5. Highlights of DENIS scientific results
5.1. Faint stars in the solar neighbourhood: red and brown dwarfs
The large range of DENIS wavelengths (from the I-band to the K-band is a factor 2.5 in wavelength) makes the survey data well suited for the detection of red dwarfs of the latest spectral types (M7 or later) about which little is known. The analysis of 220 square degrees of high galactic data led to the identification of 25 dwarfs later than M7V, doubling the known inventory of these very cool objects (Delfosse et al. 1999), and 3 brown dwarf candidates. One of these three is DENIS-P J1228-1547 (Delfosse et al. 1997); Keck spectroscopy showed the presence of a strong lithium absorption line at 680.7 nm (Martín et al. 1997; Tinney et al. 1997); in such fully convective objects lithium nuclear burning never took place and thus there has been no hydrogen burning either. In addition, this object is the first identified binary brown dwarf (Martin et al. 1999). Interestingly, one of the other two candidates DENIS-P J0205-1159 has no lithium (Martín, private communication) in spite of a significantly lower effective temperature.
Very few of these enigmatic objects are known. The extreme M dwarfs and brown dwarfs contained in the DENIS database constitute potentially the first statistically significant sample of these objects, sufficiently large to determine their local density (i.e. the disc luminosity function) and to form the basis for the calibration of the observable properties of brown dwarfs (flux calibrated spectra and distances) into more fundamental physical quantities (effective temperature, luminosity, mass).
5.2. The interstellar medium
Detailed maps of the extinction in nearby molecular clouds (in Chameleon, Ophiuchus, Orion and Serpens) can be obtained via star counts in the DENIS data. An example is the study by Cambrésy et al. (1997) of the nearby Cham I cloud, that has produced an extinction map of much higher angular resolution than before.
The DENIS survey observations make possible to obtain similar maps of other well-known and nearby clouds, e.g. those in Ophiuchus and Serpens, but also in small globules. When these extinction maps are combined with maps in the CO millimeter line emission, with 60 and 100 µm maps obtained by IRAS, the distribution of gas, dust and of the temperature will be known much better than so far.
Counting stars in the whole DENIS data base will eventually provide the first deep maps of interstellar extinction throughout the Galaxy.
5.3. Young stars in dark clouds
The exploration of DENIS data of dense molecular clouds will lead to the detection of a large number of stars in the process of formation: young stellar objects (YSOs), protostars, etc. Copet (1996) has studied DENIS data of the Orion molecular cloud and discovered many new faint YSOs which are likely to be T Tau stars.
From an analysis of similar data, Cambrésy et al. (1998) detected YSOs that probably form a new population of low-luminosity T Tau stars: a full survey of the clouds in Chameleon (an area of several tens of square degrees) and in other similar regions will improve our knowledge of the low-end of the luminosity function.
5.4. The distribution of stars in the disc and in the bulge of our Galaxy
At visual wavelength the search for stars in the inner galactic disc is limited to distances smaller than one or two kiloparsecs, while at 2 µm red giants and supergiants are detected up to galactic center distances: the extinction in the infrared is much smaller (). The DENIS large-scale digital survey in the infrared, with its homogeneous magnitude limits in its photometric bands - with the exception of a modest variation (0.6 mag) in the early data in the -band - provides an ideal support for new studies of the distribution of dust, red giants and red supergiants in our Galaxy. The newly detected supergiants will give the distribution of the star formation rate and the red giants the distribution and evolution of older stellar populations in the galactic plane.
Detailed new information are expected to be obtained about the inner spiral arms, the ring, the bar and the bulge of the Galaxy. The COBE /DIRBE data have been important in finding such large scale structures; the much higher angular resolution of DENIS (a few arcseconds instead of a few degrees with COBE ) will give more detailed information.
The DENIS data will be used to interpret the data on the inner Galaxy at 7 and 15 µm obtained through the ISOGAL program (see Pérault et al. 1996). Several other surveys of the inner Galaxy exist already, and we expect that the additional information from the DENIS and ISOGAL survey on the stellar content will be important, perhaps essential, to obtain a comprehensive model for the inner Galaxy.
The existing survey data have already significantly constrained models for the large scale structure of the Galaxy (Ruphy et al. 1996, 1997). The scale length in the galactic plane has been determined as well as the outer edge of the stellar disc, the position of the Sun relative to the galactic plane and the shape and density of some spiral arms and of the 3 kpc ring.
There is a strong interest in DENIS survey data on the bulge and the bar of our Galaxy: Schultheis (1998) analysed the DENIS measurements of 30,000 sources in the Palomar-Groningen field #3 (PG3). Among these there are 36 previously known AGB variables; carbon stars are missing, confirming earlier studies.
Planetary nebulae (PNe) are an interesting category of objects as well. Already 250 of the 750 known PNe in the southern sky have been detected by the DENIS survey (Kimeswenger et al. 1998). The survey will provide better surface photometry at its three wavelengths than previously known. It will also discover faint, red, background stars that often contaminate photometry at visual wavelengths but that also may be used as distance indicators. A further goal concerns the few thousand PNe in the inner Galaxy that must exist, but that have escaped detection so far because of interstellar extinction. Kimeswenger et al. expect that DENIS will find many new of those.
Even in medium or high latitude fields, where interstellar extinction at visual wavelengths is not a problem, the DENIS survey data will identify red giants that can be used to trace their distribution in the halo, before optical surveys (such as the Sloan) produce more complete results.
5.5. Stars in the Magellanic Clouds
With a distance modulus of for the Large Magellanic Cloud and for the SMC, DENIS records data of all stars with that have a luminosity higher than about 2500 or and thus all AGB stars except the few that are very red. There are large differences in luminosities and element abundances between populations of AGB stars in different surroundings, e.g. the SMC, the LMC, the outer disc of our Galaxy and our bulge. Most AGB stars in the SMC are carbon-rich whereas in the galactic bulge the AGB stars are all of M-type ("oxygen-rich"). The reasons for these variations are not well understood at present and the DENIS samples may provide the data for a breakthrough.
Knowledge so obtained will be of great importance in the study of AGB stars in other Local Group dwarf galaxies, within the grasp of large ground-based telescopes such as the ESO VLT.
A specific catalogue of DENIS point sources in both Magellanic Clouds is planned to become publicly available around mid 1999 (see Cioni, Loup & Habing 1999).
5.6. Galaxies and cosmology
Near infrared surveys such as DENIS offer substantial advantages for constructing samples of galaxies: the low extinction allows for a fair view of external galaxies, virtually unaffected by their interstellar media, and for samples extending to low galactic latitudes. Moreover, the near infrared domain provides galaxy samples that are more mass-weighted and less affected by recent star formation than galaxy samples in other wavebands.
Vauglin et al. (1999) have produced the first DENIS I-band extragalactic catalog, by extracting galaxies from highly compressed DENIS images, in a homogeneous manner. The I-band data are the best suited to separate stars from galaxies and to determine the parameters of extended objects. The first catalog, corresponding to DENIS observations prior to June 1997, contains 20,260 galaxies, of which roughly are new, and is complete to . The catalogue is available from CDS. The comparison of this catalogue with the sample of Mathewson et al. (1992) and Mathewson & Ford (1996) shows that the uncertainty in DENIS I-magnitude is about 0.18 mag at .
An updated version of the catalogue, containing 41,000 galaxies, and covering the observations before February 1999, is in preparation.
A special project is the search for galaxies in the Zone of Avoidance in the direction of the "Great Attractor" (Kraan-Korteweg et al. 1998). A systematic search is being made at Lyon for galaxies at latitudes smaller than . Routine analysis of the DENIS data base started in March 1997. More than 1500 extended objects have already been listed; some of these are not galaxies but newly discovered globular clusters or planetary nebulae.
It is possible to go as faint as and extract galaxies with very high () completeness and reliability, as shown by Mamon et al. (1998). The galaxy counts in I and J follow the Euclidean 0.6 slope up to the completeness limit, with no lack of bright objects, in contrast to counts performed in the optical (Heydon-Dumbleton et al. 1989; Maddox et al. 1990), and in agreement with recent optical counts by Bertin & Dennefeld (1997). A complete and reliable catalogue with 900,000 galaxies in the band, 500,000 at J and 50,000 at is expected. The homogeneity of the extraction is essential for statistical cosmological studies of galaxies such as the measurement of the angular two-point correlation function of galaxies, the derivation of the primordial density fluctuation spectrum, the building of catalogs of groups and clusters, and the study of the variation of galaxy colours with environment. So far, such studies have been based on photographic photometry, which suffers from non-linearities and large systematics in photometric calibration.
© European Southern Observatory (ESO) 1999
Online publication: August 25, 1999