Dissecting the cosmic infrared background with 3D instruments

Abstract

Context. The cosmic infrared background (CIB) consists of emission from distant, dusty, star-forming galaxies. Energetically, the CIB is very important as it contains as much energy as the extragalactic optical background. The nature and evolutionary status of the objects making up the background are, however, unclear. Aims. The CIB peaks at ∼150 µm, and as such is most effectively studied from space. The limited apertures of space-borne telescopes set the angular resolution that can be attained, and so even Herschel, with its 3.5 m diameter, will be confusion-limited at this wavelength at ∼5 mJy. The bulk of the galaxies contributing to the CIB are fainter than this, so it is difficult to study them without interferometry. Here we present the results of a preliminary study of an alternative way of probing fainter than the continuum confusion limit using far-IR imaging spectroscopy. An instrument capable of such observations is being planned for SPICA -a proposed Japanese mission with an aperture equivalent to that of Herschel and more than 2 orders of magnitude more sensitive. Methods. In this paper we investigate the potential of imaging spectrometers to break the continuum confusion limit. We have simulated the capabilities of a spectrometer with modest field of view (2 × 2 ), moderate spectral resolution (R ∼ 1-2000) and high sensitivity. Results. We find that such an instrument is capable of not only detecting line emission from sources with continuum fluxes substantially below the confusion limit, but also of determining their redshifts and, where multiple lines are detected, some emission line diagnostics. Conclusions. 3-D imaging spectrometers on cooled far-IR space telescopes will be powerful new tools for extragalactic far-IR astronomy.

The space infrared telescope for cosmology and astrophysics: SPICA A joint mission between JAXA and ESA

“…Redshifts accurate enough for immediate follow-up ground-based CO observations (tracing the molecular gas component) can be determined from the detected lines, while line ratios provide a first indication of the nature of the source powering the FIR emission. In simulations of deep, blind spectral line surveys (rms noise level of 0.4 mJy 100 h SOAP/10 h GOAL at R∼1,000) with SPICA FIR [26] it was possible to detect and determine redshifts of sources brighter than 1 mJy at 120 μm out to z∼2.5 using the five strongest FIR cooling lines (including [OI] at 63 μm), thus probing to a factor of 5 below the 120 μm continuum confusion limit. Sources at even higher redshift can be recovered by including MIR emission lines.…”

The Effectiveness of Mid/Far IR Blind, Wide Area, Spectral Surveys in Breaking the Confusion Limit

“…If the fluxes of the two sources are comparable then their spectra lines become effectively mixed, making it difficult to determine which emission lines come from which objects. The work described in Clements et al (2007) did not address this problem. In this work we try to extract redshifts from both spectrally, and spatially confused sources.…”

“…A preliminary study to explore the efficacy of blind, wide area, spectroscopic surveys in identifying FIR sources is described in Clements et al (2007). In this work an artificial 'sky' was populated with spectra based on template FIR/mid-infrared (MIR) spectral energy distributions (SEDs) with FIR emission line strengths taken from ISO-LWS observations.…”

SPICA’S Challenge: Probing High-Redshift Universe toward the Epoch of Reionization

“…To go beyond Herschel and achieve the ultimate backgroundlimited sensitivities offered by SPICA's cold telescope, it is necessary to exploit spectroscopic information (e.g., detection of strong PAH features and fine-structure/molecular hydrogen lines), which will enable us to detect sources deeper into the confusion (e.g., Clements et al, 2007).…”