HI STUDIES OF DWARF AT NANQAY

W. van Driel 1, R.C. Kraan-Kortweg 2, F.H. Briggs 3, C. Balkowski 4, B. Binggeli 5, V. Cayatte 4, P.-A. Due 6, W.K. Huchtmeier 7, J.-M. Martin 8 andT .X Thuan 9• .

1 Unite Scientifique Nan<;ay, Observatoire de Paris, Meudon, France. 2 Astronomy Department, University of Guanajuato, Mexico. 3 Kapteyn Astronomical Institute, Groningen University, The Netherlands.

4 DAEC, Observatoire de Paris, Meudon, France. 5 Astronomical Institute, University of Basel, Switzerland 6 European Southern Observatory, Garching b. Miinchen, Germany

7 Max-Planck Institut fiir Radioastronomie, Bonn, Germany 8 ARPEGES, Observatoire de Paris, Meudon, France. 9 Astronomy Department, University of Virginia, U.S.A.

Abstract To search forlow-surface brightness, HI gas-rich objects we used the Nanc;aydecimetric radio telescope in both pointed and blind surveys. Pointed observations were made of 23 optically selected candidate dwarf members of the M81 group. Only a surprisingly small percentage was detected - probably due to strong overlying HI emission of the and the M81/M82 complex. In the Cluster we observed an HI-selected complete sample of a dozen dwarfs members to study their Tully-Fisher relation. Blind HI line observations are a potentially powerful way to search fora possibly very large population of "low surface brightness" (LSB) galaxies, whose presence is important for topics like the mass density of the Universe, galaxy evolution, etc. With a blind HI line survey out to a of 2350 km/s we can detect nearby low surface brightness objects with HI masses of about 108 Me and higher at Nanc;ay. Two such surveys were made, towards the CVn region and towards the MlOl group of galaxies; the latter is still under analysis. Throughout the CVn groups, we are sensitive to Mm 3·108h-2 Me at = Vhel 600-1300 km/s. The principal result isthat no new HI-rich systems (LSB galaxies = or intergalactic clouds) were discovered in the CVn region, implying that intergalactic clouds and unseen gas-rich galaxies can form a population amounting to at most 1/30 the population in optically catalogued large galaxies, a result highly consistent with earlier work.

301 1 The N an�ay radio telescope - being renovated

The rectangular Narn,;ay decimetric radio telescope, with the equivalent collecting area of a 94- m. diameter round dish, is a transit telescope of the Kraus/Ohio State design, consisting of a fixed spherical mirror, 300 m long and 35 m high, a tiltable fiat mirror (200x40 m), and a focal carriage moving along a 90 m long curved rail track, which allows the tracking of a source on the celestial equator for about 1 hour. Located in the centre of France, it can reach declinations as low as -39°. The focal system ofthe N anr;ay telescope is presently being renovated (van Driel et al. 1997). After the installation of a dual-reflector offset Gregorian system and corrugated horns, the system temperature is expected to be about 25 K in the wavelength range of 8.5 to 30 cm. The system will thus be about 2.5 times more sensitive by the end of 1999, implying a six-foldgain in observing time efficiency at 18/21 cm wavelength (OH and Hr lines) and even a ten-fold gain at 9 cm (CH line). The installation of a new, large-bandwidth correlator will increase the efficiency even further for HI line searches of galaxies with unknown redshift.

2 Pointed HI line observations

In the case of pointed observations, the telescope is directed towards an object with known coordinates, which is then tracked (usually for 4 to 6 hours in the case of the studies presented here, resulting in a typical nns. noise of 2 mJy for a velocity resolution of 15 km/s).

2.1 Dwarf candidate members of the M81 group

The M81 group is in principle an excellent choice for dwarfgalaxy studies, as it is relatively nearby ( < V >= 101 km/s, = 114 km/s) and known to contain an important population av of dwarf galaxies, many of which are classified as Irregular. A catalogue of candidate dwarf members to a limiting absolute magnitude of � -11 for the adopted distance of 4 Mpc is available (Borngen et al. 1982). Most optically selected candidate dwarf group members do not have known , due to their low optical surface brightness. We observed 23 of them at Nanr;ay in the -529 to 1826 km/s radial velocity range, but detected only 3 of them, which were all found to be background objects with radial velocities of 600-1150 km/s (van Driel et al. 1998). The estimated 3a HI mass detection limit is only about 2 106 M0 for an assumed · line width of 50 km/s, implying upper limits to the Mm/LB ratio of 0.1 and 0.6 M0/L0,B for a non-detected 15th, respectively a 17th magnitude group member. The resulting detection rate of 3 out of 23 candidate dwarf members is thus quite low, considering that half of them are classified as Irregular dwarfs,hence expected to be relatively gas-rich. However, the mean redshift and velocity dispersion of the M81 group suggest that the HI emission of low velocity group members may still remain hidden within the strong Galactic HI emission (typically in the -150 to 115 km/s radial velocity range) or overshadowed by the very extended HI envelope encompassing the vicinity of M81 and M82 (typically in the -280 to 355 km/s radial velocity range). It appears that it will be virtually impossible to determine the radial velocities of these candidate dwarf group members through further single-dish 21-cm HI line observations; optical observations may be more effective.

2.2 Dwarf galaxies in the Hydra Cluster

The presence of gas-rich dwarf galaxies in clusters remains a mystery, as they are generally foundin regions where hot X-ray emitting gas should have stripped them of their cold gas - are

302 they the remnants of interactions of galaxies with the intercluster gas, or did they, e.g., form in tidal interactions between spirals? The Hydra Cluster has a large population of gas-rich dwarfs dwarfs, as shown by a VLA HI line study (MacMahon, 1993). Our Narn;ay observations are part of a multi-wavelength (optical, near-infrared, radio) study of a complete HI-selected sample of these dwarfs. The Nanr;ay observations of the 12-odd dwarfs considered detectable there in the HI line with a good velocity resolution (at least 15 km/s, compared to 42 km/s in the VLA study) will serve principally to determine line widths, and hence the Tully-Fisher relation (giving, e.g. an indication of their content), which will be compared with that of dwarf galaxies in the field. Optical and near-infrared observations (P.-A. Due et this Volume) have shown al., quite a range in properties of the dwarfs: they are rather blue, can be fittedby exponential disk profiles and have a wide range in surface brightness, ranging froma blue compact dwarf to low surface brightness objects. The HI line widths of the 6 galaxies observed so far show a range of about 50 to 100 km/s. We also observed 6 nearby companion spirals for the determination of accurate relative radial velocities in the pairs and to study the spirals' Tully-Fisher relation. Seven dwarfs will be reobserved at the VLA with a high velocity resolution.

3 Blind drift-scan HI line surveys - the search for LSB objects

In the case of blind, drift-scan HI line surveys, the telescope is pointed towards a certain direction on the sky and then fixed in position. As the sky turns by the HI line radio band signal is integrated continuously and, in general, read out every 16 seconds, allowing a 4' (one HPBW) sampling in Right Ascension of a 22' (HPBW) broad strip at constant Declination (Briggs et al. 1997). Observations of a certain strip can be repeated in order to obtain the rms. noise level required. Thus, a complete inventory of HI line signals detected in the area can be made within the radial velocity coverage selected, to be compared to what is known of the region covered from observations at other wavelengths. This is a potentially powerful way of searching for gas-rich, low-surface brightness objects. Knowledge of the possibly very large population of "low surface brightness" (LSB) galaxies is important for topics like the mass density of the Universe, galaxy evolution, testing large­ scale structure theories, and the interpretation of high-redshift absorption-line systems. Recent sensitive optical surveys are capable of identifying faint LSB galaxies, and since many of these are gas-rich, they can be detected with sensitive radio observations. One concern is how to appraise the mass density contained in these newly identified populations in comparison to what was already known using older catalogs based on higher surface brightness selection. New catalogues were noted (Briggs 1997a) to contain galaxies at systematically greater distances than the older catalogues, and the addition of the new objects does not significantlyalter the estimates of, e.g., luminosity functions and HI mass functions. Either the new surveys are not sensitive to nearby, large LSB disks or the brighter inner parts of the LSB disks were already included in the older catalogues. The question remains whether extreme LSB objects could exist in the nearby Universe with Mm/Ls ratios exceeding that of objects identified to this day. These are the objects that the Nanr;ay surveys can detect. Two blind drift-scan 21-cm HI line searches were made at Nanr;ayfor nearby dwarf galaxies, the first (Kraan-Korteweg et al. 1998) in a 315 square degree area in the Canes Venatici region, known to contain both loose groups and voids, and the second (still under analysis) of a 42 square degree area centered on the large nearby spiral MlOl. The strips observed at constant declination were separated by 11' (0.5 HPBW) and each strip was observed 5 times, in order to reach an rms. noise level of about 10 mJy. The radial velocity coverage is -350 to 2350 km/s,

303 in both H and V polarisation. The survey is sensitive to Mm = 109h-2 Me at 23 h-1 Mpc and to Mm = 3·108h-2 Me throughout the CVn Groups (at about 600 - 1300 km/s). The survey produced 54 spectral features, which was further reduced to a sample of 34 reliably detected galaxies by extensive pointed follow-up observations with an about two times lower rms. noise. The principal result is that no new HI-rich systems (LSB galaxies or intergalactic clouds) were discovered. The previous surveys of the CVn Groups made by deep optical surveys, followed by pointed 21-cm line observations, have effectively catalogued all the galaxies detected in our survey, which was sensitive to HI masses above 2· 108 Me throughout the CVn Groups. The logical consequence is that intergalactic clouds and unseen gas-rich galaxies can form a population amounting to at most 1/30 the population in optically catalogued large galaxies. This result is highly consistent with earlier work (Zwaan et al. 1997, Briggs 1997b). An HI mass function was estimated from the survey results, but it cannot attain a high degree of precision, as the number of galaxies is low, the volume sampled is small and known fromoptica l studies to contain groups and voids and thus cannot be considered as representative of the general population, and as there are no HI masses below 3·107 Me in the sample. We obtain a mass function consistent with that derived by Zwaan et al. (1997), using distances determined with the POTENT method to compensate for Virgocentric flow (Bertschinger et al. 1990).

References

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