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HOLMBERG IX: THE NEAREST YOUNG GALAXY

HOLMBERG IX: THE NEAREST YOUNG GALAXY. E. Sabbi, J.S. Gallagher, L.J. Smith, D.F. de Mello, & M. Mountain. ABSTRACT.

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HOLMBERG IX: THE NEAREST YOUNG GALAXY

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  1. HOLMBERG IX: THE NEAREST YOUNG GALAXY E. Sabbi, J.S. Gallagher, L.J. Smith, D.F. de Mello, & M. Mountain ABSTRACT 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. 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: AB=0.343 AU=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 kpc2

  7. Analysis • 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

  8. Results • Color-magnitude diagram shows two distinct stellar populations • Fainter stars (mF814W > 24) with red color (mF555W - mF814W > 1) are low-mass old stars in RGB phase

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