To discover Cepheids in galaxies, and then confidently determine their periods, light curves, mean magnitudes and colors is an extremely demanding task. So it is perhaps not too surprising that although Cepheids in galaxies as far away as M81 were known to Baade in the early 1960's, up until the 1980's the only major galaxy outside of the Local Group for which a definitive study had been published of its variable star content was the solitary, late-type spiral galaxy, NGC 2403 (Tammann & Sandage 1968). Only in the last decade have CCD surveys begun to come to the fore, as the last of the photographic surveys for Cepheids in galaxies easily resolved from the ground, are now being published. CCDs have small fields of view, but they have significantly higher quantum efficiency than photographic emulsions, especially at longer wavelengths. Accordingly, they are in fact allowing significant progress to be made in searching for Cepheids in the few remaining galaxies near enough to be resolved from the ground. Progress on those fronts is now reviewed.
Morphologically very similar to M33 (and its southern counterpart NGC 300) NGC 2403 is a highly resolved late-type spiral galaxy often associated with M81 and a handful of lower luminosity spiral and irregular galaxies (e.g., de Vaucouleurs 1975). Many dozens of photographic plates taken by Mount Wilson and Palomar observers were searched for variables by Tammann & Sandage (1968) resulting in the discovery of 17 Cepheids. Due to plate limitations, only the brightest Cepheids in the outer parts of the galaxy were found. Even so, only the brightest parts of the blue light curves were securely calibrated and only fragmentary information was available in the visual bandpass. Nevertheless, good periods were determined by these authors and (assuming no extinction within NGC 2403 itself) an apparent modulus was estimated from the brightness of the apparently least reddened Cepheid seen at maximum light. These data have been re-analyzed by Madore (1976) and then by Hanes (1982) and again by Rowan-Robinson (1985) with different sets of assumptions, and consequently diverging conclusions. New data were obviously required before significant progress could be made beyond arcane arguments concerning methodology.
I-band CCD observations of 8 of the known Cepheids in NGC 2403 are discussed by Freedman & Madore (1988) who derive a tentative true modulus of 27.51 mag. Their adopted error of ±0.24 mag includes the uncertainty for the still undetermined amount of reddening internal to NGC 2403 itself. Given what we now know about the difficulties inherent in doing near-infrared aperture photometry in confusion-limited cases, the modified J-band observations reported by McAlary & Madore (1984) and the conclusions drawn therein, are thus superseded.
Baade (1963) was aware that Cepheids had been detected in M81 but it was another 20 years before any results were published. Because of the high surface brightness of the disk of M81, photometry of individual stars is very difficult, making the detection of variable stars such as Cepheids extremely taxing, especially near the plate limit. Nevertheless, Sandage (1984) succeeded in determining the periods for two Cepheids in M81 using B-band photographic plate material. Freedman & Madore (1988) obtained I-band CCD photometry of these stars (based on finder charts and periods made available by Sandage (1987, private communication), and derived a true modulus of 27.59 ± 0.31 mag adopting a foreground Galactic extinction of AB = 0.15 (Burstein & Heiles 1984). This modulus corresponds to a distance of 3.30 Mpc which is still uncertain at the 15% level in as much as the sample is small and no corrections for reddening internal to M81 itself were included. HST observations by Freedman et al. (1994) yield a reddening-corrected distance of 3.6 ± 0.3 Mpc, based on 30 Cepheids discovered in two fields (one along the major axis of M81, the other positioned along the minor axis).
Sandage & Tammann (1974) attempted to find Cepheids in their 200-inch photographic plate material on M101. Their failure led them to conclude that M101 has a distance modulus in excess of 29.3 mag. Later, using CCD detectors Cook et al. (1986) did succeed in detecting at least two Cepheids at R ~ 23 mag (with preliminary periods of 37 and 47 days) in a relatively uncrowded outer region of this galaxy. Given the uncertainties in the absolute zero point of the calibration of the Cepheid PL relation and the unknown extinction internal to M101 itself these data give an apparent R-band modulus of 29.38 mag. Other fields in M101 also observed by this group (Cook et al. 1989), and their results based on 4 Cepheids (Alves & Cook 1995) give a true distance modulus of 29.08 ± 0.13 mag (6.4 ±0.4 Mpc).
More recently Kelson et al. (1996) have used HST to image an outer field in M101 and discovered 29 Cepheids, from which they derive E (B-V) = 0.03 mag and a true distance modulus of 29.34 ±0.17 mag, corresponding to a distance of 7.4 ± 0.6 Mpc. This latter distance compares very favorably with the planetary nebula luminosity function distance 7.7 ± 0.5 Mpc derived by Feldmeier et al. (1996).
Graham (1984) conducted a photographic study of NGC 300 using the Tololo 4m reflector and was able to discover 18 Cepheids, determine their periods, and estimate mean B and V magnitudes. Madore et al. (1987) subsequently observed 2 of these Cepheids at H. Walker (1988) later criticized Graham's photoelectric calibration sequence and suggested a correction to the photographic photometry of the Cepheids. Visvanathan (1987) observed three Cepheids in NGC 300 at 1.05 µm, and derived a true modulus of 25.80 maag (scaled to our adopted LMC modulus). Finally, Freedman et al. (1991) present BVRI CCD observations of 16 Cepheids in NGC 300 giving revised periods and time-averaged magnitudes and colors. A true distance modulus of 26.67 mag, corresponding to a distance of 2.16 Mpc, is derived from a multiwavelength analysis of the CCD data of the 10 Cepheids with complete light curves.
Catanzarite, Freedman, Madore & Horowitz (1998, in preparation) have found 9 Cepheids in NGC 247, which turns out to be located at a distance that is nearly a factor of two larger than that of NGC 300. The significantly larger distance, with the ensuing crowding problems and brighter absolute magnitude cut-off in the observational data made this object far harder to determine a Cepheid-based distance than was originally anticipated. The requisite data for NGC 7793 have also been obtained and are currently being reduced.
A photographic study of the Wolf-Lundmark-Melotte galaxy has been published by Sandage & Carlson (1985a) where 15 Cepheids are identified and periods determined (ranging from 3.3 to 9.6 days). The CCD study of WLM published by Ferraro et al. (1989) unfortunately included none of the Sandage & Carlson Cepheids, but the CCD photometry does indicate that the photographic photometry is too bright by +0.4 mag in both B and V (as derived from the intercomparison of the two studies given in their Table 2.
Applying the B offset to the photographic data for the Cepheids in WLM and rederiving the apparent modulus as discussed above we find (m-M) B = 25.19. Adopting AB = 0.09 mag (Burstein & Heiles 1984), gives a true modulus of 25.10 mag. From preliminary reductions of I-band CCD observations of 5 WLM Cepheids, Lee, Freedman & Madore (1993) find a true modulus of 24.89 ± 0.15 mag, corresponding to a linear distance of 0.95 Mpc.
Saha et al. (1996) reported the discovery 4 Cepheids in this galaxy and derive a true distance modulus of 24.59 ± 0.30 mag (corresponding to a distance of 830 Mpc) after correcting for E (B-V) = 0.94 mag. Wilson et al. (1996) then obtained JHK observations of these same Cepheids and derived a slightly revised true modulus of 24.57 ± 0.21 mag, giving a distance of 820 kpc. The largest uncertainty at this point must be the adopted foreground reddening correction which is considerable. A TRGB/Cepheid study of IC 10 (Sakai, Madore & Freedman 1998) is nearing completion, and should throw some light on this problematic object.
From photographic material Demers et al. (1985) report discovering 5 Cepheids with periods in the range 10 to 23 days in the nearly edge-on, Magellanic-type irregular galaxy NGC 3109. Sandage & Carlson (1988) also using photographic material, but with better plate scale, were able to observe deeper into the luminosity function and determine the periods for 29 Cepheids. The periods of their Cepheids ranged from 6 to 31 days, confirming only 2 out of the 5 earlier-reported periods. Using the data from Sandage & Carlson, and adopting a foreground extinction of AB = 0.14 mag from Burstein & Heiles (1984), we derive a true modulus of 25.94 mag for NGC 3109, corresponding to 1.54 Mpc. No correction for extinction internal to NGC 3109 itself has been applied.
Hoessel et al. (1990) report the discovery of 31 variable stars in Pegasus, with 7 being likely Cepheids. Based on these observations they derive a distance modulus of 26.22 ± 0.20 mag, corresponding to a geometric distance of 1.75 Mpc.
Hoessel et al. (1994) report the discover of 14 variable stars in Leo A of which only 5 had sufficient phase coverage for them to be classified as Cepheids. Based on these stars (corrected for foreground Galactic extinction) the authors derive a true distance modulus of 26.74 mag, corresponding to a distance of 2.2 Mpc, placing Leo A at the outermost edge of the Local Group.
Tolstoy et al. (1995) found a total of six variables in the dwarf irregular galaxy GR8, only one of which was classified as a Cepheid. After correcting for Galactic extinction the single star gave a distance modulus of (m-M)0 = 26.75 ±0.35 mag, corresponding to a linear distance of 2.24 Mpc.
In a photographic survey of Sextans A, Sandage & Carlson (1983b) discovered 5 Cepheids whose periods ranged from 15 to 25 days. In a later paper Sandage & Carlson (1985b) recalibrated the Sextans A photometry, and added data on the adjacent dwarf-irregular galaxy Sextans B, in which they discovered 4 Cepheids having periods ranging from 7 to 28 days. However, CCD data for stars in Sextans A (Hoessel et al. 1983) after transforming from the original Gunn system to the BV system give a zero-point difference between the two data sets amounting to 0.2 mag. Walker(1987) then used a CCD with standard Johnson filters to study this problem further, and concluded that the Sandage & Carlson B-band data are too faint by 0.16 mag in the color range appropriate to Cepheids. With the original Sextans A data transformed to the Sextans B photometric scale as described by Sandage & Carlson (1985b), and then further corrected using Walker's offset, we derive true moduli of 25.59 and 25.64 mag for Sextans A and B, respectively, after adopting foreground extinctions of AB = 0.06 and 0.05 mag (Burstein & Heiles 1984). The above true moduli correspond to 1.31 and 1.34 Mpc, respectively. Finally, it should be noted that for Sextans A, Visvanathan (1987) reports a true modulus of only 25.35 mag, based on single-phase 1.05 µm observations of three Cepheids.
Tolstoy et al. (1996) found 6 Cepheids out of a total of 13 variables stars in this M81 Group galaxy, NGC 2366. After correcting for foreground Galactic extinction they derive (m-M) 0 = 27.68 ± 0.20 mag, corresponding to a distance of 3.44 Mpc.
Caldwell & Schommer (1990) have been monitoring the luminous southern spiral M83 for Cepheids, while Caldwell et al. (1988) report actually having discovered Cepheids in two southern dwarf irregular galaxies, IC 5152 and the Phoenix Dwarf, as well as in M83. No details have yet been published.
Finally, it should be reported that at least two groups are attempting to discover Cepheids in the Centaurus Group. Walker (private communication) has been obtaining CTIO prime-focus CCD frames of galaxies in this cluster, but his coverage is presently insufficient to securely identify variables, and therefore no periods have yet been reported. On the other hand, Tammann et al. (1991) have a Key Project underway at ESO which has nine half nights used in early 1991 to begin a search for Cepheids in the Centaurus Group late-type galaxies NGC 5236, NGC 5264 and .