177 research outputs found
The violent interstellar medium in Messier 31
Taurus observations in the line of H alpha and Very Large Array (VLA) HI mapping of the HII complex No. 722 in M31, reveal what seems to be a spherical cavity 330 pc in diameter blown out by a stellar association of over 20(exp 6) years old. Evidence of induced star formation which was initiated less than 5(exp 6) years ago is present in the form of bright HII emission and numerous O, B and Wolf-Rayet stars which are found within the shell surrounding the cavity. The energy necessary to create the HI shell is estimated to be about 5(exp 51) erg
The critical density for star formation in HII galaxies
The star formation rate (SFR) in galaxies is believed to obey a power law relation with local gas density, first proposed by Schmidt (1959). Kennicutt (1989) has shown that there is a threshold density above which star formation occurs, and for densities at or near the threshold density, the DFR is highly non-linear, leading to bursts of star formation. Skillman (1987) empirically determined this threshold for dwarf galaxies to be approximately 1 x 10(exp 21) cm(exp -2), at a linear resolution of 500pc. During the course of our survey for HI companion clouds to HII galaxies, we obtained high resolution HI observations of five nearby HII galaxies. HII galaxies are low surface brightness, rich in HI, and contain one or a few high surface brightness knots whose optical spectra resemble those of HII regions. These knots are currently experiencing a burst of star formation. After Kennicutt (1989) we determine the critical density for star formation in the galaxies, and compare the predictions with radio and optical data
High velocity clouds in nearby disk galaxies
Clouds of neutral hydrogen in our galaxy with the absolute value of v greater than 100 km/s cover approximately 10 percent of the sky to a limiting column density of 1 x 10(exp 18) cm(exp -2). These high velocity clouds (HVCs) may dominate the kinetic energy of neutral hydrogen in non-circular motion, and are an important though poorly understood component of galactic gas. It has been suggested that the HVCs can be reproduced by a combination of three phenomena: a galactic fountain driven by disk supernovae which would account for most of the HVCs, material tidally torn from the Magellanic Clouds, and an outer arm complex which is associated with the large scale structure of the warped galactic disk. We sought to detect HVCs in external galaxies in order to test the galactic fountain model
Intergalactic Star Formation
Star formation in interacting systems may take place in various locations,
from the dust--enshrouded core of Ultraluminous Infrared Galaxies to more
unusual places such as the debris of colliding galaxies expelled into the
intergalactic medium. Determining whether star-formation proceeds in the latter
environment, far from the parent galaxies, in a similar way as in spiral disks
has motivated the multi--wavelength study presented here. We collected VLA/HI,
UV/GALEX, optical Halpha and MIR/Spitzer images of a few nearby interacting
systems chosen for their prominent "intergalactic" star formation activity.
Preliminary results on the spectacular collisional HI ring around NGC 5291 are
presented.Comment: 4 pages, 1 fig., tp appear in conference proceedings "Studying Galaxy
Evolution with Spitzer and Herschel", eds. V. Charmandaris, D. Rigopoulou &
N. Kylafi
Free-Floating HI Clouds in the M 81 Group
Recent VLA observations pointed at dwarf spheroidal (dSph) galaxies in the M
81 group reveal a hitherto hidden population of extremely low mass (~1e5 Msol)
HI clouds with no obvious optical counterparts. We have searched 10 fields in
the M81 group totalling 2.2 square degree, both targeting known dwarf
spheroidal galaxies and blank fields around the central triplet. Our
observations show that the new population of low-mass HI clouds appears to be
confined to a region toward the South-East of the central triplet (at distances
of ~100 kpc from M 81). Possible explanations for these free-floating HI clouds
are that they are related to the dSphs found to the South-East of M 81, that
they belong to the galaxies of the M 81 triplet (equivalent to HVCs), that they
are of primordial nature and provide fresh, unenriched material falling into
the M 81 group, or that they are tidal debris from the 3-body interaction
involving M 81-M 82-NGC 3077. Based on circumstantial evidence, we currently
favour the latter explanation.Comment: To appear in the Proceedings of IAU Symp. 244 on "Dark Galaxies and
Lost Baryons", eds. J. I. Davies & M. J. Disne
21-cm H I emission from the Damped Lyman-alpha absorber SBS 1543+593
We detect 21-cm emission from the Low Surface Brightness (LSB) galaxy SBS
1543+593, which gives rise to a Damped Lyman-alpha (DLA) absorption line in the
spectrum of the background QSO HS 1543+5921 (z=0.807). We obtain an accurate
measure of the velocity of the H I gas in the LSB galaxy, v=2868 km/s, and
derive a mass of 1.3e9 solar masses. We compare this value with limits obtained
towards two other z~0.1 DLA systems, and show that SBS 1543+593 would not have
been detected. Hence LSB galaxies similar to SBS 1543+593 can be responsible
for DLA systems at even modest redshifts without being detectable from their
21-cm emission.Comment: 4 pages, 1 figure, accepted for publication in A&
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
The Gas Phase in a Low Metallicity ISM
Original article can be found at: http://journals.cambridge.org/ Copyright International Astronomical Union. DOI: 10.1017/S1743921308024927We present several results from our analysis of dwarf irregular galaxies culled from The HI Nearby Galaxy Survey (THINGS). We analyse the rotation curves of two galaxies based on “bulk” velocity fields, i.e. velocity maps from which random non–circular motions are removed. We confirm that their dark matter distribution is best fit by an isothermal halo model. We show that the star formation properties of dIrr galaxies resemble those of the outer parts of larger, spiral systems. Lastly, we study the large scale (3–D) distribution of the gas, and argue that the gas disk in dIrrs is thick, both in a relative, as well as in an absolute sense as compared to spirals. Massive star formation through subsequent supernova explosions is able to redistribute the bulk of the ISM, creating large cavities. These cavities are often larger, and longer–lived than in spiral galaxies.Peer reviewe
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