Jeremy R. Mould

A nova-like red variable in M31

A late-type supergiant which appeared in the bulge of M31 in the last observing season faded 3 bolometric magnitudes in 100 days and is now no longer detectable. It is suggested that the object was a nova of an unusual type. 9 refs.

A distance scale from the infrared magnitude/H I velocity-width relation. I - The calibration

Large-aperture H (1.6 ..mu..m) magnitudes are presented for 12 nearby galaxies and are used to determine the zero point and slope of the infrared magnitude/H I velocity-width relation. By assuming only the Sandage-Tammann distances to M31 and M33, we obtain the following distance moduli to the nearest groups: Sculptor group: m-M=27.46 +- 0.2 mag, d=3.1 +- 0.3 Mpc; N2403-M81 group: m-M=27.76 +- 0.2 mag, d=3.6 +- 0.3 Mpc; M101 group: m-M=29.16 +- 0.35 mag, d=6.8 +- 1.1 Mpc.

A Cepheid distance to the Fornax cluster and the local expansion rate of the Universe

Both galaxy distances and velocities are required for the determination of the expansion rate of the Universe, as described by the Hubble constant H0. The radial velocities of galaxies arise not just from this expansion but also from random components and large-scale flows. To reach out to distances dominated by the overall cosmic expansion, it is necessary to probe large physical scales where galaxy–galaxy and galaxy–cluster interactions become lessimportant. But accurate distances of nearby galaxies and clusters (commonly measured using Cepheid variable stars) are nevertheless required to calibrate the indirect distance indicators generally used to measure these large scales. Here we report aCepheid distance of 18.6 ± 1.9 (statistical error) ± 1.9 Mpc (systematic error) for the galaxy NGC1365 in Fornax, a cluster of galaxies in the Southern Hemisphere. We find a value of H0 = 70 km s−1 Mpc−1 from Fornax alone, and 73 km s−1 Mpc−1 from the intervening galaxy flow, each corrected for infall into the Virgo cluster. These values are consistent with the Hubble constant measured in the far field using secondary methods. Our data support previous suggestions that the local small-scale velocity field has very small scatter (∼±70 km s−1).

The stellar population and luminosity function in M31 bulge and Inner Disk Fields

We report infrared photometry and stellar identifications for stars in five fields in the M31 bulge located from 2 to 11 arcmin from the nucleus. These fields have been chosen such that the bulge/disk star ratio predicted from Kents (1989) small bulge model varies from 7:1 to 1:5, allowing a study of near pure disk and near pure bulge stellar populations. We reject the hypothesis of Davies et al. (1991) that luminous stars found within 500 pc of the nucleus are due to a contaminating disk population. We find that the bulge contains stars in excess of M(sub bol) = -5 mag and that the bulge luminosity function has a distinct shape different from the disk fields. We find many stars redder than (J-K) = 2 mag, and suggest that these stars may be the counterparts of the IRAS-selected Galactic bulge Miras studied by Whitelock et at. (1991). The number of bright stars (M(sub bol) is less than -5 mag) falls off more rapidly than the r band surface brightness. By building model fields out of a bulge luminosity function and artificial stars, we are able to show that the change in the luminosity function toward the center cannot be explained simply by the mismeasurement of overcrowded star images. However, these tests also raise the possibility that the asymptotic giant branch (AGB) tip may be approximately equal to 1 mag fainter than actually measured in our most crowded field, reaching only M(sub bol) = -5. We compare observed counts of AGB stars with those predicted from theoretical lifetimes using a technique of general interest for this problem, the Fuel Consumption Theorem of Renzini & Buzzoni (1986) Spectral Evolution of Galaxies (Reidel, Dordrecht). Our methodology is generally applicable to the study of other resolved extragalactic stellar populations. The number of observed stars per magnitude up to a luminosity of M(bol) = -5.5 mag is consistent with AGB evolution of the whole population of the innermost bulge field with the standard lifetime on the AGB of 1.3 Myr/mag. We advance the possibility that the bulge of M31 may be younger than the oldest Galactic globular clusters. We note that M33 has recently been found to have an r(exp 1/4)-law spheroid consisting of intermediate-age stars; bulges can form later than the old halo population.

Infrared photometry of the semistellar nucleus of M31

New broad-band infrared JHK data and narrow-band CO and H2O indices for the semistellar nucleus of M31 are presented. The data were obtained specifically to test a prediction of a recent synthesis model by Faber and French in which the ratio of dwarf-to-giant light increases strongly in going from the bulge to the nucleus of M31. The new infrared data do not support such a model. Some alternative explanations for the behavior of the various indices are given, but the apparent conflict between the Faber-French interpretation of the strength of the Na I 8190 A feature and our data is not satisfactorily resolved.

A rich, nearby galaxy cluster in Sagittarius

The existence of a probable rich cluster, or a set of clusters in Sagittarius, seen through an extinction window in the direction of the Galactic bulge, is reported. In particular, redshifts are presented for 21 galaxies found among the objects originally identified by Terzan et al. (1978). It is suggested that the Sagittarius cluster is an entity separate from the previously discovered Ophiuchus cluster but that the two may be physically associated in a larger structure. This putative Sagittarius-Ophiuchus supercluster may be an important factor in the local supergalactic dynamics, possibly even more massive than the Coma-A1367 system, which is roughly the same distance. 19 refs.