Hi Galina: This is a revised submission of our Phase II for 10585. As compared to our earlier submission of May 10, 2005, this new Phase II contains one extra visit (visit 3) consisting of one orbit. This orbit is a recent additional allocation to this program (with the approval of the head of the Science Policies Division) to allow for additional deeper imaging in F658N and F814W. Let us know if you have any questions. ----- Submission 1 (Tue May 10 01:40:06 GMT 2005) ----- ----- Submission 2 (Fri Jun 10 18:43:45 GMT 2005) ----- none Assigned ID: 376 ----- Submission 1 (Fri Jan 21 21:05:44 GMT 2005) ----- Phase I Proposal 376 (ngc4449) successfully submitted. NGC 4449: a Testbed for Starbursts in the Low- and High-Redshift Universe We will obtain deep ACS/WFC broad-band imaging in F435W, F555W, and F814W of the Magellanic irregular galaxy NGC 4449 in order to infer the properties of the resolved (young and old) stellar population, compare the current strong star-formation activity to that at earlier epochs, trace the major episodes of star formation over cosmic time, and relate them to the past history of interaction/merging of this galaxy. NGC 4449 is one of the closest (< 5 Mpc) global starbursts with a widespread star-formation activity that could have been triggered by interaction or accretion. It is, therefore, the perfect laboratory where to address issues related to star formation in extreme environments and investigate processes connected to the formation and evolution of galaxies in the early universe. At a distance of 4.2 Mpc, the stellar content in NGC 4449 can easily be resolved with HST. Yet, up to date no comprehensive dataset has been collected that is suited to infer a global star-formation history of the galaxy and to better understand its evolution. ACS/WFC provides the angular resolution and sensitivity to easily reach magnitude limits necessary for the detection of individual stars at the tip of the red giant branch (i.e., with ages from 1 Gyr up to 12 Gyr) with the required accuracy. A mosaic of 2 pointings along the major axis will give a good coverage of the galaxy and will allow us to infer a star-formation history unbiased by local properties of peculiar regions. Narrow-band imaging in F658N (Halpha) will provide a map of the ionized gas. Finally, WFPC2 parallel observations in F555W and F814W will allow to search deep in the HI halo of NGC 4449 for evidence of stars belonging to tidal streams. Our investigation of NGC 4449 will benefit from the wealth of observations already available over almost the whole electromagnetic spectrum and will be crucial in better understanding how starbursts can be influenced by merging phenonema in both the low- and high-redshift universe.
We will use ACS/WFC to study the resolved stellar population of NGC 4449. We will perform a mosaic with 2 different pointings in a rectangle shape along the major axis of the galaxy. Our expertise suggests that the V and I filters are best suited for this type of investigation. We will thus observe in the two broad-band filters F555W and F814W. F555W is preferred to F606W because it is a more standard filter for HST stellar population studies and it is much less contaminated by nebular emission (F606W includes Halpha). We will infer the ancient star formation history of NGC 4449 from simulations tailored to reproduce the observed I vs. V-I CMD. We also require broad-band imaging in F435W in order to better characterize the hot young stars and correct each single star for intrinsic differential reddening along the line of sight with multi-band photometry. And finally, we will also do narrow-band imaging in F658N to map the distribution of the ionized gas through Halpha emission. At each of the two major axis pointings we will observe for 4 orbits in one visit. The exposure times for the different filters were chosen to reach at least one magnitude fainter than the TRGB (I = 24.1) with a photometric error below 0.1 mag (S/N > 10) in the broad-band filters. The exposures in each filter will be executed with a standard primary+secondary dithering pattern suitable to get rid of cosmic rays, remove hot/bad pixels, fill the gap between the two CCDs of the ACS/WFC, and improve the PSF sampling. Finally, parallel WFPC2 observations for each ACS pointing will be taken in F606W and F814W. It will be possible to allocate about 2 orbits per WFPC2 filter, per pointing. The goal to properly mosaic the galaxy along its major axis restricts the ORIENT of the observations. We want the CCD edges perpendicular to the major axis, which limits the ORIENT to 50 degrees, modulo 90. In addition, we want the most important parts of the galaxy to fall on WFC2, which has the smallest PSF variations over the field. We also want the chip gap to be away from the most interesting parts of the galaxy. And finally, the observations must of course be schedulable. These considerations leave ORIENT=320 degrees as the only available option. To provide sufficient flexibility for scheduling in two-gyro mode, we allow a +/- 10 degree range around this value. We also increase SCHED to 60% from its default value of 30%. This provides increased opportunities for scheduling, but with a slightly reduced visibility time (2.5 minutes) per orbit. The visit planner shows that the observations can schedule for a full 6 weeks in Oct-Dec 2005. This should provide sufficient flexibility for scheduling. The single orbit in visit 3 allows for additional deeper imaging in Halpha and F814W at a slightly different pointing. N/A N/A N/A GALAXIES 12 false 0.0 false false false false false SUPPORTED GALAXY AMORPHOUS IRREGULAR J2000 12 28 15.4900 +44 06 47.25 J2000 GALAXY AMORPHOUS IRREGULAR J2000 12 28 4.1700 +44 04 31.59 J2000 GALAXY INTERACTING GALAXY STARBURST MAGELLANIC IRREGULAR Position adjusted to fit low-surface brightness narrow-line emitting regions surrounding the main visible body of the galaxy in the FOV. 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