Does Stellar Feedback Create HI Holes? An HST/VLA Study of
Transcription
Does Stellar Feedback Create HI Holes? An HST/VLA Study of
Does Stellar Feedback Create HI Holes? An HST/VLA Study of Holmberg II ApJ, 2009, 704, 1538 Dan Weisz1, 1University Evan Skillman1, John Cannon2, Andy Dolphin3, Rob Kennicutt4, Janice Lee5, Fabian Walter6 of Minnesota, 2Macalester College, 3Raytheon Company, 4IoA Cambridge, 5OCIW, 6MPIA HI Holes in Holmberg II Nearby Dwarf Galaxies Dozens of holes, shells, and bubbles have been identified in the ISM of Ho II. Each `hole’ (green) has a measured size and expansion velocity. Non-hole control fields are used for comparison (cyan). Rhode et al. (1999; red) explored the stellar feedback connection to hole creation, finding few bright central clusters. We use deep HST/ACS imaging to explore the stellar populations of the HI holes. The Local Volume provides a wealth of diverse galaxies ideal for studying SF through resolved stellar populations. The ACS Nearby Galaxy Survey Treasury (ANGST; Dalcanton et al. 2009; Weisz et al. 2010) has multi-color photometry of individual stars in ~60 nearby dwarf galaxies (D<4Mpc). Color-Magnitude Diagrams HST/ACS Imaging of Ho II CMDs provide a fossil record of past SF activity in a galaxy. Model CMDs show how we extract the ages of stars. We can then convert these to SFHs, SFR(t,Z). Young MS and core red and blue helium burning stars (RHeBs, BHeBs) provide excellent leverage for the ages and locations of recent SF. We observed Ho II as part of the M81 Group dwarfs program (Weisz et al. 2008) in F555W and F814W. Ho II is one of the larger dIs in the M81 Group recent SFRs ~0.01-0.1 M/yr. It’s proximity to M81 makes it a typical group member, but it is not strongly interacting – suggesting an internal origin of the holes. Further, there is nothing peculiar about the stellar or HI distribution of Ho II to suggest a recent interaction. Stars Within HI Holes Deep HST/ACS imaging reveals that all HI holes in Ho II contain young, mixed age stellar populations. In many cases, the youngest stars are tens of Myr old, and fainter then detection limits of Rhode et al. (1999). Control fields show similar CMDs, suggesting small holes (<100 pc; HI resolution limit) may be forming. Star Formation Histories From the CMDs, we compute the SFHs for stars in each HI hole. Comparing the SFHs to the dynamical ages (HI expansion velocity and hole size; red dashed line), we see the majority of SF does not always take place within the dynamical age. This suggests multiple SF events, and not a single episode of SF, i.e., a cluster, is the most likely mode of HI hole creation. Spatially Resolved SFH BHeBs have a unique age-luminosity relationship. Knowing the magnitude of a BHeB equates to knowing its age. Combining the age and spatial information from the BHeBs with the CMD based SFH, we construct the spatially resolved SFH of Ho II, showing how SF has propagated in the galaxy in space, time, and amplitude over ~ 100 Myr. Z = 0.2 Z HI and Stellar Energetics The energy necessary to evacuate an HI hole with diameter, d, depends on the initial HI volume density, n0, and the observed HI expansion velocity, v, Chevalier (1974) show that: The stellar energies can then be computed using the SFHs and STARBURST99. We can then compare the stellar energy profile over the last 200 Myr to the HI hole creation energy. We find energy from SF is generally not sufficient to have created an HI hole within its dynamical age (grey dashed line). However, in all cases they do have enough energy to have created the holes on slightly longer timescales, suggesting multiple generations of SF may have been present. We also consider the effects of stellar feedback efficiencies (red:100%, green:10%, blue:1%/ Multi-Wavelength View Locations of Young Stars We use the young MS stars to study the spatial distribution of recent SF. The youngest stars (<10 Myr; red) are typically clustered on the edges of HI holes, while the older population (< 75 Myr; blue) is slightly more distributed. We do not see evidence for clustering inside HI holes. SF at multiple locations could have created small HI holes, that later merged to form the larger ones we observe. MB = -16.72 We compare the HI Hole locations to Hα, GALEX UV, and Spitzer 24μm imaging. We find Hα around the rims of holes, indicating triggered secondary SF. The UV is also clumpy, but not typically within the HI holes. Generally it is diffuse within these regions. These SF indicators are not reliable tracers of HI hole locations. Conclusions 1. The timing, energetics, SFHs, and spatial distributions of stars suggest that HI hole formation is complex and likely due to multiple generations of SF. 2. The simple model of single epoch SF resulting in HI hole formation appears to be an uncommon mode of HI hole formation. 3. Only deep CMDs of resolved stars are capable of detecting the remnant young stellar populations. Chevalier, 1974, ApJ, 188, 501; Dalcanton, 2009, ApJS, 183, 67D; Rhode et al., 1999, AJ, 118, 323; Weisz et al., 2010, In Prep. This work has been supported by the University of Minnesota and HST GO-10605