2.4. Line emission

Emission from molecular rotation and atomic fine-structure transition lines can be used to diagnose physical conditions within molecular clouds and photodissociation regions, and to trace out the velocity structure within. Some lines, such as those from CS, HCN and HCO⁺ are excited only in high-density gas, while others, including the most abundant polar species CO, trace more typical regions in the ISM.

Studies of many emission lines from molecular cloud regions in nearby galaxies are possible using existing mm and submm-wave telescopes (Wilson et al., 2000; Helfer, 2000). However, for more distant galaxies only CO lines have so far been detected in significant numbers, almost exclusively from galaxies which have been subject to strong gravitational lensing by foreground galaxies (see the summary in Combes et al., 1999). These observations are useful for deriving physical conditions within the sources, especially if multiple lines are detected (as in the case of APM 08279+5255; Downes et al. 1999b). The improved capabilities of the forthcoming mm/submm interferometer arrays - SMA, upgrades to the IRAM Plateau du Bure interferometer (PdBI), and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) ⁷ - and and ultimately the dramatically increased sensitivity of ALMA, will make high-redshift lines much easier to observe over the next decade (Combes et al., 1999; Blain et al., 2000b).

One of the most important uses of CO-line observations of distant submm galaxies found in continuum surveys is their ability to confirm an identification absolutely, by tying together an optical and submm redshift at the position of the galaxy. So far this has been achieved for only three submm galaxies (Frayer et al., 1998, 1999; Kneib et al., 2002: see Figs. 14, 18 and 19). In principle, these observations could be made for all continuum-selected galaxies. The difficulty is the narrow fractional bandwidth available for receivers and correlators. Even at the relatively low frequency of 90 GHz, the redshift of the target must be known to better than 0.5% to ensure that a 300 km s^-1 wide CO line, typical of a massive galaxy, with a width equivalent to 0.1% in redshift falls entirely within a 1-GHz band. Future cm-, mm-, and submm-wave instruments with wider bandwidths will significantly assist the the search for redshifts using molecular lines. Specially designed low-resolution, ultra-wideband dispersive spectrometers covering many tens of GHz simultaneously on single-antenna mm-wave telescopes also promise to provide redshifts for submm galaxies (Glenn, 2001).

A complementary search for redshifted cm-wave OH megamaser emission to pinpoint the redshifts and positions of ultraluminous high-redshift galaxies could be possible using radio telescopes (Townsend et al., 2001). However, there are very stringent requirements on the acceptable level of radio frequency interference from terrestrial and satellite communications. Observations of low-redshift megamasers are described by Darling and Giovanelli (2001). Megamaser emission at high redshifts is discussed by Briggs (1999) in the context of the proposed Square Kilometer Array (SKA) meter/centimeter-wave radio telescope. If it can operate at frequencies of several 10's of GHz, then the SKA is also likely to be an efficient detector of low-excitation high-redshift CO lines (Carilli and Blain, 2002).

2.4.1. Line emission contribution to continuum detections

An interesting feature of the CO line emission from low-redshift galaxies is that lines can lie in the passbands of continuum instruments, and could contribute to the continuum flux inferred. For low-redshift galaxies, the 345-GHz CO(3 2) line lies within the 850-µm atmospheric window, while the 691-GHz CO(6 5) and 230-GHz CO(2 1) lines lie in the 450-µm and 1.25-mm windows, respectively.

Assuming a reasonable template spectrum (Blain et al., 2000b), the equivalent width in frequency of the CO(3 2) line is 7.4 GHz. The passband of the current SCUBA 850-µm (353-GHz) filter is about 120 µm (50 GHz) wide, and so about 15% of the measured continuum flux density of a low-redshift galaxy in the 850-µm channel is likely to be from the CO line. The high-frequency SCUBA passband in the 450-µm atmospheric window is 75 GHz wide, while the equivalent width of the CO(6 5) transition is 3.3 GHz. Hence, a smaller 5% contribution to the continuum flux density from the line is expected at 450 µm. The CO(2 1) line has an expected equivalent width of 9.2 GHz, while the wide MAMBO passband has half-power points at 210 and 290 GHz. Contamination of the flux densities detected by MAMBO by about 10% may thus be expected.

The largest of these correction factors is comparable to the calibration uncertainty in submm-wave observations, and could be relevant to the detailed interpretation of low-redshift observations. For example, the presence of the CO(3 2) line in the 850-µm window would shift the inferred continuum emissivity spectral index in the SLUGS survey from 1.3 to 1.52. At high redshifts, any corrections are likely to be less significant, both because the relatively bright CO(3 2) line redshifts out of the 850-µm passband, and the equivalent width of lines in frequency space decreases as (1 + z)^-1.

Although the contribution to measured submm-wave flux densities from line emission could be significant at the level of order 10%, only a small fraction of the bolometric luminosity from galaxies is detected in the submm waveband. More than 99% of the bolometric luminosity still appears in the continuum, predominantly at shorter far-IR wavelengths.

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