0Assigned ID: 210
----- Attempting Submission 1 (Thu Apr 04 16:21:08 GMT 2019) -----
HST Phase I Proposal 210 (NGC253dwarfs.apt) successfully submitted.
Receipt: # 210-1
----- Attempting Submission 2 (Fri Apr 05 16:20:58 GMT 2019) -----
HST Phase I Proposal 210 (NGC253dwarfs.apt) successfully submitted.
Receipt: # 210-2FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser
FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser
FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser
FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser
FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser0----- Attempting Submission 1 (Wed Jul 24 20:57:02 GMT 2019) -----
HST Phase II Proposal 15938 (NGC253dwarfs_phaseII.apt) successfully submitted.
Receipt: # 15938-1
----- Attempting Submission 2 (Wed Jul 24 20:58:08 GMT 2019) -----
HST Phase II Proposal 15938 (NGC253dwarfs_phaseII.apt) successfully submitted.
Receipt: # 15938-2
----- Attempting Submission 3 (Fri Jul 26 00:28:37 GMT 2019) -----We are aware of the UVIS flash warnings -- it is not clear our actual exposure times are too short, but will be happy to add in a flash if it is thought necessary.
Resubmitting so that Dave Sand also gets an email -- he prepared much of the phase II.The NGC253 Satellite Luminosity Function: A Case Study for Low Density EnvironmentsThe faint end of the galaxy luminosity function is a strong test for models of galaxy formation and evolution, but can only be studied in the most constraining "ultra-faint" regime in the nearest galaxies. Deep satellite counts around a limited sample of nearby Milky Way-mass galaxies show evidence for a large variation in the number of dwarfs. These results could reflect either stochastic variations in satellite populations or systematic changes with environmental density. The origin of these variations can help elucidate the poorly-understood physical processes that contribute to the evolution of low mass galaxies, such as reionization.
This proposal requests 5 orbits of HST/ACS for two-band imaging of five new faint candidate dwarfs in the halo of NGC253 (D=3.5 Mpc), a Milky Way-mass spiral galaxy in a nearly isolated environment. This imaging will enable the measurement of distances, luminosities, metallicities, and star formation histories for these dwarfs, all of which can only be done with HST. The confirmation of these dwarfs will complete the first dwarf satellite census of NGC253 down to Mv ~ -7, testing cosmological predictions for the faint end of the satellite luminosity function in low density environments.The observations are going to be performed with ACS/WFC (primary) and WFC3/UVIS (parallel). We request imaging in F606W and F814W filters (broad V and I) for both instruments, in order to compute accurate TRGB distances for our targets and characterize their stellar content (star formation histories and metallicities), while optimizing the exposure times. The parallel pointings will allow us to observe a field region next to our targets. We will thus be able to estimate more precisely the number of background/foreground contaminants present in our ACS fields.
Our goal is to obtain a minimum SNR of ~10 at ~1 mag below the TRGB in I band, assuming that the TRGB is found at M_I = ~-4.0 and M_V = ~-2.5, in order to derive precise distance measurements, structural properties and average metallicities. With the requested SNR, the photometric errors will be ~0.05 mag in magnitude and ~0.1 mag in color at the TRGB. We will obtain distance uncertainties as low as ~0.1 mag, corresponding to ~150 kpc at the distance of NGC253.
Since the exact distances for our targets are unknown, the candidate NGC253 satellites are assumed to be at the distance of NGC253 (~3.5 Mpc). Given their low masses, these are expected to be metal-poor objects, thus having average colours of V - I ~ 1.5. At this distance, the TRGB is found at a magnitude of I ~ 24.2 and V ~ 25.7. The exposures to obtain the required SNR at 1 mag below the TRGB for both bands are ~ 1000 sec in F606W and ~ 1000 sec in F814W (as computed with the latest version of the ACS ETC), which result in one orbit per target. Each exposure will be split in two with a dither pattern in order to subsequently remove detector defects and improve the PSF sampling. We have checked that the charge transfer efficiency losses of ACS do not significantly affect our observations (at most 10% loss of flux).
This program is resilient to two-gyro mode, we believe. Our visits are 1-orbit, and we have no ORIENT requirements (we have one visit with a loose ORIENT). Nor does this program have time constraints (other than the baseline excitement of the science!). We can also achieve our science goals with a longer acquisition time, if necessary.Our team has extensive expertise with HST data reduction for ACS and WFC3, and also in point-spread-function photometry. In particular, we have a robust record of dwarf galaxy studies in the nearby universe (see Mutlu-Pakdil et al., 2018; Crnojevic et al., 2016; Sand et al., 2017), and have led several past HST observing campaigns (e.g., GO-14259, GO-13856, GO-15182, GO-15183). Our team recently performed a similar study on dwarf galaxies around the elliptical Centaurus A (Crnojevic et al. 2019), located in a denser group of galaxies. Therefore, our team has all the necessary analysis tools, ensuring a timely output from our proposed observations.Stellar Populations6falsefalsefalsefalsefalsefalsefalseSUPPORTEDGALAXYDWARF SPHEROIDALYESJ2000J2000GALAXYDWARF SPHEROIDALYESJ2000J2000GALAXYDWARF SPHEROIDALYESJ2000J2000GALAXYDWARF SPHEROIDALYESJ2000J2000GALAXYDWARF SPHEROIDALYESJ2000J2000ACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF814WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF814WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF814WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF814WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF814WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF814WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF814WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF814WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF814WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF814WPOS-TARGDITHER0.364Two-point dither. No need to fill chip gap.LINEfalsefalse247.23ACS-WFC-DITHER-LINE1falseThe scheduling data was generated by CASM 27.2.CASM was run in Phase II mode./private/var/folders/lk/kgyb23rx78z9r8t653drxmfw0000gn/T/OPVisit7457241940363322313.xml(Tic-Visit
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