As we leave the Local Group, we return to the more traditional definition of a survey, that of studying many different objects rather than many pointings on a single object. Through surveys, we are rapidly gathering enough information on molecular gas in spiral galaxies to begin making general statements directly from statistical samples.
Much of our basic understanding of the global properties of gas distributions in spirals come from single dish surveys. Single dish telescopes are able to observe a significant fraction of a galaxy in a single beam, and do not suffer the uncertainties of missing large-scale flux inherent in interferometer studies. The groundwork for our understanding of the molecular gas content of spiral galaxies was laid by earlier surveys (e.g., [23], [24], [25], to name a few), which indicated that molecular gas can be detected over a range of Hubble types, and tends to be more enhanced in the centers of galaxies than in the outskirts. Further, we find that molecular gas and star formation tracers are generally closely associated, though interpretations of the form of this association vary (e.g., [26], [27], [28], [29]).
Recent single-dish surveys (e.g., [30], [31], [32], [33], [34], [35]) highlight the fact that virtually all single-dish millimeter telescopes are actively engaged in surveys of a wider range of molecular gas properties in spirals. In recent years, these and other studies have been used to improve our measures of molecular gas in very late type spirals (Scd-Sm), construct higher resolution central rotation curves than are available from HI, and show the promise of mapping variations in molecular gas excitation and density through observations of higher rotational transitions of CO.
Single-dish telescopes have also been used increasingly in a mapping mode such as On-The-Fly (OTF) mapping. Rather that measuring only the central surface density or the variation of surface density along the major or minor axis, these studies map the detailed variation of the emission from various parts of the galaxy. An excellent example is the fully-sampled mapping of 5 nearby spiral galaxies with the NRAO 12m (now the UASO 12m), including IC 342 and M83 ([36], [37], [38]). These single dish studies discern individual arm and interarm regions, and can trace the relationship of molecular and atomic gas in detail. These studies show smooth connections between the molecular and atomic gas components, which suggests that the gas phase may be determined by local variations in turbulent pressure or dynamical conditions (e.g., [39], [40], [41], [42], [43], [44]).
Large single-dish telescopes such as the IRAM 30m and the Nobeyama 45m can achieve moderate angular resolutions of ~ 10-20", corresponding to ~ 1-2 kpc at d = 20 Mpc. Therefore, achieving kiloparsec or even sub-kiloparsec resolution in a large number of nearby spirals requires the use of interferometers. The observations possible with current millimeter interferometers provide important information for planning future surveys with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and the Atacama Large Millimeter Array (ALMA).
Circumnuclear Molecular Gas
With significantly higher resolution, interferometric studies are well suited to study central molecular gas properties, and their relationship with central activity. The progress of one of the most recent such surveys, The NUclei of GAlaxies project (NUGA), has been reported at this meeting by S. Garcia-Burillo and L. Tacconi. A study with the NRO and OVRO interferometers of CO emission in 10 barred and 10 unbarred spirals [45], [46] reveals that molecular gas is more concentrated in barred than unbarred spiral galaxies, thus providing the first statistical evidence for bar-induced inflow in transporting gas to the centers of galaxies. However, this study found no correlation between central gas concentration and type of nuclear activity, which may indicate that the resolution of the survey (4") was not sufficient to assess separately the properties of the circumnuclear gas.
Recent dissertation projects by S. Jogee
[47] and A. Baker
[48],
as well as the OVRO MAIN survey (see description in
[49],
[50])
complement these studies by targeting
particular aspects of the interaction of molecular gas with other
activities in the centers of galaxies. Jogee compared central
molecular gas mass and central star formation activity, and showed
that information about gas kinematics as well as the size of the gas
reservoir was necessary to determine whether or not a central
starburst would form. The Baker study of molecular gas in AGN with
broad H
line emission,
as well as the OVRO MAIN multi-line
survey, are being used to study central kinematic and heating conditions,
and to test which processes are most influential in determining
central activity in galaxies.
The Nobeyama Millimeter Array (NMA) has been used recently to target spirals in the Virgo cluster, using the advantage of a common distance to minimize ambiguities when comparing structures between galaxies ([51], [52], [53]), thus being able to compare directly the inferred surface mass densities and effects of interactions on molecular gas distribution and motions.
Gas in Disks
The interferometric surveys described thus far have included CO
brightness as a criterion for selection into the study. Such studies
may be biased by an unknown factor toward galaxies with more
significant, or more highly concentrated, molecular gas masses. The
BIMA Survey of Nearby Galaxies, or SONG, is the first large
interferometric CO survey to observe galaxies which were not chosen on
the basis of central CO brightness
[54],
[55].
The 44 spirals in SONG were chosen by the following criteria: Hubble type
Sa-Sd, Vsys < 2000 km s-1, i <
70°, 
>
-20°, and BT < 11.0
(1).
Observations for SONG effectively mapped a ~ 10-kpc diameter
region around each galaxy center, enabling a more accurate assessment
of the properties of gas in the inner disk. CO J = 1-0 emission was
detected in 41 of the 44 chosen galaxies (within a radius of
120"), providing a broad sample of galaxies with which
to explore the properties of gas and star formation over a range of
radii (2).
The BIMA SONG data (see Figure 1) show that the molecular gas distributions in spiral galaxies are very heterogeneous, and even the azimuthally averaged radial profiles show a range of properties with respect to the radial variation of the stellar light. A comparison of the molecular gas distributions in barred (SB/SAB) and unbarred (SA) galaxies in the full SONG sample [56] confirms and strengthens the results of the NMA-OVRO survey: the average central gas surface density of barred spirals is three times higher than that of unbarred galaxies. It also appears that in many spirals the increase in molecular gas surface density occurs over the same range of radii in which the stellar light increases above that of an exponential profile [54], [57]. As this central "excess" occurs commonly in unbarred galaxies as well as barred galaxies, there may be some other mechanism than bar inflow required to explain central gas excesses in all spiral types. The complete coverage of molecular gas in the inner disk has enabled detailed studies of the relationship of molecular gas and star formation, targeting both the validity and formulation of a star formation "law" ([58]) as well as the influence of galactic bars and gas inflow on the placement of star formation sites ([59]). In addition, the wealth of kinematic information available has enabled a systematic study of low-level streaming motions in a sub sample of ~ 20 SONG galaxies [60].
 |
Figure 1. BIMA SONG data for NGC3521, NGC5055, NGC 5248, and NGC7331 (top to bottom). (left:) integrated intensity (ICO, with 1 kpc bar at lower left); (center:) velocity field (in km s-1); and (right:) azimuthally averaged radial profiles for ICO (triangles) and optical/NIR broadband light (crosses). For the purposes of comparison, ICO has been converted to magnitudes for the radial profile (see [54]) and the optical/NIR profiles have been shifted vertically. An exponential fit to the outer part of each optical/NIR profile has been overplotted as a dot-dash line. |
Acknowledgements:
I would like to thank my fellow members of the BIMA SONG consortium, particularly T. Wong, M. Regan, and K. Sheth, for interactions that continue to enhance the scientific value of SONG. I would also like to thank R. Magee and C. Spohn-Larkins, two Bucknell University students who contributed to the research presented here.
1 SONG also imposed the criterion D25 < 70' to exclude M33, which is the subject of the Engargiola et al. (2003) survey described in Section 3.2. Back.
2 The BIMA SONG data are available at the following address: /level5/March02/SONG/SONG.html Back.