{'fraction_non_alphanumeric': 0.04368542213180577, 'fraction_numerical': 0.029070078698003106, 'mean_word_length': 4.287291253891876, 'pattern_counts': {'":': 0, '<': 4, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
The Stellar Populations of the Cetus Dwarf Spheroidal Galaxy

We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry in the V and I passbands of the recently discovered Local Group dwarf spheroidal galaxy in Cetus. Our color-magnitude diagram extends from above the first ascent red giant branch (RGB) tip to approximately half a magnitude below the horizontal branch (HB). Adopting a reddening of E(B-V) = 0.03, the magnitude of the RGB tip yields a distance modulus of (m-M)o = 24.46 +/- 0.14. After applying the reddening and distance modulus, we have utilized the color distribution of RGB stars to determine a mean metal abundance of [Fe/H] = -1.7 on the Zinn&West scale with an intrinsic internal abundance dispersion of +/-0.2 dex. An indirect calculation of the HB morphology of Cetus based on the mean dereddened HB color yields (B-R)/(B+V+R) = -0.91 +/- 0.09, which represents an HB that is redder than what can be attributed solely to Cetus' metal abundance. As such, Cetus suffers from the `second parameter effect' in which another parameter besides metallicity is controlling the HB morphology. If we adopt the conventional `age hypothesis' explanation for the second parameter effect, then this implies that Cetus is 2-3 Gyr younger than Galactic globular clusters at its metallicity.

-2on the Zinn & West scale with an intrinsic internal abundance dispersion of ∼0.2 dex. An indirect calculation of the HB morphology of Cetus based on the mean dereddened HB color yields (B − R)/(B + V + R) = −0.91 ± 0.09, which represents an HB that is redder than what can be attributed solely to Cetus' metal abundance. As such, Cetus suffers from the 'second parameter effect' in which another parameter besides metallicity is controlling the HB morphology. If we adopt the conventional 'age hypothesis' explanation for the second parameter effect, then this implies that Cetus is 2-3 Gyr younger than Galactic globular clusters at its metallicity.

Introduction
It is truly remarkable that dwarf galaxy members of the Local Group are still being uncovered over 60 years after the discovery of the first such system, Sculptor, by Shapley (1938). Ongoing efforts to search for these systems have led to our current census which includes 32 dwarf galaxies in the Local Group (Grebel 2000) distributed over a roughly 4 Mpc 3 volume. These searches have primarily been fueled by the realization that dwarfs may represent the building blocks of much larger galaxies such as spirals and ellipticals (e.g. Côté et al. 2000). Furthermore, there is the question of whether environmental effects play a role in the formation and evolution of the dwarf systems themselves. As such, understanding their properties is likely to shed light on the process of galaxy formation in general.
Within this framework, the Hubble Space Telescope (HST) cycle 8 and 9 programs entitled 'A Snapshot Survey of Probable Nearby Galaxies' PI:Seitzer) were initiated to improve our understanding of nearby dwarf galaxies -their numbers, spatial distribution, structural properties, and stellar populations, among other things (Seitzer et al. 2001). Previous papers reporting the results of our survey include Dolphin et al. (2001) and Karachentsev et al. (1999;2000a;2000b).
In the present work, we turn our attention to the Cetus dwarf spheroidal (dSph) galaxy, which was discovered by Whiting, Hau, & Irwin (1999; hereafter WHI) as part of their comprehensive program using sky survey plates to search for low surface brightness dwarf galaxy candidates. It is located at α 2000 = 00 h 26 m 11 s , δ 2000 = −11 • 02 ′ 40 ′′ in a relatively isolated region of the Local Group. Follow-up observations of Cetus with the Cerro Tololo Inter-American Observatory 1.5m telescope allowed WHI to determine a distance modulus of (m − M) 0 = 24.45 ± 0.15 based on the magnitude of the red giant branch (RGB) tip and -3a metallicity of −1.9 ± 0.2 dex from the color of the RGB. In addition, from the surface brightness profile of Cetus, WHI estimated a core radius of 1.5 ± 0.1 arcmin and a tidal radius of 4.8 ± 0.2 arcmin. Subsequent work by Tolstoy et al. (2000, see also Tolstoy 2001 presents a deep BR color-magnitude diagram for Cetus based on observations with the ESO Very Large Telescope. Their data reveal that Cetus possesses a predominantly red horizontal branch (HB) with the possible presence of a small blue HB component.
The remainder of this paper is organized as follows. The next section describes how the observations were obtained and the techniques employed to derive our color-magnitude diagram for Cetus. Sec. 3 presents an analysis of our results regarding the distance (Sec. 3.2), metallicity (3.3), and horizontal branch morphology (3.4) of Cetus. Finally, our conclusions are summarized in Sec. 4.

Observations and Data Reduction
The observations of Cetus were obtained on 16 Jan 2001 UT as part of our cycle 9 HST snapshot survey focusing on nearby dwarf galaxies (GO-8601, Seitzer et al. 2001). The data consist of two 600s exposures, one each using the F606W (∼V) and F814W (∼I) filters, with Cetus centered in the WF3 CCD. Figure 1a shows our observed WFPC2 field overlaid on the Digitized Sky Survey while Fig. 1b displays a cosmic ray cleaned mosaic of our combined F606W and F814W WFPC2 images.
The photometric reduction of these observations has already been fully described by Dolphin et al. (2001). To summarize, the calibrated images obtained from STScI were input into the HSTphot package (Dolphin 2000a). After removing cosmic rays with the HSTphot cleansep routine, simultaneous photometry was performed on the F606W and F814W frames using multiphot, with aperture corrections made to an aperture of radius 0.5 arcsec. CTE corrections and calibrations were then applied using the Dolphin (2000b) formulae, producing V I photometry for all stars detected in both images. Because of the relatively small field of the PC1 CCD, very few bright stars were available for the computation of an aperture correction, leading to larger than acceptable uncertainties in the PC1 photometry. As a result, we have excluded the PC1 results from further analysis. In the case of the WF photometry, we estimate an uncertainty of 0.02 mag in the photometric zeropoint based on the results of Dolphin (2000b). In contrast to previous papers in this series, instead of setting a lower limit of 5 for the signal-to-noise per stellar magnitude measurement, we have used a lower limit of 3.5. This has been done so as to provide the maximum amount of information on the morphology of the blue horizontal branch, which is near the limit of the photometry.  Figure 2 shows our (V,V-I) and (I,V-I) color-magnitude diagrams (CMDs) for the Cetus dSph based on photometry derived from the three WF chips for 3101 stars. The variation of the photometric errors with magnitude are also shown. The CMDs reveal a relatively steep and well-populated red giant branch (RGB) along with a predominantly red horizontal branch (HB). The tip of the first ascent RGB is located at I∼20.5 with the level of the HB being at V∼25. The CMD contains little or no indication of significant recent star formation in Cetus. For example, there are very few stars in the region where one expects to see O and B type stars (V-I < ∼ 0) indicative of a < ∼ 1 Gyr population. In addition, there is a negligible number of asymptotic giant branch stars located above the first ascent RGB, which are normally associated with an intermediate-age (1< ∼ t< ∼ 8 Gyr) population.

Reddening and Distance
Both the Burstein & Heiles (1982) and Schlegel, Finkbeiner, & Davis (1998) reddening maps indicate that Cetus is in a region of very low extinction. More specifically, the latter suggest that the reddening of Cetus is E(B − V ) = 0.03, which, following the lead of WHI, we will also adopt in the present study along with an assumed error of 0.01 mag. In addition, given that A V = 3.1E(B − V ), for the HST filters used herein, Schlegel et al. (1998) give Since the location of the first ascent RGB-tip (TRGB) appears to be well determined in our CMD, we will obtain an estimate of the distance using I(TRGB). Our RGB cumulative luminosity function is shown in Fig. 3 to which we have applied a slope finding algorithm that isolates the TRGB (dashed line). This technique yields I(T RGB) = 20.50 ± 0.10, in good accord with the WHI value. Assuming an absolute magnitude of M I (T RGB) = −4.05 ±0.10 (Da Costa & Armandroff 1990, hereafter DCA;Sakai, Madore, & Freedman 1996;Bellazzini, Ferraro, & Pancino 2001), this gives (m − M) 0 = 24.46 ± 0.14, which is in good accord with the WHI value of (m − M) 0 = 24.45 ± 0.15; to minimize confusion, we will adopt their distance modulus in the subsequent analysis.
We note in passing that the HB can in principle be used as a distance indicator as well. However, since we have no information on the presence of RR Lyrae variables in Cetus, the use of a relation between M V (RR) and [F e/H] may produce spurious results. Carretta et al. (2000) provide a calibration of the mean magnitude of the HB [M V (HB)] in terms of -5metallicity, but the only red HB clusters in their sample, 47 Tuc and NGC 362, have globular cluster-like ages. Yet, it is known that both age and metallicity affect the luminosity of a purely red HB (a.k.a. red clump;Sarajedini 1999;Girardi & Salaris 2001). Therefore, if we assume that the mean age of the Cetus population producing its red HB is not significantly different from those of 47 Tuc and NGC 362, then we can use Equation 6 of Carretta et al. (2000) and our mean metal abundance (Sec. 3.3) to calculate M V (HB) = 0.51 ± 0.08. Fitting a Gaussian to our V luminosity function, we find V (HB) = 25.01 ± 0.02 (sec. 3.4), which produces a distance modulus of (m − M) 0 = 24.41 ± 0.09, consistent with the TRGB distance obtained above.

Metallicity
At this point, since we have estimates for the reddening and distance of Cetus, it is a relatively simple matter to compare our RGB photometry to globular cluster RGB sequences. The two panels of Fig To obtain a more quantitative assessment, we have utilized equations (2) through (6) in Saviane et al. (2000) along with the relevant coefficients in their Table 5  We note that inspection of globular cluster VI CMDs and theoretical isochrones suggest that the AGB merges into the RGB at M I ∼ −2.0; this indicates that our MDF should not be significantly affected by (bluer) AGB stars simulating an excess of metal-poor stars. We will now show that the characteristics of the MDF are unaffected by the magnitude range as long as only stars brighter than M I ∼ −2.0 are used.  Fig. 5), the peak abundance is [F e/H] = −1.65 with a 1-σ spread of 0.36 dex, which when coupled with the abundance dispersion introduced purely by the photometric errors of 0.30 dex, yields an intrinsic 1-σ metallicity dispersion of 0.19 dex. Thus, we conclude that Cetus exhibits a mean metallicity of [F e/H] = −1.7. In addition, we find an intrinsic 1-σ abundance dispersion of ∼0.2 dex in Cetus; Table 2 in Grebel (2000) suggests that metal abundance dispersions of this order are quite common among the dwarf galaxies in the Local Group. With high-resolution spectroscopy, even larger dispersions are being uncovered (e.g. Shetrone, Côté, & Sargent 2000).
We note that our determination of the mean metallicity of Cetus is 0.2 dex more metalrich than that of WHI. Since we adopt the same distance and reddening as WHI, this abundance difference is likely to be the result of differences in our V-I color zeropoints. A comparison of the RGB location in the WHI CMD with our CMD suggests that the former is ∼0.05 mag bluer than the latter. The origin of this photometric zeropoint difference is unclear.
We acknowledge that our measured values of the mean metallicity and metallicity dispersion can be influenced by age effects, as many (or most) dwarf spheroidals formed their stars over extended periods of time. Based on the Girardi et al. (2000) isochrones, we estimate that for ages greater than 10 Gyr, this effect is minimal (∆t of 5 Gyr corresponds to ∆[F e/H] of roughly 0.1 dex in terms of RGB color), thus leaving our metallicity measurements largely unchanged. However, we cannot completely rule out the presence of younger stars without observing the main sequence turnoff; if stars of age ∼ 7 Gyr or younger are present, the resulting effect on our metallicity measurement is 0.2 dex or greater, also adding a large uncertainty to our metallicity dispersion measurement. Although this scenario is possible, there is no a priori reason to assume the presence of younger stars, and thus we will continue to use our measured value of [F e/H] = −1.7 with a 1-σ dispersion of 0.2 dex.
We have also utilized the data in Table 2 of Grebel (2000) to plot the mean metallicity of Local Group dwarfs as a function of their absolute magnitude (Fig. 6a) and central surface brightness (Fig. 6b), excluding the Sagittarius dwarf galaxy and M32. The plotted data represent dSphs (crosses), dIrrs (filled circles) and dEs (open circles). There is a well-known relation between mean abundance, absolute V magnitude, and central surface brightness for these systems in the sense that more luminous galaxies with higher central surface brightness tend to enrich themselves in heavy elements to a greater degree (Caldwell et al. 1998, and references therein). The large open square represents the location of Cetus based on our new mean metal abundance coupled with the luminosity and surface brightness estimates of -7 -WHI. It is clear that Cetus occupies its expected location in these diagrams.

Horizontal Branch
As first illustrated in Fig. 2, the HB of Cetus appears to be primarily populated redward of the RR Lyrae instability strip. As a comparison, Fig. 7 shows our Cetus CMD compared with the fiducial sequences of the globular clusters Rup 106 (Sarajedini & Layden 1997), M3 (Johnson & Bolte 1998) and M54 (Sarajedini & Layden 1995). While the metallicity of Cetus is similar to those of Rup 106, M3 and M54 (the RGBs match), there is no question that its HB morphology is significantly redder than the HBs of these clusters. We are probably missing some blue HB stars in Cetus (especially those that may be present along the blue HB tail, i.e. fainter than M V ∼ +1) due to photometric incompleteness, but there are unlikely to be as many blue HB stars as those on the red HB. We also point out that the (I,V-I) CMD (bottom panel of Fig. 2) shows that the photometric incompleteness along the RGB is not severe even at the faintest levels, i.e. the luminosity function of RGB stars is steadily increasing at these magnitudes. This suggests that photometric incompleteness on the HB at similar magnitude levels is not likely to be problematic.
It has been traditional to use the numbers of stars blueward of the instability strip (B) and those redward of it (R) along with stars within the strip itself (RR Lyrae variables, V) in order to construct the (B − R)/(B + V + R) index (Lee et al. 1994). However, this approach is not practical in our case because of the lack of information on the numbers of RR Lyraes in Cetus. Instead, we adopt a different procedure, one that takes advantage of the peak HB color as defined by Buonanno et al. (1997). For clusters with metallicities between −1.80 ≤ [F e/H] ≤ −1.60 as given in Table 1  The next step is to measure the peak (V −I) 0 of the Cetus HB and convert it to (B−V ) 0 . First, we construct luminosity functions of our photometry in V and I to which we fit Gaussian distributions in order to isolate the magnitude of the HB. We find V (HB) = 25.01± 0.02 and I(HB) = 24.22 ± 0.02 where the errors represent a combination of the standard errors of the mean and an estimate of the uncertainty in the fitting process. Subtracting these and including an estimated error of 0.02 mag in the photometric zeropoint along with our adopted Cetus reddening gives (V −I) 0 = 0.75±0.03. We prefer this method rather than directly measuring the peak color of the HB for two reasons. First, the color distribution -8of the HB stars is more susceptible to contamination from the color peak of the RGB stars which are slightly redward. Second, the LF peaks of the HB in V and I are more closely Gaussian so that fitting them with such a distribution is more straightforward. A conversion from (V − I) 0 to (B − V ) 0 , however, is somewhat more uncertain. Zinn & Barnes (1996) provide a horizontal branch color conversion equation; however it is based on M15 and M68, whose horizontal branches are both predominantly blue. Thus the value we calculate for red HB stars (which is what we have in Cetus) is interpolated between their blue HB stars and RGB stars at the level of the HB. Using their relation, we find ( (1998) relations is an extrapolation from RGB stars, we utilize the other two values to measure (B − V ) 0 = 0.55 ± 0.03 for the peak color of our horizontal branch. Using the relation plotted in Fig. 8, this color produces a horizontal branch index Based on its intermediate metallicity and red HB morphology, Cetus joins a long list of other Local Group dSphs that suffer from the second parameter effect (e.g. Harbeck et al. 2001), in much the same way as the Galactic globular clusters in the outer halo (Sarajedini, Chaboyer, & Demarque 1997, and references therein). That is to say, the HB morphology of Cetus is too red for its mean metal abundance. Although there is still no general agreement on the cause of the second parameter effect, the conventional explanation is based on the age hypothesis in which the stellar population of Cetus would be younger than a typical Galactic GC. Utilizing new HB models published by Rey et al. (2001, see their Fig. 4), we estimate that this age difference amounts to some 2-3 Gyr.

Conclusions
Using HST/WFPC2 in snapshot mode, we have constructed a deep CMD of the Cetus dwarf spheroidal galaxy. Based on this CMD, which extends from the tip of the RGB to about 0.5 mag below the HB, and an adopted reddening of E(B − V ) = 0.03 based on the Schlegel et al. (1998) extinction maps, we are able to determine a number of fundamental properties for this galaxy. First, we exploit the tip of the first ascent RGB to derive a distance modulus of (m − M) 0 = 24.46 ± 0.14, which agrees very well with that of Whiting et al. (1999). Having corrected for the reddening and the distance, a comparison of the This preprint was prepared with the AAS L A T E X macros v5.0.
-12 -        However, Cetus appears to have a horizontal branch (HB) that is significantly redder than the HBs of these globular clusters.