1. HOLMBERG IX: THE NEAREST YOUNG GALAXY E. Sabbi, J.S. Gallagher, L.J. Smith, D.F. de Mello, & M. Mountain Deep images taken with the Wide Field Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope provide the basis for study of the resolved stellar population of the M81 companion dwarf irregular galaxy Holmberg IX. Based on color-magnitude diagrams the stellar population toward Holmberg IX contains numerous stars with ages of red giant stars. By charting the spatial distribution of the red giant stars and considering their inferred metallicities, we concluded that most of these older stars are associated with M81 or its tidal debris. At least 20% of the stellar mass in Holmberg IX was produced in the last ~200 Myr, giving it the youngest stellar population of any nearby galaxy. The location of Holmberg IX, its high gas content, and its youthful stellar population suggest that it is a tidal dwarf galaxy, perhaps formed during the last close passage of M82 around M81. ABSTRACT arXiv:0802.4446v1 [astro-ph] 29 Feb 2008 accepted by ApJL

2.  Introduction - Who is Holmberg IX and what is a tidal dwarf galaxy?  HST Observations  Analysis and Results  Conclusions  Discussion - Can UVOT contribute to this?

3. Holmberg IX J2000: 149.383, +69.046 Distance: 3.6 Mpc Classification: Im Magnitude: V=14.10 B=14.30 U=13.90 Angular Size: 2.5  2.0 Reddening: E(B-V)=0.079 Extinction: A B =0.343 A U =0.432

4. Zwicky (1956) proposed that new stellar systems form within the tidal debris of interacting galaxies More recently this concept has received observational support and in some cases, the internal kinematics are indicative of gravitational binding Hibbard et al. 1994

5. Hibbard et al. 1994 Tidal Dwarf Galaxies (TDGs) form in dynamically cool tidal tails and favor gas-rich regions should be a mix of pre-existing stars from tidally disrupted material and a “new generation” of young stars produced as HI gas condenses TDGs should contain little dark matter (assumes a lack of DM in galaxy disks) although young TDGs may be prominent due to the burst of star formation, their long-term fate remains unclear

6. HST Observations Retrieved deep broadband F555W and F814W ACS/WFC images from the Multimission Archive at STScI (P.I. Skillman, GO-10605) Dataset consists of eight 1192s dithered exposures in both the F555W and F814W filters Data were processed through the standard ACS calibration pipeline and images were co-added using the MULTIDRIZZLE package Total exposure time is 4768s in each filter The images cover an area of 200   200  corresponding to 3.5  3.5 kpc 2

8. Analysis and Results Photometric reduction performed with DAOPHOT package in IRAF Stars detected independently in each filter using DAOFIND with detection threshold set at 4  above background Fluxes measured via aperture photometry using size of 0.15  and refined using PSF fitting from ~180 stars over the detector Applied selection criteria using DAOPHOT  2 and sharpness parameters to reject spurious and extended objects Final catalog contains 23,182 stars

9. Color-magnitude diagram shows two distinct stellar populations Fainter stars (I > 24) with red color (V-I > 1) are low-mass, old stars in RGB phase Young stellar population is represented by blue and red plumes: blue plume at V-I ~0 is comprised of upper main sequence stars + hot side of helium burning phase (blue loop) red plume at 1.2 < V-I < 2.0 is comprised of red supergiants at brighter magnitudes (I < 24) which are the cool side of the blue loop + fainter AGB stars the gap at 1.0 24 indicates a period of prolonged quiescence about 1 Gyr ago

10. Spectra of HII regions indicate a metallicity of Z ~ 0.008 for the ionized gas Used Padova isochrones (Salasnich et al. 2000) to derive ages for the two stellar components the older stellar population (RGB structure) ranges in age from 1 to 12 Gyr, also a span in metallicity a 2nd major episode of star formation started ~ 200 Myr ago according to dynamical simulations (Yun 1999) this coincides with a close encounter between M81 and M82 such an event could be the origin of Holmberg IX, thus classifying it as a TDG

11. The M81 Group (M81 / M82 / NGC 3077) Optical image from the DSS, HI contours from VLA data (D-array) Yun, Ho, & Yo 1994

12. Figure 3 shows the spatial distribution of the two populations Blue plume stars are strongly concentrated in the center of Holmberg IX, as expected for a star-forming dwarf galaxy The spatial density of the RGB stars peaks in the corner closest to M81 Experiments with artificial stars used to evaluate photometry and completeness Concluded that differences in spatial distributions not due to crowding or incompleteness in the central region

13. Conclusions CMD clearly shows an intense episode of star formation in the last 200 Myr The spatial distribution of the RGB stars and their higher metallicities are consistent with an old stellar population from the halo or disk of M81 Holmberg IX has most of its baryonic matter in the form of gas  M(HI)/M  > 40 Holmberg IX has the youngest mean stellar population of any nearby galaxy Observed properties are consistent with a TDG that formed in tidal debris ~ 200 Myr ago, the time of the closest M81-M82 approach