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This review discusses the various processes that contribute to the building of galaxies, with emphasis on the direct observational evidence for these processes. The disks, bulges, and nuclei of galaxies are all probably built up over a significantly extended period of time by the continuing addition of matter, often in discrete amounts associated with the merging of smaller units. The information about galaxy formation available from the fossil record and from look-back observations is also reviewed, and it is concluded that the epochs of disk and spheroid formation can be identified and studied directly by observations at high redshifts, except perhaps for the earliest stage of spheroid formation which probably occurs at redshifts not yet observed.

A review is given of present ideas on the evolution and stellar content ofthe Triangulum nebula = M 33 = NGC 598. A distance modulus of (m - M)₀ = 24.5 ± 0.2 (D = 795 ± 75 kpc) and a Galactic foreground reddening EB-V = 0.07, from which Mv = -18.87, are adopted throughout this paper. The disk of M 33 is embedded in a halo of globular clusters, metal-poor red giants, and RR Lyrae stars. Its nuclear bulge component is weak (Mv > -14). This suggests that the halos of galaxies are not extensions of their bulges to large radii. The ages of M 33 clusters do not appear to exhibit a hiatus in their star-forming history like that which is observed in the Large Magellanic Cloud. Young and intermediate-age clusters with luminosities rivaling the populous clusters in the LMC are rare in M 33. The integrated light of the semistellar nucleus of M 33, which contains the strongest X-ray source in the Local Group, is dominated by a young metal-rich population. At optical wavelengths the disk scale length of M 33 is 9.'6, which is similar to the 9.'9 scale length of OB associations. The ratio ofthe nova rate in M 33 to that in M 31 is approximately equal to the ratio of their luminosities. This suggests that the nova rate in a galaxy is not determined entirely by the integrated luminosity of old bulge stars. The gas-depletion time scale in the central region of M 33 is found to be ~ 1.7 × 10⁹ yr, which is significantly shorter than a Hubble time. Available data do not yet allow an unambiguous choice between the density wave and self-propagating star-formation models for the two main spiral arms of M 33.

It is shown that β, the initial postmaximum rate of SN brightness decline (in the B band) defined by Pskovskii, may have a smaller dispersion among SNe la in elliptical galaxies than in all other types of galaxies. Contamination of the sample by SNe Ib is unlikely to be the primary cause of this difference. Although the number of objects is very small, it is also possible that the velocity of SN la éjecta in elliptical galaxies is lower than in spiral galaxies. If correct, these observations provide the first direct evidence for physical differences among SNe la in different environments; reddening variations due to gas and dust are unlikely to produce most of the observed dispersion in ß among spirals. One obvious possibility is that the SNe la in spiral galaxies come from intermediate-mass stars and that differences in the metallicities, accretion rates, or other properties account for the observations. A more extreme, improbable explanation is that not all SNe la in spiral galaxies result from carbon deflagrations of carbon-oxygen white dwarfs.

There is accumulating evidence that as much as 90 percent of the mass of the universe is nonluminous and is clumped, halo-like, around individual galaxies. The gravitational force of this dark matter is presumed to be responsible for the high rotational velocities of stars and gas in the disks of spiral galaxies. At present, the form of the dark matter is unknown. Possible candidates span a range in mass of 10$^{70}$, from non-zero-mass neutrinos to massive black holes.

The telescopes and signal processing systems of the Owens Valley Millimeter Array are considered, and improvements in the sensitivity and stability of the instrument are characterized. The instrument can be applied to map sources in the 85 to 115 GHz and 218 to 265 GHz bands with a resolution of about 1 arcsec in the higher frequency band. The operation of the array is fully automated. The current scientific programs for the array encompass high-resolution imaging of protoplanetary/protostellar disk structures, observations of molecular cloud complexes associated with spiral structure in nearby galaxies, and observations of molecular structures in the nuclei of spiral and luminous IRAS galaxies. 9 refs.

It is found that anemic spiral galaxies are enormously deficient in neutral hydrogen but have normal CO emission-line strengths. This observation may be accounted for by assuming that hydrogen gas has been stripped from the outer parts of these objects, while dense molecular clouds have not been lost to ram pressure stripping.

New redshifts have been obtained for H ii regions along the major axis. These data, combined with a reanalysis of previously published work, show that optical and 21-cm determinations of the rotation curve are in excellent agreement.

A CCD study of the edge-on spiral galaxy, NGC 5170, was carried out with the CTIO 4-m telescope to search for its globular-cluster system (GCS). Three V frames near the galaxy and two removed from it (for background estimation) were analyzed. Following removal of galaxy light by median filtering and DAOPHOT analyses, a total of ~ 130 ± 20 presumed clusters were detected within 2 arc min (R = 16 kpc at D = 28 Mpc) ofthe galaxy center above a foreground of Galactic stars and a background of distant galaxies. There are two ways to interpret the luminosity function (LF) data, (a) Solving for all parameters ofthe Gaussian (assumed, from studies of other galaxies, to be the appropriate representation ofthe NGC 5170 GCLF) leads to σ = 0.73 ± 0.21, a value rather less than commonly found. In this interpretation our data reach beyond the peak ofthe GCLF and the distance modulus inferred is (m – M ) = 31.2 ± 0.3. However, this solution places strongest reliance on the faintest bins with the largest errors, (b) By fixing σ = 1.3, the derived Gaussian falls shy of the GCLF peak and (m – M) = 32.3 ± 0.3. These determinations allow the first direct comparison between the GCLF method and the Fisher-Tully method, from which 32.5 ± 0.14 has been found. Adopting parameters arising from solution (b), and making all necessary corrections, we estimate a total cluster population of ~ 815 ± 320, a specific frequency, S = 1.3 ± 0.5, or (assuming a bulge-to-disk ratio of 0.5), Sspheroidal = 3.9 ± 1.5. The S values (which are robust with respect to the assumed distance) are consistent with what is seen for other isolated disk galaxies. A power-law fit to the radial density distribution ofthe cluster candidates yields an index of – 1.69 ± 0.26, which is again consistent with parameters found by others for galaxies with total luminosity comparable to that of NGC 5170; to within the errors, the radial distribution of the halo light and clusters are indistinguishable.

Pulsars are used to trace the spiral arms of the Galaxy. Present pulsar characteristics are used to divide them into two pulsar types as defined by Huang (1987). Those pulsars of Type II are believed to be the remnants of extreme Population I stars. The ages of Type II pulsars and the galactic rotation curve are used to determine their position at birth. The spiral arm pattern velocity is used to reconstruct the arms from which the individual Type II pulsars originated. It is found that the best fit to the data requires spiral arm segments with a pitch angle of 14°.5. The present Type II pulsar positions also indicate the same local pattern.

Detailed surface photometry in four colors of the Sc spiral galaxy NGC 2903 is presented. The color structure within the spiral arms shows a gradient both across the arms, caused by age differences in star-forming regions and by local dust lanes, and across the galaxy, caused by the large-scale dust distribution. In general, all arms show indication of a difference in color from that of the disk, most clearly seen for those in the well-ordered northwest region. The magnitude of this difference depends on the luminosity of the arm. Frequently, along the minor axis, color peaks occur at progressively larger radii in (V – R), (B – V), and (U – B), respectively. The most conspicuous anomaly in the arm structure occurs in the outer northeast arm, which divides into two branches, one of which shows the characteristics of only an old population and the other of which shows evidence only of young stars and gas. The well-known \"hot spots\" in the nucleus show evidence of the presence of blue stars, dust, and gas emission.