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Assigned ID: 540
----- Attempting Submission 1 (Fri Apr 07 14:27:56 GMT 2017) -----
HST Phase I Proposal 540 (hst25_AGBhalo.apt) successfully submitted.
Receipt: # 540-1
----- Attempting Submission 2 (Fri Apr 07 15:31:56 GMT 2017) -----
HST Phase I Proposal 540 (hst25_AGBhalo.apt) successfully submitted.
Receipt: # 540-2
0
Inner stellar halos of spiral galaxies: accretion or in-situ formation?
The stellar halos surrounding spiral galaxies like the Milky Way are thought to be old and metal-poor. Indeed, we see little evidence for recent star formation in the form of main sequence stars or He burning stars in the color-magnitude diagrams (CMDs) of the stellar halos of the 16 nearby disk galaxies in our GHOSTS HST survey. It is therefore surprising, that we do see bright Asymptotic Giant Branch (AGB) stars in our CMDs, stars that are typically between 0.5 and 2.5 Gyr old. Recent simulations show two viable models for the origin of these intermediate age halo stars: 1) they have been brought in by gas-rich, star-forming, recently-disrupted satellites or 2) they have been dislodged from the main galaxy disk by recent interactions.
To constrain their origin, we propose to use WFC3 F673N narrowband imaging on two stellar halos to determine the fraction of carbon (C) stars in the detected AGB population. Classical carbon stars only form effectively at lower metallicities ([Fe/H]<-0.3) and hence will be low in number compared to normal (M-type) AGB stars if this mysterious AGB population originates from the main disk, but the C star fraction will be high if they originate from accreted metal-poor dwarf galaxies. Such measurements are hard to do in the Milky Way and M31, and NGC253 is the only disk galaxy where this halo C star fraction has been mapped before. We will triple this sample by measuring M-to-C fractions at distances 15-30kpc above the midplane of NGC891 and NGC4565. The relative distributions of C stars, M-giants and RGB stars will be compared to our suite of halo formation models to determine the relative importance of accretion vs. in situ formation.
Our aim is to identify AGB C stars using the narrowband imaging technique. Unlike similar earlier efforts where two or even three narrowband filters were used, we use the F673N narrowband filter in combination with F606W and F814W filters to separate C from M stars. We will repeat our previous F606W and F814W observations, as these AGB stars are luminosity variables and we need a time matched F606W-F673N and color measurement.
In each galaxy halo we cover one position on the minor axis at distance 17-25kpc from the galaxy center.
In NGC891 we expose 8x ~2400s in F673N and 4x300s in F606W and F814W spread over 3 separate visits. The orient is constrained such that the parallel field falls in a interesting position to measure the halo CMD in a similar fashion as our previous GHOSTS observations. Here we expose 8x1300s in F606W and 8x1100s in F814W to reach similar depths as in our previous GHOSTS observations.
In NGC4565 we observe one halo field with F673N in two orientations, one with 8 orbits and one with 7 orbits. We expose 8/7x ~2400s in F673N and 8x320s in F606W and 7x 350s in F814W spread over 3 separate visits in each orientation. The orient is constrained such that the parallel fields falls in a interesting position to measure the halo CMD in a similar fashion as our previous GHOSTS observations. Here we expose 8/7x1300s in F606W and 8/7x1100s in F814W to reach similar depths as in our previous GHOSTS observations. The orients cannot be exactly 90 degrees separate to get maximum overlap due to scheduling constraints.
Galaxies and the IGM
Stellar Populations
6
false
false
false
false
false
false
false
SUPPORTED
EXT-STAR
AGB STAR
HALO
POPULATION II
TIDAL TAIL
NO
This object was generated by the targetselector and retrieved from the SIMBAD database.
J2000
J2000
EXT-STAR
AGB STAR
HALO
POPULATION II
This object was generated by the targetselector and retrieved from the SIMBAD database.
J2000
J2000
WFC3/UVIS
Imaging
No
No
F606W
WFC3/UVIS
Imaging
No
No
F680N
WFC3/UVIS
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
WFC3/UVIS
Imaging
No
No
F606W
WFC3/UVIS
Imaging
No
No
F680N
WFC3/UVIS
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
WFC3/UVIS
Imaging
No
No
F606W
WFC3/UVIS
Imaging
No
No
F680N
WFC3/UVIS
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
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:al-overhead-at-end 120
:internal-dur-at-end 1015
:dark-offset NIL
:dark-utilization 0
:max-dark-utilization 1920
:low-sky "N"
:pcs-visibility 3238
:pcs-gap 0
)
false
3
0