The stellar initial mass function (IMF) is a critical ingredient in the interpretation of extragalactic observations and provides important clues to the physics of star formation. Unfortunately, the low-mass IMF (M<0.3 Msun) is very difficult to measure beyond the Galaxy because low mass dwarf stars contribute only a few percent to the integrated light of old stellar populations. Using high quality Keck spectra we recently found that massive elliptical galaxies in the Virgo and Coma clusters have enhanced dwarf-sensitive absorption features, indicating a very "bottom-heavy" IMF with many more low-mass stars than the IMF in our Galaxy. The main uncertainty in this result is that the subtle absorption features that we ascribe to dwarf stars may actually be due to unusual abundance patterns in giant stars. Here we propose to directly determine whether three key IMF sensitive spectral features in massive elliptical galaxies are caused by giants or by dwarfs. We will exploit the high angular resolution of ACS, in combination with its unique ramp filters, to measure pixel-to-pixel fluctuations in the light of these spectral features for one of the massive Virgo elliptical galaxies that we previously observed with Keck (NGC 4649). Pixel-to-pixel fluctuations are caused by Poisson variations in the number of giants, and by correlating the behavior of a spectral feature with the number of giants in each pixel we directly measure whether that feature comes from giants or dwarfs. We are observing a single target, the elliptical galaxy M60, in 6 ramp filters and in the broad F814W filter. The aim is that the "on" and "off" exposures for a given feature (Na I, CaT, FeH) fall approximately on the same area of the detector. The four bluest settings will fall on the WFC-1 chip; the two reddest settings (FeH "on" and FeH "off") will fall on the WFC-2 chip. In order to have broad band data for all of the filters, we split the F814W observations in two visits: one for the WFC1 ramp aperture and one for the WFC2 ramp aperture. A standard dither pattern of 4 exposures is used to improve the sampling and identify hot pixels. The data can be taken at any ORIENT but it is crucial that all 8 visits are taken at the same ORIENT. UNRESOLVED STELLAR POPULATIONS AND GALAXY STRUCTURE 12 false false 0.0 0.0 false false false SUPPORTED GALAXY ELLIPTICAL This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 ACS/WFC RampFilter No No FR853N ACS/WFC RampFilter No No FR853N ACS/WFC RampFilter No No FR853N ACS/WFC RampFilter No No FR853N ACS/WFC RampFilter No No FR1016N ACS/WFC RampFilter No No FR1016N ACS/WFC Imaging No No F814W POS-TARG DITHER 0.265 LINE 0.187 false false 69.05 4 20.67 ACS-WFC-DITHER-BOX 1 false Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 21.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12523 :visit-id "01" :proposal-version -1 :trans-version "20121031" :date "25-Jan-2013 07:08:06.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12523 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12523 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2900 :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 3233 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12523 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) true 2 0 Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 21.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12523 :visit-id "02" :proposal-version -1 :trans-version "20121031" :date "25-Jan-2013 07:07:49.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12523 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12523 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2900 :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 3233 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12523 :visit-id "02" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) true 2 0 Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 21.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12523 :visit-id "03" :proposal-version -1 :trans-version "20121031" :date "25-Jan-2013 07:07:32.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12523 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12523 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2900 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) true 1 0 Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 21.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12523 :visit-id "04" :proposal-version -1 :trans-version "20121031" :date "25-Jan-2013 07:07:19.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12523 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12523 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2900 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) true 1 0 Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 21.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12523 :visit-id "05" :proposal-version -1 :trans-version "20121031" :date "25-Jan-2013 07:07:04.0" :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 2273792 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12523 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12523 :visit-id "05" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2900 :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 3233 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12523 :visit-id "05" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3011 :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 3233 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12523 :visit-id "05" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) true 3 0 Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 21.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12523 :visit-id "06" :proposal-version -1 :trans-version "20121031" :date "25-Jan-2013 07:06:11.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12523 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12523 :visit-id "06" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2900 :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 3233 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12523 :visit-id "06" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) true 2 0 Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 21.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12523 :visit-id "07" :proposal-version -1 :trans-version "20121031" :date "25-Jan-2013 07:05:46.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12523 :visit-id "07" :obset-id "07" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12523 :visit-id "07" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2900 :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 3233 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12523 :visit-id "07" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) true 2 0 Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 21.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12523 :visit-id "08" :proposal-version -1 :trans-version "20121031" :date "25-Jan-2013 07:05:20.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12523 :visit-id "08" :obset-id "08" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12523 :visit-id "08" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2900 :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 3233 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12523 :visit-id "08" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3011 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3233 :pcs-gap 0 ) true 2 0