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Register a new userHigh-Redshift Starbursting Dwarf Galaxies Revealed by GRB Afterglows
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We present a study of 15 long-duration gamma-ray burst (GRB) host galaxies at z>2. The GRBs are selected with available early-time afterglow spectra in order to compare interstellar medium (ISM) absorption-line properties with stellar properties of the host galaxies. In addition to five previously studied hosts, we consider new detections for the host galaxies of GRB050820 and GRB060206 and place 2-sigma upper limits to the luminosities of the remaining unidentified hosts. We examine the nature of the host galaxy population and find that (1) the UV luminosity distribution of GRB host galaxies is consistent with expectations from a UV luminosity weighted random galaxy population with a median luminosity of <L(UV)>=0.1 L*; (2) there exists a moderate correlation between UV luminosity and SiII 1526 absorption width, which together with the observed large line widths of W(1526)>1.5 Ang for a large fraction of the objects suggests a galactic outflow driven velocity field in the host galaxies; (3) there is tentative evidence for a trend of declining ISM metallicity with decreasing galaxy luminosity in the star-forming galaxy population at z=2-4; (4) the interstellar UV radiation field is found ~ 35-350 times higher in GRB hosts than the Galactic mean value; and (5) additional galaxies are found at < 2 from the GRB host in all fields with known presence of strong MgII absorbers, but no additional faint galaxies are found at < 2 in fields without strong MgII absorbers. Our study confirms that the GRB host galaxies (with known optical afterglows) are representative of unobscured star-forming galaxies at z>2, and demonstrates that high spatial resolution images are necessary for an accurate identification of GRB host galaxies in the presence of strong intervening absorbers.
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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.
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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We use high-resolution ($approx 10$ pc), zoom-in simulations of a typical (stellar mass $M_starsimeq10^{10}M_odot$) Lyman Break Galaxy (LBG) at $zsimeq 6$ to investigate the stellar populations of its six dwarf galaxy satellites, whose stellar [gas] masses are in the range $log (M_star/M_odot) simeq 6-9$ [$log (M_{gas}/M_odot) simeq4.3-7.75$]. The properties and evolution of satellites show no dependence on the distance from the central massive LBG ($< 11.5$ kpc). Instead, their star formation and chemical enrichment histories are tightly connected their stellar (and sub-halo) mass. High-mass dwarf galaxies ($rm M_star gtrsim 5times 10^8 M_odot$) experience a long history of star formation, characterised by many merger events. Lower-mass systems go through a series of short star formation episodes, with no signs of mergers; their star formation activity starts relatively late ($zapprox 7$), and it is rapidly quenched by internal stellar feedback. In spite of the different evolutionary patterns, all satellites show a spherical morphology, with ancient and more metal-poor stars located towards the inner regions. All six dwarf satellites experienced high star formation rate ($rm >5,M_odot yr ^{-1}$) bursts, which can be detected by JWST while targeting high-$z$ LBGs.
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