Structure of the Galactic halo

Simulations of the V-mag surface brightness for different halo models. Galactocentric distances in kpc. Credits: “Galactic stellar haloes in the CDM model”, <a href='http://adsabs.harvard.edu/abs/2010MNRAS.406..744C'>Cooper et al. 2010, MNRAS</a>.

Simulations of the V-mag surface brightness for different halo models. Galactocentric distances in kpc. Credits: “Galactic stellar haloes in the CDM model”, Cooper et al. 2010, MNRAS.

The improvement of our knowledge on the present structure of the Milky Way will provide constraints on the Galaxy formation theories and simulations (left figure), hence the importance of having a complete picture of it. Several satellite galaxies tidal streams and debris have been discovered in the last years that are offering clues on the Milky Way formation history, as well as on that of galaxies in general.

J-PLUS will observe 8500□⁰ on the sky mainly pointing towards the portion of the galactic halo that is visible from the Northern hemisphere. This wide area, in combination with limiting magnitudes, in particular, in the set of the broadband Sloan filters (mAB=23 for a signal-to-noise ratio≥3), will provide a huge volume within the halo of the Milky Way with valuable information to deal with. The main goals pursued in this field with J-PLUS are to produce a 3D map, as much precise as possible, of the observed galactic halo, to detect overdensities in the mean halo density profile, to discover still unknown tidal streams or debris, as well as to build a metallicity map of the halo.

Of particular relevance on this concern are the more than 15,000 RR Lyrae variable stars that are estimated to be observed with J-PLUS. They play an important role when tracing halo’s structures because they are considered as standard candles given their homogeneous absolute magnitude, MV≈0.6 (if the main halo metallicity, [Fe/H]=−1.5dex, is assumed). They are evolved low-mass, old (t>10Gyr) and moderately bright stars that lay in the locus of the H-R diagram where the horizontal branch crosses the instability strip, so they pulsate, in particular, undergoing radial oscillations. They can be found everywhere in the Galaxy (undoubtedly in the halo too) as well as in the nearby galaxies. The change of their brightness, from AI≈0.1mag to AI≈1.3mag, their relatively short pulsation period, from a few hours to slightly more than a day, and their intrinsic colors (fortunately not to much contaminated with other objects) make their identification feasible. J-PLUS will have some temporal resolution because of the revisits to each field (see Fig 1), so it will also profit from this time-domain information to detect them. Besides RR Lyraes, nonetheless, main-sequence stars are also useful to trace the structure of the Galactic halo (see e.g. Jurić et al., 2008).

Simulation of an RR Lyrae star.

Figure 1. Simulation of an RR Lyrae star (modeled from data taken from the database of the OGLE-III Catalog of Variable Stars) given the spectro-temporal strategy of J-PLUS.

To illustrate this kind of studies, the Sloan Digital Sky Survey (SDSS) has already probed distances up to Galactocentric distances, dG∼110kpc with RR Lyrae stars and up to dG∼40kpc with main-sequence stars (see Sesar et al., 2010) in a particular region of the sky, the Stripe 82, employing broadband photometry. The Catalina Sky Survey (CSS) has explored the Galactic halo up to dG∼60kpc within an area of 20000□⁰ with unfiltered photometry and a sampling of more than 40 points per light-curve (see Drake et al., 2013).

J-PLUS will be approximately one magnitude deeper than SDSS and with a limiting magnitude similar than that of the Stripe 82. It will have a poorer time sampling than CSS but it will be two magnitudes deeper and with multi-band photometry. Adapted techniques, tailored to the specific features of J-PLUS, will be applied to study to the whole area of the present survey in order to obtain an improved three-dimensional picture of the Galactic halo.