0
Astro2020
Project
White Paper
ATLAS
Probe: Breakthrough Science of
Galaxy Evolution, Cosmology,
Milky Way,
and the Solar System
Lead
Author:
Name: Yun Wang
Institution: California Institute of technology
Email: wa
ng@ipac.caltech.edu
Phone: (626) 395
-‐
1415
Co
-‐
authors:
Mark Dickinson (NOAO), Lynne Hillenbrand (Caltech)
,
Massimo Robberto (STScI
& JHU
)
,
Lee
Armus (Caltech/IPAC), Mario Ballardini (Western Cape
, South Africa
),
Robert Barkhouser (J
ohns
H
opkins
U
niv.
),
James Bartlett (JPL), Peter Behroozi (U
niv. of
A
rizona
), Robert A. Benjamin
(U
niv. of
W
isconsin
W
hitewater
), Jarle Brinchmann (Porto
, Portugal;
Leiden
, Netherlands
),
Ranga
-‐
Ram Chary (Caltech/IPAC),
Chia
-‐
Hsun Chuang (Stanford),
Andrea Cimatti (
Univ. of
Bolo
gna
, Ital
y
), Charlie Conroy (Harvard), Robert Content (A
ustralia
A
stronomical
O
bs.
),
Emanuele Daddi (CEA
, France
), Megan Donahue (M
ichigan
S
tate
U
niv.
), Olivier Dore (JPL), Peter
Eisenhardt (JPL), Henry C. Ferguson (STScI),
Andreas Faisst (Caltech/IPAC),
W
esley C. Fraser
(Q
ueen’s
U
niv.
B
elfast, U.K.
), Karl Glazebrook (
Swinburne Univ
.
of Technology
, Australia
),
Varoujan Gorjian (JPL), George Helou (Caltech/IPAC), Christopher M. Hirata (O
hio
S
tate
U
niv.
)
Michael Hudson (
Univ. of
Waterloo
, Canada
), J. Davy Kir
kpatrick (Caltech/IPAC), Sangeeta
Malhotra (GSFC),
Simona Mei (
Paris
Obs.
,
Univ. of Paris
D. Diderot
, France
),
Lauro Moscardini
(
Univ. of
Bologna
, Italy
),
Jeffrey A. Newman (
Univ. of
Pittsburgh),
Zoran Ninkov (R
ochester
I
nsti.
of
T
ech.
),
Alvaro Orsi (CEFCA
, Spain),
Michael Ressler (JPL), James Rhoads (GSFC), Jason Rhodes
(JPL),
Russell Ryan (STScI), Lado Samushia (Kansas State Univ.),
Claudia Scarlata
(Univ. of
Minnesota),
Daniel Scolnic (Duke Univ.),
Michael
Seiffert (JPL)
,
Alice Sha
pley (UCLA), Stephen
Sm
ee (J
ohns
H
opkins
U
niv.
),
Francesco Valentino (
Univ. of
Copenhagen
, Denmark
),
Dmitry
Vorobiev (Univ. of Colorado), Risa H. Wechsler (Stanford)
Type of A
ctivity: Space
-‐
based P
roject
1
I.
ATLAS
Probe:
Key
Science Goals and Objectives
The observational
data from recent years have greatly improved our understanding of the Universe.
However, we are far from understanding how galaxies form and develop in the context of an evolving
“cosmic web” of dark matter, gas and stars, and the nature of dark energy re
mains a profound mystery 20
years after the di
scovery of cosmic acceleration.
Understanding galaxy evolution in the context of large
-
scale structure is of critical importance in our quest to discover how the Universe works. This requires very
large spectro
scopic surveys at high redshifts: very large numbers of galaxies over large co
-
moving volumes
for robust statistics in small redshift bins ranging over most of cosmic history. In particular, we need to map
the cosmic web of dark matter using galaxies
throu
gh most of cosmic history. This requires
a
redshift
precision of ~0.0001
(i.e.,
R=1000
slit spectroscopy
)
, and continuous IR
wavelength
coverage only possible
from space.
These observational requirements also enable definitive measurements on dark energy w
ith
minimal observational systematics by design. A very high number density wide area galaxy redshift survey
(GRS)
spanning the redshift range of 0.5<
z
<4
using the same tracer, carried out using massively parallel
wide field multi
-
object slit spectroscopy
from space, will provide definitive measurements that can illuminate
the nature of dark energy, and lead to revolutionary advances in particle physics and cosmology
.
The
currently planned projects do not meet these science goals.
JWST
has slit spectroscopi
c capability, but a
relatively small
Field of View (
FoV
)
, thus unsuitable for carrying out surveys large enough to probe the
relation between galaxy evolution and environment in a statistically robust manner. Both
Euclid
and
WFIRST
employ slitless grism sp
ectroscopy, w
hich increases background noise,
and only cover
wavelengths below 2
μ
m with fairly low spectral resolution, both of which
will limit their capability to probe
galaxy evolution science.
While
Euclid
and
WFIRST
, and the ground
-
based projects
(e.g
.,
DESI
,
PFS
,
and
LSST
)
,
will significantly advance our understanding of
the nature of dark energy,
they will
not provide
definitive measurements for its resolution, due to limits inherent to each
(see e.g., Wang et al. 2019b)
.
The
lack of
slit
spectroscop
y from space over a wide FoV
in the Near and Mid IR
is the obvious gap in
current and planned future space missions.
ATLAS
fills this gap in order to address the
fundamental questions on galaxy evolution and the dark Universe.
ATLAS
(Astrophysics Tel
escope for Large Area Spectroscopy)
is a concept for a NASA probe
-
class
space mission that will achieve groundbreaking
science in all areas of astrophysics
. It is the spectroscopic
follow
-
up space mission to
WFIRST
, boosting its scientific
return by obtain
ing deep 1
-
4
μ
m
slit
spectroscopy in three tiered galaxy
redshift surveys (wide: 2000
deg
2
; medium: 100 deg
2
; deep: 1
deg
2
) for
most of the galaxies imaged by the ~2000 deg
2
WFIRST
High Latitude Survey
(HLS)
at
z
>0.5.
ATLAS
spectroscopy will measure accur
at
e and precise redshifts for ~
200M galaxies out to
z
=
7 and beyond, and
deliver spectra that enable a wide range
of diagnostic studies of the physical
properties of galaxies over most of
cosmic history.
ATLAS
and
WFIRST
together will produce a
definitive
3D
ma
p
of the Universe over 2
000
deg
2
(Fig.1)
.
ATLAS Probe
Science G
oals
are
:
(1)
Discover how galaxies have evolved in
the cosmic web of dark matter from
cosmic dawn through the peak era of
galaxy assembly. (2) Discover the nature
of cosmic acceleration
. (3) Probe the
Milky Way's dust
-
enshrouded regions,
reaching the far side of our Galaxy. (4)
Fig. 1:
Cosmic web of dark matter (green) at z=2 traced by galaxies (red
filled circles) from
the
ATLA
S
Wide s
urvey
(left)
, which obtains spectra for
70% of galaxies in the
WFIRST
weak lensing sample, compared to
WFIRST
GRS
(right)
. The larger circles represent brighter galaxies. (Wang
et al. 2019
a
)
2
Discover the bulk compositional building blocks of planetesimals formed in the outer Solar System.
These
flow down to
the
ATLAS Probe
Scientific O
bjectives
:
(1A)
Trace the relation between galaxies and dark
matter with less than 10% shot noise on relevant scales at 1<
z
<7. (1B) Probe the physics of galaxy
evolution at 1<
z
<7. (2) Obtain definitive measurements of dark energy and tests of Gene
ral Relativity. (3)
Meas
ure the
3D structure and stellar content of the
inner
Milky Way to a d
istance of 25 kpc. (4)
Detect and
quantify the composition of 3,000 planetesimals in the outer Solar System.
ATLAS
is a 1.5m
telescope with a FoV of 0.4
deg
2
, and uses Digital Micro
-
mirror Devices (DMDs) as slit
selectors. It has a spectroscopic resolution of R = 1000, and a wavelength range of 1
-
4
μ
m
.
ATLAS
has an
unprecedented spectroscopic capability based on DMDs, with a spectroscopic multiplex factor ~6,000.
ATLAS
is designed to
fit within the NASA probe
-
class space mission cost envelope; it has a single
instrument, a telescope aperture that allows for a lighter launch vehicle, and mature technology (DMDs can
reach TRL 6 within two years). The pathfinder for
ATLAS
,
ISCEA
(Infrared
SmallSat for Cluster Evolution
Astrophysics), has been selected by NASA for a mission concept study. We anticipate
ATLAS
to be launch
ready by 2030.
ATLAS
will lead to
transformative science over the entire range of astrophysics.
We will
briefly
summarize
ATLAS
science below
.
Wang et al. (2019
a
)
presents ATLAS Probe in detail.
Astro2020 science white papers
by
Behroozi
et al. (2019)
,
Dickinson et al.
(2019)
,
Pisani et al. (2019)
,
and
Wang et al.
(2019
b
)
address
ATLAS science in
galaxy evol
ution and cosmology
.
(i)
De
coding Galaxy Evolution Physics Using
ATLAS
Probe
In today’s era of precision cosmology, we believe that we understand the growth of structure in a
universe of cold dark matter and dark energy, and we can map this over cosmic time with sophisticated
nu
merical simulations. However, the galaxies that we see are not simply dark matter halos. Baryonic
physics makes them far more complex, and we are still far from understanding how galaxies form and
develop in the evolving context of large scale structure.
A
TLAS
Probe will provide wide field, highly
multiplexed, densely sampled spectroscopy at high redshifts, producing a “time
-
lapse
SDSS
” spanning
most of cosmic history.
ATLAS
will carry out three nested surveys
(Wide/Medium/D
eep):
2000 deg
2
to
the
line flux
limit of 5
×
10
-
18
erg/s/cm
2
; 100 deg
2
to AB~25
; 1 deg
2
to AB~26
. These
will sample the galaxy
luminosity function over a wide range of redshift
s. Spectroscopic detection of H
α
(among other
lines) out to
z=5, and [OIII]+H
β
in the late reionization era, to z
=7, will relate each galaxy to its place in the cosmic web
with precision that cannot be achieved with photometric redshifts or
slitless spectroscopy (Fig.2
). Detection
of multiple emission lines will provide diagnostics of ISM excitation, metal abundance
and du
st reddening.
The Medium and Deep
survey
s
will also measure absorption line redshifts for quiescent galaxies.
Galaxy properties correlate with those of the underlying dark matter halos: their masses, spins
, positions,
and environments.
ATLAS
sur
veys will allow statistical derivation of dark matter halo masses from
clustering of galaxies binned by other observable/inferable parameters such as stellar mass, star
formation
rate or morphology.
This information will be used to derive the
stellar mass
—
halo mass relationship
(SMHMR, Moster et al. 2013, Behroozi et al. 2013), which measures the efficiency with which galaxies turn
gas into stars, and is a key probe of the strength of feedback from stars and supermassive black holes.
ATLAS
will measure the
SMHMR as a function of galaxy properties and its evolution over cosmic time.
Additional constraints will come from spectroscopic
group catalogs
and
local environmental density
.
ATLAS
can also measure average dark matter accretion rates for galaxies via
the detection of the splash
-
back radius (a.k.a., turnaround radius) of their satellites (More et al. 2016) out to
z
∼
5.
The shape of the SMHMR implies a characteristic mass at which galaxies most effec
tively convert gas
into stars.
At lower and higher masses, “feedback” is invoked to prevent gas from cooling onto galaxies, or
to expel gas, thus suppressing star for
mation efficiency. Supermassive black holes power active galactic
nuclei (AGN), which may regul
ate star formation and growth.
ATLAS
spectroscopy will identify vast
samples of high
-
redshift AGN using standard nebular excitation diagnostics (e.g., Baldwi
n, P
hillips, &
3
Terlevich 1981
)
.
[OIII] luminosities will provide a measure of the accretion luminosities and black hole
growth rates.
ATLAS
will connect AGN activity to local and large
-
scale environment with exquisite statistical
accuracy that is only possible
today in the local universe.
Current observations indicate that the intergalactic medium (IGM) completed its transitio
n from neutral to
ionized around
z ~ 6
, but the processes resp
onsible are poorly understood.
Reionization may have been
highly inhom
ogeneous, with expanding bubbles driven by strongly clustered young galaxies that are highly
biased tracers of dark matter structure. Future radio facilities (LOFAR, HERA, SKA
, ASKAP
) will map (at
least statistically) the distribution of neutral hydrogen
i
n the epoch of reionization.
ATLAS
surveys will
provide complementary maps
of the spatial distribution of
the (potentially) ionizing galaxies themselves
over the s
ame volumes, detecting [OIII]+H
β
emission lines at 5<
z
<
7 over very wide sky areas. An accurat
e
measurement of 3D clustering will strongly constrain theoretical models that can then be extrapolated to
higher redshifts, earlier
in the epoch of reionization.
There is already evidence that Ly
α
may be
inhomogeneously suppressed by the neutral IGM at z > 7, (e.g., Tilvi et al. 2014), and that its escape may
correlate with galaxy overdensities that can more effectively ionize large vol
umes (Castellano et al. 2016).
ATLAS
spectroscopy can detect o
r set limits on Ly
α
emission from vast numbers of galaxies selected
photometrically from deep
WFIRST
surveys, providing additional constraints on the reionization process.
The
ATLAS
surveys will enable a wide range of additional investigations. They wi
ll provide spectroscopic
identification and confirmation for hundreds of massive galaxy clusters at “cosmic noon” (z
≈
2), as well as
early groups and proto
-
clusters in the
AT
LAS
deep survey out to z
≈
7.
Galaxy emission line widths can be
interpreted in c
oncert with
WFIRST
imaging and structural properties to infer galaxy velocity functions.
Velocity shifts between ISM absorption lines (e.g., MgII 2800
Å
, NaI 5890,5896
Å
) and galaxy systemic
redshifts (e.g., from H
α
or [OIII] emission) can be used to trace
gas flows around star
-
forming galaxies.
Densely
-
sampled spectroscopy can also be used to study galaxy pairs and the evolution of the me
rger
fraction and merger rate.
Overall,
ATLAS
will provide rich and abundant spectroscopy to fully exploit the
wealth of
information from the
WFIRST
,
Euclid
and
LSST
imaging surveys, firmly connecting hundreds of
millions of galaxies to the evolving cosmic web.
(ii)
Definitive Measurements of Dark E
nergy
from
ATLAS
Probe
Given our ignorance of the nature of dark energy,
it is critical that we obtain measurements on dark
ene
rgy that are model
-
independent (
cosmic expansion history
H
(
z
) &
growth of large scale structure
f
g
(
z
)
as free functions)
and definitive (high precision and accuracy) over the entire redshift range over
which dark
energy influences the expansion of the Universe (i.e., 0<
z
<4).
ATLAS
Wide
covers
2000 deg
2
at 0.5<
z
<4,
with a galaxy surface number density ~12 times that of the
WFIRST
GRS and ~50 times that of
Euclid
, with
sp
ectroscopic redshifts for 183M
gala
xies
(see Fig.1
).
Galaxy clustering data from 3D distributions of
galaxies is the most robust probe of cosmic acceleration.
The baryon acoustic oscillation (BAO)
Fig.2: The spatial distribution of H
α
-
emitting gala
xies at z=2 from the semi
-
analytical galaxy formation model GALFORM. Each
panel illustrates a different survey of the same galaxy distribution, with redshift accuracy
σ
z
/(1+z) equal to (a) 10
-
2
(most
optimistic photo
-
zs); (b) 10
-
3
(slitless spectroscopy);
and (c) 10
-
4
(
ATLAS
slit
spectroscopy).
4
measurements provide a direct
measurement of
H
(
z
) and angular
diameter distance
D
A
(
z
) (Blake
&
Glazebrook 2003; Seo & Eisenstein
2003),
and
the
redshift
-
space
distortions
(RSD)
enable
measurement of
f
g
(z) (Guzzo et al.
2008; Wang 2008).
ATLAS
Wide
provides multiple galaxy tracers of
BAO/RSD (red galaxies, different
emiss
ion
-
line selected galaxies
,
and
WL shear selected galaxies) over
0.5<
z
<4, with each at high number
densities. These enable robust
modeling of BAO/RSD (e.g., the
removal of the nonlinear effects via
the reconstruction of the linear
density field), and significantly tightens constrain
ts on dark energy and modified gravity by evading the
cosmic variance when used as multi
-
tracers (McDonald & Seljak 2009).
ATLAS
Wide
enables detailed
study of
the galaxy
-
formation systematics (feedback; assembly bias; conformity) that are potential
system
atics in modeling RSD
(Tojeiro et al., 2017).
It
measures
H
(
z
),
D
A
(
z
), and
f
g
(
z
) over
the wide redshift
range of 0.5<
z
<
4 (see Fig.3
), with high precision over 0.5<
z
<3.5.
If
early dark energy remains
viable in
the
2020s
, it can be measured by enha
n
cing
ATLA
S
Wide
with
a
high z survey
targeting
H
α
emission line
galaxies
at 3<
z
<4 selected from
WFIRST
HLS imaging.
The very high number density galaxy
samples from the
ATLAS
Wide
survey
provide
the ideal data set for
studying higher
-
order statistics of galaxy
clustering. For a galaxy sa
mple with number density
n
,
shot noise
scales as 1/
n
for
2pt
, and 1/
n
2
for
3pt statistics
. Fig.2 shows that
ATLAS
Wide
3pt
statistics
gives definitive
measurements on dark energy, outperforming all other measurements
(Samushia et
al.
2019)
.
Since the
3pt statistics provides information not contained in the 2pt statistics, the combination of these is needed to
optimally extract the cosmological information from galaxy clustering
data (see, e.g., Gagrani & Samushia
2017), and enable
s the direct measurement of bias
b
(
z
).
In a
dd
ition, the cross
-
3pt function,
gala
xy
-
galaxy
-
lensing shear
,
will help break degeneracies between galaxy bias and cosmological parameters. It will be
measured with high signal
-
to
-
noise for the sample sizes discus
sed here.
While the use of galaxy clustering
2pt statistics is now standard in cosmology, the use of the 3pt statistics is still limited due to a number of
technical challenges (see, e.g., Yankelevich & Porciani 2019).
ATLAS
Wide
in the next decade will ta
ke
advantage of the anticipated future advances in galaxy 3pt statistics
to deliver game
-
changing science
.
(iii)
Probing the Dust
-
Obscured Inner Milky Way With
ATLAS
Probe
The
ATLAS
Wide Survey at high Galactic latitude will also probe the foregroun
d stellar content of the
Milky Way to unprecedented spectroscopic depth.
In addition, a dedicat
ed
ATLAS
Galactic Plane Survey
will
unveil and characterize objects at all evolutionary phases, from deeply embedded class
-
0 protostars to
the most elusive
dusty
Luminous Blue Variables.
With SNR>30 spectra for 95M sources having AB<18.2
mag
, and SNR>5 to AB=21.5 for hundreds
of
millions
more stars, covering 700 deg
2
in 0.4 years of
observing time, this survey will advance our understanding of the structure, star
-
forming history, and stellar
content of the Milky Way.
Currently, we know more about the structure and the spatially resolved star formation histories of
galaxies in and even beyond the Local Group, than we do about our own Galaxy.
ATLAS
will advanc
e our
Fig.3. Expected
H
(
z
) and
f
g
(
z
) from futu
re surveys. “2pt” refers to
galaxy
powe
r
spectrum, “3pt” refers to
galaxy
bispectrum.
Constraints are derived following
Wang et al. (2013) & Samushia et al. (2019). The constraints on
D
A
(
z
) (not
shown to avoid cluttering) provide a cross
-
check on
H
(
z
). The bias between
galaxy and matter distributions is
b
(
z
).
ATLAS
overlaps ground
-
based projects
0.5<z
≲
1 for key cross
-
check and mitigation of systematic effects).
5
understanding of the 3D Milky Way beyond the ongoing revolution provided by ESA/Gaia, especially in the
inner Galaxy (|
l
|<65
°
and|
b
|<1
°
) where 98.5% of the Galactic plane has G
-
band extinction >7.5 mag. Going
to the infrared drops the high extinction
fraction to only 10.4% of the Galactic plane in the K band.
ATLAS
’
infrared range thus opens up large regions of the Galaxy for spectral investigation.
ATLAS
will cover both
the inner and outer Galaxy, and thus very different regions in terms of galactic
structure, star formation,
evolved star properties, and interstellar dust.
ATLAS
spectroscopy will follow on the advances of
2MASS
,
WISE
, and
Spitzer
in identifying candidates
of particularly interesting object classes based on colors alone, and the
expected return from
WFIRST
. For
WFIRST
photometry in particular, stellar photospheric temperatures and interstellar extinction effects will
be almost completely degenerate due to the filter choices.
ATLAS
spectra will also complement the
planned
SDSS
-
V
spectroscopic survey programs
which will cover
brighter Milky Way objects (
H
<11.5 mag).
For
Galactic Structure
,
ATLAS
spectroscopy can produce unique information on the bar(s) of the
Galaxy, the nuclear region, the stellar disk, and spiral arms, relati
ve to existing and planned photometric
surveys that use star counts techniques and statistical analysis of color
-
color and color magnitude
diagrams. Not only will
ATLAS
be able to provide spectral information on all of the (nearby) stars observed
in the op
tical by
Gaia
, it will be able to see much of the substantial stellar population that is not detectable
in the optical. Reconstructing the 3D structure of the Galaxy will allow progress on a number of
fundamental questions regarding e.g. the scale
-
length o
f the Galactic disk, whether the stellar warp and the
gas warp coincide, and the existence of stellar streams across the Galactic plane.
For
Star Formation,
the
Spitzer
/GLIMPSE surveys produced an unprecedented picture of star formation
in our Galaxy
by unveiling hundreds of new star forming regions.
ATLAS
will quantify the Star Formation
Rate (SFR) of the Galaxy, its variation with Galactocentric radius, and its association with various
dynamical features in the Galaxy, testing theories of star format
ion both on a global scale and at the
molecular cloud level.
ATLAS
will characterize Young Stellar Objects (YSO) and their surrounding
environments, analyzing the energy budget and mapping the spatially resolved star formation history of the
Galaxy over th
e past <50
-
100 Myr.
For
Interstellar Extinction
, by measuring the extinction law over tens of millions of lines of sight,
ATLAS
will provide a unique dataset to study in detail the variation of the extinction curve out to the edge of the
Galactic disk
. Variations with Galactic longitude have been identified and attributed to small variations in
ISM density, mean grain size, or disk metallicity gradient. Variations of chemical composition of dust grains
reflect the abundance/depletion of metals in the I
SM, and hence the cooling mechanisms that control the
efficiency of star formation.
For
Substellar Objects
,
ATLAS
will study brown dwarfs as they
cool through the L
-
T
-
Y spectral
sequence with age. These objects provide important information on the sh
ape and low
-
mass cutoff of the
field mass function. They also stand as proxies for exoplanets, given their atmospheres of similar effective
temperatures. At the 3
σ
limit of
J
≈
23.5 mag AB,
ATLAS
will detect 5
-
Gyr
-
old field brown dwarfs as low in
mass as 10 MJup (Teff
∼
300K, an early
-
Y dwarf) at 10 pc and 35 MJup (Teff
∼
700K, late
-
T) at 100 pc.
For
Evolved Stars,
ATLAS
will detect Red Giant Clump star standard can
dles with
L
mag=
−
1.75 (0.71 in
AB mag) at 10kpc for
Av
= 30 mag, reaching the heavily obscured regions of the Galactic Center or the
outer edges of the Milky Way over at least the two outer quadrants. Red Supergiants can be studied across
the entire Galactic
disk. AGB stars (initial mass 4
-
8M
⊕
) are sufficiently short lived that they can be used to
trace the spiral arms of the Galaxy. Their number and distribution provides a fossil record of the recent
history of star formation in the Milky Way.
(iv)
Explori
ng the Outer Solar
System
With
ATLAS
Probe
Despite more than 2 decades of spectroscopic observations of Kuiper Belt Objects (KBOs), very little is
known about their surface compositions; beyond water
-
ice and methanol, no materials have been
6
confidently
ident
ified in the spectra of small (
D
≲
500
km
) KBOs. This is largely the result of the lack of
identifying absorption fe
atures in the
λ
≲
2.5
μ
m
region, beyond which current facilities are insufficiently
sensitive to gather observations of these bodies. This i
s unfortunate, as many anticipated materials exhibit
strong absorption features at these longer IR wavelengths (Parker et al., 2016).
Notably, no silicate materials, commonly identifi
ed by Fe/Mg absorptions at ~1
μ
m
, and deep hydroxl
feature at 3.0
μ
m
,
have ever been detected in the spectra of KBOs. This represents one of the big
outstanding gaps in our compositional knowledge of these bodies. From our best proxy of the spectra of
KBOs
-
Phoebe
-
it is clear that
ATLAS
Probe
has the potential to
detect such features, as it will provide the
requisite SN
R throug
hout the important
1
-
4
μ
m
wavelength range.
Spectra of KBOs will come from two separate
ATLAS
surveys. The
ATLAS
W
ide S
urvey will gather
useful IR reflectance spectra of roughly 300 KBOs.
These spectra will provide NIR spectral slopes, as well
as a measure of water
-
ice absorption. For the brightest targets, s
ilicate detection is possible. The
ATLAS
Solar System Survey, with pointed observations of known bodies, would gather more than 3000 s
pectra,
down to a prac
tical brightness limit of
r
~23.2
, for
on
-
target integrations of 2
500
s. All such bodies have, or
will be detected and trac
ked by the
LSST
, or the
Pan
-
STA
R
RS
surveys, and the resultant spectra will have
higher SNR than typical from the
spectra gathe
red during the
ATLAS
W
ide S
urvey.
II.
ATLAS
Probe:
Technical Overview
To meet its science objectives,
ATLAS
requires a
~1.5m space telescope in an L2 orbit
(Fig.4)
, with a
multi
-
object spectrograph with R~1000 over a FoV ~0.4 deg
2
,
wi
th spectrosc
opic multiplex of ~6
000, and
the wavelength coverage of 1
-
4
μ
m.
(i)
ATLAS
Probe
Instrument
Optical Design:
ATLAS
has only one instrument
consisting of 4 identical modules
, compact and modular;
it fits below the primary mirror structure into a cylindrical
envelop
e only slightly larger than 1.5
m in diameter
(the size of the primary) and
~65
cm in height
(see Fig.5
)
. The
instrument
size can be reduced in a future
design
phase
.
The camera optics image a square 0.75
ʺ″
field onto a little less than 2 x 2 pixels on
the
detector, deliver
ing
a scale of about 0.385
ʺ″
pixel. T
he footprint of
the DMD on each NIR detector is about
4,000 x 2,100 pixels. The scale can easily be changed in the spectral direction to at least 2.1 pixel/micro
-
mirror because of the unused space on
the
4k
×
4k
detector.
The instrument is maintained at temperatures
Fig.5:
A full view of the preliminary optical design for
the
ATLAS
Probe instrument. The large gray circle is
the back of the primary.
Fig.4
:
ATLAS
Probe
orbit.
7
~50K to keep thermal noise below zodiacal light
level.
Our preliminary
optical
design already has
excellent image quality in the spectrograph. The
Gaussian Equivalent Full Width at Half Maximum
(GEFW
HM) is about 1/2 of a
DMD
mic
ro
-
mirror
image. The image quality of the fore
-
op
tics on
the DMD is 1/2 of a
micro
-
mirror, satisfactory at
this point of the preliminary design.
Table
1
lists
the main p
arameters of our system.
For a
detailed discussion of the
ATLAS
in
strument, see
Wang et al. (2019
a
).
Target Selection Mechanism:
The
ATLAS
i
nstrume
nt r
equirement of a spectroscopic
multiplex factor of
~60
00 drives the adoption
of
DMDs
as the target selection mechanism. DMDs
have been invented for digital display/projec
tion
applications by Texas Instrum
ents.
ATLAS
baselines
the 2k CINEMA model with
2048
×
1080
micro
-
mirrors,
13.7
μ
m on a side.
Each micro
-
mirr
or of the DMD’s can tilt ±12
°
to separate the
reflected “ON” vs. “OFF” beam.
Therefore a
DMD, perpendicularly illumina
ted, must receive a beam slower than f/2.4 to prevent overlap between the
input and output beam
s
,
setting an upper limit to the scale per micro
-
mirror, and therefore to the total
FoV
of the
spectrograph.
Detec
tors and ASICs:
Our baseline detector is the Te
ledyne H4RG
-
10, the same type currently under
development for
WFIRST
. The long
-
wavelength cutoff
s
of our spectroscopic channels (2.1
μ
m and 4
μ
m)
are compatable with
the standard
~2.35
μ
m and ~5.37
μ
m cutoff of
WFIRST
and
JWST
devices
respectively. Fine
-
tuning
the Hg vs. Cd stoichiometric ratio can further reduce
the long wavelength cutoff
and therefore the
dark current, allowing warmer operating temperatures. H4RG arrays test
ed by
WFIRST
have typical QE>90
% in the 0.8
-
2.3
5
μ
m range, readout noise ~
15e in Double
Correla
ted Mode and mean
dark current <0.01
e/s/pixel.
JWST
de
vices have similar performance.
The architecture of the multiplexer
allows multiple non
-
destructive reads of each pixel during a single exposure
(“sampling up the ramp”
)
; this
enables mitigation
against cosmic rays and reduction in read noise: typical readout noise with 16
non
-
destructive
samples drops to about 5e. Teledyne has developed SIDECAR ASICs (Application Specific
Integrated Circuit) to manage all aspects of FPA operation and output digit
ization in cold environment. By
keeping analog signal paths as short as possible to reduce noise and output capacitance loading, ASICS
improve power consumption, speed, weight and performance. A Teledyne ASIC device is currently driving
HST
/ACS and several
Teledyne ASICs will soon fly on 3 out of 4
JWST
instruments; we will adopt such
devices for
ATLAS
.
(ii)
ATLAS
Probe
Mission Architecture:
Mission Implementation:
We assume a 5 year
ATLAS
mission in a halo L2 orbit similar to
that of
JWST
(
Fig.
4
)
, enabl
ing long observations in a very stable thermal and radiation environment; the narrow Sun
-
Spacecraft
-
Earth angle facilitates passive cooling to the
~
50K operating temperature needed for the long
wavelength channel detectors. Absolute pointing requirements a
re relaxed due to t
he versatility of the
DMDs; this results in a
pointing stability
requirement
of 0.1
ʺ″
over the 1000s exposure time
which
is
a
dequate
for the 0.39
ʺ″
pixels.
The temperature of the instrument
is
low enough to make it immune to
TELESCOPE
Type
modified Ritchey
-
Chrétien
Primary: diameter & focal ratio 150cm; f/1.6
Primary: central obscuration 19% diameter (3.7% area)
Secondary: diameter 29 cm
Telescope focal ratio f/11.
2
PYRAMID MIRROR 4 rectangular faces
Size 4
×
13.6 cm
×
7.4 cm
Field of view 4
×
25.60
ʹ′
×
13.50
ʹ′
FORE OPTICS
f/# (off axis) f/2.3
×
f/2.5
Scale on DMD (slit size) 0.75
ʺ″
×
0.75
ʺ″
/micro
-
mirror
COLLIMATOR
Elements 4 mirrors (+ 1 dichroic)
DISPERSING ELEMENTS
Wavelength ranges
1
-
2.1
μ
m (NIR); 2.1
-
4
μ
m (MIR)
Resolving powe
r R ~ 1000
Type prism
CAMERAS
Elements 1 mirror (each camera)
Sampling on detector 0.38
ʺ″
×
0.39
ʺ″
(pixel scale)
Table 1:
ATLAS
main optical parameters.
8
changes relate
d to the spacecraft attitude.
Note that for a survey telescope the attitude
can be more easily maintained within optimal
range than for a general observatory (like
e.g.
HST
or
JWST
) where the pointing and
orientation are driven by the science
programs,
imp
acting
optimal scheduling.
Table 2 summarizes
ATLAS
mission
implementation requireme
n
ts.
We have
a
ssumed
that
solar panels can be integrated
with bottom sun shield / bottom deck of
spacecraft.
Thermal solutions
adopted
for
similar class missions
are adequa
te
for
ATLAS
’s 4 spectrometers.
Mass Estimates
:
ATLAS
instrument mass
were derived by s
caling
results from
studies
of similar class missions. A
s
ingle spectrometer mass and costs
is
scaled by
the
number of detectors
, for
a t
otal
of 4 identical s
pe
ctrometers
using 2 H4RG detectors each
. The
focal plane mass/costs
are scaled
from
a
similar study with H2RG detectors
.
Rules of thumb
were
applied to
calculate spectrometer mass and costs
from focal plane mass/cost
. The
t
elescope mass
was
calculated
from
telescope diameter
, using a
l
inear fit
derived from similar
study results to estimate
mass
as
a
function of diameter
. The
e
rror in
the
fit
was
used to
calculate max and min probable
mass
. The p
ayload mass
is
given as the
sum of 4
spectrometers
plus
the
tel
escope
,
with
estimated
minimum, mode, and maximum
payload mass
, see
Table 3
.
ATLAS
spacecraft
mass
estimate is based on scaling
of
similar
missions using the total
payload mass (Telescope + 4 Spectrometers)
, see Table 4
.
Estimated minimum, mode, and m
aximum
mass are given, as well as 70th percentile confidence spacecraft mass
.
Payload mass estimates are used
to generate spacecraft bus estimates using historical average Mass/Payload ratios
.
Bus cost estimates are
generated using another relationship bet
ween astrophysics spacecraft mass to cost ratios
.
The bus costs
have been increased
by $10M to accommodate out
-
of
-
family
pointing requirements
.
Mission
•
Astrophysics IR all sky observer (galaxy
redshift survey)
•
L2 Or
bit
•
Class B Mission
•
Dual string spacecraft bus
Constraints
•
Tight pointing stability
–
Driven by slit size of 0.75”
–
Requires +/
-
0.375” 3
-
sigma
•
~40
-
60 K detector temperature (in family with
other passively cooled
similar missions
)
•
Sunlight cannot contact te
lescope
•
Long exposures (up to days)
Measurement
•
4 identical spectrometers
cover
ing 1
-
4
μ
m
Data Volumes
•
~600Mb ever
y 500 seconds, for 170 samples
per d
ay
•
~186 Tb over 5 years
Commanding
Weekly commanding cycle once on orbit.
Table 2:
.
ATLAS
Probe mis
sion implementation requirements.
Detectors
(#)
Focal Plane
mass (kg)
Spectrometer
min mass (kg)
Spectrometer
mode mass (kg)
Spectrometer
max mass (kg)
2
2.4
26.6
39.9
79.7
Telescope
Diameter (m)
Telescope
min mass (kg)
Telescope mode
mass (kg)
Telescope max
ma
ss (kg)
1.5
214.8
350.1
485.5
Payload
Payload min
mass (kg)
Payload mode
mass (kg)
Payload max
mass (kg)
321.1
509.6
804.4
Table 3: E
stimate
d
instrument mass
for
ATLAS
Probe
.
Payload Mass
Estimate
Bus Mass
Estimate
Bus Cost
Estimate
Minimum Payload Mass
321 kg
618 kg
$140.05
Mean Payload Mass
510 kg
981 kg
$216.41
Maximum Payload Mass
804 kg
1548 kg
$335.82
70% Values
598 kg
1150 kg
$252.08
Max
imum mass
estimate used for
margin calculations
EELV Capability (kg)
3400
LV margin uses an average L/V
adapter mass of 30 kg and
propellant mass of 161.7 kg
Total ATLAS MEV Mass (kg)
2352
NASA Margin (%)
48%
Table 4:
Estimated spacecraft mass a
nd cost for
ATLAS
Probe.