WFIRST-AFTA: Local Group Science

WFIRST-AFTA: Local Group Science
Dan Weisz
Hubble Fellow
University of Washington
sdfg
Image Credit: Robert Gendler
Overview
•
Basic terminology
•
Importance of Stellar Pops
•
Possible scientific applications
of WFIRST in the Local Group
Why
HST?
Ground
HST
PSF Stability: Precision + deblending
K-band (Keck AO)
KKH 98, Keck AO: J. Melbourne+ 2008
F475W+F814W!
(ACS)
Brief Intro to Stellar Populations
inRHeBs
the Local Group
BHeBs &
(25-500 Myr)
MS
Brighter
AGB
RGB
HB
Lower MS
Information is Rich
and Redundant
ancient
MSTO/SGB
Hotter
Brief Intro to Stellar Populations in the Local Group
Optical CMD
Broad Color Range
Near-IR CMD
Narrow Color Range
Brief Intro to Stellar Populations in the Local Group
Optical CMD
Near-IR CMD
HLS Depth*
1 Mpc
0.5 Mpc
50 kpc
*assuming no crowding
Broad Color Range
Narrow Color Range
Importance of Stellar Pops
•
Ground truth for “subgrid physics”
- age, energy input, ISM, metallicity, SFR, dust,
integrated luminosity, SN remnant masses, IMF
• Non-dissipative tracer of large-scale
interactions!
• Necessary to test connections between
baryonic and dark matter in small galaxies
The Stellar IMF in LG Dwarfs
The Astrophysical Journal, 763:110 (8pp), 2013 February 1
Kalirai et al.
SMC
121 ACS orbits
F606W
(GO-11677; PI: H. Richer)
The Astrophysical Journal, 763:110 (8pp), 2013 February 1
Best Fit
Salpeter
𝛘2 / DOF
47 Tuc
F606W-F814W
Figure 2. CMD of all stars along the line of sight in the HST/ACS observations (left). The photometry reveals three distinct populations; the main sequence and white
dwarf cooling sequence of the foreground Milky Way globular cluster 47 Tuc (gray points), and the main sequence of the outskirts of the background SMC dwarf
galaxy (e.g., the middle sequence—darker points). With a 50% completeness limit of F 606W = 29.9 (see Section 3.3), this imaging represents the deepest probe of
the SMC’s populations to date. The inset panel presents a closer view of the faintest stars on the lower SMC main sequence (dark points) and 47 Tuc white dwarf
cooling sequence (gray points). The red curve illustrates a stellar isochrone with [Fe/H] = −1.1 and t = 7 Gyr (Dotter et al. 2008), at a distance of (m − M)0 =
19.1 ± 0.1 (determined directly, as described in Section 3.2). The proper-motion selected SMC population, down to the limit of the data, is shown as darker points in
the left panel, and reproduced down to F 606W = 28.6 in the middle panel. As discussed in Section 3.4, we simulate the SMC population by drawing stars from an
IMF and convolving them with (1) a range of mass–luminosity relations appropriate for the SMC halo, (2) binaries, (3) photometric scatter in the observations, and
(4) incompleteness in the observations. An example of the simulated CMD, for an IMF with form dN/dM ∝ M −1.90 , is shown in the right panel.
Figure 4. Distribution of χ 2 from the fit of power-law IMFs to the ob(A color version of this figure is available in the online journal.)
+0.15
Kalirai+ 2013
Same science in
the Near-IR : ~12 orbits
the SMC based on RR Lyrae stars (e.g., top-left panel of Figure 5
IMF Slope
served SMC luminosity function. The minimum occurs at α = −1.90 (−0.10 )
(3σ error), and the χ 2 per degree of freedom for this fit is 0.86.
in our derivation of the stellar IMF, our analysis below uses
M = 0.37–0.93 M⊙ (black curve). Figure 5 also illustrates two
m8
The Stellar IMF in LG Dwarfs
26
28
18
Hercules
Leo IV
UMa I
22
M92
M92
24
814
F814W
m (STMAG)
20
26
M92
Low-Mass
IMF
Slope varies
with sigma, [Fe/H]
28
-0.8 -0.6 -0.4 -0.2 0.0
Geha+ 2013 m606-m814 (STMAG)
-0.8 -0.6 -0.4 -0.2 0.0
m606-m814 (STMAG)
F606W-F814W
-0.8 -0.6 -0.4 -0.2 0.0
m606-m814 (STMAG)
7
Figure 1. The
CMD of each UFD in our sample (black points). For reference, we show the empirical ridge line for the MS, SGB, and RGB in M92 (green
Brown+
2012,2014
curve), along with the HB locus in M92 (green points). The M92 fiducial has been placed at the distance and reddening for each galaxy (Table 1), matching
IMF Slope
the luminosity of HB stars and the color of the lower MS stars. Because the CMD of each galaxy looks, to first order, like that of a ancient metal-poor globular
cluster, the stellar population of each galaxy is dominated by ancient metal-poor stars. The CMDs of these galaxies are all extremely similar to one another,
implying they have similar stellar populations and star formation histories.
Current data not deep enough to rule out 2.2.3. Metallicities
MW Chabrier
IMF to We determine the best-fitting T and [Fe/H] values simultaneously
fromconfidence
χ minimization of the pixel-by-pixel flux difhigh
fitting, we degrade the synthetic spectra to the DEIMOS resolution. We excise wavelength regions affected by telluric contamination, strong sky emission lines, and regions improperly
synthesized due to NLTE effects (Ca triplet and Mg I λ8807).
2.2.4. Membership
We determined the membership status of stars in each sample using a refined version of the approach adopted by Simon & Geha (2007) and Simon et al. (2011), in which all of
the available data for each star, including its velocity, color,
magnitude, metallicity, position, and spectrum, were examined by eye. Photometry was extracted from Sloan Digital
eff
Sky Survey (SDSS) Data Release 9 (Ahn et al. 2012) or Data
2
Release 10 (Ahn et al. 2014) for each galaxy, and for the parFig. 5.— The
power law slope, α, plotted against galaxy velocity dispersion (left), metallicity [Fe/H] (middle) and the observed stellar mass
ference between the observed spectra and the synthetic
grid,
ticularly
IV line
andin the
CVn
II,panel
weissupplemented
range (right). Data are taken from
the samesparse
sources asUFDs
Figure 4. Leo
The dotted
middle
an empirical relationship suggested by
using only spectral regions sensitive to variations
in Fe
abunKroupa
(2001)
with a zero-point
shift
to
fit
these
data.
Indirect
measurements
are
shown
for
elliptical
galaxies
fromSand
Conroy & van Dokkum
the
SDSS
data
at
faint
magnitudes
with
photometry
from
(2012). A trend is seen in the sense of shallower, more bottom-light IMF slopes towards less massive and more metal-poor galaxies.
dance. We separately fit [α/Fe] using regions sensitive to Mg,
σ
[Fe/H]
Stellar Mass Range
m8
WFIRST: Local Group Dwarfs
26
28
18
Hercules
Leo IV
M92
M92
24
!
26
Detect and characterize
entire dwarf galaxies
814
F814W
m (STMAG)
20
22
Wider field
Get more stars faint
stars for
M92 IMF
UMa I
28
-0.8 -0.6 -0.4 -0.2 0.0
m606-m814 (STMAG)
-0.8 -0.6 -0.4 -0.2 0.0
m606-m814 (STMAG)
F606W-F814W
Coma Berenices
-0.8 -0.6 -0.4 -0.2 0.0
m606-m814 (STMAG)
Figure 1. The CMD of each UFD in our sample (black points). For reference, we show the empirical ridge line for the MS, SGB, and RGB in M92 (green
Geha+
2013
curve), along
with the HB locus in M92 (green points). The M92 fiducial has been placed at the distance and reddening for each galaxy (Table 1), matching
NGC 6822
the luminosity of HB stars and the color of the lower MS stars. Because the CMD of each galaxy looks, to first order, like that of a ancient metal-poor globular
cluster, the stellar population of each galaxy is dominated by ancient metal-poor stars. The CMDs of these galaxies are all extremely similar to one another,
implying they have similar stellar populations and star formation histories.
fitting, we degrade the synthetic spectra to the DEIMOS resolution. We excise wavelength regions affected by telluric contamination, strong sky emission lines, and regions improperly
synthesized due to NLTE effects (Ca triplet and Mg I λ8807).
2.2.3. Metallicities
We determine the best-fitting Teff and [Fe/H] values simultaneously from χ2 minimization of the pixel-by-pixel flux difference between the observed spectra and the synthetic grid,
using only spectral regions sensitive to variations in Fe abundance. We separately fit [α/Fe] using regions sensitive to Mg,
2.2.4. Membership
We determined the membership status of stars in each sample using a refined version of the approach adopted by Simon & Geha (2007) and Simon et al. (2011), in which all of
the available data for each star, including its velocity, color,
magnitude, metallicity, position, and spectrum, were examined by eye. Photometry was extracted from Sloan Digital
Sky Survey (SDSS) Data Release 9 (Ahn et al. 2012) or Data
Release 10 (Ahn et al. 2014) for each galaxy, and for the particularly sparse UFDs Leo IV and CVn II, we supplemented
the SDSS data at faint magnitudes with photometry from Sand
Optical
BHeB
NIR
RHeB
AGB
RHeB
MS+BHeB
MS
RGB
TRGB
Red Clump
(Red HB)
V-I vs I
AGB
J-H vs H
Cool Stars can dominate rest-frame near-IR flux at early times
nal, 748:47 (26pp), 2012 March 20
Melbourne et al.
Few RGB stars
at high-z
AGB & RHeBs account for
~70% of rest-frame near-IR flux
at high redshifts
11, only now we include corrections to the model predictions for TP-AGB and RHeB fluxes. RHeB flux contributions are increased by
P-AGB contributions are reduced by a similar factor compared to the 2010 Padova models. The overall contribution of massive stars to
Melbourne+ 2012
Rare Phases of Evolution: AGB & RHeB Stars
SMC-SAGE Spitzer + 2Mass
Short lifetimes: challenging to find
Rapid Evolution:
challenging to model
LG survey for rare, IR-bright stars =
great WFIRST science case
Fig. 5.— Top:
J − KS CMD showing the separation of carbonBoyer+
2011
rich and oxygen-rich AGB stars in the SMC, following Cioni et al.
(2006a,b). The J − Ks color is also used to select RSG and RGB
stars. See text. Bottom: Same as upper panel, with the selection of
HST’s Wide Area UV-Opt-IR Map of M31
432 WFC3/IR Pointings
2 WFIRST-AFTA Pointings
mage: Dustin Lang
astrometry.net
>117,000,000 stars,
multiple properties
measured at up to 17 different times
http://archive.stsci.edu/prepds/phat/
~50% of the
number of
stars in SDSS
Filters
Spectra of Stars
JWST
“Crowding” varies with Radius
Outer Bulge
of M31
Outer Disk of
M31
Depth varies with Radius
F814W
F475W
F160W
F110W
Crowding
limited* in NIR
& optical
*Maximum possible
# of stars per
arcsec2 = 0.5 million
per ACS image
Leverage full multi-camera data
IR data only
Williams+ 2014
Same IR data, but including
higher res optical data in PSF fitting
Spatially Resolved SFH of M31
5
Simones et al.
42 30
Alexia Lewis (UW)
23
dec
21
22
19
17
20
42 00
RA
15
18
16
13
14
11
9
12
7
10
41 30
Measure SFHs of
~9000 independent
100x100pc regions
from optical-only
CMDs
5 kpc
5
8
6
4
brick 2
2
41 00
40 30
12 30
12 00
11 30
Figure 2. Here we show example results for two di↵erent regions, one in Brick 15 sitting on a bright star-forming region (cyan, lower)
and the other in B17 in a more quiescent spot (red, upper). We have overlaid the outlines of Bricks 15 and 17 on top of a GALEX FUV
image (Gil de Paz et al. 2007). The region outlines are also shown. For each region, we show the CMD, with the color-magnitude space
we do not fit shaded in red, and the SFH back to 1 Gyr. The SFHs have been binned into 25 Myr increments below 100 Myr and 50 Myr
11 00
10 30
10 00
Simones et al.
42 30
23
dec
21
22
19
42 00
RA
17
20
15
18
16
13
14
11
9
12
7
10
41 30
Spatially Resolved SFH of M31
5 kpc
5
8
6
4
brick 2
2
41 00
40 30
12 30
12 00
11 30
11 00
10 30
10 00
of the PHAT survey area. The 21 PHAT bricks analyzed in this study are outlined and numbered. Each brick was
regions on a 15 ⇥ 30 grid, as shown for brick 2 in the inset panel. Lewis et al. (2014) derived the SFHs for all of the
arcsec regions.
M31 using the Hubble Space Telescope (HST). The
SFHs of LocalinPHAT
survey area is shown in Figure 1. Lewis et al.
(2014) used photometry in the two optical bands (F475W
Group galaxies
and F814W, observed with the Advanced Camera for
y, we have used the PHAT CMD-based
Surveys, ACS, instrument) to derive spatially-resolved
ar population synthesis to create maps of
SFHs for the northeast quadrant of M31, excluding the
would
benefit
from
violet (UV) flux at sub-kpc resolution for
crowded bulge area. To summarize their work, each
quadrant of M31. We then compared the
brick (using PHAT terminology; see Figure 1) in the
maps with observations
GALEX. We
PHAT survey (except bricks 1 and 3) was divided into
wide,frombluer
filter
sed on GALEX FUV and NUV (far and
450 regions on a uniform 15 ⇥ 30 grid, with each region
on data from PHAT make it possible to
ype of analysis to include a wide variety of
n the most prominent local group spiral
ugh this work can easily be extended to
gth regimes. In §2, we describe the SFH
⇠ 24 arcsec⇥27 arcsec in size. The F475W, F814W CMD
of each region was then fit using the SFH history code
Lewis+ (2015)
Stellar Mass Surface Density
Long Live the Rings
RGB stars > 1 Gyr old
Radius [kpc]
Lewis+ (2015)
Mapping the Dusty ISM
“How I took a
winding path to
somewhere I didn’t
expect to go”
~ Julianne Dalcanton
w/ Morgan Fouesneau
NIR Color-Magnitude Diagrams
RGB Extended to the Red
Dalcanton+ 2015
Red Giant Branch is Doubled
F110W-F160W
CMD of Single
WFC3/IR frame,
subdivided in
4x4 grid
Subregions
of single
WFC3/IR
frame
Complicates
CMD
interpretation,
but great for
quantifying dust
Unreddened
peak
Reddened
peak
Foreground
stars
Background
stars
6.6” = 25 pc
pixels
2MASS Extinction map: Lombardi et al 2010
25 pc
Median AV
Residual
structure: 1.5%
flat fielding errors
in WFC3/IR!
Optical
HI
AV
(Extinction)
AV
(Emission)
Draine+
2013
Superb Agreement w/ Dust
Morphology from Emission
Extinction
Emission
(Draine+ 2013)
But, normalizations don’t agree.
Emission-based dust masses high by factor of ~2
Extinction
Emission
(Draine+ 2013)
Comparison w/ CO
Schruba+ 2013
CO
• General morphological
agreement
• But not perfect....
Extinction
Extinction
Contours on CO
Mapping M31 with WFIRST
PHAT Depth CMDs in ~5000s
PHAT Area
with WFIRST
PANDAs Survey: McConnachie+
Summary
•
•
•
Near-IR: efficient for (luminous) cool stars
Potentially Transformative Science in LG:
-
IMF and low-mass stars outside the MW complete census of luminous cool stars
ISM, dust mapping, dust tomography
large scale mapping of accretion in M31
Inner MW, deeper into the disk and halo
find and characterize dwarf galaxies and halos
proper motions and internal stellar kinematics
Good potential synergy with future high
resolution imaging