Stellar populations and inhomogeneities of the galactic ... - CiteSeerX

Astron. Astrophys. 326, 597–607 (1997)

ASTRONOMY AND ASTROPHYSICS

Stellar populations and inhomogeneities of the galactic plane from DENIS star counts? S. Ruphy1 , N. Epchtein1 , M. Cohen2 , E. Copet1 , B. de Batz1 , J. Borsenberger3 , P. Fouqué1,5 , S. Kimeswenger4 , F. Lacombe1 , T. Le Bertre1 , D. Rouan1 , and D. Tiphène1 1 2 3 4 5

Observatoire de Paris, F-92195 Meudon, France Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA Institut d’Astrophysique de Paris, 98 bis Bd. Arago, F-75014 Paris, France Institut für Astronomie der Leopold-Franzens Universität, Technikerstraβe 25, A-6020 Innsbruck, Austria European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received 17 April 1997 / Accepted 23 May 1997

Abstract. This paper reports on the analysis of 3-colour (IJKs ) near infrared star counts obtained during the commissioning phase of the Deep Near Infrared Southern Sky Survey (DENIS) project. Observing methods, data processing, source extraction and calibration are briefly outlined. Nine strips of 12’ of RA x 30◦ in declination crossing the galactic plane at different longitudes ranging from 217◦ to 385◦ , covering approximately 50 square degrees are analyzed. One strip has been measured in 3 colours, the others in J and Ks only. More than 250 000 objects are detected and calibrated. The completeness limits are 17.5, 15.5 and 13.5 in the I, J and Ks bands, respectively. Colourcolour diagrams are shown to be efficient tool for breaking out dwarf and giant star populations especially at low latitude, where the interstellar extinction strongly affects the far away red giants. Source counts are compared to the SKY model developed by Wainscoat et al. (1992). This small sample of DENIS data is tentatively used to investigate the existence and the spatial distribution of the stellar populations associated with the molecular ring at RG.C. = 4 kpc. Key words: galaxy: structure – galaxy: stellar content – infrared: stars

1. Introduction The new large near-infrared surveys currently in progress, 2MASS (Skrutskie et al., 1997) in the US, and DENIS (Epchtein, 1997) in Europe, open a new field of investigation of galactic structure, and especially of the spatial distribution of Send offprint requests to: [email protected] ? Based on observations collected at the European Southern Observatory, La Silla, Chile

stars in obscured areas of our Galaxy. Most of our knowledge on the distribution of stellar populations in our Galaxy has been, so far, essentially based on star counts carried out in the optical range using digitized Schmidt plates or deep CCD exposures restricted to small areas of the sky (e.g. Robin, 1994a). With the advent of large scale and relatively deep surveys in the nearinfrared range, astronomers are provided with new extremely valuable data, since the 1-2.5 micron range offers unique advantages over the optical one: the interstellar extinction is much weaker and it corresponds to the peak emissivity of cool stars (red dwarfs, evolved red giants and supergiants). It allows probing hidden stellar populations in the bulge and highly obscured molecular clouds, and taking a census of evolved giants throughout the Galaxy. Near-infrared surveys (e.g., Price, 1997, Garzón et al., 1993, Kent, 1996) have been so far restricted to a limited area of the sky or to very bright sources by lack of sensitivity (Two Micron Sky Survey, Neugebauer & Leighton, 1969, Two Micron Galactic Survey, Garzón et al., 1993). The DENIS survey (Deep Near-Infrared Survey of the Southern Sky) provides a digitized survey of all the southern sky, with a 3σ detection limit of 18, 16 and 14, in the I, J and Ks bands respectively. The survey started in December 1995, but the instrument (Epchtein et al., 1994) has been in operation since September 1994, for a “protosurvey” phase in the J and Ks bands, and a significant fraction of the sky has already been surveyed in these two bands. Thanks to DENIS, relatively deep near-infrared star counts are now available for the first time on a large scale. The basic tool to interpret star counts in terms of galactic structure is comparison with predictions given by models of the point source sky. The star counts test the overall consistency of models of the Galaxy and enable us to infer various global parameters such as, for instance, the scale length and the distance of the cutoff of the thin disc, or the scale length and scale height of the thick disc. Another major interest, which is one

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S. Ruphy et al.: Stellar populations and inhomogeneities of the galactic plane

of the purposes of the present paper, is to investigate some localised aspects of the stellar distribution in the plane, namely inhomogeneities such as the spiral arms and the molecular ring. Here we present the first comprehensive set of deep nearinfrared star counts and colour diagrams issued from DENIS, in different directions along the galactic plane. The possibility to break out dwarfs from giants in I-J/J-Ks colour-colour diagrams is first discussed. The set of data is then compared with star counts predicted by the Wainscoat et al. (1992) model (hereafter the SKY model) and allows us to test the model, for the first time extensively in the near-infrared. We show that the DENIS star counts in the inner part of the galactic plane put new constraints on the shape of the molecular ring and the luminosity function of the spiral arms.

the Netherlands). Each of them involves distinct stages in the data processing pipeline and makes use of dedicated software. The first step, performed at PDAC, essentially consists of flat– fielding, subtraction of bias and darks and deglitching of the raw images. The second step, performed at LDAC, basically consists of source extraction, astrometric and photometric calibration and “small source” data archiving in a database. Although at the time of the preparation of this work, the first step was running in nominal conditions, the second one was not fully in operation. Therefore, the source extraction, photometric and astrometric calibrations have been performed by the authors using the methods briefly described in Sect. 3.2. More details can be found in Ruphy (1996). 3.1. Darks, bias and flat fielding

2. Observations The observations presented here were made over the period April-June 1995, as part of the “protosurvey”, using the DENIS camera equipped with two operational channels, in J (1.25 µm) and Ks (2.15 µm), attached to the focus of the ESO 1m telescope at La Silla, Chile. They consist of a set of 8 strips crossing the galactic plane at different longitudes. An additionnal strip was observed in all three bands (I-Gunn, J and Ks ) in December 1995. A strip is the DENIS basic unit of observation used to scan the sky in a step and stare mode, by steps of 10’, at constant right ascension, near the meridian, along declination arcs of 30◦ . A strip is made up of 180 elementary images of 12’×12’ (256 × 256 pixels) and covers an area of 12’×30◦ . The data were acquired in the standard conditions of the survey, namely with a 10 s integration time and a 3 arcsec spatial resolution. A detailed description of the instrument and a complete report on its performance can be found in Epchtein (1997), and Copet (1996). The areas of the sky covered by the observations reported in this article are listed in Table 1. This selection of strips was motivated by both scientific and practical arguments. First, the directions were chosen to provide new observational inputs for specific questions of galactic structure. Strips 1903 and 3064 correspond to the anticenter direction and have permitted us to investigate the radial scale length and the distance of the cutoff of the stellar disc (Ruphy et al., 1996). Strips 2501, 2502 and 4034 cross the ring, and strips 2292, 2299, 2225 and 2305 pass through the local spur and the Sagittarius arm and sample the galactic plane at two different longitudes. Then, for each position, in order to improve the statistics of the star counts, two adjacent strips were selected for their high quality among the data available, and then processed and calibrated as detailed in next section. 3. Data reduction The nominal DENIS processing involves 2 specially designed data analysis centers (Deul et al., 1995), one located in Paris (the Paris Data Analysis Center, or PDAC, housed at Institut d’Astrophysique de Paris), the other in Leiden (the Leiden Data Analysis Center, or LDAC, housed at Leiden Observatory in

Dark is first subtracted in each image. The mean level and standard deviation of the background E(t) are then estimated using an a priori flat Si,j and a clipped mean: E(t) = hmi,j (t)/Si,j iclipped where mi,j is the intensity minus dark for the pixel (i,j). Images with the lowest standard deviation in the night are selected, together with a set of images taken at sunrise. A two parameter iterative regression is performed on these selected images: M in

N X

(mi,j (t) − Si,j E(t) − Bi,j )2

t=1

from which the flat Si,j and the bias Bi,j are derived. The previous procedure is then iterated with the new flat and bias estimations. The bias obtained by the regression is only an average bias for the night. It does not take into account some slight variations of the pixels’ zero values during a night. Therefore, a final bias is determined for each strip, using a clipped mean: Bi,j = hmi,j (t) − Si,j E(t)iclipped The whole image processing is presented in detail by Borsenberger (1997). 3.2. Source extraction In order to improve the detection limit and the deblending, the images are pre-smoothed by a “Mexican Hat filter” shape function following the wavelet analysis method developped by Coupinot et al. (1992); we adopt for the filter a spatial dimension of 11×11 pixels, and we subtract the background on each frame before the convolution. This background is estimated locally on each image in a grid of 32×32 pixels, by software developed by Bertin & Arnouts (1996). This software is also used for the actual extraction on the convolved images: the program searches for sets of contiguous pixels above a given threshold. Experimentation has proven that a threshold of 3σ above the rms of the convolved image and a minimum area of 4 connected pixels


599

Table 1. Areas of the sky observed by DENIS ∆δcentre J2000 +2◦ to −27.9◦

∆(l,b)

l for b = 0◦

J & Ks

αcentre J2000 6 h 59 min 20 s

(211.8◦ , 3.3◦ ) to (238.8◦ , −10.4◦ )

' 217◦

3064

I, J & Ks

7 h 42 min 00 s

+2◦ to −27.9◦

(216.9◦ , 12.15◦ ) to (242.48◦ , −2.4◦ )

' 239◦

2225 2305

J & Ks J & Ks

10 h 54 min 40 s 10 h 56 min 00 s

−57.7◦ to −87.6◦ ”

(287.8◦ , 1.7◦ ) to (301.6◦ , −25.0◦ ) ”

' 289◦ ”

2292 2299

J & Ks J & Ks

12 h 53 min 20 s 12 h 54 min 40 s

−57.7◦ to −87.6◦ ”

(303.2◦ , 5.2◦ ) to (302.9◦ , −24.7◦ ) ”

' 303◦ ”

2501 2502

J & Ks J & Ks

16 h 19 min 13 s 16 h 20 min 00 s

−57.8◦ to −27.9◦ ”

(327.8◦ , −5.3◦ ) to (349.2◦ , 15.7◦ ) ”

' 333◦ ”

4034

J & Ks

16 h 26 min 9 s

−57.8◦ to −27.9◦

(328.5◦ , −6.0◦ ) to (350.3◦ , 14.6◦ )

' 335◦

Strip number 1903

Bands

5

is optimal for the selection of faint sources and the rejection of fake sources.

4 3

For each frame, the positions are first corrected for pointing offsets, using cross-identification with the Guide Star Catalog (GSC). This pairing also provides the coefficients of secondorder polynomials to correct for the distortion. For each object in the J catalog, the closest source in the Ks catalog within 5” is selected and assumed to be its Ks counterpart. Since the positions in the J and Ks catalogs are derived from the GSC catalog independently, the 1σ positional error derived from the J/Ks correlation corresponds to an upper limit for the DENIS position accuracy. Its typical value is 0.7 arcsecond. This error is only relative to the GSC and does not take into account the uncertainty in the GSC positions. Fig. 1 shows the difference in RA and declination for all objects (' 30000) cross-correlated in J and Ks in one strip (strip 1903). The same procedure is applied to the I catalog and gives a similar precision in the positions. A source of inhomogeneity in the astrometric accuracy is the number and the distribution of the GSC stars within a field, which varies from one field to the other and, in a few cases, does not provide an accurate distortion matrix. 3.4. Photometric calibration Photometry was done at fixed aperture on the smoothed images. An aperture diameter of 6” was adopted, to fit the smoothing 2.1” FWHM. To perform the photometric calibration of a strip, one standard star, chosen as close as possible to its start point, was observed at the beginning of the strip. For the strip 3064, an additional standard star was observed at the end of the strip. Each standard star was observed eight times, the zero-point was determined for each frame, and then the average value of these eight consecutive observations was applied to the whole strip. The typical scatter of the zero-points about these average values is less than 0.1 mag, slightly varying from one standard star to

2

deltaJ-deltaK (arcsecond)

3.3. Astrometric calibration

1 0 -1 -2 -3 -4 -5 -5

-4

-3

-2

-1 0 1 alphaJ-alphaK (arcsecond)

2

3

4

5

Fig. 1. Difference in RA and declination for sources detected in both J and Ks in the strip 1903. The 1σ positional error is equal to 0.7.

the other. For each band, the polynomials of the 1σ photometric error as a function of magnitude were obtained by comparing the magnitudes of all stars in the eight fields observed for the standard star. These polynomials are plotted in Fig 2. Furthermore, the overlap between two adjacent strips allows us to compare the magnitudes for a given object observed in two different strips at two distinct dates. This is very useful to check the adopted zero point value and the photometric conditions for each strip, provided that an adjacent strip is available. The agreement of the magnitudes of objects in common in two adjacent strips was satisfactory for all the strips presented in this paper (consistent within +/- 0.1 magnitude). 4. Results 4.1. Completeness limit Before any comparison of the star counts with point source sky models can be made, the completeness limit of the DENIS catalogs must be determined. A quantitative analysis of this param-

600

S. Ruphy et al.: Stellar populations and inhomogeneities of the galactic plane 0.3

Table 2. Maximum distances of detection of stars with DENIS (extinction is not taken into account). The absolute magnitudes are from Wainscoat et al. (1992). MI is derived from MJ using the I-J colour indices given by Johnson (1968a).

0.25

sigma

0.2

Type

0.15

0.1

0.05

0 6

8

10

12 magnitudes I, J, K

14

16

18

Fig. 2. 1σ photometric internal error as a function of magnitude, in each DENIS band. 2: KS band; : J band; +: I band

eter would require, for instance, the use of simulated images or multiple exposures of the same field, and depends on the extraction process. Since the extraction process involved here is not the nominal DENIS process, the completeness limit is simply estimated from differential star count histograms. Apart from slight variations due to the observing conditions, the only significant variation comes from crowding, which causes blending of individual sources, increases the background level above which a source is detected at 3σ, and, consequently, decreases the completeness limit. A conservative estimation of this limit, for uncrowded fields (i.e. less than 2000 sources per frame) is: 17.5 in I, 15.5 in J and 13.5 in Ks . 4.2. Separation of dwarfs and giants To get a rough idea of the kind of stars observed with DENIS, the maximum distance at which a star is detected in each DENIS band can be straightforwardly estimated from the maximum distances calculated without extinction and listed in Table 2, assuming that a typical value for AV in the outer part of the Galaxy is 2 at 4 kpc and beyond, and assuming that AI = 0.473 AV , AJ = 0.245 AV and AK = 0.091 AV (Johnson, 1968b). For instance, the most numerous kind of dwarfs in the Galaxy, namely the M dwarfs, are detected in the 3 DENIS bands only up to a few hundreds of parsecs, whereas the numerous K giants are detected up to the edge of the galactic disc. A first step in the analysis of the different types of stars is to be able to separate giants from dwarfs. This separation is very useful since, for instance, the dwarf to giant ratio depends on the initial mass function and on the star formation rate. Therefore, as explained by Robin (1994b), the determination of this ratio at different latitudes and magnitudes should constrain the star formation rate. Another interest is to be able to select some samples of specific stars, such as giants of a given spectral type, in order to investigate the spatial distribution of these stars at large distance from the Sun. The reliability of this separation, using only the DENIS bands, depends on several parameters: the magnitude range and the amount of extinction along the line of

MI

B0,1 V B8-A0 V F8 V K4-5 V M4-5 V F8-G2 III K0,1 III M0 III M7 III

-3.5 0.3 3.4 5.76 9.13 1.3 -0.06 -2.52 -7

MI -MJ MJ -MK d (kpc) d (kpc) d (kpc) for I=18 for J=16 for K=14 -0.35 -0.15 200 67 25 -0.01 -0.02 30 14 5.4 0.23 0.29 8.3 3.7 3.2 0.41 0.74 2.8 1.3 0.75 0.80 0.90 0.8 0.3 0.2 0.30 0.55 22 10 5 0.39 1.05 41 20 10.6 0.65 1.07 127 68 42 0.67 1.37 1000 540 400

Table 3. Field characteristics: area in square degree, position, number of sources detected in the 3 bands in the field. Field 1 2

Area (deg2 ) 0.48 0.79

∆(l,b)

N

(237.7◦ , 0.55◦ ) to (239.0◦ , 1.55◦ ) (219.7◦ , 8.6◦ ) to (223.7◦ , 10.8◦ )

3800 2600

sight, as a function of longitude and latitude. We first consider this separation in a direction of relatively low extinction, the anticenter direction and then we look at more central regions of the Galaxy. 4.2.1. I-J/J-Ks diagrams in the anticenter direction A detailed analysis of two fields, one in the galactic plane, and one at intermediate latitude, has been conducted to investigate the question of the dwarf/giant separation in the outer part of the galactic plane. The general characteristics of the two fields (field 1 corresponding to 15 DENIS images, and field 2 to 25 images) are given in Table 3. We checked on the I images that obscuration was apparently uniform, or did not, at least, present any obvious inhomogeneity in these two fields. Fig. 3 displays I-J/J-Ks colour-colour diagrams of DENIS sources in field 1, for different ranges of magnitudes. Due to the saturation limits of the detectors, stars brighter than 7 in Ks , 8 in J and 10 in I are ignored. Two distinct concentrations of stars are clearly seen on the first diagrams, namely for sources brighter than 13 in Ks . A rough separation can be set at J-Ks = 0.6 and I-J = 0.7. The effect of decreasing photometric accuracy contributes to the increasing dispersion of the distribution at faint magnitudes in Ks and blurs the separation between the two concentrations in the last bin. Therefore, only the first four diagrams are considered to be significant, and we will restrict further analysis to sources brighter than 13 in Ks . J-K and I-J colour indices of dwarfs and giants given by Bessel and Brett (1988) indicate that the bluer concentration should correspond to the dwarfs, and the redder to the giants. Since no giant is intrinsically bluer than J-Ks = 0.45, which corresponds to the bluest giants G0, and since any giant fainter than 10 in Ks should be at least about 1 kpc from the Sun and will have, in

S. Ruphy et al.: Stellar populations and inhomogeneities of the galactic plane 7 < K < 10

601

10 < K < 11

2

2

1.5

1.5

I-J

2.5

I-J

2.5

1

1

0.5

0.5

0

0

-0.5 -0.5

0

0.5

1

1.5

-0.5 -0.5

2

0

0.5

J-K

J-K

11 < K < 12

12 < K < 13

1

1.5

2

1

1.5

2

1

1.5

2

2.5

2.5

2

2 Av = 2

1.5

I-J

I-J

1.5

1

1

0.5

0.5

0

0

-0.5 -0.5

0

0.5

1

1.5

-0.5 -0.5

2

0

0.5

J-K

J-K

13 < K < 13.5

13.5 < K < 14

2

2

1.5

1.5

I-J

2.5

I-J

2.5

1

1

0.5

0.5

0

0

-0.5 -0.5

0

0.5

1

1.5

2

J-K

-0.5 -0.5

0

0.5 J-K

Fig. 3. I-J/J-Ks colour-colour diagrams of DENIS sources at about l= 218◦ (field 1), for different ranges of magnitudes (from left to right and top to bottom): 7 ≤ Ks ≤ 10, 10 ≤ Ks ≤ 11, 11 ≤ Ks ≤ 12, 12 ≤ Ks ≤ 13, 13 ≤ Ks ≤ 13.5, 13.5 ≤ Ks ≤ 14.

the direction of field 1, a dust reddening in J-Ks larger than 0.1, the bluer concentration cannot contain any giant. But the redder concentration may contain K and M dwarfs, specially at faint magnitudes. Nevertheless, simulations obtained with a model of synthetic stellar populations of the Galaxy (the Besançon model, Robin et al. (1986)) in that field indicate that the proportion of dwarfs to giants in that part of the diagram should be

only a few% for the 10 ≤ Ks ≤ 11 bin and could reach ' 10% for the 12 ≤ Ks ≤ 13 bin. So, at Ks = 13, giants can be easily separated from dwarfs in the galactic plane, using J-Ks /I-J colour-colour diagrams. Is this separation due to the reddening effect of extinction on the giants in the galactic plane, or to the intrinsic colour distribution of each population? In order to estimate the intrinsic J-Ks colour distribution of the DENIS sources

602


Fig. 4. J-Ks distribution for sources in field 1 at about l= 218◦ (from left to right and top to bottom): 7 ≤ Ks ≤ 10, 10 ≤ Ks ≤ 11, 11 ≤ Ks ≤ 12, 12 ≤ Ks ≤ 13; dotted line: observed distribution, solid line: dereddened distribution. The two distributions differ only for sources having a J-Ks ≥ 0.45, namely sources that are likely to be giants.

having a J-Ks ≥ 0.6, we applied a simple dereddening procedure: for each source assumed to be a giant, the J-Ks gives a first estimation of the spectral type, based on the colour indices versus spectral type given by Bessel and Brett (1988), this spectral type is then converted into an absolute K magnitude, using the luminosity function parameters at 2.2 µm of Garwood and Jones (1987). A first estimation of the distance of the source is derived from MK . The interstellar extinction toward each source is evaluated using an exponential model of dust distribution, similar in form to the disk stellar distribution. According to Jones et al. (1981), the radial scale length for the extinction is 4 kpc, and a value of 100 pc is used for the scale height. We assumed that the dust absorption in the solar neighbourhood is 0.7 mag/kpc in V, which corresponds to 0.064 mag/kpc in K (Johnson, 1968b), and we adopted a solar galactocentric distance of 8.5 kpc. Assuming that EJ−K = 0.154AV (Johnson, 1968b), the J-Ks is corrected from the colour excess, and a new distance is derived. This procedure converges in most cases after a few iterations. The same process could have been applied to the I-Ks colour indice, but since EI−K = 0.378AV ,

the dereddened distribution would have been more dependant on the adopted value for AV . As discussed previously, the expected proportion of dwarfs with a J-Ks ≥ 0.6 should not be negligible in the last bin of magnitude, as it is in the first three bins. This is confirmed by the fact that after dereddening, a small fraction of sources (' 10%) has a J-Ks inferior to 0.45. These sources are obviously dwarfs that should not have been dereddened. Therefore, we kept the observed colours of these sources unchanged in the dereddened distribution. In the first three bins of magnitude, only one source has a J-Ks inferior to 0.45 after dereddening, which is compatible with the very low proportion of dwarfs with a J-Ks ≥ 0.6 expected at those magnitudes. Fig. 4 compares the observed and the dereddened J-Ks distribution, in each bin of magnitude, for sources in field 1. The following straightforward conclusion can be drawn from these comparisons: except for the few sources brighter than ' 11 in Ks , for which dwarfs and giants are separable thanks to their intrinsic J-Ks colour indice, the separation of the two populations is made possible mainly by the effect of extinction on the colour indices of giants. According to the dereddened distribution presented on Fig. 4, G8/K0 are the dom-


Fig. 5. J-Ks distribution for sources in field 2 in the 12 ≤ Ks ≤ 13 interval.

inant spectral types of the giants detected by DENIS in the outer part of the galactic plane. Concerning the proportion of main sequence stars to giants, as the magnitude is fainter, this proportion increases, since giants of earlier spectral type become quickly population bounded and no longer contribute to the star counts. When looking at higher latitude, for instance at b ' 9◦ (field 2), the reddening of the giants is too weak to allow separation of dwarfs and giants: the J-Ks distribution for sources with 12 ≤ Ks ≤ 13, seen in Fig. 5 is no longer bimodal, as it is in the galactic plane (field 1, see Fig. 3). 4.2.2. Ks /J-Ks magnitude-colour diagrams at different longitudes along the galactic plane Ks /J-Ks diagrams are presented in Fig. 6 for four different longitudes in the galactic plane (The I data were not yet available in those fields). When going closer to the inner part of the Galaxy, the red branch dominated by the giants is progressively spread by the increasing reddening. The lack of sources in the lower right parts of the diagrams is due to the limit in sensitivity in the J band. Such diagrams indicate that the separation of dwarfs to giants based on the J and Ks colours is no longer straightforward for l ≥ 300◦ . Furthermore, no simple dereddening process as the one previously used can be applied in those directions, because of the likely inhomogeneity of the dust and gas distribution along the line of sight. Further analysis, combining data at other wavelengths (in I band and mid-infrared bands) will be necessary to disentangle the contributions of the different stellar populations in such diagrams. 4.3. Comparisons with the SKY model The DENIS star counts presented in the previous sections were used to test a model of the point source sky -the SKY model-, for the first time extensively in the near-infrared, and to derive new constraints on the modeling of the molecular ring and the spiral arms. The SKY model is based on a realistic representation of the Galaxy, including features such as disc, spiral arms, local

603

spur, molecular ring, bulge and halo, and delivers star counts in many filters lying within a large spectral range, from far-UV to mid-infrared. The basic model is described by Wainscoat et al. (1992), and was considerably enhanced by Cohen (1994, 1995). The model has been successfully compared with DENIS star counts at high and intermediate latitude (Ruphy et al., 1995). The model is tested here for the first time in J and Ks on a large scale along the galactic plane. Comparisons were made on the whole strips listed in Table 1, by steps of 3◦ in declination at intermediate latitudes (b≥10◦ ), and by steps of 1◦ at low latitudes (b≤10◦ ). In order to improve the statistics of the counts, when two adjacent strips were available, we averaged the star counts in the two strips. We scaled all counts to an area of 1 deg 2 , but retained the true Poisson errors associated with the observed counts. We show in this paper only a selective sample of these comparisons. A more comprehensive set of comparisons can be found in Ruphy (1996). 4.3.1. Comparisons SKY/DENIS at l=303◦ Fig. 7 illustrates the comparison of predicted differential star counts (log N/mag/deg2 ), at l=303◦ for different latitudes (strip 2299 and 2292). The effect of extinction when crossing the Chameleon cloud (l=303◦ , b=-14.63◦ ) is clearly seen on the J counts which are below the expected number, whereas the Ks counts are not affected. In that direction, the contribution of the disc largely outnumbers the contributions of the other components (ring, spiral arms, halo).The agreement between the model and the data is quite satisfactory across the whole range of latitudes, and is also verified at other nearby longitudes (strips 2225 and 2305 crossing the plane at l=289◦ ). Nevertheless, in those directions, the model systematically underestimates the number of bright sources (KS ≤ 10) observed at low latitudes (|b| ≤ 3◦ ). Preliminary analyses suggest that this discrepancy is due to an imperfect luminosity function for the spiral arms in the model and can be alleviated by inserting a population of K-bright supergiants into the model’s spiral arms. The same phenomenon can be seen in counts at brighter K magnitudes, like the Two Micron Galactic Survey, hereafter TMGS, (Garzón et al., 1993)), after removing disc stars according to SKY’s predictions. This is discussed elsewhere (Cohen et al., 1997, in preparation). We note here merely that the combination of DENIS and TMGS counts can serve as a valuable guide to the requisite modifications to SKY’s representation of the cores of spiral arms. 4.3.2. Modeling of the molecular ring In our Galaxy, the existence of a broad annulus of molecular gas (commonly called the “molecular ring”) was established independently by Burton et al. (1975) and by Scoville & Solomon (1975). The exact distribution of gas in this structure, and its physical conditions, are rather poorly known. Indeed, substantive differences occur between the CO mapped in the northern (Clemens et al., 1988) and southern (Robinson et al., 1988) hemispheres. The CO is more highly clumped than the HI in

604


7

7

8

8

9

9

10

10

K

6

K

6

11

11

12

12

13

13

14

14

15

15 -1

0

1

2 J-K

3

4

0

5

1

2

3

4

5

6

3

4

5

6

J-K

7

7

8

8

9

9

10

10

K

6

K

6

11

11

12

12

13

13

14

14

15

15 0

1

2

3

4

5

6

J-K

0

1

2 J-K

Fig. 6. J-Ks distributions at different longitudes along the galactic plane. Upper left: (l = 238◦ , b = 1◦ ), upper right: (l = 289◦ , b ' 0◦ , lower left: (l = 303◦ , b ' 0◦ ), lower right: (l = 335◦ , b ' 0◦ )

the inner Galaxy (Bania, 1977) although the inferred overall gas distribution, projected onto the Galactic plane, is somewhat suggestive of a clumped, elliptical structure (Clemens et al., 1988). There are several cogent reasons why one might prefer to model the “molecular ring” by a non-uniformly dense elliptical, rather than a uniform circular, pattern. Such a distribution provides a natural explanation for: i) the north-south asymmetries in integrated CO intensity and galactocentric range over which CO is observed (Robinson et al., 1988), ii) the face-on global distribution of gas in the first quadrant (Clemens et al., 1988), and iii) the wider range of latitudes over which molecular gas is detected in the first quadrant (Dame et al., 1987), suggesting that any such ellipse has the nearest end of its major axis lying within the first quadrant. Also in the inner Galaxy, both HI (e.g, Cohen & Davies, 1976) and CO (Bania, 1977) observations delineate a structure known as the “3 kpc expanding arm”. This entity is traceable and kinematically distinct from gas moving at conventional velocities only in the longitude range 336◦ to about 10◦ . Several analyses of the HI gas associated with this feature were written within the framework of the density-wave theory (e.g., Shane, 1972; Simonson & Mader, 1973; Peters, 1975), seeking a so-

called “dispersion ring”, where stars travel in elliptical orbits near the inner Lindblad resonance. In this interpretation, the near-side of the ellipse has a tangential direction near longitude 25◦ , with the corresponding fourth quadrant tangent near longitude 338◦ . The far-side of this structure (beyond the Galactic Centre) was also identified by Simonson & Mader, (1973) with “feature II” in the HI observations (e.g., Shane, 1972; Cohen & Davies, 1976). Bania (1977) argued that one does not infer the existence of a continuous ring of neutral gas from the HI data: any such ring must have considerable clumps of ionized or molecular gas. Bania (1977) also demonstrated from CO measurements near longitude 24◦ (the approximate tangent to the putative ring) that too little emission is seen for any continuous ring, especially a uniform one. However, the ellipse suggested in detail by Simonson & Mader (1973) is not uniform and has strong density enhancements in its two “ansae”. The significance of the above discussion is that, although the gas distribution of the inner Galaxy is complex and the “3 kpc expanding arm” is not identical to the ring, this feature nevertheless must lie immediately interior to the molecular ring, if not in direct interaction with the molecular gas. One might reasonably imagine that any such contiguity would have forced the CO gas to con-

605

Log N / mag


J

K

Fig. 7. Comparison of SKY predictions and DENIS differential star counts in J (left column) and Ks (right column), in a sample of fields extracted from strips 2299 and 2292 (l =303◦ , at latitude b = 2.37◦ , 0.37◦ and -14.63◦ , from top to bottom). Diamonds, observed star counts; solid line, total model prediction; dots (very close to the solid line), disc; long dashes, spiral arms; long dash-dot, halo.

form to the same gross pattern of density inhomogeneties as are present in the expanding feature. Thus, any stars formed within the expanding arm, and those formed in regions of star formation in the ring that were triggered by the encounter with this arm, would be expected to mimic the gaseous non-uniformities in the “3 kpc expanding arm”. Under this assumption, we have chosen to build a new molecular ring into the SKY model, replacing

the earlier circular structure by an ellipse with the same proportions and density enhancements as that proposed by Simonson & Mader (1973: who used 10 kpc as the distance from the Sun to the Galactic Centre), but translated to the currently believed physical location of the actual molecular ring gas. Indeed, the normalization of the new ring within SKY is almost identical with that of the old when areally integrated, weighted by the


Log N / mag

606

K

K

Fig. 8. Comparison of SKY predictions and DENIS differential star counts in Ks , in a sample of fields extracted from strip 4034. Right: elliptical ring; left: circular ring. Top: l = 331◦ , b = -3.9◦ ; Bottom: l = 334◦ , b = -1.12◦ ; Diamonds, observed star counts; solid line, total model prediction; dots (very close to the solid line), disc; long dashes, spiral arms; long dash-dot, halo; short dashes, bulge; short dash-dot, molecular ring.

radially decaying stellar populations (cf. Eq. (14) of Wainscoat et al., 1992). We have also preserved the direction of the major axis of the Shane (1972) and Simonson & Mader (1973) ellipse. This new construct is a better approximation to the face-on map of the CO offered by Clemens et al. (1988) than was the previous circular ring. It offers a compromise between ad hoc complexity (i.e., representing the real clumps of CO gas in SKY) and analytic simplicity for the model that still provides the requisite degree of asymmetry in the inner Galaxy to interpret those observations which some feel argue for a highly elongated bar. The presence of the dense ansae also represents a first step toward clumpiness in our simulated ring. There is independent support for ellipticity of the inner Galaxy from the recent nonparametric light and density distributions of Binney, Gerhard & Spergel (1997), constructed from COBE/DIRBE maps, while elliptical rings are not uncommon in other spiral galaxies. The detailed geometry has been probed by comparing predicted and observed star counts in directions crossing the supposed circular ring. Fig. 8 shows a sample of the comparisons in strip 4034. Similar conclusions can be drawn from comparisons along strips 2501 and 2502. An elliptical shape gives a better fit than a circular shape: the systematic excess of faint sources

(11 ≤ K ≤ 13) seen with a circular shape is significantly reduced with an elliptical shape. Furthermore, a non-uniform ellipse orientated with major axis offset by 30o from the line of sight to the Galactic Centre and these dense ansae (Simonson & Mader, 1973) naturally creates an excess of young and massive stars around the l = 20–27o direction, which is qualitatively in agreement with peaks in bright star counts seen in the TMGS data (Hammersley et al., 1994). Nevertheless, this preliminary result has to be confirmed over a larger interval of longitudes -namely the directions for which the contribution of the ring is significant (335o ≤ l ≤ 345o )when more DENIS data are available. We emphasize that the primary value of DENIS data is to establish the geometry of the ellipse rather than to probe the dense ansae because, in these longitudes, our line of sight travels along the low gas density regions of the ring, between the ansae. It remains for the future to see whether we will have sufficiently finely sampled star counts with DENIS, that we can actually sculpt the stellar populations associated with the ring clumps portrayed by Clemens et al. (1988) and Robinson et al. (1988) and, with our assumption of the relevance of the ex-


panding arm, confirm Bania’s (1977) suggestion that this HI arm must have substantial regions of HII or molecular gas. 5. Conclusion The conclusions of the present paper are twofold: first, we show that dwarfs and giants are separable in DENIS colour diagrams only in the outer part of the galactic plane (roughly l ≥ 300o ). In the inner parts, the identification of the different stellar populations requires complementary data to disentangle the effects of extinction and the contributions of localised inhomogeneities such as the molecular ring and the spiral arms. In that respect, the cross-identification of the DENIS data with the observations at 7 and 15 µm obtained by the ISOGAL program on small areas of the galactic plane should greatly help, and preliminary results have been already obtained (Pérault et al., 1996). Second, the extensive comparisons between DENIS star counts and the SKY model have revealed the ability of nearinfrared observations to produce an accurate representation of the galactic disc, and even first-order representations of inhomogeneities such as the molecular ring and the spiral arms. In specific directions, the star counts put new constraints on the modeling of the molecular ring, for which an elliptical shape seems a more accurate representation than a circular shape. In the near future, massive processing of the whole set of DENIS data, will completely renew our description of the galactic structure and more specifically of the inhomogeneities of the galactic disc. Acknowledgements. The DENIS team is warmly thanked for making this work possible and in particular the operations team at La Silla. The DENIS project is supported by the SCIENCE and the Human Capital and Mobility plans of the European Commission under grants CT920791 and CT940627, the European Southern Observatory, in France by the Institut National des Sciences de l’Univers, the Education Ministery and the Centre National de la Recherche Scientifique, in Germany by the State of Baden–Wurttemberg, in Spain by the DGICYT, in Italy by the Consiglio Nazionale delle Ricerche, in Austria by the Science Fund (P8700-PHY, P10036-PHY) and Federal Ministry of Science, Transport and the Arts, in Brazil by the Fundation for the development of Scientific Research of the State of São Paulo (FAPESP). We are grateful to W. B. Burton for his careful reading of the manuscript and his useful comments.

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