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Substructure revealed by RR Lyraes in SDSS Stripe 82
MNRAS in press (arXiv:0906.0498)

Laura Watkins, Wyn Evans & Vasily Belokurov
Institute of Astronomy, University of Cambridge llw25@ast.cam.ac.uk

Abstract
Here we present the results of a study to extract a high-quality set of RR Lyraes in SDSS Stripe 82. We find that the RR Lyraes in Stripe 82 are predominantly associated with one of three substructures which we identify as the Hercules-Aquila Cloud, the Sagittarius Stream and the previously-unknown Pisces Overdensity. We also find evidence for a break in the RR Lyrae distribution at ~25 kpc.

Introduction

SDSS Stripe 82 is the region spanning from 20h to 4h in right ascension and from ­1°.25 to 1°.25 in declination (see left panel). Stripe 82 was repeatedly imaged (see middle panel) over an 8year baseline, making it ideal for the study of variability and proper motions.

Bramich et al. (2008) compiled two catalogues to best exploit the Stripe 82 data set. The Light-Motion-Curve Catalogue (LMCC) contains light-motion curves for all objects in the Stripe; the Higher-Level Catalogue (HLC) contains 229 derived quantities (positions, mean magnitudes, 2, RMS, Stetson indices etc).

RR Lyraes are a class of variable star (a typical light curve is shown in the middle panel); the brightest RR Lyraes can be seen out to ~130 kpc and even the faintest can be seen out to ~100 kpc (see right panel), as such they are excellent tracers of substructure in the halo.

Substructure
To the left, we show the distribution of RR Lyraes in right ascension and distance. There are three clear overdensities: The greatest concentration of RR Lyraes is in the fields coincident with the Hercules-Aquila Cloud (Belokurov et al. 2007). Not all of these RR Lyraes are necessarily associated with the Cloud, as there may be contamination from the underlying smooth population associated with the Galactic Spheroid, however we are unable to disentangle the populations with the current data set. The plane of the orbit of the Sagittarius dwarf galaxy crosses Stripe 82, and there is a visible overdensity of RR Lyraes at this location ( ~ 2h) which we associate with the Sagittarius Stream. There are few RR Lyraes at large distances (D > 50 kpc), of which 28 lie in a clump at ~ 23.5h. We term this structure the Pisces Overdensity. Distance uncertainties are shown as vertical bars; although the errors for the distant RR Lyrae are large enough to be visible, they cannot be responsible for the overdensity. Neither can the overdensity be explained away by the properties of the survey. The Pisces Overdensity was recently confirmed using spectroscopic data by Kollmeier et al. (2009). We also see that there is a sharp drop in the number of RR Lyraes beyond 40 kpc. Above we show the number density of RR Lyraes plotted in the plane of Galactic longitude versus r-band magnitude. Once again, the three substructures show up very clearly, together with some isolated hot pixels that may be indicators of real objects. We find that nearly 60 per cent of all the RR Lyraes in Stripe 82 are associated with the Hercules-Aquila Cloud alone, emphasising the arguments made by Belokurov et al. (2007) as to the importance of this structure.

RR Lyrae density distribution
Here we analyse the density distribution of our RR Lyraes. Classical models for a smooth halo predict that RR Lyraes are distributed as a power-law like ~ r-n with n ~ 3.1 (Wetterer & McGraw 1996). With no substructure present in Stripe 82, we would see a steady fall-off in numbers with distance, instead of the sharp drop observed beyond ~40 kpc, which is real and cannot be attributed to properties of the survey. The presence of an edge to the RR Lyrae distribution in the stellar halo at r~50 kpc has been proposed before by Ivezi et al. 2000, using a sample of 148 RR Lyraes in SDSS commissioning data. However, the same authors later applied their method to a larger area of the sky and found no break until at least 70 kpc (Ivezi et al. 2004). Vivas & Zinn (2006) also found no break before the limit of their survey at ~60 kpc. So, "edge" may be too strong a term, but the number density profile of the RR Lyraes does seem to be best matched by a broken power-law, as first advocated by Saha (1985). Adjusting by the fraction of Galactic volume sampled by our survey, and assuming that our efficiency is ~ 1, we find that the sphericallyaveraged number density of RR Lyrae is: n(r) = 2.6 (23 kpc / r)2.4 n(r) = 2.6 (23 kpc / r)
4.5

Pisces Overdensity
Here we present a summary of the properties we have found for the Pisces Overdensity. Galactic longitude, ~ [63°, 93°] Galactic latitude, b ~ [-60°, -46°] Heliocentric distance, D ~ 79.9 ± 13.9 kpc Galactocentric distance, r ~ 79.4 ± 14.1 kpc Metallicity, [Fe/H] ~ -1.48 ± 0.28 Mass, M ~ 104 Msolar

References
Belokurov V. et al., 2007, ApJ, 657, L89 Bramich D. M. et al., 2008, MNRAS, 386, 887 Ivezi, Z. et al., 2000, AJ, 120, 963 Ivezi, Z. et al., 2004, In "Satellites and Tidal Streams", eds F. Kollmeier J. et al., 2009, ApJL submitted, arXiv:0908.1381 Miceli, A. et al., 2008, ApJ, 678, 865 Saha A., 1985, ApJ, 289, 310 The break at ~25 kpc is a consequence of the fact that over 70 per cent of the RR Lyraes in Stripe 82 are associated with either the Hercules-Aquila Cloud or the Sagittarius Stream substructures, which lie within 40 kpc of the Galactic centre. A similar conclusion regarding the importance of substructure is reached by Sesar et al. (2007), who divide their RR Lyrae distribution into 13 clumps, of which they suggest at least seven correspond to real substructures; their "Structure J" corresponds to the Pisces Overdensity. Sesar B. et al., 2007, ApJ, 134, 2236 Vivas, A. K., Zinn, R., 2006, AJ, 132, 714 Watkins L. L. et al., 2009, MNRAS in press, arXiv:0906.0498 Wetterer C. J., McGraw J. T., 1996, AJ, 112, 1046

5 < r < 23 kpc 23 < r < 100 kpc

out to ~100 kpc, beyond which our data is incomplete. We must stress that this formula is merely a convenient parametrisation of the data, as the RR Lyrae distribution is neither spherically symmetric nor smooth, but dominated by the three structures in the Stripe. The inner power-law slope is almost the same as that found by Miceli et al. (2008) ­ namely n=2.43 ­ in a very large sample of RR Lyrae stars closer than 30 kpc in the LONEOS survey; while our break radius of 23 kpc is very close to the break at r = 25 kpc proposed by Saha (1985).