SAFIRE: Scientific Capability
The Submillimeter and Far-InfraRed Experiment (SAFIRE) on the SOFIA
airborne observatory is designed to be a wide-field imaging spectrometer
with moderate spectral resolving power. It will achieve a resolution of
about 200km/s, continuously tunable over the 100μm-700μm range. In this
section, we describe the highest priority science projects for SAFIRE which
have helped motivate the design of the instrument.
SAFIRE is envisioned as a wide-field spectrometer for measuring line
emission from galaxies, specifically concentrating on the far-infrared
(100μm-300μm) atomic fine-structure lines (both locally and redshifted) and
on submillimeter lines (300μm-650μm) which are not easily accessible from
ground-based facilities. It
will be a powerful tool for studying such topics as:
- Powering of Ultraluminous Infrared Galaxies
- ISM
cooling traced by FIR fine structure lines
- Evolution of Matter in Universe
- Diagnostics of Active Galactic Nuclei
- Star
formation in the Galaxy
- Star
formation out to high redshifts
1. Far-Infrared Line Emission
Many fine-structure and molecular-transition lines serve as probes of the
physical properties of the ISM of the Milky Way and other galaxies. One
example of this is shown by the COBE/FIRAS average line spectrum of our
galaxy, which shows several important lines (below). As an example, we list below
several far-infrared lines and the physical investigations which can be
conducted by observing them.
- Oi lines probe the physical
conditions of gas in PDRs.
- Nii lines trace the warm ionized
- Cii line traces PDRs, atomic clouds,
and warm ionized medium.
- Nii (with N style='font-variant:
small-caps'>iii ) gives the effective temperature of stellar or AGN
UV radiation fields.
- Sii
line indicates the presence of dissociative J-shocks.
- CO rotational lines trace shocked gas found in warm dense gas of PDRs.
- OH lines trace shocked gas in cool dense gas.
- OH, CH, and NH3 together constrain molecular cloud chemistry.
- Ci traces star formation, atomic
clouds.
Figure 10. COBE/FIRAS average line spectrum of the Milky Way, covering
the SAFIRE bands (15-100 cm-1) and showing the stronger lines.
The brightest emission lines from star forming galaxies are the
fine-structure lines of common species: Cii at 158μm, Oi at 145μm, and Nii at 122μm and 205μm.
These lines dominate the cooling of several phases of the ISM, which
comprise much of the mass. These lines also trace a variety of source
types, including Hii regions,
atomic clouds, and photon dominated regions. These lines can be used as a
diagnostic of the interstellar radiation field (both its hardness and its
intensity) and can probe the density, mass, and metallicity of the ISM. Of
particular note among the fine-structure lines is the brightest, the
158μm Cii line, which
typically accounts for between 0.1 and 1% of the total far-IR luminosity of
star forming Galaxies. Furthermore, several bright
shorter wavelength lines will be visible by SAFIRE when redshifted; these
include O i 63μm (~50%
Cii flux); O iii 52μm and 88μm (~50% C
ii flux); N ii 122μm (~20% C ii flux); Nii 205μm (~10% C ii flux).
2. Galactic Center
The Galactic center region is an interesting laboratory of physics, showing
a broad range of phenomena in the interstellar medium at angular and
temperature scales relevant to investigation with SAFIRE on SOFIA. SAFIRE
can map the entire thermal and nonthermal arches region in a wide variety
of spectral lines and at high angular resolution (Figure 11). One
outstanding question is simple: where are the stars? While it is clear that
the thermal arches are excited by stellar radiation, it is not clear where
these stars are. Mapping in Cii, Oi, Nii, and mid-J CO lines
delineates PDRs, ionized, and molecular gas, and locates excitation
gradients, thereby locating the sources of excitation. The Sickle is also
heated by starlight, but its morphology is distorted by the strong magnetic
fields of non-thermal filaments. The ionized surfaces of molecular clouds
may be the electron source for these non-thermal filaments. One can expect
shock excited lines (e.g. mid- to high-J CO and HCN) there.
SAFIRE's field of view is large enough to take a snapshot of the
circumnuclear ring. Its spectral coverage will allow a variety of
investigations. One possible project is to determine the mass and physical
size of the circumnuclear ring. One approach is to use the Ci line ratios, which trace gas
column density and temperature, but are insensitive to gas density. It is
estimated that this gas only remains in the ring for ~104 years,
so where is the gas reservoir that replenishes the ring? SAFIRE can observe
the Cii 158μm and Oi 145μm lines, which trace gas
excitation and kinematics at high spatial resolution over broad regions.
What is the excitation mechanism for the bright molecular line emission
from the ring: radiation from stellar cluster, or shocks? Mid- and
high-J CO line ratios trace excitation of
molecular gas; shock excited material will be broadly distributed, whereas
UV excitation should be seen only near the exciting stars. Another
diagnostic is to observe the OH 163μm line, which is radiatively
pumped; when compared with CO, this measurement can decouple shock from UV
excitation.
KAO/KWIC 38μm image of the Galactic center, with inset enlargement
of the circumnuclear ring.
3. Local & Normal Galaxies
As a diagnostic of local galaxies, SAFIRE's great strength is in its large
instantaneous spatial coverage. A nearby galaxy such as M83 is of order 7'
in diameter, but can be imaged in only a few SAFIRE fields-of-view. A map
of M83 made in the C ii
158μm lines using the FIFI instrument aboard the KAO is shown below.
Using SAFIRE on SOFIA, the angular resolution is about 3 times better,
sufficient to resolve the spiral structure at very high contrast ratios.
Furthermore, the sensitivity of SAFIRE will allow the KAO map to be
duplicated at the SOFIA resolution in about one hour of observing time.
Bright sources such as Cen A can be observed at a signal-to-noise of ~10
with a single pointing lasting only seconds.
Observations of Cii
158μm emission made by the FIFI instrument on the KAO (contours)
overlaid on a visible image. SAFIRE's array footprint is shown, oriented
along the major axis of the galaxy. The angular resolution of SAFIRE will
permit the separation of nuclear, arm, and interarm components of emission.
An image does not represent sufficiently the wealth of information acquired
by an imaging spectrometer. Consider instead the nearby starburst galaxy
M82. Four tunings of the SAFIRE Fabry-Perot, spaced by 150km/s, results in
the image stack shown at right in the figure below. The central source of ~1' in
diameter can be seen to move across the field, showing the dynamics of the
galaxy. If we choose one of the brightest regions and slew the Fabry-Perot
over a broad range, we can build up a complete spectrum of that position
(and all others in the field of view). A simulation of this is shown at
left in the figure below.
Simulations of SAFIRE 3D observations of M82. (Left) cut through the
data cube at one spatially averaged region, showing the spectrum of that
area. (Right) Stack of images separated by 150km/s (approximate resolution
of SAFIRE) showing the motion of the galaxy. SAFIRE can detect M82
extremely quickly and much fainter sources with reasonable integrations.
4. Diagnostics of ULIRGs
In the far-infrared, Ultraluminous Infrared Galaxies (ULIRGs) often have
prominent fine-structure lines. These lines can trace the radiation fields
in the cores of ULIRGs, measure the gas properties, and probe abundances.
Typically, these lines dominate the gas cooling. However, in the
prototypical ULIRG Arp220, which has very strong molecular emission, the
158μm line of ionized carbon is an order of magnitude weaker than
expected. By contrast, Arp299, which is also a ULIRG, has a bright Cii line. In fact, less than half of
the ULIRGs have severe Cii
deficits, despite being similar in luminosity. It can be seen that the
far-infrared spectrum of such galaxies can differ substantially (below).
ISO's sensitivity did not permit it to measure a large sample of ULIRGs; it
was also not able to detect line emission from sources at even quite small
redshifts. SIRTF does not have spectroscopic capability in this wavelength
region. SAFIRE will be the most sensitive instrument for measuring the
wealth of far-infrared fine-structure lines until Herschel becomes
available.
ULIRG spectra from ISO.
5. High Redshift Sources
The far-infrared continuum is a good diagnostic of the total luminosity of
dusty galaxies, and has been measured out to high redshifts by ground-based
instruments. SOFIA's facility camera, HAWC, will achieve very high point
source sensitivity across the far-infrared, and will detect large numbers
of galaxies at redshifts out to z~3. Since strong C style='font-variant:
small-caps'>ii emission is often associated with star formation,
SAFIRE can conduct a companion survey of C ii in the distant universe. SAFIRE's
wavelength range covers z=0 to z=3.4, which encompasses the great change in
star formation rate per unit in co-moving volume. Figure 15 shows the
measured star formation rate density compared with several models,
highlighting SAFIRE's ability to measure this parameter. This will help to
provide an answer to two important questions: What powers high redshift,
dusty galaxies? How strong are starbursts in the early universe?
Star formation rate density
measurements and models, from Blain et al. (1999)
In the case of surveying nearby galaxies, there is much to learn about the
Cii emission: there is a
tendency for higher luminosity sources to have a smaller Cii line to far-infrared continuum
ratio. This change in line ratio is a physical diagnostic Ð to first order,
the ratio reflects the strength of the ambient interstellar radiation
field, G0. SAFIRE can trace this line in ULIRGs out to z>0.3
with integration times of about one hour. This is shown schematically
below at left. There is also a trend of changing C ii luminosity as a function of the
far-infrared color (and hence temperature) of galaxies; SAFIRE can measure
this variation in normal galaxies out to z~0.2 and in ULIRGs out to z~3
(below right).
6. Complementarity to other facilities
Since SAFIRE will achieve first light on SOFIA after 2010, it follows
Spitzer and will coexist with Herschel. It is therefore useful to consider
how SAFIRE adds scientific capability for the astronomical community.
Spitzer has supreme sensitivity, but is limited by a relatively small 85cm
aperture and a cutoff wavelength (in broadband imaging alone) at
170μm. SAFIRE will have the benefit of SOFIA's 2.5m aperture, but will
operate at longer wavelengths, so the diffraction-limited resolution will
be comparable. SAFIRE has both imaging and spectroscopic capability, so
serves as a strong diagnostic tool for studying sources discovered by
Spitzer. Furthermore, SAFIRE's spatial and spectral resolution are comparable
to that of Spitzer/IRS, so that SAFIRE can study highly redshifted sources
and compare that data with Spitzer's observations of nearby sources.
Herschel is a warm telescope like SOFIA, but is larger (3.5m) and does not
have any atmospheric absorption lines (which, though modest at SOFIA
altitudes, is not negligible). SAFIRE provides a useful capability when
compared to Herschel's instrument complement. Its spectral resolution is
finer that SPIRE's, while the two instruments cover similar wavelength
ranges. SAFIRE has a spectral coverage and areal coverage greater that
HIFI, permitting more flexibility and more rapid mapping of extended bright
regions. SAFIRE's capabilities are similar to PACS, but SAFIRE's coverage
extends to longer wavelengths; this will allow SAFIRE to extend PACS
studies of galaxies out to higher redshifts.
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