----- Submission 1 (Wed May 04 12:30:10 GMT 2005) ----- Assigned ID: 69 ----- Submission 1 (Thu Jan 20 16:25:16 GMT 2005) ----- The Formation History of the M81 Spheroid Spheroidal stellar populations (elliptical galaxies, bulges, and halos) contain a significant fraction of all stars and metals in the local universe. The mechanisms responsible for their formation are ultimately the ones which governed galaxy formation during early epochs. To begin understanding the M81 spheroid, we are currently studying the globular cluster population using HST/WFPC2 multiband imaging and ground based optical spectroscopy. To complete this effort, we propose to use ACS/WFC to obtain deep (I, V-I) color magnitude diagrams (to the horizontal branch) of two fields in M81 - one dominated by thick disk stars and the other halo stars. These observations will provide tight constraints on the formation timescales and chemical enrichment history of the field star population. Combined with results on the globular clusters, we will reconstruct the early formation history of M81, and compare with those found for other nearby, massive galaxies. Because M81 is the earliest type spiral galaxy (Sab) available for such a detailed study, it provides a unique opportunity to probe the connection between elliptical halos and lower mass spiral spheroids.
We will use the ACS/WFC to obtain deep V-I color magnitude diagrams of the halo of M81, the closest (3.6 Mpc) early-type (Sab) spiral galaxy. Since M81 has two close interacting companions (M82 and NGC 3077), we have carefully chosen our target field to avoid the HI bridges and other interaction material. Basen on the M81 structural analysis performed by Tikhonov et al. (2004), we have selected a halo field which is outside of the radius found by their analysis. We will use the F606W and F814W filters to make color magnitude diagrams, due to the additional sensitivity of the F606W filter over F555W. These filters have previous observations of Galactic globular clusters with the ACS/WFC which can be used for calibration. Based on previous work, we need to achieve a minimum S/N of 10 for stars in the red clump (M_V=0). Assuming the Cepheid distance modulus of 27.8 and A_V=0.2 for M81, we need to reach V=28 and I=27. In addition to stars in the red clump, we are interested in the morphology of horizontal branch stars. Because these are bluer (hotter) than red clump stars, additional time in the F814W filter relative to that in the F606W is required. Therefore, we observe the M81 halo field for 5 orbits in F606W, and 9 total orbits in F814W. The F814W orbits will be broken into 2 separate visits or 4 and 5 orbit durations. In addition to the primary ACS/WFC primary observations, coordinated parallels with WFPC2 were approved. These will be taken with the same filters. We have specified a range of possible spacecraft roll angles in order to optimize the location of the coordinated WFPC2 parallels. Our range of orientations is schedulable, and has been chosen to sample a somewhat more distant M81 halo field, so that we can explore metallicity gradients. STELLAR POPULATIONS 12 false 0.0 false false false false false SUPPORTED GALAXY HALO J2000 09 53 3.2000 +68 52 3.60 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFPC2 Imaging No F606W WFPC2 Imaging No F814W WFPC2 Imaging No F606W WFPC2 Imaging No F814W POS-TARG DITHER 0.265 LINE 0.187 false false 69.05 4 20.67 ACS-WFC-DITHER-BOX 1 false (Tic-Visit :proposal-id 10604 :visit-id "01" :proposal-version -1 :trans-version "20050323" :date "02-May-2005 12:23:37.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 2632320 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10604 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10604 :visit-id "01" :orbit-number 1 :pcs-time 367 :si-vis-utilization 2868 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "01" :orbit-number 2 :pcs-time 367 :si-vis-utilization 2786 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "01" :orbit-number 3 :pcs-time 367 :si-vis-utilization 2786 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "01" :orbit-number 4 :pcs-time 367 :si-vis-utilization 2766 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) false 4 0 /home/rupali/proposals/hst05/m81_cmd/OP-cache/10604-v01.xml (Tic-Visit :proposal-id 10604 :visit-id "02" :proposal-version -1 :trans-version "20050323" :date "02-May-2005 12:24:31.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 3290400 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10604 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10604 :visit-id "02" :orbit-number 1 :pcs-time 367 :si-vis-utilization 2868 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "02" :orbit-number 2 :pcs-time 367 :si-vis-utilization 2786 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "02" :orbit-number 3 :pcs-time 367 :si-vis-utilization 2786 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "02" :orbit-number 4 :pcs-time 367 :si-vis-utilization 2786 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "02" :orbit-number 5 :pcs-time 367 :si-vis-utilization 2766 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) true 5 0 /home/rupali/proposals/hst05/m81_cmd/OP-cache/10604-v02.xml (Tic-Visit :proposal-id 10604 :visit-id "03" :proposal-version -1 :trans-version "20050323" :date "02-May-2005 12:25:11.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 3290400 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10604 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10604 :visit-id "03" :orbit-number 1 :pcs-time 367 :si-vis-utilization 2868 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "03" :orbit-number 2 :pcs-time 367 :si-vis-utilization 2786 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "03" :orbit-number 3 :pcs-time 367 :si-vis-utilization 2786 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "03" :orbit-number 4 :pcs-time 367 :si-vis-utilization 2786 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10604 :visit-id "03" :orbit-number 5 :pcs-time 367 :si-vis-utilization 2766 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3281 :pcs-gap 0 ) true 5 0 /home/rupali/proposals/hst05/m81_cmd/OP-cache/10604-v03.xml