We present first results from a narrowband imaging program for intermediate redshift emission-line galaxies using the newly commissioned FourStar infrared camera at the 6.5m Magellan telescope. To enable prompt identification of Halpha emitters, a pa
ir of custom 1% filters, which sample low-airglow atmospheric windows at 1.19 mu m and 2.10 mu m, is used to detect both Halpha and [OII]lambda 3727 emission from the same redshift volume at z=2.2. Initial observations are taken over a 130 arcmin^2 area in the CANDELS-COSMOS field. The exquisite image quality resulting from the combination of the instrument, telescope, and standard site conditions (~0.55 FWHM) allows the 1.19 mu m and 2.10 mu m data to probe 3sigma emission-line depths down to 1.0e-17 erg/s/cm^2 and 1.2e-17 erg/s/cm^2 respectively, in less than 10 hours of integration time in each narrowband. For Halpha at z=0.8 and z=2.2, these fluxes correspond to observed star formation rates of ~0.3 and ~4 Msun/yr respectively. We find 122 sources with a 1.19 mu m excess, and 136 with a 2.10 mu m excess, 41 of which show an excess in both bands. The dual narrowband technique, as implemented here, is estimated to identify about >80% of z=2.2 Halpha emitters in the narrowband excess population. With the most secure such sample obtained to-date, we compute constraints on the faint-end slope of the z=2.2 Halpha luminosity function. These narrow-deep FourStar observations have been obtained as part of the larger NewHalpha Survey, which will combine the data with wide-shallow imaging through a similar narrowband filter pair with NEWFIRM at the KPNO/CTIO 4m telescopes, to enable study of both luminous (but rare) and faint emission-line galaxies in the intermediate redshift universe. [Abridged]
A great deal of our understanding of star formation in the local universe has been built upon an extensive foundation of H-alpha observational studies. However, recent work in the ultraviolet (UV) with GALEX has shown that star formation rates (SFRs)
inferred from H-alpha in galactic environments characterized by low stellar and gas densities tend to be less than those based on the UV luminosity. The origin of the discrepancy is actively debated because one possible explanation is that the stellar initial mass function is systematically deficient in high mass stars in such environments. In this contribution, we summarize our work on this topic using a dwarf galaxy dominated sample of ~300 late-type galaxies in the 11 Mpc Local Volume. The sample allows us to examine the discrepancy between H-alpha and UV SFRs using a statistical number of galaxies with activities less than 0.1 Msun/yr. A range of potential causes for such an effect are reviewed. We find that while the IMF hypothesis is not inconsistent with our observations, alternate explanations remain that must be investigated further before a final conclusion can be drawn.
An unresolved question in galaxy evolution is whether the star formation histories of low mass systems are preferentially dominated by starbursts or modes that are more quiescent and continuous. Here, we quantify the prevalence of global starbursts i
n dwarf galaxies at the present epoch, and infer their characteristic durations and amplitudes. The analysis is based on the H-alpha component of the 11 Mpc H-alpha UV Galaxy Survey (11HUGS), which is providing H-alpha and GALEX UV imaging for an approximately volume-limited sample of ~300 star-forming galaxies within 11 Mpc. We first examine the completeness properties of the sample, and then directly tally the number of bursting dwarfs and compute the fraction of star formation that is concentrated in such systems. Our results are consistent with a picture where dwarfs that are currently experiencing massive global bursts are just the ~6% tip of a low-mass galaxy iceberg. Moreover, bursts are only responsible for about a quarter of the total star formation in the overall dwarf population, so the majority of stars in low-mass systems are not formed in this mode today. Spirals and irregulars devoid of H-alpha emission are rare, indicating that the complete cessation of star formation generally does not occur in such galaxies and is not characteristic of the inter-burst state, at least for the more luminous systems with M(B)<-15. The starburst statistics presented here directly constrain the duty cycle and the average burst amplitude under the simplest assumptions where all dwarf irregulars share a common star formation history and undergo similar bursts cycles with equal probability. Uncertainties in such assumptions are discussed in the context of previous work.
