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Register a new userDynamics of Starbursting Dwarf Galaxies. II. UGC 4483
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UGC 4483 is a nearby Blue Compact Dwarf (BCD) galaxy. HST observations have resolved the galaxy into single stars and this has led to the derivation of its star formation history and to a direct estimate of its stellar mass. We have analysed archival VLA observations of the 21 cm line and found that UGC 4483 has a steeply-rising rotation curve which flattens in the outer parts at a velocity of ~20 km/s. Radial motions of ~5 km/s may also be present. As far as we know, UGC 4483 is the lowest-mass galaxy with a differentially rotating HI disk. The steep rise of the rotation curve indicates that there is a strong central concentration of mass. We have built mass models using the HST information on the stellar mass to break the disk-halo degeneracy: old stars contribute ~50$%$ of the observed rotation velocity at 2.2 disk scale-lengths. Baryons (gas and stars) constitute an important fraction of the total dynamical mass. These are striking differences with respect to typical dwarf irregular galaxies (dIrrs), which usually have slowly-rising rotation curves and are thought to be entirely dominated by dark matter. BCDs appear to be different from non-starbursting dIrrs in terms of their HI and stellar distributions and their internal dynamics. To their high central surface brightnesses and high central HI densities correspond strong central rotation-velocity gradients. This implies that the starburst is closely related with the gravitational potential and the concentration of gas. We discuss the implications of our results on the properties of the progenitors/descendants of BCDs.
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A subgroup of dwarf galaxies have characteristics of a possible evolutionary transition between star-forming systems and dwarf ellipticals. These systems host significant starbursts in combination with smooth, elliptical outer envelopes and small HI content; they are low on gas and unlikely to sustain high star formation rates over significant cosmic time spans. We explore possible origins of such starburst transition dwarfs using moderately deep optical images. While galaxy-galaxy interactions could produce these galaxies, no optical evidence exists for tidal debris or other outer disturbances, and they also lack nearby giant neighbors which could supply recent perturbations. Colors of the outer regions indicate that star formation ceased > 1 Gyr in the past, a longer time span than can be reasonably associated with the current starbursts. We consider mechanisms where the starbursts are tied either to interactions with other dwarfs or to the state of the interstellar medium, and discuss the possibility of episodic star formation events associated with gas heating and cooling in low specific angular momentum galaxies.
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We present new WFC3/UVIS observations of UGC 4483, the closest example of a metal-poor blue compact dwarf galaxy, with a metallicity of $Z simeq 1/15 Z_{odot}$ and located at a distance of $D simeq 3.4$ Mpc. The extremely high quality of our new data allows us to clearly resolve the multiple stellar evolutionary phases populating the color-magnitude diagram (CMD), to reach more than 4 mag deeper than the tip of the red giant branch, and to detect for the first time core He-burning stars with masses $lesssim 2$ M$_{odot}$, populating the red clump and possibly the horizontal branch (HB) of the galaxy. By applying the synthetic CMD method to our observations, we determine an average star formation rate over the whole Hubble time of at least $(7.01pm0.44) times 10^{-4}$ $mathrm{M_{odot}/yr}$, corresponding to a total astrated stellar mass of $(9.60pm0.61)times 10^6$ $mathrm{M_{odot}}$, 87% of which went into stars at epochs earlier than 1 Gyr ago. With our star formation history recovery method we find the best fit with a distance modulus of DM = $27.45pm0.10$, slightly lower than previous estimates. Finally, we find strong evidence of an old ($gtrsim 10$ Gyr) stellar population in UGC 4483 thanks to the detection of an HB phase and the identification of six candidate RR Lyrae variable stars.
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