The UMass/INAOE Galaxy Survey
Paglione, T.A.D. (Michigan), Wall, W.F. (INAOE) & Heyer, M.H. (FCRAO)

Abstract
We propose using the unique mapping capability and sensitivity of SEQUOIA to survey the CO and ¹³CO J=1-0 emission in the disks of galaxies. Our goal is to determine if the ¹²CO/¹³CO ratio varies with galactocentric radius, galaxy type, star formation, etc. As an array, SEQUOIA can detect such gradients with negligible uncertainties in relative calibration and pointing. Variations in this ratio from galaxy to galaxy, and within galaxies, may indicate changes in the conversion factor relating CO intensity and H₂ mass. A better understanding of the CO-to-H₂ conversion factor in galaxies will yield more accurate mass determinations critical for understanding important galactic problems such as dynamics, structure and star formation, to name but a few. We had great success last season in the quality of both the data and the undergraduates working on the project. The results were presented at the June AAS meeting, and are currently being compiled for publication.

Introduction
Knowing the distribution of molecular mass in a galaxy is fundamental to understanding its evolution, structure, kinematics and star formation. Unfortunately, H₂ is not visible in typical cloud regions. CO, the most abundant interstellar molecule next to H₂, is relatively easy to excite, and its J=1-0 emission is readily observable with ground-based telescopes. Therefore, CO is detectable in nearly all molecular regions, and the integrated CO J=1-0 intensity is commonly used to measure molecular gas column density. However, because CO J=1-0 emission is often optically thick and arises in nearly all parts of every molecular cloud, using a ``standard'' CO-to-H₂ conversion factor everywhere may be inappropriate, especially if it depends on the physical conditions of the gas (e.g., Sakamoto 1996). This problem may worsen when studying the unresolved, overlapping clouds of external galaxies. Further, it is unclear whether X(CO), the empirical conversion factor derived from Galactic disk clouds, is valid in other galaxies, or in the dense, metal-rich clouds of our own Galactic center (Maloney & Black 1988; Sodroski et al. 1995; Paglione et al. 1998).

One way to test for variations in X(CO) in galaxies is to observe optically thin ¹³CO emission. If the CO/¹³CO ratio varies with position or velocity, then the (most likely saturated) CO line is not a constant measure of gas mass. A few such studies have been done (Rickard & Blitz 1985; Young & Sanders 1986; Sage & Isbell 1991). The basic results of these small surveys were the following:

1. There was no systematic change in CO/¹³CO with radius. In direct conflict with the later surveys, Rickard & Blitz found low values of CO/¹³CO in galactic disks. However, just 13 of the combined sample of only 19 galaxies were observed at more than one position.
2. No significant correlations were seen between CO/¹³CO and many galactic parameters except dust color temperature. The relation to temperature is expected assuming LTE. With CO optically thick, ¹³CO optically thin, and both lines thermalized, then T_B(CO) is roughly T_gas, and T_B(¹³CO) is roughly 1/T_gas. However, even this one correlation relied mostly on only two data points.
3. The average CO/¹³CO ratio in galaxies was higher than in the Milky Way. They concluded the higher ratio was due to a different value of X(CO). However, the large beams may simply be sensitive to a more prevalent diffuse component visible mostly in CO (Polk et al. 1988).

   With the sensitivity and mapping ability of SEQUOIA, we can unambiguously find any variations with position because uncertainties in relative pointing and calibration are minimized with an array. A comprehensive survey of many galaxy types will bring out any relations between CO/¹³CO and galactic parameters if they exist. Measuring CO/¹³CO in a large, representative sample of galaxies will indicate any deviation of the mean X(CO) from the Galactic value. Large-scale surveys of CO and ¹³CO in the Milky Way are also available for comparison if indeed CO/¹³CO depends on the spatial scale of the observations.

   Results of First Season
   In the shortened 1998/1999 season we mapped the 8 brightest and most extended galaxies in the survey in ¹³CO and CO emission. We also mapped the CO line in several others. Emission from both CO and ¹³CO was detected at nearly every mapped position along the major axes of these galaxies, in some cases as far as 5' from the nucleus (see figure). We achieved average r.m.s. levels of ~3 mK for ¹³CO and ~10 mK for CO. The average nuclear CO/¹³CO ratio for the observed galaxies is 15+/-1. Extreme ratios of 5 and 30 were found for M51 and M82, respectively. Though some of the galaxies exhibit a slight drop in the CO/¹³CO ratio with galactocentric distance, the ratio mostly remains constant through the disk.

   Over a dozen students were involved in the survey last year, and 4 of those earned summer FCAD internships. Two students presented the survey results at the June AAS meeting. In addition to taking data, this year the students will lead efforts for all data archiving, reduction and some analysis.

   Time Request and Justification
   We propose mapping the CO and ¹³CO emission in 44 galaxies. We will take advantage of the undersubscribed LST range of the FCRAO covering roughly 9 to 16 hours. Undergraduates will make the majority of the observations, mostly remotely. They are also responsible for proper data archiving, reduction, and some analysis. A well-trained team of about 10 students is ready.

   The survey galaxies are from the FCRAO extragalactic CO survey of Young et al. (1995). The selection criterion was that the galaxies have peak CO antenna temperatures of at least 60 mK in more than one position (yielding a 3-sigma detection of the ¹³CO line given CO/¹³CO ~ 10 and a 2 mK r.m.s.). In this, our second season of operation, we are targeting the next brightest and extended sources in the full survey list. Last season, many of these sources were observed in CO.

   The r.m.s. antenna temperature is ~2 mK for ¹³CO after 6 hours, given T_sys 350 K and a 5 MHz channel width. For CO, with T_sys <1000 K, we should achieve better than 10 mK r.m.s. after 1-2 hours. These estimates are conservative since the system temperatures are usually better, and we can smooth the data in velocity for more sensitivity. Depending on the weather, manpower, telescope schedule, and the extent of the emission, each galaxy will require approximately 2-3 weeks to map to sufficient signal-to-noise in both lines (130 hours). The array will be oriented along the major axis of each galaxy, and we will sample at half-beam spacing along this axis. Thus gain variations are reduced by observing each map position with all four pixels of a row in the array.

   We propose mapping the CO and ¹³CO emission in 54 galaxies. We will take advantage of the undersubscribed LST range of the FCRAO covering roughly 9 to 16 hours, given the available manpower to take the observations. Observing remotely with the FCRAO makes manpower a smaller issue than in the past, though we intend for undergraduates to contribute to this effort significantly, including trips to Quabbin, data reduction and analysis.

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