Molecules, Dust and Free-Free Emission

With the discovery of dust and CO emission at , it is clear that a part of the electromagnetic spectrum and the associated physical processes is likely to be visible at the VLA's operating wavelengths. The enhanced VLA would provide almost complete wavelength coverage between 4.5 and 50 GHz, as well as much improved system temperatures and bandwidth. This upgrade combined with the existing large collecting area is just what is needed to study thermal continuum and dust emission at high z, as well as molecular lines as they are shifted into the VLA window.

In the cm-wavelength continuum, the VLA is now primarily sensitive to synchrotron emission for the dusty, star-forming galaxies. In the galaxy rest frame somewhere shortward of a few mm, dust emission dominates the spectrum. In between the long wavelength synchrotron emission and the short wavelength dust spectrum, free-free emission should dominate, possibly between 40 and 100 GHz. This free-free emission as well as the tail of the dust emission should be shifted into the VLA window as z increases.

IRAS revealed a large number of exceptionally luminous galaxies, and made the starburst phenomenon a commonplace of astrophysical discussion. The most direct and reliable estimate of the star-formation rate in these systems is the high-frequency radio emission from the ionized gas, since that is unaffected by the strong dust absorption expected in these systems. Combining this with lower-frequency measurements gives the ratio of synchrotron to thermal emission, itself an indicator of the high-mass end of the initial mass function; comparing this with more quiescent galaxies tells us whether starbursts preferentially form high-mass stars, as might be expected if the star-forming gas is exceptionally turbulent. Resolving the thermal radio emission also gives the physical scale of the starburst. Clearly one wants sensitive, multi-frequency observations both to detect the thermal emission, and to determine where the thermal/synchrotron break occurs in these sources. A reasonable starburst would have an intrinsic thermal luminosity of (100 times stronger than a typical, normal galaxy); this corresponds to 10 Jy at 1 Mpc, or a few microJy at z = 1. 40 GHz is redshifted at z = 1 to 20 GHz, so this is a strong argument for the highest possible bandwidths at and above 15 GHz: the experiment would still take about 12 hrs, but the possible payoff is tremendous.

At , dust emission at 200 GHz is shifted to the enhanced VLA bands. These high-redshift galaxies have been detected in CO with incredibly high estimates of the gas mass ( solar masses ); the dust reservoir accompanying the gas (itself with a mass of solar masses ) is observable by its thermal emission in many high-redshift galaxies and QSOs. The MMA will be the best instrument for detecting the dust at higher frequencies, but the VLA's 50 GHz limits or detections would be valuable both in looking at the coldest dust, which may not emit at all at higher frequencies, and in determining the spectral shape at relatively long wavelengths. The former is important to estimating the total (rather than just the warm) dust mass, while the latter constrains the microscopic properties of the dust. Further, the dust-ionized gas break in the spectrum could only be observed with the enhanced VLA. This would be a tough but do-able experiment: solar masses of dust, corresponding (with local gas/dust ratios) to solar masses of hydrogen, would show up at about K) microJy at . This would take 12 hrs with the proposed upgrade, or 1.3 hrs with the super-wide 10 GHz bandwidth, for a detection. Again this argues strongly for the highest possible sensitivity at frequencies above 20 GHz, as well as high resolution to image the dust.

Detections of CO emission at high redshift have been reported for about ten objects. Of these, the strongest lines have been observed from the infrared luminous source IRAS FSC10214+4724 at z = 2.3, and the lensed quasar known as the Cloverleaf at z = 2.5. Other detections include radio galaxies and damped Lyman- systems. For all detections the inferred mass of molecular hydrogen is in the range - solar masses .

Although the nature of the sources is not clear and may well exist in great variety, it is clear that there is an abundance of molecular gas at high redshift. These gas-rich systems present us with examples of various stages of galaxy formation and evolution that can be imaged. Because the emission is in spectral lines, it carries all the usual spectroscopic information, most notably, information about the kinematics. Radio spectroscopy is not only uniquely capable of observing the molecular gas that dominates these systems, in doing so it provides a wealth of information.

The task of studying the high redshift molecular line sources falls most naturally to the Millimeter Array. From what little is known about the temperature in the sources, higher lying rotational lines are expected to be stronger, so that even at the observed redshifts these fall in the millimeter bands. However, the CO transition at 115 GHz has been detected in FSC10214 and should be observable with an enhanced VLA in many sources. It should also be mentioned that models of early universe chemistry indicate that molecular oxygen may be more abundant than CO. The enhanced VLA will be used to determine the mass in O from observations of the redshifted 60 GHz line and, for 1.4, of the redshifted 119 GHz line. Together with the CO observations, the results on O would allow direct tests of chemical evolution with cosmic epoch. The contribution of the enhanced VLA, beyond the detection capability (which the Green Bank Telescope will also provide), will be to image the lines so that their kinematics can be determined from both spatial and velocity structure.

The VLA Development Plan must address two basic requirements for these observations. First, low noise receivers are needed for continuous frequency coverage from 20-50 GHz. This provides complete coverage for redshifts z = 1.3 to 4.8 for the CO(J=10) transition at 115 GHz and for redshifts z = 0.16 to 4.9 for one of the O lines. Second, the correlator must have a minimum bandwidth of 500 MHz. The lines can be wide; 1000 km/s full width is not unreasonable. With an equal amount of baseline, that translates to over 300 MHz bandwidth at 50 GHz observing frequency. Uncertainties in the redshift and the fact that the gas need not have the same exact redshift argue for 500 MHz as the required bandwidth. For searches in redshift even more bandwidth is necessary.

As an example of the enhanced VLA capabilities for imaging these sources, consider the following: a CO(J=10) line strength of 3 mJy peak and a full width of 800 km/s spread over an area of diameter at z = 2.3. With 100 km/s channels in 12 hrs in the D configuration, the enhanced VLA would achieve 15:1 signal-to-noise per channel at resolution on the line peak. If the emission is more peaked spatially or spectrally, higher spatial and spectral resolution will probably also be useful.

As more high redshift objects are found, the enhanced VLA and the MMA together will allow us to construct a complete picture of the physical conditions in dusty, otherwise, obscured objects. This is an entirely new astrophysical arena which the enhanced VLA and the MMA will open up together.