dust temperature, and decreases systematically with increasing spatial scale. The effect of cosmic ray transport is negligible as cosmic ray electrons were only injected Myr ago. The average magnetic field strength () of G in the disc is comparable to that of large star-forming galaxies. The local magnetic field is strongly correlated with local star formation rate () as , indicating a star-burst driven fluctuation dynamo to be efficient ( per cent) in amplifying the field in IC\,10. The high spatial resolution observations presented here suggest that the high efficiency of magnetic field amplification and strong coupling with SFR likely sets up the radio--FIR correlation in cosmologically young galaxies.Peer reviewedFinal Accepted Versio
Impact of Simulated 1/f Noise for HI Intensity Mapping Experiments
Cosmology has entered an era where the experimental limitations are not due
to instrumental sensitivity but instead due to inherent systematic
uncertainties in the instrumentation and data analysis methods. The field of HI
intensity mapping (IM) is still maturing, however early attempts are already
systematics limited. One such systematic limitation is 1/f noise, which largely
originates within the instrumentation and manifests as multiplicative gain
fluctuations. To date there has been little discussion about the possible
impact of 1/f noise on upcoming single-dish HI IM experiments such as BINGO,
FAST or SKA. Presented in this work are Monte-Carlo end-to-end simulations of a
30 day HI IM survey using the SKA-MID array covering a bandwidth of 950 and
1410 MHz. These simulations extend 1/f noise models to include not just
temporal fluctuations but also correlated gain fluctuations across the receiver
bandpass. The power spectral density of the spectral gain fluctuations are
modelled as a power-law, and characterised by a parameter . It is found
that the degree of 1/f noise frequency correlation will be critical to the
success of HI IM experiments. Small values of ( < 0.25) or high
correlation is preferred as this is more easily removed using current component
separation techniques. The spectral index of temporal fluctuations ()
is also found to have a large impact on signal-to-noise. Telescope slew speed
has a smaller impact, and a scan speed of 1 deg s should be sufficient
for a HI IM survey with the SKA.Comment: 22 pages, 15 figures, 2 table
Small Bites: Star formation recipes in extreme dwarfs
We study the relationship between the gas column density (Sigma_HI) and the
star formation rate surface density (Sigma_SFR) for a sample of extremely small
(M_B ~ -13, Delta V_50 ~ 30 km/s) dwarf irregular galaxies. We find a clear
stochasticity in the relation between the gas column density and star
formation. All gas with Sigma_HI >~ 10 M_sun/pc^2 has some ongoing star
formation, but the fraction of gas with ongoing star formation decreases as the
gas column density decreases, and falls to about 50% at Sigma_HI ~ 3
M_sun/pc^2. Further, even for the most dense gas, the star formation efficiency
is at least a factor of ~ 2 smaller than typical of star forming regions in
spirals. We also find that the ratio of H-alpha emission to FUV emission
increases with increasing gas column density. This is unlikely to be due to
increasing dust extinction because the required dust to gas ratios are too
high. We suggest instead that this correlation arises because massive (i.e.
H-alpha producing) stars are formed preferentially in regions with high gas
density.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS
Letters. The definitive version is available at www.blackwell- synergy.co
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
Atomic hydrogen, star formation and feedback in the lowest mass Blue Compact Dwarf galaxies
We present the results from a search for HI emission from a sample of newly
discovered dwarf galaxies in the M81 group. HI is detected in three galaxies,
all of which are classified as BCDs. The HI masses of these galaxies are ~ 10^6
M_sun, making these some of the lowest mass BCDs known. For these three
galaxies FUV images (from GALEX) and H-alpha images (from the Russian 6m BTA
telescope) are available.The H-alpha emission is very faint, and, in principle
could be produced by a single O star. Further, in all cases we find offsets
between the peak of the FUV emission and that of the H-alpha emission. Offsets
between the most recent sites of star formation (i.e. those traced by H-alpha)
and the older sites (i.e. those traced by FUV) would be natural if the star
formation is stochastic. In spite of the expectation that the effects of
mechanical feedback from star formation would be most directly seen in the
smallest galaxies with low gravitational potentials, we only see tentative
evidence of outflowing HI gas associated with the star forming region in one of
the galaxies.Comment: 9 pages, 5 figures, 5 tables. Accepted for publication in MNRAS. The
definitive version is available at www.blackwell-synergy.co