Article - GCNEWS, Vol. 7, March 1998


A Newsletter for Galactic Center Research
This Volume was edited by Angela Cotera & Heino Falcke

Volume 7, March 1998 - ARTICLE

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Surveys of molecular gas in the Galactic center region: On the usefulness of tedious projects with moderate-sized telescopes

Susanne Hüttemeister
Radioastronomisches Institut, Bonn University

A different type of molecular ISM

The molecular interstellar medium (ISM) in the central region of our Galaxy is distinctly different from what is found in the Galactic disk. There, molecular clouds are usually classified as Giant Molecular Clouds (GMCs, H2-density ~102cm-3, temperature ~20K), often with central denser and warmer `hot cores' associated with regions of massive star formation, or quiescent and cold `dark clouds' (density >= 104cm-3, temperature ~10K). The molecular ISM in the inner ~500pc of the Galaxy, called the Central Molecular Zone (CMZ) by e.g. Morris & Serabyn (1996), is on average , denser, warmer and more turbulent than a typical disk GMC. This is evident from linewidths that are about a factor of 10 wider than those of disk clouds, highly excited molecular states and rare molecular species that are readily detected in the GC clouds. Even though the mass estimate recently had to be corrected downward, the gas in the CMZ is still the largest and densest concentration of molecular material in the Galaxy.

Star formation activity in the Galactic center (GC) region is presently not impressive, at least outside a few special locations, first and foremost SgrB2. Still, the ISM in this region may be a nearby `laboratory for starburst galaxies' just as the rather quiescent SgrA^* has been called a laboratory for AGN (Mezger et al. 1997), sharing at least some characteristics with its far more active extragalactic cousins. Thus, understanding the unique properties of the ISM in the GC region is not only important in its own right but also has the potential to influence and challenge our conclusions about objects that are too far away to study at high spatial resolution and/or to observe in lines from rarer molecular species.

Tedious but necessary: Surveys

Surveys of the GC region, even at the very moderate resolution of a few arcminutes, take a lot of time: comparatively small telescopes are used for months or even years of routine observations. The resulting data are, however, the base on which our understanding of the behaviour of the ~3 * 107Mo of material in the CMZ and, if we take the extragalactic analogy seriously, the gas in an increasing number of galactic bulges, is relying.

Since the main constituent of molecular clouds, H2, lacks a permanent dipole moment, molecular gas that is not hot enough to excite vibrational (infrared) H2 transitions -- only very exceptional clouds manage this -- has to be detected in `tracer molecules'. The most abundant of those, and thus the workhorse of molecular spectroscopists, is the main isotope of carbon monoxide, 12CO, with a relative abundance to H2 of ~10-4. Most maps of molecular gas distributions in the Milky Way and, even more pronounced, in external galaxies, are taken in the rotational J = 1 -> 0 transition of this molecule at 115GHz (2.6mm). CO also has the advantage of being easily excited even at low gas densities of only ~102cm-3. Thus, it traces almost all molecular gas.

Why isn't it, then, sufficient to survey the CMZ -- and external galaxies -- once in 12CO and be done with it? Inspection of the images obtained by surveys in a number of molecules and transitions readily provides the answer: The gas distribution looks very different, depending on what species and what excitation state is used. 12CO 1 -> 0 maps (see Bitran et al. and Jackson et al. 1996 for recent examples) tell us the morphology of molecular gas of all kinds -- and little else. It is striking how little contrast is seen in 12CO: These images let the observer question the concept of distinct molecular clouds in the GC. And this is, indeed, an important insight: 12CO picks up (among other things) a diffuse `intercloud medium', that, in the GC, is molecular rather than atomic. To get to the physical meat of the matter, the density and temperature distribution of the gas, multilevel studies, isotope studies and surveys in rarer, weaker molecular species are required. Since a field of at the very least 5o * 2o has to be covered, this makes for fairly tedious work, and it sometimes seems that more data have been accumulated for the nuclear regions of some external galaxies than for our GC.

The target: physical conditions and dynamics

Both multilevel and isotopomer studies are easiest in CO, and have, therefore, so far only been done in CO lines, at least for the entire CMZ: Oka et al. (1996, 1998) have observed the 12CO 2 -> 1 transition with a resolution exactly matching the 1 -> 0 line mapped in the famous Columbia/Tololo `Mini' Surveys of the molecular Milky Way, of which the Bitran et al. data (published independently a little belatedly) are a part. The Northern and Southern `Mini' telescopes have a diameter of 1.2m. Consequently, the telescopes built for the 2 -> 1 survey (again, there is a Northern and a Southern one) have a diameter of 60cm -- thus countering the notion that new telescopes have, necessarily, to be larger than their predecessors. In this case, small is quite beautiful! 13CO 1 -> 0 data have been provided by Bally et al. (1987), who also mapped CS (at the 7m Bell Labs telescope). Dahmen et al. (1997) provided a survey of the rare isotopomer C18O, again carried out with the Southern `Mini'.

As an example for the usefulness of such multiline surveys, let us consider the case of the `conversion factor': An analysis of line ratios clearly demonstrates the traps one can fall into if one considers only one line. A `standard' factor to convert integrated 12CO 1 -> 0 intensity to H2 column density has been established (by Strong et al. 1988 and in intense discussions in the subsequent years, for clouds in the disk of the Galaxy). This infamous quantity has been very widely used to determine gas masses in external galaxies, often in the central regions -- in many cases, out of desperation since measurements of lines other than 12CO 1 -> 0 are exceedingly difficult. In an analysis of line ratios, we (Dahmen et al. 1998) could show that not only has the `average' conversion factor to be lowered for the GC region by a factor of almost 10 -- the situation is even worse: There is no simple causal connection between 12CO 1 -> 0 line intensity and H2 column density. It is the otherwise convenient fact that the 12CO 1 -> 0 transition is so easily excited even at low gas densities that wreaks havoc here: A very significant amount of the total brightness of the emission arises from a thin, warm, unbound gas component that contributes little to the mass and does not exist in Galactic disk clouds. The presence of this component is tied to the special conditions in the GC region: large tidal forces and tri-axial (i.e. bar) potentials lead to shocks and cloud-cloud collisions that disrupt the material and leave part of it in a `diffuse', unbound state. We have every reason to believe that these processes also work in the centers of external galaxies -- only very desperate people should turn to `standard conversion' to determine gas masses in these regions anymore. It was only the combination of large scale surveys of the GC region, where the clouds can be resolved on pc scales, that made clear not only that `standard' CO-H2 conversion breaks down under Galactic center conditions, but also why this is the case.

Of course, molecular surveys are also used to trace the dynamics of the gas in the potential of the GC region, which is defined by the stellar component. Over the last years, a bar potential has become the by far most popular model to explain the unique kinematics (e.g. the large non-circular motions) observed. Again, surveys in different molecular species and transitions help to clarify matters, by pointing out regions of high and low density, helping to locate shocks and regions of peculiar chemistry.

What should be next?

Higher resolution? Very high resolution surveys of the entire CMZ on the scale of a few arcseconds resolution are still out of reach at this time, though the advent of array receivers for millimeter spectroscopy might change that situation in the future (see the 12CO/HCN survey of Jackson et al. (1996) obtained with the 15-element QUARRY receiver for a first taste of the capabilities of arrays in the future). The gas in the CMZ only reveals its true complexity at scales of 1-2pc or smaller (angular resolution of <= 40''). On these scales, multilevel studies of many molecules uncover exotic cool cores with hot core-like chemistry, embedded in a warm, thin molecular medium and subject to shocks, a type of ISM seen nowhere else in the Galaxy, intimately tied to the large scale dynamics of the the CMZ (Hüttemeister et al. 1998). What we would see if we could survey the entire CMZ on this scale is unknown -- and will remain so for at least some years to come.

More molecules? Molecules other than CO (CS, HCN, and -- on a slightly smaller scale -- SiO (Martin-Pintado et al. 1997) and HNCO (Dahmen et al. 1997) have already been surveyed) need higher gas densities to become excited and thus trace different components of the ISM. Some species, like SiO and HNCO, that are very rare or are seen only under very special conditions in confined region in the Galactic disk are widespread and abundant in the CMZ. It is, of course, tempting to imagine having at least medium resolution data of them all: the CO isotopomers for general gas properties, CS and HCN as `all purpose' tracers of gas at higher densities (>= 104cm-3), HCN's isomer HNC as a tracer of cool and quiescent gas, CH3OH as an indicator of hot, dense material, SiO, SO and SO2 for shocks, the radical CN to locate region of enhanced UV field, i.e. photodissociation regions, the molecular ion HCO+ to point out regions with high cosmic ray flux ... the list could go on for quite a while. And of course one can only really analyze the data if one has more than one transition for each molecule.

In the absence of a dedicated survey telescope in the ~10m class with an array receiver, this will clearly remain a dream for many years to come. However, one should keep in mind that for some external galaxies, e.g. the prototypical starburst NGC253, we are approaching a situation where most of the molecules above mentioned have been observed, many interferometrically at high resolution. To have a `baseline' for understanding these observations, further efforts in mapping the CMZ are certainly needed, even if the projects are time-consuming and, at least at first look, `routine'. As time passes, more data will become available -- till then, it will help the efforts of everyone working in this field if survey data are published timely and readily made available in digital form to the community (at least on request). A single one-line survey will provide only very limited new information -- pointing out for yet another time that the gas distribution is asymmetric, dynamically consistent with a bar and that the gas is, on average, rather dense and warm, is not very original anymore. The true value of new surveys will become apparent in an analysis in the context of what already exists.


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