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تسجيل مستخدم جديدFour and one more: The formation history and total mass of globular clusters in the Fornax dSph
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We have determined the detailed star formation history and total mass of the globular clusters in the Fornax dwarf spheroidal using archival HST WFPC2 data. Colour magnitude diagrams are constructed in the F555W and F814W bands and corrected for the effect of Fornax field star contamination, after which we use the routine Talos to derive the quantitative star formation history as a function of age and metallicity. The star formation history of the Fornax globular clusters shows that Fornax 1, 2, 3 and 5 are all dominated by ancient (>10 Gyr) populations. Cluster Fornax 1,2 and 3 display metallicities as low as [Fe/H]=-2.5 while Fornax 5 is slightly more metal-rich at [Fe/H]=-1.8, consistent with resolved and unresolved metallicity tracers. Conversely, Fornax 4 is dominated by a more metal-rich~([Fe/H]=-1.2) and younger population at 10 Gyr, inconsistent with the other clusters. A lack of stellar populations overlapping with the main body of Fornax argues against the nucleus cluster scenario for Fornax 4. The combined stellar mass in globular clusters as derived from the SFH is (9.57$pm$0.93)$times$10$^{5}$ M$_{odot}$ which corresponds to 2.5$pm$0.2 percent of the total stellar mass in Fornax. The mass of the four most metal-poor clusters can be further compared to the metal-poor Fornax field to yield a mass fraction of 19.6$pm$3.1 percent. Therefore, the SFH results provide separate supporting evidence for the unusually high mass fraction of the GCs compared to the Fornax field population.
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We use measurements of nitrogen abundances in red giants to search for multiple stellar populations in the four most metal-poor globular clusters (GCs) in the Fornax dwarf spheroidal galaxy (Fornax 1, 2, 3, and 5). New imaging in the F343N filter, ob
tained with the Wide Field Camera 3 on the Hubble Space Telescope, is combined with archival F555W and F814W observations to determine the strength of the NH band near 3370 AA. After accounting for observational errors, the spread in the F343N-F555W colors of red giants in the Fornax GCs is similar to that in M15 and corresponds to an abundance range of Delta([N/Fe])=2 dex, as observed also in several Galactic GCs. The spread in F555W-F814W is, instead, fully accounted for by observational errors. The stars with the reddest F343N-F555W colors (indicative of N-enhanced composition) have more centrally concentrated radial distributions in all four clusters, although the difference is not highly statistically significant within any individual cluster. From double-Gaussian fits to the color distributions we find roughly equal numbers of N-normal and N-enhanced stars (formally about 40% N-normal stars in Fornax 1, 3, and 5 and 60% in Fornax 2). We conclude that GC formation, in particular regarding the processes responsible for the origin of multiple stellar populations, appears to have operated similarly in the Milky Way and in the Fornax dSph. Combined with the high ratio of metal-poor GCs to field stars in the Fornax dSph, this places an important constraint on scenarios for the origin of multiple stellar populations in GCs.
Recently, high-dispersion spectroscopy has demonstrated conclusively that four of the five globular clusters (GCs) in the Fornax dwarf spheroidal galaxy are very metal-poor with [Fe/H]<-2. The remaining cluster, Fornax 4, has [Fe/H]=-1.4. This is in
stark contrast to the field star metallicity distribution which shows a broad peak around [Fe/H]=-1 with only a few percent of the stars having [Fe/H]<-2. If we only consider stars and clusters with [Fe/H]<-2 we thus find an extremely high GC specific frequency, SN=400, implying by far the highest ratio of GCs to field stars known anywhere. We estimate that about 1/5-1/4 of all stars in the Fornax dSph with [Fe/H]<-2 belong to the four most metal-poor GCs. These GCs could, therefore, at most have been a factor of 4-5 more massive initially. Yet, the Fornax GCs appear to share the same anomalous chemical abundance patterns known from Milky Way GCs, commonly attributed to the presence of multiple stellar generations within the clusters. The extreme ratio of metal-poor GC- versus field stars in the Fornax dSph is difficult to reconcile with scenarios for self-enrichment and early evolution of GCs in which a large fraction (90%-95%) of the first-generation stars have been lost. It also suggests that the GCs may not have formed as part of a larger population of now disrupted clusters with an initial power-law mass distribution. The Fornax dSph may be a rosetta stone for constraining theories of the formation, self-enrichment and early dynamical evolution of star clusters.
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ive data. Light curves and accurate periodicities were obtained for 30 RR Lyrae stars and 1 SX Phoenicis variable. The RR Lyrae sample includes 15 fundamental-mode pulsators, 13 first-overtone pulsators, 1 candidate double-mode pulsator and one RR Lyrae star with uncertain type classification. The average and minimum periods of the ab-type RR Lyrae stars, <Pab>=0.590 days, P(ab,min)=0.53297 days, and the position in the horizontal branch type--metallicity plane, indicate that the cluster has Oosterhoff-intermediate properties, basically confirming previous indications by Mackey & Gilmore (2003b), although with some differences both in the period and type classification of individual variables. The average apparent magnitude of the Fornax 5 RR Lyrae stars is <V(RR)>=21.35 +/- 0.02 mag (sigma=0.07 mag, average on 14 stars more likely belonging to the cluster, and having well sampled light curves). This value leads to a true distance modulus of mu0=20.76 +/- 0.07 (d=141.9 (+4.6;-4.5) kpc) if we adopt for the cluster the metal abundance by Buonanno et al. (1998; [Fe/H]=-2.20 +/- 0.20), or mu0=20.66 +/- 0.07 (d=135.5 (+4.4;-4.3) kpc), if we adopt Strader et al.s (2003) metal abundance ([Fe/H]=-1.73 +/- 0.13).
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