Deep Near-Infrared Survey of the southern sky

First scientific results

1. Faint stars in the solar neighbourhood: red and brown dwarfs

The range of DENIS wavelengths is large (from the I-band to the K-band is a factor 2.5 in wavelength) and this 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., 1998), 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. 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 obvious priority now is to extend this analysis to a much larger sky area. Extrapolation from the above results shows that the already observed data contain 300 extreme M dwarfs and 30 brown dwarfs. Together these will provide a first statistically significant sample of these objects, sufficiently large to determine their local density (i.e. the disk 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).

2. The interstellar medium; extinction

Detailed maps of the extinction in nearby molecular clouds (in Chameleon, Ophiuchus, Orion and Serpens) can be obtained via starcounts in the DENIS data. An example is the study by L. Cambrésy et al. (1997) of the nearby Cham I cloud that has produced an extinction map of much higher angular resolution than before (see the enclosed plate that shows the extinction map derived from the DENIS star count in the J band and translated into visual extinction scale. There is an excellent coincidence of the 'cold' dust emission represented by the isophots in white as measured by IRAS at 100-micron) . The DENIS survey observations will 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.

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 or YSOs, protostars, etc.). E. Copet (thesis, 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. Because young stars sometimes are strong X-ray emitters the DENIS data are compared with ROSAT surveys results; this work is in collaboration with R. Neuhauser (MPE Garching).

4. The distribution of stars in the inner disk and in the bulge of our Galaxy

At visual wavelength the search for stars in the inner galactic disk 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 (AK~AV /10). DENIS will make the first large-scale digital survey in the infrared; in addition it has homogeneous magnitude limits in its photometric bands, with the exception of a modest variation (0.6 mag) in the early data in the K-band. Thus new studies are possible of the distribution of dust, red giants and red supergiants in our Galaxy; these studies tackle the question what are the distributions of different kinds of stars 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 will 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 and will solve issues that could not be solved with the COBE/DIRBE data. The distribution of different kinds of stars in our Galaxy, that has been a major topic of research earlier in this century, is becoming an important topic again, mainly due to new and larger surveys, of which DENIS is an example, but also the surveys that search for the microlensing effects (MACHO, EROS etc.)

The DENIS data will be used to interpret the data on the inner Galaxy at 7 and 15 µm obtained through the ISOGAL program, one of the longest ISO programs. The DENIS and ISO teams of Co-investigators have many astronomers in common, e.g. the Principal Investigators (N. Epchtein for DENIS and A. Omont for ISOGAL). Several other surveys of the inner Galaxy exist already, e.g. radiocontinuum studies, 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, 1996, Ruphy et al, 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; we look forward to cross-identify DENIS data on the ring and the bulge with those of ISOGAL. DENIS adds data that reveal the presence of stars that otherwise would have been missed.

There is a strong interest in DENIS survey data on the bulge and the bar of our Galaxy; again: there is considerable overlap between ISOGAL and DENIS survey. Schultheis and Hron (Vienna) 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 by V. and B. Blanco. Whitelock (Cape Town) is studying red giants in two areas where we see the near and the far end of the galactic bar.

Planetary nebulae (PNe) are an interesting category of objects as well. PNe emerge from AGB stars and represent the same galactic population. The study of PN properties thus complements that of the AGB stars; in fact, they are probably the best objects from this population to derive the abundances of the various elements and the variation of these abundances over the face of the Galaxy. Already 250 of the 750 known PNe in the southern sky have been detected by the DENIS survey. The survey will provide better surface photometry at its three wavelengths than previously known. It also will discover faint, red, background stars that often mess up photometry at visual wavelengths but that also may be used as distance indicators (Kimeswenger, Kerber, Roth et al., 1998). 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, Kerber, Roth et al. expect that DENIS will find many new PNe.

Even at 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, well before optical surveys produce results (e.g. the SLOAN survey).

5. Stars in the Magellanic Clouds

The Large Magellanic Cloud has a distance modulus of (m–M)=18.6 and the SMC has (m–M)=19.1. Therefore DENIS records data of all stars with (V–I)> 3 that have a higher luminosity than about 2500 L or Mbol< –4.0 and thus all AGB stars except the few that are very red. Earlier photometric and even spectroscopic surveys have produced incomplete samples and the DENIS data will largely correct for this incompleteness. There are large differences in luminosities and element abundances between populations of AGB stars in different surroundings, e.g. between the SMC, txe LMK,`the outer disk of our Galaxy and our bulge. Most AGB stars in the SMC are carbon-rich whereas in the 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 give 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, studies that are within the grasp of the VLT. Besides DENIS data reveal upper MS stars and SGs that together with RGB and AGB stars allow to investigate the important ratios for stellar population studies. A first goal is to publish a catalogue of point sources in both Clouds; the catalogue is planned to be publicly available around mid 1999. In several small fields in the LMC and SMC ISO detects the only group of AGB stars that DENIS will miss, redder objects that are believed to experience a great mass loss (Loup et al. 1997). A large consortium, started by C. Loup, is studying LMC and SMC data of DENIS and ISO.

6. Galaxies and cosmology

The DENIS survey makes it possible to construct large samples of galaxies that are not limited by uncertain interstellar extinction effects on the cut–off magnitude. 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; this will have several cosmological implications.

Vauglin and Paturel (both from Lyon) extract galaxies from the survey measurements 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 systematic routine to extract bright galaxies from the I-band images has begun in Lyon in February 1996 and has already led to a catalog of about 36,000 galaxies, of which roughly 2/3 are new. The comparison with another existing sample (that of Mathewson) shows that the uncertainty in DENIS I-magnitude is 0.17 mag.

A special project is the search for galaxies in the Zone of Avoidance in the direction of the ``Great Attractor". Schr&ouml;der, Kraan-Korteweg, Mamon &amp; Ruphy thus discovered new galaxies at latitudes 1° < b < 5°. A systematic search is being made at Lyon. Routine analysis of the DENIS data base started in March 1997. More than 200 extended objects have already been listed; some of these are not galaxies but newly discovered globular clusters.

It is possible to go as faint as I = 16.5 and extract galaxies with very high (>95%) completeness and reliability, as shown by G. Mamon and colleagues. The galaxy counts in the I– and J– bands are Euclidean and thus require no very recent galaxy evolution (in contrast to counts performed in the optical). A catalogue with 900,000 galaxies in the I–band, 500,000 at J and 50,000 in K is expected. The homogeneity of the extraction is essential for statistical cosmological studies of galexies"sech 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. Previous studies were based upon optical photographic photometry, which suffers from non-linearities and large systematics in photometric calibration.

The galaxies discovered in the DENIS survey are expected to be the input for three spectroscopic surveys in the southern sky: one in which radial velocities will be measured (in 2001–2002) for roughly 100,000 galaxies using the UK Schmidt Telescope and a robotic multifibre spectroscope (6dF; PI's M. Colless and G. Mamon); the second (in 1999-2002; PI: G. Paturel) measuring the 21 cm line of roughly 5000 inclined spiral galaxies with the Nançay telescope (which is limited to δ> –39); and the third for roughly 20,000 inclined spirals as well as early type galaxies using the 6dF instrument at high spectral resolution (2003–2005; PI: G. Mamon).

Excerpt from DENIS status report, H. Habing & N. Epchtein, May 1998
Updated November 1998