APEX SV projects

CO 3-2 emission and metallicity in the face-on galaxy M33

Coordinator: C. Kramer,F. Israel, S. Lord, M. Roellig, M. Dumke

Abstract:
We aim at studying the effect of varying metallicities on the CO 3-2 emission along the major axis of the large spiral galaxy M33 at only 840 kpc distance. M33 shows a particularly strong gradient of metallicity with galactic radius. PDR models predict that the energy balance and the relative importance of different line tracers of gas cooling are a strong function of the metallicity. However, the detailed dependence of CO intensities on varying metallicities remains to be studied. This will also help in the interpretation of low-metallicity objects like nearby dwarf galaxies or galaxies in the early universe.

Data:
Program is available and data products can be downloaded

Scientific justification:

Newborn massive stars illuminate and heat their surrounding natal dense molecular clouds thereby creating photon dominated regions (PDRs). Their bright FIR and submillimeter cooling lines are thus key tracers of star formation activity throughout the cosmological evolution of galaxies. Hence, proper modeling of PDR emission is of central importance for the interpretation of the observations in order to derive the physical parameters and the chemical state of the ISM in external galaxies. Next to the [CII] and [OI] lines at 158 and 63 ľm, a major contribution to the gas cooling stems from the [CI] fine structure lines at 609 ľm and 370 ľm, and from the rotational transitions of CO (e.g. Fixsen et al. 1999, Kramer et al. 2005). To better understand the evolution of matter, starting with low-metallicity material of cosmological origin, the effect of different metallicities on the resulting PDR emission has to be studied.
Previous observations of CI/CO, CI/CII, and e.g. CO 3-2/1-0 at solar metallicity have shown the great potential of these lines to study the excitation conditions of galaxies (Kramer et al. 2005, Israel & Baas 2003, Gerin & Phillips 2000, Bayet et al. 2004). At low metallicities, observations have shown enhanced [CII]/CO line ratios (e.g. Smith & Madden 1997, Bolatto et al. 1999, Hunter et al. 2001). Observations of the LMC by Stark et al. (1997) show that the extent of [CI] emission is also enhanced.
In a recent modelling effort, Roellig et al. (2005) have analyzed the variation of CII emission with metallicity using the spherical symmetric KOSMA- PDR model. They confirm that low metallicities affect not only the size of the photo dissociated outer layer of a PDR but have also profound effects on the chemical network and temperatures. They find e.g. that the geometrical depth of the CII layer scales inversely with metallicity. The models confirm the observed rise of the CII/CO ratio with decreasing metallicity. They also predict a drop of CO 3-2 intensities with decreasing Z (Fig. 1).
However, the interpretion of the observed CII flux is difficult since a significant fraction may stem from the diffuse ionized and the diffuse cold neutral medium (Hollenbach & Tielens 1999). In contrast, CI and CO emission stem entirely from the surface regions of dense molecular clouds, viz. PDRs. The large nearby spiral galaxy M33 (NGC598) shows a strong radial metallicity gradient (Fig. 2) and thus offers the unique chance to study systematically gradual changes in the physical conditions of the ISM with galacto-centric distance, viz. metallicity. The metallicity gradient of M33 variies between solar in the nucleus and 1/10 solar in the outer disk (Vilchez et al. 1988). Its net metal abundance is close to the LMC's (1/3 solar).
We suggest to observe CO 3-2 along the major axis of M33 using APEX. We have already obtained CO 2-1 JCMT data (Fig. 3) and ISO/LWS CII, NII(122) line and continuum data along the major axis (Israel priv. comm., see also Higdon et al. 2003). Adding CO 10 BIMA data (Engargiola et al. 2003, Rosolowsky et al. 2003, Heyer et al. 2004) will allow a detailed excitation analysis of the CO emitting gas. In addition, we will be able to study the CO cooling power in comparison to the CII emission stemming from PDRs. The fraction of CII emission stemming from the ionized gas shall be estimated from the NII line fluxes. We also plan to use the recently published GALEX UV data (Thilker et al. 2005) to estimate the extinction corrected UV flux which is an important parameter in all PDR models. In a study which nicely complements ours, the SINGS team (Kennicutt et al. 2004) is combining Spitzer NIR and MIR observations of M33 with UV data from GALEX to probe the behavior of the dust (PAH features, hot dust, cold dust) as a function of metallicity.

*Table 1: Basic properties of M33. D and i are the distance and inclination respectively (Engargiola et al. 2003)*
Source	RC3-Type	V_LSR(km.s-1)	D(Mpc)	scale(10")	i(deg)	PA(deg)
M33	SA(s)cd	-180	0.84	41 pc	51	21

Technical Justification
We plan to conduct pointed position switched observations in CO 3-2 with APEX at 18" resolution along the major axis of M33. The linewidths observed in CO 2-1 (Figure 3) and in CO 1-0 (Engargiola et al. 2003) are in general 12 km.s-1 and less at angular resolutions of 21" to 13".
Peak CO 2-1 temperatures are 500 mK (T*_A ) (Fig. 3). From spiral arm observations in M83 and M51 (Kramer et al. 2005), we expect the CO 32 line to be upto a factor 2 weaker. A signal-to-noise ratio of at least 10 should be sufficient to detect most of the emission visible in the PV-diagram shown in Figure 3. We thus aim at an rms of 25 mK (T*_A ) at 2 km.s-1 channel spacing. M33 rises to almost 40 elevation. Assuming a system temperature of 250 K as suggested in the call for proposal is valid for good weather conditions in the Atacama. We need 90 sec on+off integration time per position. To obtain a fully-sampled cut at 10 grid spacing between -12' and +12' (Fig. 3) we will observe 144 positions along the major axis.
We thus ask for a total of 3.6 hours of on+off observing time in the CO 3-2 line.

References

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