Representative light curves for 9 of the 52 Cepheid candidates discovered in NGC 1365 are given in Figure 31. The phase coverage in all cases is sufficiently dense and uniform that the form of the light curves is clearly delineated. This allowed 37 of these variables to be unambiguously classified as high-quality Cepheids with their distinctively rapid brightening, followed by a long linear decline phase. Periods, obtained using modified Lafler-Kinman algorithm (Lafler & Kinman 1965), are judged to be statistically good to a few percent, although in some cases ambiguities larger than this do exist as a consequence of the narrow observing window and the restricted number of cycles (between 1 and 5) covered within the 44-day window.
Figure 31. Representative V-band light
curves for 9 of the 37 Cepheid variables found in the Fornax cluster galaxy,
NGC 1365. Magnitudes are plotted along the vertical
axis. Two repeated cycles in phase are plotted along the horizontal axis.
The resulting V and I period-luminosity relations for the complete set of 37 Cepheids are shown in the upper and lower panels of Figure 32, respectively. The solid line is a minimum ^{2} fit to the fiducial PL relation for LMC Cepheids (Madore & Freedman 1991), corrected for E (B-V) _{LMC} = 0.10 mag, scaled to an LMC true distance modulus of µ_{0} = 18.50 mag, and shifted into registration with the Fornax data. [Recent results from the Hipparcos satellite bearing on the Galactic calibration of the Cepheid zero point (Feast & Catchpole 1997; Madore & Freedman 1997) indicate that the LMC calibration is confirmed at the level of uncertainty indicated in Table 4, with the possibility that a small (upward) correction to the LMC reddening is in order.] The derived apparent moduli are µ _{V} = 31.67 ± 0.05 mag and µ _{I} = 31.57 ± 0.04 mag. Correcting for a derived total line-of-sight reddening of E (V-I) _{N1365} = 0.10 mag (based on the Cepheids themselves) gives a true distance modulus of µ _{0} = 31.43 ± 0.06 mag. This corresponds to a distance to NGC 1365 of 18.0± 0.6 Mpc. The quoted error at this step in the analysis quantifies only the statistical uncertainty generated by photometric errors in the ALLFRAME data combined with the intrinsic magnitude and color width of the Cepheid instability strip.
Figure 32. V and I-band
Period-Luminosity relations for the full set of 37 Cepheids monitored in
NGC 1365. The fits are to the fiducial relations
given by
Madore & Freedman (1991),
shifted to the apparent distance modulus of NGC
1365. Dashed lines indicate the expected intrinsic (2-sigma) width of
the relationship due to the finite temperature width of the Cepheid
instability strip.
Source of Uncertainty on the Mean | Description of Uncertainty | Percentage Error | |
FORNAX CLUSTER | EXPANSION VELOCITY AND INFERRED DISTANCE | ||
(L) Velocity Dispersion | ± 31 km/sec = ± 310 / N-1 (No. of gals = 111) at | 2% | |
(M) Geometry of Cluster | ± 0.4 Mpc at 18.6 Mpc | 2% | |
[N] Virgocentric Flow | ± 22 km/sec on -44 km/sec along the Local Group line of sight | 2% | |
[O1] Bulk Flow | ± 300 km/sec | 24% | |
Random Errors | (J) + (L) + (M) combined in quadrature | 10% | |
Systematic Errors | [K] + [N] + [O1] combined in quadrature | 26% | |
H_{0}= 68 km/sec/Mpc ± 7 (random) ± 18 [systematic] | |||
LOCAL FLOW | M81, M101, NGC 1023, NGC 2090, NGC 3621, NGC 7331, LEO, VIRGO, FORNAX | ||
(P) Random Motions | ± 4 km/sec/Mpc = ± 12 / N-1 (No. of samples = 9) | 6% | |
[O2] Bulk Flow | ± 300 km/sec at V (max) = +1,400 km/sec | 21% | |
Random Errors | (P) = total observed scatter on the mean | 6% | |
Systematic Errors | [SC] + [Z] + [O2] combined in quadrature | 23% | |
H_{0} = 72 km/sec/Mpc ± 4 (random) ± 17 [systematic] | |||
DISTANT FLOW | I. TULLY-FISHER: 16 CLUSTERS TO 10,000 km/sec | ||
(S) Observed Scatter | ± 0.04 mag = ± 0.16 / N-1 (No. of clusters = 16) | 2% | |
[R] TF Zero Point | (mean) = ± 0.13 mag = ± 0.40 / N-1 (No. of calibrators = 11) | 6% | |
[O3] Bulk Flow | ± 300 km/sec evaluated at 10,000 km/sec | 3% | |
Random Errors | (S) | 2% | |
Systematic Errors | [SC] + [Z] + [R] + [O3] combined in quadrature | 11% | |
H_{0} = 76 km/sec/Mpc ± 2 (random) ± 8 [systematic] | |||
DISTANT FLOW | II. HYBRID METHODS: 17 CLUSTERS TO 11,000 km/sec | ||
(U) Observed Scatter | ± 0.02 mag = ± 0.06 / N-1 (No. of clusters = 17) | 2% | |
[O4] Bulk Flow | ± 300 km/sec evaluated at 11,000 km/sec | 3% | |
[T] FORNAX Distance | [SC] + [Z] combined in quadrature | 9% | |
Random Errors | (U) | 2% | |
Systematic Errors | [T] + [Z] combined in quadrature | 10% | |
H_{0} = 72 km/sec/Mpc ± 1 (random) ± 7 [systematic] | |||
DISTANT FLOW | III. Type Ia SN: 20 EVENTS OUT TO 20,000 km/sec | ||
(T1) Peak Luminosity | ± 0.11 mag = ± 0.45 / N-1 (No. of SNIa = 16) | 6% | |
(V1) Random Motions | ± 300 km/sec at 5,000 km/sec | 6% | |
[O5] Bulk Flow | ± 300 km/sec at 20,000 km/sec | 2% | |
[Q1] SNIa Zero Point | (mean) = ± 0.18 mag = ± 0.45 / N-1 (No. of calibrators = 7) | 9% | |
Random Errors | (T1) + (V1) combined in quadrature | 8% | |
Systematic Errors | [SC] + [O5] + [Q1] combined in quadrature | 12% | |
H_{0} = 68 km/sec/Mpc ± 5 (random) ± 8 [systematic] |