The elusive old stellar halo of low-mass spirals: RR Lyrae stars in NGC55

0 Assigned ID: 67 ----- Attempting Submission 1 (Mon Apr 03 14:03:19 GMT 2017) ----- HST Phase I Proposal 67 (bernard_ngc55_c25.apt) successfully submitted. Receipt: # 67-1 ----- Attempting Submission 2 (Fri Apr 07 20:31:26 GMT 2017) ----- This is a resubmission after the HST stop for the gyroscope problem. In order to ensure observability, I shifted the ACS pointing and changed the orientation angle. New POS TARG orientation has been added in the second orbit of each visit. Note that the two images of the same orbit have the same POS TARG -1284859600 ----- Attempting Submission 1 (Wed Jul 19 15:48:42 GMT 2017) ----- ----- Attempting Submission 2 (Thu Sep 14 14:46:37 GMT 2017) ----- ----- Attempting Submission 3 (Fri Nov 16 16:29:51 GMT 2018) ----- The elusive old stellar halo of low-mass spirals: RR Lyrae stars in NGC55 Spiral galaxies dominate the stellar mass density in the Universe, yet the details of their formation and evolution are still greatly debated. The simulations based on the current cosmological model show that these galaxies grow in mass through accretion and mergers. One of the consequence of this galaxy assembly process is the formation of an old, metal-poor stellar halo. However, to date the presence of a spheroidal component in low-mass spirals is elusive and controversial, mostly due to the difficulty to detect such a faint, diffuse component amid the foreground/background contamination. We propose a novel method to detect and characterize such a halo in the nearby, edge-on spiral galaxy NGC 55, that by-passes these limitations by using the pulsational properties of its RR Lyrae stars. These observations will allow us to i) firmly establish the presence of an old stellar halo, ii) quantify its early chemical evolution, and iii) determine the nature of its building blocks. We will use the resulting deep color-magnitude diagrams reaching 1.5 magnitudes below the horizontal-branch to test halo and thick disk formation models. The goal of this proposal is to obtain accurate light curves of RR Lyrae variable stars and a relatively deep color-magnitude diagram for a field in the halo of the nearby dwarf spiral galaxy NGC55. The two cameras used (ACS and WFC3) will probe slightly different radial distances from the center of the galaxy, hence the orientation is important. Our visits span 2 orbits. For the first orbit, one F475W and one F814W exposure (with no CR-splits) are obtained with the ACS. For the second orbit, one F475W and one F814W exposure are obtained at the same pointing, but the order of the filters is reversed to provide better sampling of RR Lyrae variable star light curves. The images in the same visit are kept with the same POS TARG in order to have two images in the same position, which is supposed to help in the CR removal. Small ditherings are performed between visits in order to remove hot pixels and to smooth the detector response. We don't intend to cover the interchip gap. Ditherings are performed using POS-TARG, and following the basic UDF dither pattern, shifting the pointings by ~2 pix. Coordinated parallel exposures following the same strategy with WFC3 are requested. The first visit is requested within a given range of orientations. This range has been chosen to cover a range of distances from the center of the galaxy and to avoid bright stars in the main and the parallel field. Subsequent visits are requested to be obtained with the same ORIENT as the first visit, to maximize the sky area covered at full depth both in the prime and the parallel fields. Since the time sampling is crucial, we request the visits to be executed sequentially sometime after 4 hours but within 3 days of the previous visit, and grouped within a maximum time span of 5 days. We also request that visits separated by 8 orbits or multiples of 8 orbits be avoided. SAA impacted orbits are also to be avoided. This strategy proved successful in our previous programs (PID=13028, 13739 & 10505). This will ensure optimal sampling of the light curves of the RR Lyrae variable stars, which are the main targets of these observations. Galaxies and the IGM Stellar Populations 6 false false false false false false false SUPPORTED GALAXY HALO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F814W 1546876465479 Timing link suitability includes constraints ([02 After 01 by 0.2 to 3.0 days]) Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 26.2. CASM was run in Phase II mode. /private/var/folders/0b/qbp4bysx6vdbfxvp_xw3dyxw00023y/T/OPVisit4742633422369243315.xml This is the first visit to our requested field. We request a range in orientations in this first visit with the same orientation requested in subsequent visits to this target. This requirement will ensure that the parallel WFC3 fields of view will match exactly across all visits to this target. We have requested timing constraints such that (1) all visits to NGC55 be executed sequentially sometime after 4 hours but within 3 days of the previous visit, and (2) all visits to NGC55 should be executed within 5 days. In addition, we also request that visits separated by 8 orbits or multiples of 8 orbits be avoided. SAA impacted orbits are to be avoided. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. The orientation must be the same as in Visit 1 to ensure that the parallel WFC3 fields of view will match exactly across all visits to this target. 1545685970000 75.4339801039969 (Tic-Visit :proposal-id 15063 :visit-id "03" :proposal-version -1 :trans-version "20180927" :date "16-Nov-2018 16:29:33.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15063 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15063 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2927 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3260 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15063 :visit-id "03" :orbit-number 2 :pcs-time 242 :si-vis-utilization 3018 :al-overhead-at-end 116 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3260 :pcs-gap 0 ) true 2 0 1542385775106 Timing link suitability includes constraints ([04 After 03 by 0.2 to 3.0 days] [05 After 04 by 0.2 to 3.0 days]) Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 26.2. CASM was run in Phase II mode. The orientation must be the same as in Visit 1 to ensure that the parallel WFC3 fields of view will match exactly across all visits to this target. 1545703141000 75.4339801039969 (Tic-Visit :proposal-id 15063 :visit-id "04" :proposal-version -1 :trans-version "20180927" :date "16-Nov-2018 16:29:34.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15063 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15063 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2927 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3260 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15063 :visit-id "04" :orbit-number 2 :pcs-time 242 :si-vis-utilization 3018 :al-overhead-at-end 116 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3260 :pcs-gap 0 ) true 2 0 1542385776432 Timing link suitability includes constraints ([05 After 04 by 0.2 to 3.0 days] [06 After 05 by 0.2 to 3.0 days]) Hst Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 26.2. 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CASM was run in Phase II mode. 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We request a range in orientations in this first visit with the same orientation requested in subsequent visits to this target. This requirement will ensure that the parallel WFC3 fields of view will match exactly across all visits to this target. We have requested timing constraints such that (1) all visits to NGC55 be executed sequentially sometime after 4 hours but within 3 days of the previous visit, and (2) all visits to NGC55 should be executed within 5 days. In addition, we also request that visits separated by 8 orbits or multiples of 8 orbits be avoided. SAA impacted orbits are to be avoided. These timing constraints provide optimal sampling of RR Lyrae light curves. 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