Star Formation in LITTLE THINGS Dwarf Galaxies

Abstract

In this thesis we test and expand our current knowledge of Star Formation Laws (SF laws) in the extreme environment of dwarf irregular galaxies. We focus on the SF characteristics of our 18 galaxies sample, extending current investigations of the Schmidt-Kennicutt law to the low luminosity, low metallicity regime. The Hi data used in this project have been observed, calibrated and imaged according to the LITTLE THINGS Survey prescription to which I brought my own contribution as a member of the team. Apart from high resolution, VLA data in B, C and D array con gurations, this project makes use of an extensive set of multi- wavelength data (H , FUV, 24 m, 3.6 m, V-band and K-band). Molecular gas in dwarfs is very di fficult to observe, mainly because due to the low metallicity environment, we lose our only molecular tracer, the CO which becomes under luminous. Therefore the gas distribution is represented by Hi gas only. We create our Star Formation Rate (SFR) maps mainly based on FUV maps because our analysis shows that FUV is the SF tracer that allows us the most extensive sampling of the SFR surface density (SFRD) and Hi surface density relation. The main results of our study are: Whereas in spiral galaxies Bigiel et al. (2008) have found a one to one relation between star formation rate and molecular gas and no relation between the SFR and the neutral gas, in a small sample of dwarfs as well as in the outskirts of spiral galaxies Bigiel et al. (2010b) has found that SFRD does correlate with Hi surface density. We con rm the existence of the SFRD vs. Hi surface density relation in dwarf irregular galaxies and a linear fitting through all our data (all 18 galaxies combined) yields a power law relation SFR / 1:87 0:3 HI . We find that the interiors of Hi shells, at 400 pc scales, become resolved and show up in SFRD versus Hi surface density plots although within the shell interior we have SFRD values but no Hi surface density related to them. Thus, the points originating from those regions contribute significantly to the increase of the scatter in the plot. We show that by excluding those points the correlation between SFRD and Hi surface density improves between 10% and 20%. Eight of the 18 galaxies in our sample have Hi maxima higher than the 10M pc-2 value found by Bigiel et al. (2008) for spiral galaxies. Krumholz et al. (2011) predicted that the 10M pc-2 threshold is metallicity dependent in galaxies with sub-solar metallicity, however the theoretically predicted values for our galaxies only match the observed Hi maxima in one case (DDO168). We find that metallicity cannot be the only factor setting the Hi to H2 transition. In fact, we find evidence that the higher the interstellar radiation field (ISRF), the higher the Hi maximum is, hence we suggest that the ISRF should also be taken into consideration in predicting the Hi to H2 transition threshold. We find that even tighter than the SFRD vs. Hi surface density relation is the SFRD vs. V-band surface density relation. Unlike the SFRD vs. Hi surface density relation the SFRD vs. V-band surface density relation follows a power law and can be written as follows: SFR / (10 V )ð€€€0:43 0:03. The SFRD vs. V-band surface density relation suggests that the existing stars also play a role in the formation of the next generation of stars. Within our sample of dwarf galaxies the average pressure per resolution element and the SFRD are in a 1:1 linear relation: SFR / P1:02 0:05. A similar relation has been found by Blitz & Rosolowsky (2006) for the low-pressure regimes of spiral galaxies. In conclusion we find that in the extreme environments of dwarf galaxies the metal deficiency and the lack of the classic SF stimulators (spiral arms, shear motions) do not impede the star forming process. In these galaxies, dust-shielding becomes predominantly self-shielding and there is plenty of Hi available to achieve this additional task. Existing stars assume the role of pressure enhancers, which in turn will stimulate SF without the need of spiral arms or shear motion.