A&A 453, 635–651 (2006) Astronomy DOI: 10.1051/0004-6361:20053842 & c ESO 2006 Astrophysics Modelling the Galactic interstellar extinction distribution in three dimensions D. J. Marshall1,A.C.Robin1,C.Reylé1, M. Schultheis1, and S. Picaud1,2 1 CNRS UMR 6091, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France e-mail: [email protected] 2 Instituto de Astronomia e Geofísica (IAG/USP), São Paulo, Brazil Received 18 July 2005 / Accepted 25 March 2006 ABSTRACT Aims. The Two Micron All Sky Survey, along with the Stellar Population Synthesis Model of the Galaxy, developed in Besançon, is used to calculate the extinction distribution along different lines of sight. By combining many lines of sight, the large scale distribution of interstellar material can be deduced. Methods. The Galaxy model is used to provide the intrinsic colour of stars and their probable distances, so that the near infrared colour excess, and hence the extinction, may be calculated and its distance evaluated. Such a technique is dependent on the model used, however we are able to show that moderate changes in the model parameters result in insignificant changes in the predicted extinction. Results. This technique has now been applied to over 64 000 lines of sight, each separated by 15, in the inner Galaxy (|l|≤100◦, |b|≤10◦). We have projected our three dimensional results onto a two dimensional plane in order to compare them with existing two dimensional extinction maps and CO surveys. We find that although differences exist due to the different methods used or the medium traced, the same large scale structures are visible in each of the different maps. Using our extinction map, we have derived the main characteristics of the large scale structure of the dust distribution. The scale height of the interstellar matter is found to +17 be 125−7 pc. The dust distribution is found to be asymmetrically warped, in agreement with CO and HI observations of the ISM. However, the slope of the dust warp and the galactocentric distance where it starts are found to be smaller than the values measured for the external HI disc: for positive longitudes the angle is θ = 89◦, it starts at 8.7 kpc from the Galactic center and grows with a slope of 0.14, while at negative longitudes, the angle of the maximum is at θ = 272◦, the starting radius 7.8 kpc and the slope 0.11. Finally, the presence of dust is detected in the Galactic bulge. It forms an elongated structure approximately 5.2 kpc long and lies at an angle of 30 ± 5◦ with respect to the Sun-Galactic centre direction. This may be interpreted as a dust lane along the Galactic bar. This resulting extinction map will be useful for studies of the inner Galaxy and its stellar populations. Key words. ISM: dust, extinction – ISM: structure – Galaxy: structure 1. Introduction invaluable step to identifying low extinction windows at low Galactic latitude. Interstellar extinction remains a serious obstacle for the obser- vation of stars in the Milky Way, and for interpreting these ob- Several studies have been undertaken to further our knowl- servations in terms of Galactic structure. It has two main effects, edge of the distribution of the extinction in our Galaxy. Most that of dimming sources and reddening them. Any stellar study of them provide a two dimensional map of the extinction in near the Galactic plane would need to correct for this effect to en- different regions of the Galaxy in which they calculate either able a proper interpretation of the observations. Recent infrared the mean extinction or the total extinction for various lines surveys provide a superb tool to probe further into the densest of sight: Froebrich et al. (2005) use accumulative star counts parts of the Galaxy as the extinction suffered in the Ks band is from the 2MASS survey to construct a relative extinction map approximately one tenth of that suffered in the visible. of the Galactic plane; Sumi (2004) has created a two dimen- Furthermore, extragalactic studies often shun the so called sional map of the mean extinction towards the Galactic bulge “Zone of Avoidance” at around |b| ∼< 10◦ and towards the in- using red clump giants from the Optical Gravitational Lensing ner Galaxy due to the amount of extinction and the high source Experiment II; Dutra et al. (2003) and Schultheis et al. (1999) confusion. The determination of interstellar extinction in this determined the mean extinction towards the same region us- zone, which covers about 10% of the infrared sky, is thus an ing 2MASS data and DENIS data, respectively; Schlegel et al. (1998) used the dust IR emission as observed by DIRBE to cre- ate an all sky two dimensional map of the total extinction in- The three dimensional results (Table 1) are available in elec- tegrated along each line of sight. However, according to Arce tronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr et al. (1999), the Schlegel et al. (1998) map overestimates the (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/ reddening by a factor of 1.3–1.5 in regions of smooth extinc- qcat?J/A+A/453/635 tion with AV > 0.5, and underestimates it in regions of steep Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20053842 636 D. J. Marshall et al.: Modelling interstellar extinction extinction gradients. Cambrésy et al. (2005) also noticed that the parameters is estimated and limits and possible bias are dis- correlation of the near-infrared extinction to the far-infrared op- cussed in Sect. 6. Our results are summarised in Sect. 7. tical depth shows a discrepancy of about 30% for AV > 1mag, probably due to the ubiquitous presence of fluffygrains. 2. 2MASS point source catalogue Other authors have determined the distribution of extinction along different lines of sight using various methods: Fitzgerald 2.1. Description of the survey (1968) provided the local distribution of extinction up to a few The Two Micron All Sky Survey (2MASS) is a ground based kiloparsecs using photoelectric photometry. Later on, Neckel survey which uniformly scanned the entire sky in three near- et al. (1980) measured extinction and distances towards indi- infrared bands (J, H and Ks). Amongst its final products is vidual O, B and A stars. These studies provide limited resolu- the point source catalogue (PSC) which includes point sources tion for the extinction distribution due to the scarceness of the brighter than about 1 mJy in each band, with signal-to-noise ra- distribution of hot stars, often related to clouds. They do not ff tio (SNR) greater than 10, using a pixel size of 2.0 . Each star provide the distribution of the di use extinction. The study of has accurate photometry and astrometry as defined in the level 1 Berdnikov et al. (1990) is limited to a sample of 615 stars in requirements (Cutri et al. 2003). 9 Kapteyn Selected Areas and shows considerable dispersion in The quality of the data was destined to meet or surpass all of extinction among the stars in a given direction. Arenou et al. the level 1 requirements. We list three of them below: (1992) mapped the local extinction from a collection of spectral and photometric data. The validity of this map is limited to a dis- – photometric sensitivity of 10-σ at 15.8, 15.1, 14.3 mag at J, ◦ ◦ tance of about 1 kpc, hence it cannot be used at |b| < 10 ,where H, Ks respectively for |b| > 10 ; a significant part of the extinction is situated at distances larger – completeness >0.99 at 10-σ sensitivity limits; than 1 kpc. In addition, the resolution of the map is of the order – reliability >0.9995. of several degrees. Cambrésy (1999a) determined detailed maps of extinction in nearby Giant Molecular Clouds from USNO star These requirements were met for the entire unconfused sky. counts, which are very useful for studies of these clouds, but However, the 2MASS survey has no level 1 requirements for the Galactic Plane – these specifications apply only to high Galactic not outside. He also considered the whole sky extinction from ◦ USNO star counts, but still with a two dimensional approach, latitudes. Within approximately a 6 radius of the Galactic cen- ◦ tre, the average magnitude for a SNR = 10 source is up to ∼2mag and with a limited spatial resolution of 6 (Cambrésy 1999b). = A great deal of other determinations can also be found in the brighter than the typical SNR 10 source in unconfused regions. This is also seen for various regions in the Galactic plane at |b| ∼< literature which concern one or a few lines of sight. Putting all ◦ ± ◦ these maps together leads to inhomogeneous maps with poorly 3 and 60 from the Galactic centre. defined accuracy. They also have the problem of matching dif- As well as containing default magnitudes and associated un- ferent results at the boundaries between authors (Hakkila et al. certainty for each star, the 2MASS PSC also contains a number 1997). of flags to aid the user in selecting stars appropriate for the study. We make use of the Read Flag (rd_flg), which indicates the The relationship between infrared colour excess and extinc- method used to determine the default magnitude of the source, tion is well documented, and has been in use for some time and the Photometry Quality Flag (ph_flg) which gives an indi- (Jones et al. 1984; Lada et al. 1994). In this type of study, stars cation of the quality of the photometric determination (signal to that are reddened due to their location behind a cloud are com- noise ratio, measurement error).