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The Far-Infrared Surveyor Mission
study: paper I, the genesis
M. Meixner, A Cooray, R. Carter, M. DiPirro, A. Flores,
et al.
M. Meixner, A Cooray, R. Carter, M. DiPirro, A. Flores, D. Leisawitz, L.
Armus, C. Battersby, E. Bergin, C. M. Bradford, K Ennico, G. J. Melnick, S.
Milam, D. Narayanan, K. Pontoppidan, A. Pope, T. Roellig, K. Sandstrom,
K. Y. L. Su, J. Vieira, E. Wright, J. Zmuidzinas, S. Alato, S. Carey, M.
Gerin, F. Helmich, K. Menten, D. Scott, I. Sakon, R. Vavrek, "The Far-
Infrared Surveyor Mission study: paper I, the genesis," Proc. SPIE 9904,
Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter
Wave, 99040K (29 July 2016); doi: 10.1117/12.2240456
Event: SPIE Astronomical Telescopes + Instrumentation, 2016, Edinburgh,
United Kingdom
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The Far-Infrared Surveyor Mission Study: Paper I, the Genesis
M.Meixner
*1a,b
, A. Cooray
c
,R. Carter
d
, M. DiPirro
d
, A. Flores
d
, D. Leisawitz
d
, L. Armus
e
, C.
Battersby
f
, E. Bergin
g
, C.M. Bradford
h
, K. Ennico
i
, G. J. Melnick
f
, S. Milam
d
, D. Narayanan
j
, K.
Pontoppidan
a
, A. Pope
k
, T. Roellig
i
, K. Sandstrom
l
, K.Y.L. Su
m
, J. Vieira
n
, E. Wright
o
, J.
Zmuidzinas
p
, S. Alato
q
, S. Carey
e
, M. Gerin
r
, F. Helmich
s
, K. Menten
t
, D. Scott
u
, I. Sakon
v
, R.
Vavrek
w
a
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218,
b
Dept. of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles St., Baltimore,
MD 21218,
c
Dept. of Physics and Astronomy, University of California, Irvine,
d
NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771 USA,
e
NASA
Infrared Processing and Analysis Center,
f
Harvard-Smithsonian Center for Astrophysics,
g
Dept. of
Astronomy, University of Michigan,
h
NASA Jet Propulsion Laboratory,
i
NASA Ames Research
Center,
j
University of Florida, Gainsville,
k
Department of Astronomy, University of Massachusetts,
LGRT-B 619E, Amherst, MA 01003, USA,
l
University of California, San Diego,
m
Steward
Observatory, University of Arizona,
n
University of Illinois, Urbana-Champaign,
o
University of
California, Los Angeles,
p
California Institute of Technology,
q
SNSB,
r
CNES,
s
SRON,
t
DLR,
u
CAS,
v
JAXA,
w
ESA
ABSTRACT
This paper describes the beginning of the Far-Infrared Surveyor mission study for NASA’s Astrophysics Decadal 2020.
We describe the scope of the study, and the open process approach of the Science and Technology Definition Team. We
are currently developing the science cases and provide some preliminary highlights here. We note key areas for
technological innovation and improvements necessary to make a Far-Infrared Surveyor mission a reality.
1. INTRODUCTION
The Far-Infrared (Far-IR) Surveyor concept originates most recently in the NASA 2013 Roadmap
1
, but has a history
within the community. This mission builds upon decades of success in space/airborne infrared astronomy with IRAS
2
,
KAO
3
, ISO
4
, Spitzer
5
, Herschel
6
, SOFIA
7
and soon to be launched, JWST
8
. The Roadmap envisaged a space observatory
with a large gain in sensitivity over the Herschel Space Observatory
6
, better angular resolution sufficient to overcome
spatial confusion in deep cosmic surveys and new spectroscopic capabilities. A white paper from the 2015 Pasadena Far-
Infrared Community workshop
9
, provides a recent starting point that summarizes some key questions in the field and a
description of a potential, single aperture telescope.
The Science and Technology Definition Team (STDT) were selected to represent the US astronomy community’s
demographics, geography, and expertise areas and include: Margaret Meixner, Asantha Cooray, David Leisawitz, Lee
Armus, Cara Battersby, Ted Bergin, Matt Bradford, Kimberly Ennico, Gary Melnick, Stefanie Milam, Desika
Narayanan, Klaus Pontoppidan, Alexandra Pope, Tom Roellig, Karin Sandstrom, Kate Su, Joaquin Vieira, Ed Wright,
and Jonas Zmuidinas. This group, which is at the nexus of the mission concept study, is co-led by the two community
co-chairs, Asantha Cooray and Margaret Meixner, who are responsible for delivery of the reports to NASA headquarters
(HQ) and the Decadal Committee. The STDT community co-chairs direct the engineering study that will be performed
by NASA Goddard Space Flight Center with study manager, Ruth Carter, study scientist, David Leisawitz, study
technologist, Mike DiPirro, and study mission systems engineer, Anel Flores. However, we anticipate assistance from
other NASA centers including Ames, JPL and Marshall. The STDT draws upon input from the astronomical community
through the Far-IR Science Interest Group (SIG), NASA government labs, (Goddard, JPL, Ames), Academia, and
*
meixner@stsci.edu; phone 1-410-338-5013
Invited Paper
Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave, edited by
Howard A. MacEwen, Giovanni G. Fazio, Makenzie Lystrup, Proc. of SPIE Vol. 9904,
99040K · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2240456
Proc. of SPIE Vol. 9904 99040K-1
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Industry. The NASA HQ Program Scientists, Kartik Sheth and Dominic Benford, NASA Cosmic Origins Program
Scientists, Susan Neff and Deborah Padgett, and IPAC support staff scientist, Sean Carey, observe and support the study
as ex-officio, non-voting representatives. Several ex-officio non-voting international organizational representatives are
observing and contributing to the process including: Susanne Alato (SNSB), Maryvonne Gerin (CNES), Frank Helmich
(SRON), Karl Menten (DLR), Douglas Scott (CAS), Itsuki Sakon (JAXA), Roland Vavrek (ESA). Finally, the STDT
chairs have established a Senior Advisory board with past experience with the Decadal process, to be a technical
sounding board and a review board for drafts of the report. This senior advisory board includes Marcia Rieke, Jean
Turner, Sarah Lipscy, Harry Ferguson, Charles Lawrence, Harvey Moseley, George Helou, John Carlstrom, Jon
Arenberg, Meg Urry, George Rieke and John Mather.
The STDT relies on a number of working groups to prioritize the science identification and science drivers of the
mission architecture. The work related to mission concept development is coordinated through a separate working group
whose primary expertise on issues involving technology. While members of the STDT lead each of the working groups
they are primarily composed of international community members. The five science working groups (with leads
identified) are: Reionization and Cosmology (Matt Bradford, Joaquin Vieira), Evolution of Galaxies and Blackholes
(Alexandra Pope and Lee Armus), Milky Way, ISM and Local Volume of Galaxies (Cara Battersby, Karin Sandstrom),
Protoplanetary disks and Exoplanets (Klaus Pontoppiddan, Kate Su), and Solar System (Stefanie Milam). The Mission
Concept Development Group is coordinated by Tom Roellig. The mission development relies on the identification of
primary science drivers and establishing the technical requirements (e.g., spatial and spectral resolution, continuum or
spectral sensitivity, survey area or number of targets, wavelength coverage, among others). Key science themes
identified by the science working groups are outlined in Section 2.
The main deliverable is a report for the Decadal 2020 committee that provides a concept maturity Level 4 design for the
Far-IR Surveyor from the engineering study and the science case which justifies the cost of the mission. There will be
interim reports and deliverables along the way. The process is intended to be open and transparent to the community
which is international in nature. Biweekly telecons are open to everyone and talks from the face-to-face meetings are
posted on our public websites: asd.gsfc.nasa.gov/firs.
2. SCIENCE CASE
The science pursued by such a Far-Infrared Surveyor must be posed in the 2030 context. This scientific landscape will
have been shaped by 15+ years of Atacama Large Millimeter Array (ALMA) observations, the discoveries by the James
Webb Space Telescope (JWST), the Stratospheric Observatory for Infrared Astronomy (SOFIA), and the Wide-Field
Infrared Survey Telescope (WFIRST) and the results from newly operational 25-35 m ground based telescope facilities.
The Far-IR Surveyor is envisioned to be a general purpose space observatory with a broad science case that we highlight
in five areas below.
2.1 Cosmic Dawn and Reionization
Rest-frame UV and optical lines allow studies on the ISM and gas-phase metallicities. With Hubble, such
studies have been attempted at z~1 to 2 and will soon be extended to z~6 with JWST. In the post-JWST era a far-
infrared space telescope with at least a four-order of magnitude sensitivity improvement over Herschel will enable
studies on the gas properties, AGN activity and star- formation within galaxies at redshifts z = 6 to 12. This redshift
range covering the epoch of reionization is especially important for our understanding of the cosmic origins, formation
of first stars, galaxies and blackholes, and the onset of large-scale structure we see today. In particular, 20 to 600 μm
spectroscopic observations can: (1) disentangle the complex conditions in the ISM of primordial galaxies by measuring
the gas densities and excitation, and the prevalence of shock heating; (2) use spectral line diagnostics to study the first-
epoch of AGN activity, including the formation of first massive blackholes; and (3) detect, measure, and map out
molecular hydrogen rotational line emission from primordial cooling halos that are the formation sites of first stars and
galaxies at z > 10. Related to (3), molecular hydrogen is now understood to be the main coolant of primordial gas
leading to the formation of first galaxies. It is also the most abundant molecule in the universe. Molecular hydrogen
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bserved wavelength (nm)
0
0
0
0
0
Peak of the star formation rate density
I I I I I
ILJ_
0
10
[0I] ,[NII] ,[CII]
[NeII],[eV],[SiIl]
H2(S(5),S(3),S(1),S(0))
PAH(6.2,7.7,11.3µm)
spectrum
z=12
o
2 4 6
8
10 12
Redshift
cooling in primordial dark matter halos will likely be the primary tracer to study the transition from dark ages at z > 20,
when no luminous sources exist, to reionization at z < 10. At a metallicity of 10
-3.5
Z
solar
gas cooling will transit from H
2
to atomic fine-structure lines. At z < 8, when primordial molecular hydrogen is easily destroyed by UV radiation, the
prevalence of shocks in the ISM may provide ways to form a second and later generations of molecular hydrogen. The
rotational lines of molecular hydrogen span across a decade of rest wavelengths from 2 to 30 μm. The detection of
molecular hydrogen cooling requires an exteremely sensitive telescope (line sensitivity down to 10
-23
W m
-2
) and will
likely be the primary driver of spectral line sensitivity of the Far-IR Surveyor.
Figure 1: Key MIR and FIR spectral lines for diagnosing star formation and black hole growth are uniquely
shifted to the Far-IR Surveyor spectral coverage window over much of cosmic time.
2.2 Evolution of Galaxies and Blackholes
With a huge leap in sensitivity and access to a rich astrophysical window of diagnostic features (Figure 1), the Far-IR
Surveyor will uniquely address fundamental questions about how galaxies and black holes are born, how they evolve
over cosmic time, and how gas, dust and metals are created and enrich the interstellar and intergalactic medium. With a
large, cold telescope in space, the Far-IR Surveyor will be able to measure a wide variety of emission and absorption
lines from atomic and molecular gas, and dust in thousands of galaxies, independent of the level of obscuration,
providing our first, clear and unbiased picture of galaxy evolution from the local Universe to the epoch of Reionization.
Through sensitive spectroscopy over the large range in wavelengths inaccessible to JWST and ALMA, the Far-IR
Surveyor will be able to measure star formation rates and black hole accretion rates simultaneously. Spectral lines over
this wavelength range also probe the build up of metals and the dynamic physical processes that regulate feedback and
how galaxies grow over time. At early cosmic time, the Far-IR Surveyor can uniquely measure molecular hydrogen to
trace turbulent formation of the first structures in the Universe. Through wide-area surveys, the Far-IR Surveyor will
probe these processes in all physical environments, from the field to dense clusters, and provide a complete census of the
dusty, hidden Universe.
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2.3 Local Volume of Galaxies, Milky Way, Interstellar medium, Evolved stars and Star Formation
Studying the interstellar medium in the Milky Way and nearby galaxies provides the link between the formation of stars
and planetary systems and the evolution of galaxies over cosmic time. Most of the fundamental diagnostic tools for such
studies are found in the far-IR: the peak and long-wavelength tail of the dust spectral energy distribution (SED) and the
dominant cooling lines for most ISM phases ([CII] 158 um, [OI] 63 um, [OIII] 88 um, [NII] 122 & 205 um, etc.). In
addition to pushing into new regimes of sensitivity and angular resolution with these fundamental tracers of ISM
energetics and physical state, the Far-IR Surveyor will provide an array of powerful new tools for studying the ISM and
star formation. Lines such as HD and low-lying H
2
O rotational lines that were detected in only the brightest peaks of
nearby star-forming regions like Orion with previous IR telescopes will become routine tracers of the ISM in the era of
the Far-IR Surveyor.
The Far-IR contains key diagnostics for studying star formation and feedback across a wide-range of environments.
From detailed studies of Galactic-disk star-forming regions, to feedback-driven galactic outflows in nearby galaxies, the
Far-IR Surveyor will allow us to constrain how star formation and feedback are regulated in galaxies and the dependence
of these processes on environment. Measurements of the peak of the SED for dust and far-IR cooling lines will enable
radical progress in our understanding of star formation, in particular: (1) by measuring the amplitude and variability of
accretion onto protostars with the far-IR SED to determine how they gain mass and what sets the initial mass function
and (2) by measuring the phase structure of the ISM and tracing the cycling of gas between phases. The Far-IR uniquely
enables detailed kinematic studies of the main cooling lines of the ISM and higher-energy molecular transitions, tracing
the energetics of gas from protostellar cores through galactic winds. These measurements are critical for studying
feedback processes in the local universe and their interactions with the wider environment.
The Far-IR is also host to a variety of diagnostics that trace the abundance and characteristics of dust and molecules in
the interstellar medium. The evolution of dust in the ISM and the properties of molecular gas remain critical questions
for studying galaxy evolution and star formation at all redshifts. The peak of the dust SED and the Rayleigh-Jeans tail at
long wavelengths are critical for measuring dust masses. The Far-IR Surveyor will enable measurements of the dust SED
and mass in the local ISM and nearby galaxies at very low surface brightness, probing environments where the dust life-
cycle is very different from the local area of the Milky Way (i.e. low metallicity dwarf galaxies, the diffuse atomic
outskirts of spiral galaxies). The far-IR will also provide spectroscopic measurements of the long-wavelength dust
emission to constrain the growth of grains in the Milky Way molecular clouds. The greatly enhanced sensitivity of the
Far-IR Surveyor compared to other infrared telescopes also provides access to unique tracers of the molecular ISM,
which have never been systematically studied. These include the far-IR rotational lines of the HD molecule, which
provides a unique handle on tracing molecular gas masses in the Milky Way and nearby galaxies; the high-J rotational
lines of CO, which trace energetics in shocked or highly irradiated molecular gas; and the low-lying H
2
O lines, which
can track the formation of H
2
O in the diffuse ISM through its incorporation into proto-planetary disks and eventually
planets.
2.4 Protoplanetary Disks and Exoplanets
A next-generation mid- to far-infrared space observatory will uniquely answer questions fundamental to our
understanding of planet formation and of how ingredients for life are delivered and incorporated into habitable planets.
We aim to measure the total gas mass across all evolutionary stages of planet formation in disks across the entire stellar
mass range, using the strong ground-state line of deuterated molecular hydrogen, HD at 112 um. Only a single
unambiguous gas-mass measurement of a protoplanetary disk was made with Herschel, while a cold far-infrared space
telescope will be able to measure the mass of thousands of disks. The Far-IR Surveyor will create a comprehensive
census and mass inventory of both water vapor and water ice, as well as other volatile molecules, in planet-forming
regions, using the 43 um water ice band, and a multitude of far-infrared water lines. The power of a Far-IR Surveyor can
be leveraged to accurately measure the total water content, and its spatial distribution, in disks around stars of all masses
(including brown dwarfs) and at distances large enough to include massive star-forming regions, similar to those our
own Sun formed in.
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