Reports from Observers The ARAUCARIA Project – First Observations of Blue Supergiants in NGC 3109 Chris Evans1 Fabio Bresolin Miguel Urbaneja Grzegorz Pietrzyn´ski 3,4 Wolfgang Gieren 3 Rolf-Peter Kudritzki 1 United Kingdom Astronomy Technology Centre, Edinburgh, United Kingdom Institute for Astronomy, University of Hawaii, USA 3 Universidad de Concepción, Chile 4 Warsaw University Observatory, Poland NGC 3109 is an irregular galaxy at the edge of the Local Group at a distance of 1.3 Mpc. Here we present new VLT observations of its young, massive star population, which have allowed us to probe stellar abundances and kinemat- ics for the first time. The mean oxygen abundance obtained from early B-type supergiants confirms suggestions that NGC 3109 is a large Magellanic Irregular Figure 1: Part of the V-band FORS pre-image of our NGC 3109 is very metal poor. In this at 1.3 Mpc, which puts it at the outer edge most western field, with the targets encircled. NGC 3109 is approximately edge-on and the FORS context we advocate studies of the stel- of the Local Group. Using FORS in the targets are well sampled along both the major lar population of NGC 3109 as a com- configurable MOS (multi-object spectros- and minor axes. pelling target for future Extremely Large copy) mode, we have observed 91 stars Telescopes (ELTs). in NGC 3109. These were observed in Example spectra are shown in Figure . 4 MOS configurations, using the 600 B Of our 91 targets, 1 are late O-type stars, grism (giving a common wavelength cov- ranging from O8 to O9.5 – such high- The ARAUCARIA Project is an ESO Large erage of l3900 to l4750 Å). The cumula- quality observations of resolved O-type Programme using FORS on the VLT. tive exposure time for each field was stars (note the He II emission ‘bump’ at Its principal motivation is to provide im- roughly 3 hours. Part of our most western l4686 Å in the spectrum of star #33) be- proved distances to galaxies in the Local field is shown in the FORS pre-image in yond 1 Mpc are really quite remarkable. and Sculptor Groups, via the period-lu- Figure 1, with our targets encircled. From minosity relationship of Cepheid variables published photometry it has been sug- (Gieren et al. 005). A secondary com- gested that red giants in NGC 3109 have #33 09 If V = 19.6 ponent of the project is to characterise metal abundances that are similar to tens of blue supergiants (typically B- and those found in stars in the Small Magel- A-type stars) in each of the target gal- lanic Cloud (SMC), i.e. very metal poor #09 B0.5 Ia V = 18.8 axies. Blue supergiants are the most vis- when compared to the solar neighbour- ually luminous ‘normal’ stars, thereby hood. With this in mind, we classified the ux fl #37 B2.5 Ia V = 19.7 enabling direct studies of stellar popula- FORS spectra using criteria that have d tions in galaxies that are otherwise already tackled the issue of low metallic- unreachable with 8-m telescopes. From ity (e.g. Evans et al. 004). Our sample Normalise comparisons with theoretical spectra, is primarily composed of late-O, B and A #05 B8 Ia V = 18.5 we can investigate physical parameters spectral types – this is the first spectral such as temperatures and chemical exploration of this galaxy. As an aside, we #01 A2 Ia V = 17.8 abundances of our targets, obtaining es- note that the first large-scale CCD sur- timates of the metallicity of the host vey of NGC 3109 was reported in this systems. Moreover, blue supergiants have publication by Bresolin et al. (1990) – the also been advanced as an alternative acquisition of high-quality spectroscopy 3800 3900 4000 4100 4200 4300 4400 4500 4600 4700 4800 4900 method of distance determination via the in this galaxy some 16 years later illus- Wavelength (Å) flux-weighted gravity luminosity relation- trates the considerable advancement in ship (Kudritzki et al. 003). studies of extragalactic stellar popula- Figure 2: FORS spectra of five of our targets in NGC 3109. The quality of the data is particular- tions over that period. ly impressive when one remembers that the stars are at distances of over 1 Mpc. The Messenger 126 – December 006 5 Reports from Observers Evans C. et al., The ARAUCARIA Project However, in terms of quantitative analy- 1.6 Figure 3: FORS spec- sis, the early B-type supergiants in our NGC 3109 #22 B1 Ia trum (black line) of star #22, classified as sample are of more immediate interest – B1 Ia. A FASTWIND 1.4 these stars have a wide variety of strong model spectrum (Teff = metallic lines in their absorption spectra, FASTWIND model 22 000 K, logg = .60) is providing an excellent tool for investigat- shown above in red, smoothed to the same ing chemical abundances of young stellar 1.2 resolution as the FORS populations. data. VLT-FORS We have analysed a subset of eight of our 1.0 early B-type spectra using the FAST- WIND model atmosphere code (Puls et al. 005). From comparisons with theo- 0.8 retical spectra we can obtain physical parameters such as temperatures, gravi- ties, and, of most interest in a broader 0.6 context, chemical abundances. An ex- 4000 4100 4200 4300 4400 4500 4600 4700 4800 4900 ample FASTWIND model matched to one Wavelength (Å) of the observed spectra is shown in Fig- ure 3. The mean oxygen abundance in our eight stars is found to be log(O/H) + 1 = 7.76 ± 0.07, in excellent agreement –200 Figure 4: Differential ra- dial velocities as a func- with results from H II regions. This is only –150 tion of radius along the ~ 1 % of the oxygen abundance found major axis of NGC 3109 in the solar neighbourhood, and is lower –100 – typical uncertainties than the oxygen abundances found in the are of order ± 20 km/s. ] Also shown are rotation SMC (cf. log(O/H) + 1 = 8.13, Trundle –1 –50 curves from H I (solid line) and Lennon, 005). We also obtain upper [kms and H (dotted line). ) a s 0 limits to the magnesium and silicon abun- sy v – r v dances, which are comparable to those ( v 50 found for stars in the SMC – the exact ∆ abundance of the alpha-elements will re- 100 quire higher-resolution spectroscopy, but it is clear that stars in NGC 3109 have 150 metal abundances that are very deficient 200 when compared to the solar neighbour- 400 300 200 100 0 –100 –200 –300 –400 hood, and likely even lower than in the Radius [arcsec] SMC. We have also used our FORS spectra velocities of the young population largely Meanwhile, lower-resolution spectros- to investigate the stellar rotation curve of trace those of the gas, with a fair amount copy could trace the kinematics of the NGC 3109. H I observations suggest of scatter. Further observations of this non-supergiant population (e.g. via the a dominant dark-matter halo (Jobin and sort would be of value to ascertain wheth- Calcium Triplet), probing the outer struc- Carignan 1990), that cosmological N- er the stellar results are revealing genuine ture of this dark-matter dominated body cold dark matter simulations have sub-structures in the disc, or whether dwarf and providing crucial input for cos- struggled to reproduce (Navarro et al. we are simply limited by the small sample/ mological simulations. 1996). The spectral resolution from FORS spectral resolution. (R ~ 1,000) is somewhat limiting for stud- ies of stellar kinematics, but from simple Plans for the next generation of large References measurements of line-centres of hydro- ground-based telescopes, the so-called Blais-Ouellette S., Amram P. and Carignan C. 001, gen and helium lines, we estimated radial Extremely Large Telescopes (ELTs), are AJ 11, 195 velocities for the majority (84) of our stars. now gaining momentum. In this context Bresolin F., Capaccioli M. and Piotto G. 1990, The mean 1-sigma (internal) uncertainty we suggest NGC 3109 as an exciting op- The Messenger 60, 36 is of order 0 km/s. Figure 4 shows differ- portunity to study many stages of stel- Evans C. J. et al. 004, MNRAS 353, 601 Gieren W. et al. 005, The Messenger 11, 3 ential radial velocities for each of our lar evolution in a very metal poor environ- Jobin M. and Carignan C. 1990, AJ 100, 648 stars, compared with published results ment. A large primary aperture would Kudritzki R.-P., Bresolin F. and Przybilla N. 003, from H I radio maps and Ha imaging enable high-resolution spectroscopy of ApJ 58, 83L (Jobin and Carignan 1990, Blais-Ouellette the young, massive population, and Navarro J. F. et al. 1996, ApJ 46, 563 Puls J. et al. 005, A&A 435, 669 et al. 001). As one might expect, the of stars on the asymptotic giant branch. Trundle C. and Lennon D. J. 005, A&A 434, 677 6 The Messenger 126 – December 006.