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Interstellar H2 in M 33 Detected with FUSE

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A&A 398, 983–991 (2003) Astronomy DOI: 10.1051/0004-6361:20021725 & c ESO 2003 Astrophysics ? Interstellar H2 in M 33 detected with FUSE H. Bluhm1,K.S.deBoer1,O.Marggraf1,P.Richter2, and B. P. Wakker3 1 Sternwarte, Universit¨at Bonn, Auf dem H¨ugel 71, 53121 Bonn, Germany 2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy 3 University of Wisconsin–Madison, 475 N. Charter Street, Madison, WI 53706, USA Received 30 October 2002 / Accepted 18 November 2002 Abstract. Fuse spectra of the four brightest H ii regions in M 33 show absorption by interstellar gas in the Galaxy and in M 33. On three lines of sight molecular hydrogen in M 33 is detected. This is the first measurement of diffuse H2 in absorption in a Local Group galaxy other than the Magellanic Clouds. A quantitative analysis is difficult because of the low signal to noise ratio and the systematic effects produced by having multiple objects in the Fuse aperture. We use the M 33 Fuse data to demonstrate in a more general manner the complexity of interpreting interstellar absorption line spectra towards multi-object background 16 17 2 sources. We derive H column densities of 10 to 10 cm− along 3 sight lines (NGC 588, NGC 592, NGC 595). Because of 2 ≈ the systematic effects, these values most likely represent upper limits and the non-detection of H2 towards NGC 604 does not exclude the existence of significant amounts of molecular gas along this sight line. Key words. galaxies: individual: M 33 – ISM: abundances – ISM: molecules – ultraviolet: ISM 1. Introduction NGC objects. The galaxy is inclined at 55◦ and shows signs of bending as derived from 21 cm radio synthesis observations Observations with the Copernicus satellite have shown that (Rogstad et al. 1976). In the direction of two of the bright H ii the ISM of the Milky Way contains large amounts of molecular regions CO has been found in emission (Wilson & Scoville hydrogen. The mass in H2 may even dominate that in atomic 1992). The metallicity of M 33 is subsolar with a radial abun- form (see Savage et al. 1977). Since 1996 other galaxies also dance gradient which differs for different elements. For ex- have become accessible for investigation of the content of H2, ample, neon abundances in the observed H ii regions vary be- using Orfeus and Fuse far ultraviolet spectra. Both the Large tween 0.2 and 0.3 dex subsolar (see Willner & Nelson-Patel and the Small Magellanic Cloud appear to have substantial 2002). Vibrationally excited H2 has been found in emission by amounts of H2 as found from absorption line studies (de Boer Israel et al. (1990) towards NGC 604, most likely from back- et al. 1998; Richter et al. 1998; Richter 2000; Tumlinson et al. ground gas. 2002). In this paper we investigate Fuse spectra of four bright ob- Fuse has sufficient sensitivity to record spectra of bright jects in M 33. Each of these consist of a number of young and sources in other nearby galaxies such as M 31 and M 33. It thus UV-bright stars surrounded by an H ii region. In 3 of the is of importance to extend our knowledge about the content 4 spectra H2 is detected in absorption in M 33. in molecular gas to galaxies other than the Milky Way and In Sect. 2 we give some information about the instrument, the LMC and SMC. These galaxies have different histories, dif- the targets, and the data reduction, in Sect. 3 the difficulties ferent stellar contents and different abundances. An intercom- with the interpretation of interstellar absorption spectra towards parison of these galaxies may lead to clues about the interplay multi-object background sources are described. The measure- of molecular cloud and star formation with chemical content ments are presented in Sect. 4, followed by a discussion of the and radiation field. results in Sect. 5. M 33 is the smallest spiral galaxy of the Local Group at a distance of 800 kpc. It has a ragged optical appear- ∼ ance and bright H ii regions, some of them catalogued as 2. Data Send offprint requests to: H. Bluhm, 2.1. The instrument e-mail: [email protected] ? Based on observations made with the NASA-CNES-CSA Far Fuse is equipped with four coaligned telescopes and Rowland Ultraviolet Spectroscopic Explorer, available in the public archive. spectrographs. The detectors are two microchannel plates. For FUSE is operated for NASA by the Johns Hopkins University under a description of the instrument and its on-orbit performance NASA contract NAS5-32985. see Moos et al. (2000) and Sahnow et al. (2000). Information Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20021725 984 H. Bluhm et al.: Interstellar H2 inM33 Fig. 1. The Fuse 30 30 arcsec LWRS aperture overlaid on the four H ii regions/star clusters. For NGC 588, NGC 595, and NGC 604 HST WFPC 2 images (central× wavelength 1730 Å) are shown, for NGC 592 only a DSS1 image was available. Table 1. Data on Fuse pointings on M 33 H ii regions. The position angle is the angle of the Fuse y axis (east of north). N20 is the number of stars having an UV flux (at 1730 Å) of at least 20% of the brightest source in the area covered by the Fuse aperture. For NGC 592 N20 is unknown because no public HST image is available (see Fig. 1). Object Observation ID α2000:0 δ2000:0 Pos. angle Exp. time N20 NGC 588 a0860404001 01 32 45.50 +30 38 55.00 334:9◦ 2819 s 8 a0860404002 2401 s NGC 592 a0860202001 01 33 12.30 +30 38 49.00 334:9◦ 2164 s ? a0860202002 1800 s NGC 595 a0860303001 01 33 33.60 +30 41 32.00 322:4◦ 3620 s 12 NGC 604 a0860101001 01 34 32.50 +30 47 04.00 323:1◦ 3633 s 9 a0860101002 3520 s H. Bluhm et al.: Interstellar H2 inM33 985 50 50 40 40 Ly R(0), 2-0 Ly R(0), 1-0 O I (1039) 40 40 30 30 O I (1039) 30 30 2d* 1f 2d 20 1a* 1a 20 2d* 2d 2e* 1b* 20 1b 20 10 10 10 10 1f 0 0 0 0 50 50 40 40 Ly P(1), 2-0 Ar I (1048) Ly P(1), 4-0 Ar I (1048) 40 40 4c* 30 30 4c* 4c 4c 30 30 0a* 2a 5b 20 1d 2c* 2c 20 0a* 2a* 20 1b 20 5b 10 10 10 10 0 0 0 0 50 50 40 40 Ly P(2), 4-0 Fe II (1121) Ly P(2), 4-0 Fe II (1121) 40 5c 40 30 5c 30 3a* 3a* 30 3a 30 3a 20 20 20 20 mc* mc mc* mc Counts Counts 10 10 10 10 0 0 0 0 50 50 40 40 Ly P(3), 3-0 P II (1152) Ly P(3), 4-0 P II (1152) 40 40 30 30 30 mb 30 4a* 20 20 20 20 10 10 10 10 0 0 0 0 50 40 -400 -300 -200 -100 0 100 200 -400 -300 -200 -100 0 100 200 Ly P(4), 3-0 -1 Ly P(4), 7-0 -1 40 vLSR [km s ] 30 vLSR [km s ] 30 20 20 NGC 588 10 NGC 592 10 0b* 0 0 -400 -300 -200 -100 0 100 200 -400 -300 -200 -100 0 100 200 -1 -1 vLSR [km s ] vLSR [km s ] 30 30 500 500 Ly R(0), 2-0 O I (1039) Ly R(0), 1-0 O I (1039) 400 400 1f 2d 20 20 1f 2e* 1a 1b* 1b 2d* 2d 300 300 10 10 200 200 100 100 0 0 0 0 30 30 500 500 Ly P(1), 5-0 4c* Ar I (1048) Ly P(1), 4-0 Ar I (1066) 400 400 20 20 5a 300 1d 2c 300 2b 1e 2a* 2a 4c 0a* 5b 10 10 200 200 100 100 0 0 0 0 30 30 500 500 Ly R(2), 3-0 Fe II (1144) Ly P(2), 7-0 Fe II (1125) 400 400 20 1c* 20 1c 5a 300 300 10 10 200 200 Counts Counts 100 100 0 0 0 0 30 30 500 500 Ly P(3), 4-0 P II (1152) Ly P(3), 7-0 P II (1152) 400 400 20 4b* 20 mb 300 300 md* 10 10 200 200 100 100 0 0 0 0 30 500 -400 -300 -200 -100 0 100 200 -400 -300 -200 -100 0 100 200 Ly P(4), 4-0 -1 Ly P(4), 3-0 -1 vLSR [km s ] 400 vLSR [km s ] 20 300 200 10 NGC 595 NGC 604 100 0 0 -400 -300 -200 -100 0 100 200 -400 -300 -200 -100 0 100 200 -1 -1 vLSR [km s ] vLSR [km s ] Fig. 2. For each line of sight a sample of five H2 and four metal lines is shown. Because of peculiarities in the spectra (sight line structure, noise features, structures in the background source spectrum) the choice of transitions is not the same for all four sight lines. The dotted lines mark the radial velocities of Galactic and M 33 gas. Identified other transitions within the range of the plots are labeled in the spectra as follows: H2 Lyman: 0a) R(0),4–0; 0b) R(0),6–0; 1a) R(1),1–0; 1b) R(1),2–0; 1c) P(1), 3–0; 1d) R(1),4–0; 1e) R(1),5–0; 2a) R(2),2–0; 2b) P(2),3–0; 2c) R(2),4–0; 2d) R(2),5–0; 2e) P(2),5–0; 3a) R(3),4–0; 3b) P(3),6–0; 4a) R(4),3–0; 4b) R(4),4–0; 4c); P(4),5–0; 5a) P(5),4–0; 5b) R(5),5–0; 5c) P(5)(5–0).
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