FIRST: Star Formation, Galaxy Formation, and More!

What is FIRST?

The Far Infrared and Submillimeter Telescope (FIRST) will be the only space facility to cover the far-infrared and submillimeter wavelength range. With a 3.5 m diameter telescope and three sensitive instruments covering the wavelength range from 80 to 670 microns, this cornerstone ESA mission will provide full and sensitive access to a poorly-explored range of the electromagnetic spectrum.

In comparison to other recent and planned facilities in this wavelength range, FIRST will have a larger and cooler telescope than SOFIA, which will cover a similar wavelength range. SOFIA, which is mounted aboard an aircraft, will also suffer from residual atmospheric absorption and emission and will not be available for observing 24 hours per day. ISO and SIRTF only cover the short end of this wavelength range (up to 200 microns); compared to those telescopes, FIRST has a larger (but warmer) telescope. SIRTF is a cryogenically cooled 0.85 m telescope that is due for launch in December 2001.

FIRST will be a true ``observatory'' in the same sense as NASA's ``Great Observatories'', which are the Hubble Space Telescope in the optical and near-infrared (and eventually NGST), AXAF in the X-ray band, the Gamma Ray Observatory, and SIRTF in the infrared. Like these other space observatories, FIRST is expected to open a major new window on the universe and provide results which are exciting for scientists as well as the general public. Effective access to FIRST will be crucial for Canadian scientists working on topics that need to be studied in the far-infrared and submillimeter wavelength region.

There will be three instruments on FIRST. HIFI is a heterodyne spectrometer capable of spectral resolutions of 10³ - 10⁷ and covering wavelength ranges from 240-625 microns, 157-213 microns, and 111-125 microns. SPIRE is a bolometer array that offers simultaneous imaging in broad continuum bands centered at 250, 350, and 500 microns; it also has an FTS mode for low-resolution spectroscopy. PACS provides continuum imaging and medium-resolution spectroscopy in the 80-210 microns band. More details on the individual instruments are given in the last section.

Science Goals of FIRST:

In the broadest terms, FIRST will be used to study the formation and death of stars and the evolution of the interstellar medium, both in the Milky Way and in other galaxies in a wide range of redshifts. The three major science objectives of FIRST are (1) the formation of galaxies in the early universe and their evolution; (2) the formation of stars and the physics of the interstellar medium; and (3) the interaction between star formation and galaxy formation. A schematic view of the complex interaction between star formation and evolution and the interstellar medium is given in the figure below. Changes in the abundances of different molecules during core formation and collapse, which can be measured with high-resolution spectroscopy, are expected to provide critical information for understanding the star formation process.

Because of the versatile nature of the instruments on FIRST, there are a wide-range of astrophysical problems that will be studied with this observatory. These topics include: the chemical composition of comets; the temperature structure of the interstellar medium; the physical conditions in circumstellar disks; dust disks around main sequence stars; mass loss in evolved stars; the interstellar medium in nearby galaxies; physical conditions in active galactic nuclei; and the star formation history of the early universe. To illustrate the capabilities of FIRST, we will focus on two topics: early universe studies and protostars in molecular clouds.

FIRST and the Early Universe: One project that will certainly be undertaken with SPIRE is large-area deep surveys for high-redshift galaxies in the 200-500 micron range. Follow-up work will include low-resolution spectroscopy with the FTS on SPIRE to produce crude redshifts, as well as higher resolution spectroscopy with HIFI. The scientific goals of such a survey include characterizing the star formation history of the universe from z=1-5, searching for dusty galaxies at high-z that may be missed by optical and near-infrared searches, and studying the large-scale structure of the high-redshift universe.
SPIRE is significantly more sensitive than the JCMT at 450-500 microns, and thus should be able to detect at 450 microns many of the high-redshift galaxies found at 850 microns by recent surveys. The ability to obtain simultaneous, sensitive images in the three wavelength bands should provide crude estimate of the redshift of the sources using the 250/500 micron ratio. HIFI is sensitive enough to detect the [CII] line at 158 microns at redshifts from 1 to 3. The [CII] line is the most luminous cooling line in star-forming galaxies and provides a sensitive tracer of the global star formation rate that can be compared with the results from the far-infrared continuum measurements.
Protostars in molecular clouds: Wide-field surveys of nearby molecular clouds with SPIRE will provide an important tool to investigate the first stages of protostellar collapse. This is particularly true for the earliest stages of collapse, since these extremely young and cold objects are often invisible at wavelengths shorter than 100 microns i.e. the range investigated by IRAS. These surveys will allow us to search for dense cold clumps as small as 0.03 Msun and will allow us to obtain reliable estimates of the lifetimes of the isothermal and the dense embedded phase of protostellar collapse. It is these earliest phases in the formation of a star which provide the strongest constraint on theoretical models of star formation.
The star formation process is accompanied by an evolution in chemical complexity. Water plays a crucial role in both the physics and the chemistry of star forming regions. Water is one of the major reservoirs of oxygen, as well as one of the main coolants of star forming clouds. The relative intensities of different far-infrared water lines provide powerful diagnostics of the density, temperature, and thermal radiation from dust in star forming regions. The ground-state line of water at 540 microns has been detected for the first time in a number of star-forming regions with the SWAS satellite. The HIFI instrument on FIRST will have a much higher sensitivity than either SWAS or ODIN, as well as access to many more transitions of water than either of the two smaller missions.

Details of the Instruments:

HIFI provides very high resolution spectroscopy (R = 10³ - 10⁷), over the wavelength range from 240-625 microns (480-1250 GHz), 157-213 microns, and 111-125 microns. This heterodyne instrument uses pairs of double sideband SIS mixers in dual polarization for the long wavelength band, and a single hot electron bolometer for each of the short wavelength bands. It has both wide-band (1 MHz) and narrow-band (0.14 or 0.25 MHz) spectrometers which can be operated simultaneously, with bandwidths up to 4 GHz. The instrument is diffraction limited, with a beam of 6-36". The system temperatures are estimated to vary from 70-500 K in the long-wavelength band (with better system temperatures at the long wavelength end of the band) to 650-800 K in the short-wavelength bands. For comparison, the JCMT has a system temperature of 300 K at 650 microns and a spatial resolution of 10". Thus, at this wavelength, HIFI will be four times more sensitive than the JCMT, but will have 3.5 times worse spatial resolution. However, observations at 650 microns are possible for less than 25% of the time at the JCMT, and observations at 540 microns are impossible from the ground, due to the strong water vapor line in the Earth's atmosphere. The ODIN satellite will access this ground-state water line, but with a system temperature of 2000 K and a telescope diameter of 1.1 m, ODIN will be much less sensitive and have poorer resolution than HIFI.
SPIRE has a three-band imaging photometer which is optimised for deep surveys. Three detector arrays observe simultaneously in three bands centered on 250, 350, and 500 microns. The field of view is 4' with an angular resolution of 18" at 250 microns and 36" at 500 microns. The one sigma rms noise in one hour of integration will be 1.5-2 mJy in each of the three bands in a fully-sampled 4'x4' field. In comparison, the rms noise achieved by SCUBA on the JCMT at 450 microns in one hour is 35 in a 2.5'x2.5' field of view. In addition, observations at this wavelength are possible less than 25% of the time at the JCMT. However, the spatial resolution of SPIRE will be approximately four times worse than that of the JCMT. SPIRE also contains a Fourier transform spectrometer (FTS) operating between 200 and 670 microns, with a spectral resolution of 20-1000 at 250 microns. It has a field of view of 2' and is optimized for spectral surveys.
PACS is a photoconductor array that provides imaging photometry and medium resolution spectroscopy in the 80 to 210 micron range. It can perform simultaneous dual-band imaging in the 90 and 180 micron bands with fields of view of 55"x85" and 109"x170". In its spectrometer mode, it can perform simultaneous spectroscopy over 5x5 pixels (9.4") in the 80-110 and 110-210 micron bands with a velocity resolution of 100-250 km/s and an instantaneous bandwidth of 1300-3000 km/s. Its 5 sigma point source detection limit in one hour integration is 5 mJy or 2 x 10^-18 W/m².

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