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

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

We aim at studying the effect of varying metallicities on the CO 3-2emission along the major axis of the large spiral galaxy M33 at only840 kpc distance. M33 shows a particularly strong gradient ofmetallicity with galactic radius. PDR models predict that the energybalance and the relative importance of different line tracers of gascooling are a strong function of the metallicity. However, the detaileddependence of CO intensities on varying metallicities remains to bestudied. This will also help in the interpretation of low-metallicityobjects like nearby dwarf galaxies or galaxies in the early universe.

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Scientific justification:

Newborn massive stars illuminate and heat their surrounding nataldense molecular clouds thereby creating photon dominated regions(PDRs). Their bright FIR and submillimeter cooling lines are thus keytracers of star formation activity throughout the cosmologicalevolution of galaxies. Hence, proper modeling of PDR emission is ofcentral importance for the interpretation of the observations in orderto derive the physical parameters and the chemical state of the ISM inexternal galaxies. Next to the [CII] and [OI] lines at 158 and 63µm, a major contribution to the gas cooling stems from the [CI]fine structure lines at 609 µm and 370 µm, and from therotational transitions of CO (e.g. Fixsen et al. 1999, Kramer et al.2005). To better understand the evolution of matter, starting withlow-metallicity material of cosmological origin, the effect ofdifferent metallicities on the resulting PDR emission has to bestudied.
Previous observations of CI/CO, CI/CII, and e.g. CO 3-2/1-0 atsolar metallicity have shown the great potential of these lines tostudy the excitation conditions of galaxies (Kramer et al. 2005, Israel& Baas 2003, Gerin & Phillips 2000, Bayet et al. 2004). At lowmetallicities, 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 theextent of [CI] emission is also enhanced.
In a recent modelling effort, Roellig et al. (2005) have analyzedthe variation of CII emission with metallicity using the sphericalsymmetric KOSMA- PDR model. They confirm that low metallicities affectnot only the size of the photo dissociated outer layer of a PDR buthave also profound effects on the chemical network and temperatures.They find e.g. that the geometrical depth of the CII layer scalesinversely with metallicity. The models confirm the observed rise of theCII/CO ratio with decreasing metallicity. They also predict a drop ofCO 3-2 intensities with decreasing Z (Fig. 1).
However, the interpretion of the observed CII flux is difficultsince a significant fraction may stem from the diffuse ionized and thediffuse cold neutral medium (Hollenbach & Tielens 1999). Incontrast, CI and CO emission stem entirely from the surface regions ofdense 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 gradualchanges in the physical conditions of the ISM with galacto-centricdistance, viz. metallicity. The metallicity gradient of M33 variiesbetween solar in the nucleus and 1/10 solar in the outer disk (Vilchezet al. 1988). Its net metal abundance is close to the LMC's (1/3solar).
We suggest to observe CO 3-2 along the major axis of M33 using APEX. Wehave 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. Inaddition, we will be able to study the CO cooling power in comparisonto the CII emission stemming from PDRs. The fraction of CII emissionstemming from the ionized gas shall be estimated from the NII linefluxes. We also plan to use the recently published GALEX UV data(Thilker et al. 2005) to estimate the extinction corrected UV fluxwhich is an important parameter in all PDR models. In a study whichnicely complements ours, the SINGS team (Kennicutt et al. 2004) iscombining Spitzer NIR and MIR observations of M33 with UV data fromGALEX to probe the behavior of the dust (PAH features, hot dust, colddust) 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 pointedposition switched observations in CO 3-2 with APEX at 18" resolutionalong the major axis of M33. The linewidths observed in CO 2-1 (Figure3) and in CO 1-0 (Engargiola et al. 2003) are in general 12 km.s-1 andless at angular resolutions of 21" to 13".
Peak CO 2-1 temperatures are 500 mK (T*_A ) (Fig. 3). From spiralarm observations in M83 and M51 (Kramer et al. 2005), we expect the CO3­2 line to be upto a factor 2 weaker. A signal-to-noise ratio ofat least 10 should be sufficient to detect most of the emission visiblein 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 250K as suggested in the call for proposal is valid for good weatherconditions in the Atacama. We need 90 sec on+off integration time perposition. 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.


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