1. 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

2. 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 ; 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)

3. 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.

4. 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)

5. SEGUE/SDSS: a local Halo sample.

6. SEGUE/SDSS: the dual Halo.

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

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

9. 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

10. THIN+THICK
DISC
stars
HALO
stars

11. 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 ) surveys.
Breaking the 5-10 kpc local halo and reaching out to 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

12. SKYMAPPER filter set: tuned for photo-metallicity

13. SKYMAPPER: Sensitivity

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

15. 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

17. 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