The Galactic Stellar Halo imaged by VST S. Zaggia, Y. Momany INAF-Oss. Astronomico di Padova P. Bonifacio CFIST-Obs. de Paris A. Grado INAF-Oss. Astronomico di Capodimonte

The Galactic Halo: recent updates The Milky Way Halo has been completely revolutionized in the last 15 years by the advent of large multi-wavelength photometric surveys (SDSS/SEGUE). A smooth Halo has been supplemented by a structured system populated by: a plethora of ghosty streams of dead dwarf galaxies (e.g. Belokurov et al. 2007; Grillmair et al. 2006, 2008), dying globular clusters (Odenkirchen, 2002) over-densities of-yet-unknown-origin (e.g. Newberg et al. 2002) the discovery of a dual (inner-outer) halo with different shape, chemical distribution and kinematics (Carollo et al 2008)

The “Field of Streams”, from Bielokurov et al ‘06 Ursa MajorII CVn II CVn I Coma Bootes I & II BULGE ANTICENTER Leo A Several new faint dwarf galaxies are marked together with over-densities (compared to a smooth Halo), the Sagittarius stream, and few globular clusters streams. Grillmair et al. (2008) have recently found other 4 new streams and a new galaxy in Bootes in this same field.

The Galactic Halo: formation We don’t have yet a clear picture of the origin of the galactic Halo and of the real nature of its true stellar population. OLD SCENARIO: dichotomy between the classical monolithic collapse (Eggen et al 1962) and the accretion of smaller fragments (Searle and Zinn 1978) EMERGING SCENARIO: a small part of the Halo may indeed have formed through a dissipative collapse while the majority of the Halo is the result of the accretion of satellite dwarf galaxies or proto-galaxies (Chiba and Beers 2000) This picture is supported by the results of modern N-body simulations of galaxy formation in a L Cold Dark Matter Universe (e.g. Abadi et al. 2006)

SEGUE/SDSS: a local Halo sample.

SEGUE/SDSS: the dual Halo.

M31: not all Halos are the same. from Ibata et al. 2007

Metal Poor Stars When the first stars formed? What is their chemical composition? What are the cosmological implications?

Early formation models of the Galaxy predict a Metal Poor Stars Early formation models of the Galaxy predict a large number of stars at [Fe/H]<-4.0: NONE are found

THIN+THICK DISC stars HALO stars

PROPOSAL for the VST: a dedicated stellar survey for the metal poor stars Use of a technique similar to the U excess adopted in the HK (Beers ate al 1999) and HES (Wisotzki et al. 2000) surveys. Breaking the 5-10 kpc local halo and reaching out to 50-100 kpc (whole galaxy) Properly mapping statistically the low metallicity tail of the metallicity distribution. Extend the metallicity sensitivity of broadband filters below [Fe/H]=-1.8

SKYMAPPER filter set: tuned for photo-metallicity

SKYMAPPER: Sensitivity

VST Filters: problematic and expensive Each filter cost ~50K€ and take a lot of time to be delivered

Pilot project at WFI We used the CaK filter centered at l= 390nm and Dl= 12nm 2 SDSS fields of 10 sqdeg covered data reduced with ALAMBIC photometry with DAOPHOT

VST-CAK survey Procurement of the CaK filter ~50K€ (applied for FIRB money 1M€ !) VST+CaK filter 8x more efficient of WFI+CaK filter 1000 sqdeg, KIDS SGP will be perfect or SDSS equatorial fields detection of 1000 stars with [Fe/H]<-3.5 25 nights needed many other surveys will benefit from this filter