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Register a new userProperties of Bulgeless Disk Galaxies II. Star Formation as a Function of Circular Velocity
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Authors
Linda C. Watson
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We study the relation between the surface density of gas and star formation rate in twenty moderately-inclined, bulgeless disk galaxies (Sd-Sdm Hubble types) using CO(1-0) data from the IRAM 30m telescope, HI emission line data from the VLA/EVLA, H-alpha data from the MDM Observatory, and PAH emission data derived from Spitzer IRAC observations. We specifically investigate the efficiency of star formation as a function of circular velocity (v_circ). Previous work found that the vertical dust structure and disk stability of edge-on, bulgeless disk galaxies transition from diffuse dust lanes with large scale heights and gravitationally-stable disks at v_circ < 120 km/s (M_star <~ 10^10 M_sun) to narrow dust lanes with small scale heights and gravitationally-unstable disks at v_circ > 120 km/s. We find no transition in star formation efficiency (Sigma_SFR/Sigma_HI+H2) at v_circ = 120 km/s, or at any other circular velocity probed by our sample (v_circ = 46 - 190 km/s). Contrary to previous work, we find no transition in disk stability at any circular velocity in our sample. Assuming our sample has the same dust structure transition as the edge-on sample, our results demonstrate that scale height differences in the cold interstellar medium of bulgeless disk galaxies do not significantly affect the molecular fraction or star formation efficiency. This may indicate that star formation is primarily affected by physical processes that act on smaller scales than the dust scale height, which lends support to local star formation models.
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We present GMRT 1280 MHz radio continuum observations and follow-up optical studies of the disk and nuclear star formation in a sample of low luminosity bulgeless galaxies. The main aim is to understand bulge formation and overall disk evolution in these late type galaxies. We detected radio continuum from five of the twelve galaxies in our sample; the emission is mainly associated with disk star formation. Only two of the detected galaxies had extended radio emission; the others had patchy disk emission. In the former two galaxies, NGC3445 and NGC4027, the radio continuum is associated with star formation triggered by tidal interactions with nearby companion galaxies. We did follow-up Halpha imaging and nuclear spectroscopy of both galaxies using the Himalayan Chandra Telescope (HCT). The Halpha emission is mainly associated with the strong spiral arms. The nuclear spectra indicate ongoing nuclear star formation in NGC3445 and NGC4027 which maybe associated with nuclear star clusters. No obvious signs of AGN activity were detected. Although nearly bulgeless, both galaxies appear to have central oval distortions in the R band images; these could represent pseudobulges that may later evolve into large bulges. We thus conclude that tidal interactions are an important means of bulge formation and disk evolution in bulgeless galaxies; without such triggers these galaxies appear to be low in star formation and overall disk evolution.
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We explore how the star formation efficiency in a protocluster clump is regulated by metallicity dependent stellar winds from the newly formed massive OB stars (Mstar >5 Msol). The model describes the co-evolution of the mass function of gravitationally bound cores and of the IMF in a protocluster clump. Dense cores are generated uniformly in time at different locations in the clump, and contract over lifetimes that are a few times their free fall times. The cores collapse to form stars that power strong stellar winds whose cumulative kinetic energy evacuates the gas from the clump and quenches further core and star formation. This sets the final star formation efficiency, SFEf. Models are run with various metallicities in the range Z/Zsol=[0.1,2]. We find that the SFEf decreases strongly with increasing metallicity.The SFEf-metallicity relation is well described by a decaying exponential whose exact parameters depend weakly on the value of the core formation efficiency. We find that there is almost no dependence of the SFEf-metallicity relation on the clump mass. This is due to the fact that an increase (decrease) in the clump mass leads to an increase (decrease) in the feedback from OB stars which is opposed by an increase (decrease) in the gravitational potential of the clump. The clump mass-cluster mass relations we find for all of the different metallicity cases imply a negligible difference between the exponent of the mass function of the protocluster clumps and that of the young clusters mass function. By normalizing the SFEs to their value for the solar metallicity case, we compare our results to SFE-metallicity relations derived on galactic scales and find a good agreement. As a by-product of this study, we also provide ready-to-use prescriptions for the power of stellar winds of main sequence OB stars in the mass range [5,80] Msol in the metallicity range we have considered
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