----- Submission 1 (Fri May 13 22:35:36 GMT 2005) ----- Phase II Proposal 10580 (ngc4038_IIbox2) successfully submitted. ----- Submission 2 (Sat Jun 11 00:51:20 GMT 2005) ----- Assigned ID: 107 ----- Submission 1 (Thu Jan 20 22:43:15 GMT 2005) ----- A tip of the red giant branch distance to NGC 4038/39 ("The Antennae") We propose to use ACS to determine a tip of the red giant branch (TRGB) distance to the merger system archetype NGC4038/39 ("The Antennae"). This system is the closest major merger to us, but its distance remains a point of debate, with proposed values ranging from 14 to 33 Mpc. Our previous HST/WFPC2 V,I band observation targeted the star forming (SF) regions near the tip of the southern tail of NGC4038. These data revealed a background population of red stars far from SF regions which we identify as RGB stars. The TRGB was detected near our completeness limit at about 26.5 mag, suggesting a distance of 13.8+/-1.7 Mpc, 30% lower than the most accepted distance of 21 Mpc, and less than half the distance of 29 Mpc adopted to characterize the "ultraluminous X-ray binaries" (UXB) sources discovered in recent Chandra observations by Fabbiano et al. (2001). The lower distance has profound implications for the mass and luminosities of all sources associated with The Antennae, such as the Tidal Dwarf candidates, the globular cluster system, and the UXB population. With its greater sensitivity and higher resolution, ACS can resolve this issue in just a few orbits. We therefore request seven orbits to obtain V,I band images of the southern tail of NGC4038/39 to reach 1.6 mag lower than the TRGB, for our proposed distance modulus. Even if the larger distance is the correct one, our proposed imaging would still give a robust distance constraint from the TRGB.
Our aim is to test the range of plausible distances in the literature. If we are seeing the RGB tip in our data, the field of ACS will enable us to study the structure of Schweizer's object in star counts. On the other hand, if we use the Fabbiano et al. (2001) distance, 29 Mpc, the RGB tip should be at I = 28.1. A photometric precision comparable to our current study of the Antennae can be reached for S/N = 3, and according to the ACS/ETC, less than 3 hours are needed to reach it for the F606W filter (splitting into 4 exposures), and less than 2 hours for the F814W filter (splitting into 3 exposures) of the WFC camera (for a K4V star, and EB-V = 0.046). These exposure times will allow us to obtain photometry with a precision of 0.4 mag (1-sigma), i.e. comparable to that of our current WFPC2 data set. The same exposure times will allow us to reach the tip at S/N > 12 if our distance modulus is correct (i.e. the precision of our photometry will be around 0.08 mag). The visibility of the Antennae is 52 minutes, but, according to the ACS manual, overheads will reduce the effective observing time. If the observations are done in one visit, then 5 minutes per orbit will be taken by the guide star acquisition (6 min the first orbit). The ACS overhead is 4min for the first exposure, and 2.5 min for the rest. Using a small dithering pattern, the telescope overhead for the movements is 0.7 min per exposure, i.e. 3 min is the effective overhead per exposure (except the first one). During the first orbit we thus have 52 - 6 - 4 = 42 min effective observing time. After the first orbit, the effective observing time is 52 - 5 - 3 = 44 min, if we take 1 CR-SPLIT exposure per orbit. In 4 orbits we then get 2.9 hours which are sufficient for the F606W filter to reach S/N ~ 3. We will follow with another 3 orbits for the F814W filter, with 1 CR-SPLIT exposure per orbit. The effective observing time will be 2.2 hours, enough to reach S/N , 3 for the F814W filter. Our total request is then for 7 orbits, split into two visits. This will allow us to study the RGB in some detail if our short distance is correct, and to detect the RGB tip if the longer distance is valid, thus placing a reasonable limit on the Antennae distance modulus. STAR FORMATION 12 false 0.0 false false false false false SUPPORTED GALAXY TIDAL TAIL J2000 12 01 55.0000 -18 52 44.00 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W /home/isaviane/proposals/HST/cycle14/OP-cache/ngc4038_II_deep-v01.xml Collect 4 F606W exposures in 4 orbits, and apply a minimal dither pattern, to cover the inter-chip gap and provide some PSF subpixel sampling (Tic-Visit :proposal-id 10580 :visit-id "01" :proposal-version -1 :trans-version "20050323" :date "10-Jun-2005 19:38:15.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10580 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10580 :visit-id "01" :orbit-number 1 :pcs-time 367 :si-vis-utilization 2890 :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 3257 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10580 :visit-id "01" :orbit-number 2 :pcs-time 367 :si-vis-utilization 2890 :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 3257 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10580 :visit-id "01" :orbit-number 3 :pcs-time 367 :si-vis-utilization 2890 :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 3257 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10580 :visit-id "01" :orbit-number 4 :pcs-time 367 :si-vis-utilization 2890 :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 3257 :pcs-gap 0 ) true 4 0 /home/isaviane/proposals/HST/cycle14/OP-cache/ngc4038_IIbox2-v01.xml /home/isaviane/proposals/HST/cycle14/OP-cache/ngc4038_II_deep-v02.xml Collect 3 F606W exposures in 3 orbits, and apply a minimal dither pattern, to cover the inter-chip gap and provide some PSF subpixel sampling (Tic-Visit :proposal-id 10580 :visit-id "02" :proposal-version -1 :trans-version "20050323" :date "10-Jun-2005 19:38:21.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 852672 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10580 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10580 :visit-id "02" :orbit-number 1 :pcs-time 367 :si-vis-utilization 2890 :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 3257 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10580 :visit-id "02" :orbit-number 2 :pcs-time 367 :si-vis-utilization 2890 :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 3257 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10580 :visit-id "02" :orbit-number 3 :pcs-time 367 :si-vis-utilization 2890 :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 3257 :pcs-gap 0 ) true 3 0 /home/isaviane/proposals/HST/cycle14/OP-cache/ngc4038_IIbox2-v02.xml