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تسجيل مستخدم جديدUnmixing the Galactic Halo with RR Lyrae tagging
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We show that tagging RR Lyrae stars according to their location in the period-amplitude diagram can be used to shed light on the genesis of the Galactic stellar halo. The mixture of RR Lyrae of ab type, separated into classes along the lines suggested by Oosterhoff, displays a strong and coherent evolution with Galactocentric radius. The change in the RR Lyrae composition appears to coincide with the break in the halos radial density profile at ~25 kpc. Using simple models of the stellar halo, we establish that at least three different types of accretion events are necessary to explain the observed RRab behavior. Given that there exists a correlation between the RRab class fraction and the total stellar content of a dwarf satellite, we hypothesize that the field halo RRab composition is controlled by the mass of the progenitor contributing the bulk of the stellar debris at the given radius. This idea is tested against a suite of cosmological zoom-in simulations of Milky Way-like stellar halo formation. Finally, we study some of the most prominent stellar streams in the Milky Way halo and demonstrate that their RRab class fractions follow the trends established previously.
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We present the analysis of 12227 type-ab RR Lyrae found among the 200 million public lightcurves in the Catalina Surveys Data Release 1 (CSDR1). These stars span the largest volume of the Milky Way ever surveyed with RR Lyrae, covering ~20,000 square
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eq R_{mathrm{gc}} leq 141 ; mathrm{kpc}$ with a distance precision of 3% and thus is able to trace the halo out to larger distances than most previous studies. After excising stars that are attributed to dense regions such as stellar streams, the Galactic disc and bulge as well as halo globular clusters, the sample contains ${sim}11,000$ sources within $20 ; mathrm{kpc} leq R_{mathrm{gc}} leq 131 ; mathrm{kpc}$. We then apply forward modeling using ellipsoidal stellar density models $rho(l,b,R_{mathrm{gc}})$ both with a constant and a radius-dependent halo flattening $q(R_{mathrm{gc}})$. Assuming constant flattening $q$, the distribution of the sources is reasonably well fit from $20 ; mathrm{kpc}$ to $131 ; mathrm{kpc}$ by a single power law with $n=4.40^{+0.05}_{-0.04}$ and $q=0.918^{+0.016}_{-0.014}$. The distance distribution is fit comparably well by an Einasto profile with $n=9.53^{+0.27}_{-0.28}$, an effective radius $r_{mathrm{eff}}=1.07 pm 0.10 ; mathrm{kpc}$ and a halo flattening of $q=0.923 pm 0.007$. If we allow for a radius-dependent flattening $q(R_{mathrm{gc}})$, we find evidence for a distinct flattening of $q{sim}0.8$ of the inner halo at ${sim} 25 ; mathrm{kpc}$. Additionally, we find that the south Galactic hemisphere is more flattened than the north Galactic hemisphere. The results of our work are largely consistent with many earlier results, e.g. cite{Watkins2009}, cite{Iorio2017}. We find that the stellar halo, as traced in RR Lyrae stars, exhibits a substantial number of further significant over- and underdensities, even after all known overdensities have been masked.
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