The Case for Space-borne Far-Infrared Line Surveys

1. The Case for Space-borne Far-Infrared Line Surveys J. J. Bock1, C.M. Bradford2, L. Earle3, J. Glenn3, B. Naylor2, H. T. Nguyen1, J. Zmuidzinas2 1Jet Propulsion Laboratory, Pasadena, CA 91109 2California Institute of Technology, Pasadena, CA 91125 3University of Colorado, Boulder, CO 80309 Measured line strengths • Far-IR Spectroscopy Beyond SIRTF/HSO • Sensitive direct detectors and a cooled aperture promise orders of magnitude improvement in the sensitivity for far-IR/submm spectroscopy • Advances in direct detector technology will enable spectroscopy at the background limit available only from space at these wavelengths • . The confusion limit for spectroscopy is significantly lower than for photometry, reducing the need for high spatial resolution and large aperture telescopes • A broadband spectrometer (R ~ 1000) coupled to a modest (~3.5 m) cooled aperture provides exciting scientific opportunities • conduct a far-infrared line survey 106 times faster than planned capabilities even with existing detector technology • this improves to107 to 109 times faster with background-limited sensitivity • A space-borne spectrometer can measure redshifts and line fluxes for far-IR/submm sources • break the current spectroscopic bottleneck in the study of far-infrared galaxies • rapidly follow up SIRTF/HSO far-infrared galaxies • SCIENCE WITH SPECTRAL LINE SURVEYS • FAR-INFRARED GALAXIES • The far-infrared extragalactic background has an energy density equal to that of the optical/UV background. Studies of far-infrared galaxies remain in their infancy, with only dozens of objects detected to date. Even so, the current bottleneck for studying far-infrared galaxies is a systematic determination of their redshift. The lack of spectroscopic information will become more apparent as large planned photometric surveys (SIRTF, ASTRO-F, Herschel) detect tens of thousands of galaxies. SIRTF and Herschel will not have sufficient sensitivity to measure redshifts on a significant sample of far-infrared galaxies. ALMA will have high sensitivity but in limited bands confined to atmospheric windows. A space-borne spectrometer is required, combining complete spectral coverage and high sensitivity. • Systematically measure z - even modest objects can be detected at high redshift • Determine activity, metallicity, and physical properties • DETECTION OF REDSHIFTED H2, HD LINE EMISSION • The first objects to form in the early universe gravitationally collapsed from over-dense regions. Energy from these structures was predominantly radiated by molecular hydrogen, because metals were not present to provide more efficient cooling. The onset of the first star formation heats the surrounding gas, eventually dissociating H2 and ionizing the IGM by z=5. During the first phase of star formation and their supernovae, H2 line emission may be bright. These objects can be identified by the distinct 6.9 mm, 9.7 mm, 17 mm and 28 mm lines of molecular hydrogen. • Survey for redshifted line emission from the first objects Line sensitivity for a Wideband Far-Infrared Spectrometer (WaFIRS) on a 3.5 m telescope cooled to 5 K, compared to other planned facilities. Two optical efficiencies are plotted, both for a single polarization only. The background-limited sensitivity is calculated based on the DIRBE background light measurements (Kelsall et al., 1988 and Schelegel, Finkbeiner & Davis, 1998). This calculation is for detection of a line at a known frequency, which discounts the advantage of continuous wavelength coverage. TECHNOLOGY FOR SPECTROSCOPY MODEST (~3.5 m) COOLED APERTURE The confusion limit for spectroscopy is much lower than the more well-known confusion limit encountered in photometry. The spectral dimension increases the number of independent detection pixels by (c/s) ln(nmax/nmin) ~ 1000, where s is the intrinsic velocity linewidth. Surveying the same patch of sky with the same aperture diameter, a confusion-limited spectrometer can detect ~1000 lines where a confusion-limited photometer would detect one galaxy. WIDE INSTANTANEOUS BANDWIDTH A spectrometer with modest resolution of l/Dl = c/s ~ 500 is optimal for detection of broad (s ~ 500 km/s) lines from galaxies. Surveying the entire far-infrared and sub-millimeter simultaneously is the best strategy for detecting lines from objects at an unknown redshift. SENSITIVE DIRECT DETECTORS Unlike heterodyne systems which are ultimately limited by fundamental quantum-mechanical noise, direct detectors can achieve background-limited sensitivity under arbitrarily small photon levels. With a cooled telescope, the background-limited sensitivity is set by the astrophysical sky. A direct detector with NEP ~ 5e-20 W/Hz will be background-limited. ALMA ALMA WaFIRS WaFIRS Time required to conduct a survey for a line in the available bandwidth of each instrument, to a sensitivity of 1e-21 W/m2. This is the appropriate figure of merit for detecting lines at unknown redshift. WaFIRS has more than 7 orders of magnitude speed advantage over existing instruments from 40-300 microns The ULIRG (L = 1.1e12 Lsol) Arp 220 is detectable to z = 10, based on measured (solid) and estimated (dashed) line strengths. The continuum emission is reduced by 104 to emphasize the line intensities. We use a LCDM, WL = 0.7, Wm = 0.3 cosmology. The ALMA line sensitivity assumes the atmospheric windows are surveyed with an instantaneous bandwidth of 4 GHz. Arp 220 is notable for having extremely weak atomic lines, and represents a difficult object for line detection. The starburst galaxy M82 is detectable at z = 5, based on measured (solid) and estimated (dashed) line strengths. The continuum emission is reduced by 103 to emphasize the line intensities. We use a LCDM, WL = 0.7, Wm = 0.3 cosmology. The ALMA line sensitivity assumes the atmospheric windows are surveyed with an instantaneous bandwidth of 4 GHz. M82 is not an extremely luminous (L = 3.3e10 Lsolar) far-infrared galaxy compared to objects currently being detected by SCUBA at cosmological distances. WaFIRS Sensitivity Results from the ISO LWS reveal significant variability in the fine structure and molecular line strengths due to the sources’ individual properties. Deep spectroscopic studies of the galaxies found by SIRTF/HSO will measure redshifts and probe ISM conditions. (figure from J. Fischer, et al., astro-ph/9911310, 1999) [CII] line emission from any object detected photometrically by SIRTF or Herschel is readily detected by the space-borne spectrometer. We assume a line-to-continuum ratio L(line)/L(bol) = 1 x 10-3. Molecular hydrogen line emission driven by the first generation of star formation, calculated according to the model of Ciardi and Ferrara (2000). Line emission is detectable out to large redshift (z = 20) in moderate integration time. Waveguide-coupled diffraction grating spectrometer covers a wide spectral band lmax/lmin ~ 1.6 with modest spectral resolving power of 100 – 1000. This format allows a wide spectral band to be covered simultaneously for conducting spectral line surveys.