1. The Dwarf Galaxy Duty Cycle: Measurements from a Complete Sample of the Local Volume Janice C. Lee 1, Robert C. Kennicutt 1, Sanae Akiyama 1, Jose G. Funes 1,2, Shoko Sakai 3 Abstract We have carried out an H  and R-band imaging survey of a volume-limited sample of 340 spiral and irregular galaxies within a distance of 11 Mpc. Our goal is to fully character- ize the star formation properties of complete samples of galaxies in the local universe. In particular, we are using this dataset to understand the importance of starbursts in dwarf galaxies. A preliminary analysis suggests that about 1 out of 10 dwarf galaxies is currently forming stars at a rate that is at least two times higher than its average past rate. These bursty systems are responsible for 20-30% of the total current star formation in dwarfs. 1 Steward Observatory, University of Arizona, Tucson, AZ, USA 2 Vatican Observatory Research Group, University of Arizona, Tucson, AZ, USA 3 Division of Astronomy and Astrophysics, UCLA, Los Angeles, CA USA Construction of the Sample To build a sample of the local volume, we began by compiling spiral and irregular galaxies from: ● the Tully Nearby Galaxies Catalog (NBG) with d < 11 Mpc, where the tabulated NBG distances are based on the flow model of Tully & Shaya (1984) ● NED, with the constraints that the object lie outside of the galactic plane (|b| > 20°), with B < 15.0 and v LG < 825 km/s. After excluding probable Virgo Cluster and Coma Group members and collecting all available direct distance estimates, our resulting catalog contains 340 galaxies within 11 Mpc, where H o = 75 km/s/Mpc has been adopted to convert flow-corrected velocities to distances for galaxies without direct distance estimates. Observations Over a four year period (2000-03), galaxies in our 11 Mpc catalog have been observed through Kron- Cousins R-band and narrow-band (FWHM~70Å) H  filters at the Bok 2.3m (KPNO), the VATT 1.8m (MGIO) and the CTIO 0.9m. Completeness?? Ultimately our dataset is a composite of numerous cata- logs with diverse selection criteria. Thus, there is no a priori selection function that can be adopted. Therefore, we will seek to establish the completeness of our sample in a variety of ways, as illustrated to the left. Figure 2 - Our 11 Mpc catalog fully samples the HI mass function (based on HI blind surveys) down to 7.5.e8 M sun. By comparing our HI mass densities to those from HIPASS we can estimate the completeness of our sample to be 90% at 2.4e8 and 83% at 1.3e8. Figure 1 - By construction, our sample is limited at B~15 (the limit of the parent catalogs from which we extract the majority of our sources). Thus, our data- set should be complete to M B ~-15 for d<11 Mpc. This is confirmed by comparison to independently deter- mined B-band luminosity functions. Comparison also shows that our 7 Mpc sub- sample is complete to M B ~- 13.5, and that the 11 Mpc sample can be reasonably corrected down to that magnitude. Figure 3 – The relation between b and EW(H  ) as predicted by the KTC94 models (red), and from a coarse empirical calibration between the continuum flux and the R-band apparent mag for two choices of constant M/L R that bracket the typical range (black). This type of analysis can be used to set upper limits on the # of bursty systems, as well as their fractional contribution to the SFR. Figure 6 – The contribution of bursty systems to the total current SF. For the entire sample, systems with b >2 are responsible for ~6% of the total current SF, while they account for 20-30% of the SF in dwarfs. Figure 4 – Distributions of b by M B (solid curves) and t-type (dotted curves). The median b for the entire sample is 0.5, and increases from 0.3 when only the most luminous galaxies are considered, to 0.5 for the low-luminosity samples. The median b also increases with later Hubble type. Figure 5 – The fraction of bursty systems in our sample. For both the entire sample and the low-luminosity samples about 7% have b >2. If we take this as a threshold over which a galaxy is considered bursting, and assume that all dwarfs have an equal probability of bursting, then the duty cycle for dwarf galaxies (i.e. the fraction of time each system spends in the bursting state) is also about 7%. Clearly, much work lies ahead. We will: ● use updated synthesis models, and also include the effects of dust ● incorporate incompleteness corrections, and assess errors ● use the entire sample to quantify the incompleteness in the SFR density based on UV or emission-line selected surveys ● take advantage of the wealth of 2-D information available in our images to understand how asymmetries, concentrations, and radial extents vary with burstiness and overall star formation. ● During the current cycle, we will obtain FUV and NUV images for a complete subset (N=110) of our catalog to probe star-formation on a longer timescale. This will provide powerful constraints on systematic errors in Ha inferred star formation related quantities. …a GALEX Legacy Figure 7 – The top line shows the distribution of MB for the complete 11 Mpc sample. The filled histogram shows the current coverage of this volume by the GALEX PI NGS and MIS surveys and illustrates the roughly 5-fold under-representation of galaxies fainter than M B =-17. The middle histogram shows the total coverage of the local population when our GALEX Legacy program targets are combined with the NGS and MIS surveys. What is a burst? For the purposes of this first analysis, we will use the Scalo “ b ” (birthrate) parameter as a measure of the burstiness of a galaxy where b = current SFR/ past average SFR To derive b, we use the relationship between EW(H  ) and b predicted by the synthesis models of Kennicutt, Tamblyn & Congdon (1994) (KTC94). What is a dwarf? Here we use the blue luminosity of a galaxy as an observable indicator of mass. While B-band light is greatly affected by dust and recent star formation, preliminary analyses using stellar masses computed from the models of KTC94 indicate that these results based on samples defined by M B are consistent with dwarf samples defined by mass with M  <2e9M sun. U8091 (GR8), a bursting dwarf galaxy with b> 2 is shown in the background. The 11 Mpc H  Imaging Survey The Role of Bursty Systems: A Preview Coming Soon…