DENIS observations in the J (1.2 micron) and K_S (2.15 micron) bands together
with isochrones calculated for the RGB and AGB phase are used to draw an
extinction map of the inner Galactic Bulge. The uncertainty in this method is
mainly limited by the optical depth of the Bulge itself. A comparison with
fields of known extinction shows a very good agreement. We present an
extinction map for the inner Galactic Bulge (approx. 20 sq. deg.)Comment: 4 pages, 4 figures, accepted for publication in A&A as a letter, see
  also http://www-denis.iap.fr/articles/extinction

Alard, C.

Borsenberger, J.

Copet, E.

Epchtein, N.

Fouque, P.

Ganesh, S.

Habing, H.

Omont, A.

Schultheis, M.

Simon, G.

English

arXiv

Fouqué, P.

Leiden University Scholary Publications

Astron. Astrophys. 349, L69–L72 (1999) ASTRONOMYANDASTROPHYSICSLetter to the EditorInterstellar extinction towards the inner Galactic Bulge?M. Schultheis1, S. Ganesh1,2, G. Simon3, A. Omont1, C. Alard3,1, J. Borsenberger1, E. Copet4,1, N. Epchtein6, P. Fouqúe5,and H. Habing71 Institut d’Astrophysique de Paris, CNRS, 98bis Bd Arago, F-75014 Paris, France2 Physical Research Laboratory, Navarangpura, Ahmedabad 380009, India3 DASGAL CNRS UMR 8633, Observatoire de Paris, France4 DESPA CNRS-UMR 8632, Observatoire de Paris, France5 ESO, Santiago, Chile6 O.C.A., Nice, France7 Leiden Observatory, Leiden, The NetherlandsReceived 22 June 1999 / Accepted 28 August 1999Abstract. DENIS observations in the J (1.25µm) and KS(2.15µm) bands together with isochrones calculated for theRGB and AGB phase are used to draw an extinction map of theinner Galactic Bulge. The uncertainty in this method is mainlylimited by the optical depth of the Bulge itself. A compari-son with fields of known extinction shows a very good agree-ment. We present an extinction map for the inner Galactic Bulge(∼ 20 deg2)Key words: stars: AGB and post-AGB – infrared: stars – ISM:dust, extinction1. IntroductionStudying the stellar populations in the Galactic Bulge requiresknowledge of the interstellar extinction. Previous studies (e. g.Catchpole et al. 1990, Frogel et al. 1999, Unavane et al. 1998)have shown that it can vary fromAV = 5m up toAV = 35mtowards the Galactic Centre region. As in most parts of theGalactic Bulge, interstellar absorption is not homogeneous butoccurs in clumps, a detailed extinction map of the Bulge istherefore essential.Catchpole et al. (1990) mapped the interstellar extinctionaround the Galactic Centre (∼ 2 deg2) using the red giant branchof 47 Tuc as a reference. Stanek et al. (1996) mapped the inter-stellar extinction of Baade’s window using OGLE photometryof red clump stars. Frogel et al. (1999) determined the inter-stellar reddening for a few fields in the inner Galactic Bulgeusing the red giant branch of Baade’s window as a reference.AK values ranging from 0.27 up to 2.15 mag were found by thelatter.Send offprint requests to: schulthe@iap.fr? Based on observations collected at the European Southern Obser-vatory, La Silla ChileThe DENIS survey (Epchtein et al. 1997, Persi et al. 1995)with 2MASS (Skrutskie et al. 1997) is the first attempt to carryout a complete survey of the southern sky. The limiting mag-nitudes in the three near-IR photometric bands (I = 0.8µm, J= 1.25µm andKS = 2.15 µm) for point sources are18m, 16mand13m respectively. The photometric accuracy (rms) is betterthan 0.1 mag and the astrometric accuracy better than 1 arcsec.These numbers are for uncrowded fields.DENISKS/(J − KS) colour magnitude diagrams (CMDs)in regions of low extinction show a well defined RGB and AGBsequence in the Galactic Bulge (see Fig. 1). Due to their highluminosities (up toMbol ∼ −4.5 for non-saturated sources inDENIS) these stars are ideal tracers of the stellar populationsin the Bulge and are found even in highly extincted regions.In this paper we present a method to derive interstellar extinc-tion using isochrones from Bertelli et al. (1994) in combina-tion with DENIS CMDs. We show that this method is appro-priate for low as well as for highly extincted regions. Finallywe present a map of the extinction in the inner Bulge between-8<`<8 and -1.5<b<1.5◦. The finer details of the features seenin the map will be discussed in a subsequent paper.2. ObservationsThe near infrared data were acquired in the framework of theDENIS survey, in a dedicated observation of a large Bulge field,simultaneously (Summer 1998) in the three usual DENIS bands,Gunn-I (0.8µm), J (1.25µm) andKS (2.15µm). For the sourceextraction we used PSF fitting optimised for the crowded fields(Alard et al. in preparation). For the astrometry, the individ-ual DENIS frames were cross-correlated with the PMM catalog(USNO-A2.0). The absolute astrometry is then fixed by the ac-curacy of this catalog (∼ 1′′). The internal accuracy of the DE-NIS astrometry, derived from the identifications in the overlapsis of the order of 0.5′′.LETTERL70 M. Schultheis et al.: Interstellar extinction towards the inner Galactic BulgeFor the determination of the zero point all standard stars ob-served in a given night have been used. The typical uncertaintyof the zero points has been derived from the overlapping regionsand is about 0.05, 0.15 and 0.15 mag in the I,J andKS bandsrespectively.3. Extinction determination using isochronesColour-Magnitude Diagrams were constructed for sources ina small window (radius of 2 arcmin) in the field. The modalvalue of the distribution of theAV required to move the starsin the CMD to the zero extinction isochrone was taken as thevalue of the extinction for this window. The interstellar ex-tinction law (AV : AJ : AKS = 1 : 0.256 : 0.089) from Glass(1999) was used. The window was then displaced laterally inuniform steps to construct an image of the spatial distributionof the extinction over the whole field. We have found that to-wards the Galactic Bulge a sampling window of radius 2 arcminprovides a sufficient number of stars to form a sequence en-abling a reliable estimate of the extinction in such a window.We presently do not use the DENIS data withKS < 7 due tothe saturation of the detectors. We only use those sources whichhave been detected in J as well as inKS with KS brighter than11 mag in order to be as complete as possible in J. This cri-terion is also quite important in order to rule out fake sourcesat the fainter end of the luminosity function and to guaranteean RGB/AGB identification. However, in the regions with veryhigh extinction (AV > 25) we find that a large proportion of thesources detected atKS do not have counterparts at the shorterwavelengths. The location of these ‘missing’ J sources is con-centrated in regions with high extinction (AV > 25m). Hencefor regions with large extinction, where the number of sourcesdetected with the above criterion is smaller than in regions withlower extinction, we only get a lower limit on the extinction.Results with such a sampling window and with displacementsteps of 1 arcmin are discussed below.Isochrones from Bertelli et al. (1994) placed at 8 kpc dis-tance for a 10 Gyr stellar population with Z=0.02 has been usedas a reference system. The isochrones were calculated for theESO filter system by convolving the near-infrared bands withthe spectral energy distributions from Kurucz (1992) for temper-atures higher than 4000 K. At lower temperatures the effectivetemperature scale from Ridgway et al. (1980) for the late Mgiants and the Lanc¸on & Rocca-Volmerange (1992) scale forthe early M giants has been used. The lack of very red stan-dards limits the near-infrared colour transformation (Bressan &Nasi 1999) and causes the colours of the Z=0.02 isochrone to‘saturate’ around(J − K)0 = 1.35. Therefore a new, empiri-calTeff -(J–K) colour relation has been derived by making a fitthrough theTeff/(J − K)0 data available for cool giants [seeSchultheis et al. 1998 and Ng et al. (in preparation)]. Schultheiset al. (1998) demonstrated the good agreement of the isochroneswith the newTeff -colour relation using NIR photometry of asample of Miras and Semiregular Variables in a field locatedat the outer Bulge. However, the upper part above the RGB tip(KS ∼ 8.0) remains nevertheless more uncertain.Fig. 1. DENIS KS/(J − K)S diagram for Baade’s window (Sgr I).Superimposed are isochrones with Z=0.008, Z=0.02 and Z=0.05, re-spectively (see text). Saturated stars (KS < 7) are not displayed.Based on observed near infrared spectra for a sample of Mgiants and Mira Variables, kindly provided by A. Lanc¸on, wehave found the difference between K andKS to be small, in theorder of 0.04–0.05 mag.3.1. Uncertainties of the isochrone methodFig. 1 shows a DENIS CMD in a part of the Baade’s Window(SgrI,∼ 0.06 deg2). Overlaid are the isochrones with differentmetallicities (forAV = 0). The isochrone with Z=0.02 is foundo follow well the observed CMD and is shown atAV = 0 andalso atAV = 1.35 which is the modal value of the extinctionfound for the field.It is a well known fact that in the Galactic Bulge one dealswith a wide metallicity and age range. Using isochrones with adifferent Z would give us different extinction values. In orderto estimate the effect of metallicity on the derived extinctionvalues, isochrones with Z=0.05 and Z=0.008 have been consid-ered. Between the isochrone with Z=0.008 and Z=0.05 there isa ∆(J − K) ∼ 0.2 (see also e. g. Bressan et al. 1998) whichcorresponds to a∆AV ∼ 1m. This is the typical uncertainty inthe determination of the extinction assuming solar metallicity.I contrast to the effect of metallicity the isochrones are hardlyaffected by an age-range (see e. g. Schultheis et al. 1998).Analysis of repeated observations (1996 & 1998) showshat the internal dispersion in the photometry, in the crowdedregions, is less than 0.15 mag forKS < 11. Compared to theLETTERM. Schultheis et al.: Interstellar extinction towards the inner Galactic Bulge L71Fig. 2. Observed CMDs with isochrones at derived extinction values (left panel) and histograms of the extinction values for individual sources(right panel) for sample fields (radius=2′) in Baade’s window (filled circles, solid line), a field located at`=-0.37 andb = 0.5 (filled triangles,dotted line) and around the Galactic Centre (open circles, dashed line).distance spread of the Galactic Bulge (∼ 0.35 mag) the errorsin the photometric accuracy as well as the errors coming fromthe isochrones are negligible.3.2. CMDs in sampling windowsIn Fig. 2 we show the CMDs for sampling windows at three dif-ferent locations, namely a field with low extinction in Baade’swindow, one with intermediate extinction at` = −0.37o andb = 0.5o and one at the Galactic Centre (` = 0, b = 0). The lowextinction field considered here is a small part of the Sgr I fieldwhose CMD is presented in Fig. 1. While the RGB/AGB branchin Baade’s window shows a narrow RGB/AGB sequence, for thetwo other fields one sees clearly a wide spread in the(J − KS)colour. The Baade’s window is known to have low and wellbehaved extinction and this is seen as a sharp peak in the distri-bution ofAV (Fig. 2). For the field at̀ = −0.37, b = 0.5 andaround the Galactic Centre one does not find a single well de-fined peak but perhaps two different peaks which indicates thatthere maybe two or more distinct layers of extinction causingmaterial along this line of sight. Alternatively the absorbing mat-ter may show clumpiness on a scale smaller than the 4 arcminutediameter of the sampling window. As mentioned earlier, our Jand K detections are complete in regions with low extinction (upto AV < 25) and a large proportion of K sources do not have Jcounterparts (up to 60–90%) in the regions with high extinction(GC). It is unfortunately not possible to use the results from theregions with low extinction to assign a completeness limit forthe obscured ones because the controlling factors are confusionin the first case and the detector sensitivity in the second. Amap of the ‘missing’ J sources shows that they are a significantsource of uncertainty in regions withAV > 25.Table 1.Mean value ofAV (in 0.1◦ × 0.1◦ box) along different linesof sight towards the inner Galaxyb ` = 6◦ ` = 3◦ ` = 0◦ ` = −3◦ ` = −6◦0.75◦ 6.8 8.9 12.0 11.9 13.70.00◦ 11.7 16.3 25.7 22.8 11.6−0.75◦ 14.7 10.9 10.9 13.5 7.74. ResultsThe whole map for the inner Galactic Bulge is shown in Fig. 3.The map has a resolution of 4 arcminutes (the diameter of thesampling window used). Note the clumpy and filamentary be-haviour of the distribution of extinction especially close to theGalactic plane. There are also small pockets with uniform ex-tinction of aboutAV = 6 magnitudes (for example the ISOGALfield ` = 0, b = 1 studied by Omont et al. 1999). Table 1 givesthe values of the mean extinction in various locations in the in-ner Bulge. The distribution of extinction along the line of sightthrough the Bulge will be discussed in detail in a subsequentpaper (Ganesh et al. in preparation).In Fig. 4 we present a contour map of the extinction aroundthe Galactic Centre. The distribution of extinction in this re-gion has been studied earlier by Catchpole et al. (1990) usingH&K data. We find a good agreement in the observed struc-tures keeping in mind the higher resolution results presentedhere. The difference inAV compared to Catchpole et al. is typ-ically smaller than 3. The average interstellar extinction alongthe Galactic plane (see Fig. 4) is≈ 24m in AV although thismight be a lower limit (as seen by the presence of patches withAV > 30) due to the effect of the ‘missing’ J sources discussedearlier. Along the minor axis theAV decreases to15m towardsLETTERL72 M. Schultheis et al.: Interstellar extinction towards the inner Galactic Bulge1.50.0-4.0-1.5Galactic Longitude (degrees)Latitude (deg)8.0 4.0 0.0 -8.0Fig. 3. Map of the extinction in the inner Galactic Bulge. The image grey scale represents anAV range in magnitudes from 0 (white) to 35(black). A coloured high resolution image can be found at http://www-denis.iap.fr/articles/extinction/21280.5LatitudeGalactic Longitude0.00.5 0.0 -0.53432333714 2121141414-0.5Fig. 4. Contour plot of the extinction around the Galactic Centre withcontours plotted in the range from 0 to 35 at 7 mag intervals inAV.The major contours are labeled in large italic fonts. Local maxima (withAV > 30) are also marked with the localAV values in small fonts.the edges, starting from nearly25m near the Centre. The derivedextinction values from Fig. 4 are also qualitatively in agreementwith the values found by Wood et al. (1998) near the GalacticCentre.5. ConclusionDENIS observations in the J andKS band together withisochrones by Bertelli et al. (1994) are an excellent tool to mapthe interstellar extinction. A comparison with the field studiedby Catchpole shows a very good agreement although we seemore details due to the better resolution. Several fields withrelatively low (AV ∼ 6) and homogeneous extinction can beidentified on the basis of the extinction map. This identificationshould help further detailed investigation of the stellar popula-tion in these windows. A study of the three-dimensional distri-bution of the material responsible for the interstellar extinctionshould be facilitated by the availability of the extinction map.Acknowledgements.We want to thank I. S. Glass, J. van Loon andY. K. Ng for useful discussions. The referee, G. P. Tiede, is thanked foruseful comments and suggestions. SG was supported by a fellowshipfrom the Minist́ere des Affaires Etrangeŕ s, France. MS acknowledgesthe receipt of an ESA fellowship. The DENIS project is partially fundedby European Commission through SCIENCE and Human Capital andMobility plan grants. It is also supported, in France by the InstitutNational des Sciences de l’Univers, the Education Ministry and theCentre National de la Recherche Scientifique, in Germany by the Stateof Baden-Ẅurtemberg, in Spain by the DG1CYT, in Italy by the Con-siglio Nazionale delle Ricerche, in Austria by the Fonds zur Förderungder wissenschaftlichen Forschung und Bundesministerium f¨ r Wis-senshaft und Forschung, in Brazil by the Foundation for the develop-ment of Scientific Research of the State of Sao Paulo (FAPESP), andin Hungary by an OTKA grant and an ESOC&EE grant.ReferencesBertelli, G., Bressan, A., Chiosio, C., Fagotto, F., Nasi, E., 1994,A&AS, 301, 381Bessell, M. S., Brett, J. M., 1988, PASP 100Bressan, A, Nasi, E., 1995, priv. communicationBressan, A, Granato, G. L., Silva, L., 1998, A&A 332, 135Catchpole, R. M., Whitelock, P. A., Glass, I. S., 1990, MNRAS 247,479Epchtein N. et al. 1997, Messenger 87, 27Frogel, J. A., Tiede, G. P., Kuchinski, L. E., 1999, AJ, 117, 2296Glass, I. S., 1999, Handbook of Infrared Astronomy, Cambridge, Univ.PressKurucz, R. L., 1992, “The stellar populations of galaxies”, IAUsymp. 149, 225Lançon, A., Rocca-Volmernage, B., 1992, A&AS 96, 593Omont, A., Ganesh, S., Alard, C., et al., 1999, accepted for publicationin A&APersi, P., Burton, W. B., Epchtein, N., Omont, A. (eds.), 1995, Proc. ofthe “1st Euroconference on near-infrared sky surveys, San Miniato,Italy”, Mem. S. A. It 66Ridgway, S. T., Joyce, R. R., White, N. M., Wing, R. F., 1980, ApJ235, 126Schultheis, M., Ng. Y. K., Hron, J., Kerschbaum, F., 1998, A&A 338,581Skrutskie, M., Schneider, S. E., Stiening, R. et al., 1997, “The impactof Large Scale Near-IR Sky Surveys”, eds. F. Garzon et al., p. 25.,Dordrecht, KluwerStanek, K. Z., 1996, AJ 460, L37Unavane,M., Gilmore, G., Epchtein, N., Simon, G., Tiphene, D., deBatz, B., 1998, MNRAS 295, 119Wood, P. R., Habing, H. J., McGregor, P. J., 1998, A&A 336, 925LETTER

Interstellar extinction towards the inner Galactic Bulge

4 pages, 4 figures, accepted for publication in A&A as a letter, see also http://www-denis.iap.fr/articles/extinction/DENIS observations in the J (1.2 micron) and K_S (2.15 micron) bands together with isochrones calculated for the RGB and AGB phase are used to draw an extinction map of the inner Galactic Bulge. The uncertainty in this method is mainly limited by the optical depth of the Bulge itself. A comparison with fields of known extinction shows a very good agreement. We present an extinction map for the inner Galactic Bulge (approx. 20 sq. deg.

Simon, Grégory

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Interstellar extinction towards the inner Galactic Bulge

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