Young stars in the disk of M82

Abstract

We present CCD V l c photometry in a field 1.9 x 1 . 3 square arcmin and positioned about 3 arcmin north-east of the centre of M82. More than 100 stars have been identified and measured above the photometric limits V = 23m5, I = 22m. Most of the objects fainter than V Starlike as well as 2-3 somewhat diffuse objects -candidates of globular clusters -preferentially occupy yellow colours in the colour-magnitude diagram; extreme blue and red objects are missing. The colour-magnitude diagram indicates continuing star formation in this part of the disk. The absence of blue stars is caused by reddening; no convincing explanation is found for the missing red supergiants. Young stars are distributed throughout the whole field, the youngest being concentrated in associations at the SW edge of the field. To cope with the very irregular background created by the galaxy's main body and with the low contrast of many of the stars against this background, a modified photometric method is introduced in which the background is estimated using the intensities in a ring drawn very tightly around the star in the wings of the point spread function.Key words: interacting galaxies -young stars -M82 A A A subject classification: 158 20" should be members of M82 as expected from the statistics of foreground stars. IntroductionThe galaxy M82, a member of the M81 group of galaxies, has been a favourite object to observers ever since Lynds and Sandage (1963) found evidence of energetic events in its central region, now known to be the seat of a huge starburst. Despite the vigorous starforming activity at its centre, the outer parts of M82 seem to show no sign of recent star formation; no HI1 regions, no bright young objects, and no spiral arms are seen. This led Sandage and Brucato (1979) to include it as a member of the small morphological group of "Amorphous Galaxies" showing no structure in their main body. The whole central M81 group is embedded in a huge cloud of intergalactic HI. It is now generally agreed that this cloud is the result of a passage of one or two galaxies (M82, NGC3077) near M81, leading to tidal disturbances especially in the interstellar medium of these galaxies. Tidal tails of HI are now seen joining the three galaxies (Appleton et al. 1981, Yun et al. 1994. The time passed since these events has been estimated by numerical modelling of the interaction; for M82 values between. 2...5 . lo8 yr have been found (for instance Kilian 1980, Brouillet et al. 1991).It has been suggested already by O'Connell and Mangano (1978) that the absence of young stars in the outer parts of M82 may be the result of this interaction, which severely disturbed the interstellar medium in M82, making further star formation impossible. Scattered star formation occured, however, in some limited regions in the disk, probably triggered by interaction with backfalling intergalactic material. If this scenario applies, the age 16 Astron. Nachr. 317 (1996) 4 226 Astron. Nachr. 317 (1996) 4 and the distribution of the youngest stars in the outskir...

Photometry of young starlike objects in the dwarf galaxy NGC 2976— age and metallicity

“…The metallicity of the disk population is estimated as solar (z∼ 0.02), in contrast to the results got earlier for M82 (Notni et al 1996), where the data are better represented by theoretical isochrones for z∼ 0.004. This high metallicity may indicate matter enrichment during previous star-forming activities.…”

“…The very irregular background of the central disk made the use by the 'background-interpolating' method most promising (Notni et al 1996). In this procedure, the background in the immediate neighbourhood of an object is interpolated inside a circle centred on the object and made as small as possible, possibly even lying inside the outer range of the point-spread-function (PSF).…”

BVR photometry of the resolved dwarf galaxy Ho IX

“…DAOPHOT uses rings of pixels around the photometric aperture while UMAG uses just the peripheral pixels of the same aperture and does not need additional background space. A similar method of photometry was applied independently by Notni et al (1996) in the crowded field of the galaxy M 82. A comparison of the UMAG and DAOPHOT PSF photometry of stellar images in the crowded fields of 5 resolved dwarf galaxies is given by Makarova et al (1997).…”

“…This software is especially developed for photometry in crowded fields of galaxies where the number of star-like objects may be comparable to or lower than the number of slightly diffuse ones. Under these conditions PSF photometry is not as efficient as usual (Georgiev 1995b;Notni et al 1996;Georgiev et al 1997b).…”

The central region of NGC 3077: ground‐based photometry and age of bright ‘starlike’ objects

“…The very irregular background of the central disk made photometry by the 'background-interpolating' method most promising (Notni et al 1996). In the central regions, best results were obtained interpolating the background from outside a radius of 5 pixels (1 1993, 1 .4 1996) for the SAOsets and 4 pixels (2 ) for the CA-set and integrating the excess light inside this full aperture; the point-spread-functions determined from some well isolated bright stars were used to correct to infinite radius.…”