Showing papers by "Deidre A. Hunter published in 2011"

Deep Radio Continuum Imaging of the Dwarf Irregular Galaxy IC 10: Tracing Star Formation and Magnetic Fields

Abstract: We exploit the vastly increased sensitivity of the Expanded Very Large Array to study the radio continuum and polarization properties of the post-starburst, dwarf irregular galaxy IC 10 at 6 cm, at a linear resolution of {approx}50 pc. We find close agreement between radio continuum and H{alpha} emission, from the brightest H II regions to the weaker emission in the disk. A quantitative analysis shows a strictly linear correlation, where the thermal component contributes 50% to the total radio emission, the remainder being due to a non-thermal component with a surprisingly steep radio spectral index of between -0.7 and -1.0 suggesting substantial radiation losses of the cosmic-ray electrons. We confirm and clearly resolve polarized emission at the 10%-20% level associated with a non-thermal superbubble, where the ordered magnetic field is possibly enhanced due to the compression of the expanding bubble. A fraction of the cosmic-ray electrons has likely escaped because the measured radio emission is a factor of three lower than what is suggested by the H{alpha}-inferred star formation rate.

Outside-in Shrinking of the Star-forming Disk of Dwarf Irregular Galaxies

Abstract: We have studied multi-band surface brightness profiles of a representative sample of 34 nearby dwarf irregular galaxies (dIrrs). Our data include GALEX FUV/NUV, UBV, H\alpha, and Spitzer 3.6 \mum images. These galaxies constitute the majority of the LITTLE THINGS survey. By modeling the azimuthal averages of the spectral energy distributions with a complete library of star formation (SF) histories, we derived the stellar mass surface density distributions and the SF rate averaged over three different timescales: the recent 0.1 Gyr, 1 Gyr and a Hubble time. We find that, for \sim 80% (27 galaxies) of our sample galaxies, radial profiles (at least in the outer part) at shorter wavelengths have shorter disk scale lengths than those at longer wavelengths. This indicates that the star-forming disk has been shrinking. In addition, the radial distributions of the stellar mass surface density are well described as piece-wise exponential profiles, and \sim 80% of the galaxies have steeper mass profiles in the outer disk than in the inner region. The steep radial decline of SF rate in the outer parts compared to that in the inner disks gives a natural explanation for the down-bending stellar mass surface density profiles. Within the inner disks, our sample galaxies on average have constant ratios of recent SF rate to stellar mass with radius. Nevertheless, \sim 35% (12 galaxies, among which 7 have baryonic mass < 10^8 M\odot) of the sample exhibit negative slopes across the observed disk, which is in contrast with the "inside-out" disk growth scenario suggested for luminous spiral galaxies. The tendency of SF to become concentrated toward the inner disks in low mass dIrrs is interpreted as a result of their susceptibility to environmental effects and regulation through stellar feedback.

VII Zw 403: HI Structure in a Blue Compact Dwarf Galaxy

Abstract: We present optical (UBVJ), ultraviolet (FUV, NUV), and high resolution atomic hydrogen (HI) observations of the nearby blue compact dwarf (BCD), VII Zw 403 We find that VII Zw 403 has a relatively high HI mass-to-light ratio for a BCD The rotation velocity is nominally 10-15 km/s, but rises to ~20 km/s after correction for the ~8-10 km/s random motions present in the gas The velocity field is complex; including a variation in the position angle of the major axis going from the NE to the SW parts of the galaxy Our high resolution HI maps reveal structure in the central gas, including a large, low-density HI depression or hole between the southern and northern halves of the galaxy, coincident with an unresolved x-ray source Although interactions have been proposed as the triggering mechanism for the vigorous star formation occurring in BCDs, VII Zw 403 does not seem to have been tidally triggered by an external interaction, as we have found no nearby possible perturbers It also doesn't appear to fall in the set of galaxies that exhibit a strong central mass density concentration, as its optical scale length is large in comparison to similar systems However, there are some features that are compatible with an accretion event: optical/HI axis misalignment, a change in position angle of the kinematic axis, and a complex velocity field

VII Zw 403: H I structure in a blue compact dwarf galaxy

Abstract: We present optical (UBVJ), ultraviolet (FUV, NUV), and high-resolution atomic hydrogen (H I) observations of the nearby blue compact dwarf (BCD), VII Zw 403. We find that VII Zw 403 has a relatively high H I mass-to-light ratio for a BCD. The rotation velocity is nominally 10-15 km s–1, but rises to ~20 km s–1 after correction for the ~8-10 km s–1 random motions present in the gas. The velocity field is complex, including a variation in the position angle of the major axis going from the northeast to the southwest parts of the galaxy. Our high-resolution H I maps reveal structure in the central gas, including a large, low-density H I depression or hole between the southern and northern halves of the galaxy, coincident with an unresolved X-ray source. Although interactions have been proposed as the triggering mechanism for the vigorous star formation occurring in BCDs, VII Zw 403 does not seem to have been tidally triggered by an external interaction, as we have found no nearby possible perturbers. It also does not appear to fall in the set of galaxies that exhibit a strong central mass density concentration, as its optical scale length is large in comparison to similar systems. However, there are some features that are compatible with an accretion event: optical/H I axis misalignment, a change in position angle of the kinematic axis, and a complex velocity field.

Mapping the Extended HI Distribution of Three Dwarf Galaxies

Abstract: We present large field HI-line emission maps obtained with the single-dish Green Bank Telescope centered on the dwarf irregular galaxies Sextans A, NGC 2366, and WLM We do not detect the extended skirts of emission associated with the galaxies that were reported from Effelsberg observations (Huchtmeier et al 1981) The ratio of HI at 10^19 atoms cm^-2 to optical extents of these galaxies are instead 2--3, which is normal for this type of galaxy There is no evidence for a truncation in the HI distribution >/=10^19 atoms cm^-2

Mapping the Extended H I Distribution of Three Dwarf Galaxies

Abstract: We present large field H I-line emission maps obtained with the single-dish Green Bank Telescope centered on the dwarf irregular galaxies Sextans A, NGC 2366, and WLM. We do not detect the extended skirts of emission associated with the galaxies that were reported from Effelsberg observations. The ratio of H I at 10{sup 19} atoms cm{sup -2} to optical extents of these galaxies is instead 2-3, which is normal for this type of galaxy. There is no evidence for a truncation in the H I distribution {>=}10{sup 19} atoms cm{sup -2}.

Radial variations of the SFHs of dwarf irregular galaxies

Abstract: The LITTLE THINGS project† has compiled multi-wavelength data (including VLA hi-line emission maps, GALEX FUV/NUV imagery, UBV, narrow-band Hα, and Spitzer images) for a representative sample of nearby dwarf irregular (dIrr) galaxies. The broadband data are used to constrain the radial variations of the star formation (SF) rate (SFR) averaged over the past 0.1 Gyr, 1 Gyr and a Hubble time, with a complete library of model SF histories (SFHs). The recent SF of more than ~ 80% of the dIrrs in our sample has been concentrated in the inner disk, and the SF in the outer disk has been markedly suppressed. This outside-in shrinking of the star-forming disk leaves a down-bending (double exponential) stellar mass surface density (Σ*) distribution. Our findings in dIrrs are in contrast to the inside-out disk growth scenario suggested for luminous spiral galaxies.