1. STEP Survey- the SMC in Time: Evolution of a Prototype interacting dwarf galaxy
V. Ripepi
INAF-Osservatorio Astronomico di Capodimonte, Napoli
Co-I:
M. Marconi, I. Musella, M. Dall’Ora,M. Capaccioli (INAF-OAC)
G. Clementini, M. Tosi, D. Romano, M. Cignoni (INAF-OABo)
M.R. Cioni (Univ. Hertfordshire)
E. Cappellaro (INAF-OAPD)
E. Brocato, G. Raimondo, M. Cantiello (INAF-OATeramo)
A. Nota, M. Sirianni (STSCI)
J.S. Gallagher (Univ. Wisconsin)
E. Grebel (Max-Planck Institute)

2. The Magellanic System
The Magellanic system is composed by LMC and SMC plus the Magellanic Bridge and Stream.

3. Why the SMC?
Dwarf Galaxies (DGs): dominant present-day population of groups and clusters
In hierarchical  CDM models DGs are possible “building blocks” of larger galaxies (including the MW)  understanding DG star formation and chemical enrichment histories is fundamental to constrain models of Galaxy formation.
Local Group DGs are ideal laboratories for understanding DG properties: we can explore in detail ages, metallicities and spatial distributions of their stellar populations.
Among Local Group DGs, the Small Magellanic Cloud (SMC) is the closest late-type dwarf (60 Kpc).
The SMC is a metal poor galaxy (Z  0.004)  best local analog of the large majority of DGs and blue compact galaxies.
Tidally interacting with its neighbours: Wing and Bridge towards the LMC  probe galaxy interaction models.

4. Why the VST? High photometric accuracy Good spatial resolution
Large field of view
High photometric accuracy
Good spatial resolution
Survey limiting magnitudes fully within the reach of VST capabilities

5. Main Aims of the Survey
1. (Core Survey) To study in a homogeneous way the SFH of the SMC and of its stellar cluster component through deep (1-2 mag fainter than the TO of the oldest population) CMD over the whole SMC body.
Trends of SFH with position
Compare Field and Cluster SFH
Cluster and Field Age-Metallicity relations (VLT follow-up)
Low Metallicity massive Stellar Cluster with ages 2-9 Gyrs: evolution properties of stars in the mass range 1-2 M
Cluster SBF: age-metallicity degeneracy.
OGLE III
STEP

6. 2. (Core Survey) To study for the first time systematically the Wing and Bridge stellar populations:
Inventory of variable stars in the Bridge.
SFH of intra-cloud population by using both CMDs and variable stars (e.g. Cepheids, RR Lyrae,  Scuti etc.) as population tracers.
Solve the problem of the formation and evolution of the stellar components in the Bridge (VLT follow-up): tidally stripped or locally formed (Bekki & Chiba 2007, Harris, 2007)?
3. (Core + Hα-r’ Survey) To study the young (PMS) stellar component:
Help to detect intermediate- and low-mass PMS stars (Herbig Ae/Be, T Tauri + PNs, CVs, Be etc.)
Study of IMF at low metallicity

7. Survey description DEEP SURVEY (Core-first 2 years):
Imaging of 35 sq. deg at B~25, V~24.5, i’~ with S/N=10 ( ~ 30 on the body, 3 on the Bridge, 2 in the directions of the Magellanic Stream)
Seeing < 1.0” and grey time (Moon < 7 days) required ~81 h
SHALLOW SURVEY (Core-first 2 years):
Time series photometry (30 phase points in V, 10 in B and i’) on 30 sq. deg properly placed along the wing and the bridge: B~20, V~19.5, i’ (expected magnitude of RR Lyrae) with S/N=100.
Summing up the exposures  same limiting magnitudes as above but with S/N=5.
Seeing <1.2” and bright time (Moon < 10 days) required ~89 h
Hα-r’ SURVEY (third year):
Imaging of 65 sq. deg. at r’~21.5 with S/N~30 (S/N~10 in Hα).
Seeing < 1.0”-1.2” and bright time (no Moon restrictions) required ~ 25 h

8. Survey description

9. Survey description

10. Complementary surveys
HST photometry on selected fields, old and young clusters (P.I. J. Gallagher/A. Nota). Glatt et al. 2008a, 2008b, 2009, Sabbi et al. 2009
VLT spectroscopy (CaII triplet) on several SMC fields (P.I. E. Grebel). Data analysis on-going.
HST/VLT Fields

11. Complementary surveys
ESO Public survey (P.I: M.R. Cioni): YHKs photometry of the Magellanic System (LMC, SMC, Bridge, Stream): 184 sq. deg. at Ks=20.3 mag in five years.
V. Ripepi, G. Clementini, and M. Marconi are Co-I of
Survey

12. Survey Data Plans: Data Reduction
data flow ~ 1 TByte/year
pre-reduction/astrometry: Astrowise/VSTube
image subtraction analysis: VODIA
PSF photometry: Allstar/Allframe
light curve analysis: custom programs

13. Survey summary and management plan
Core Survey Summary:
65 Squared degrees imaged in BVi at V24.5 mag with S/N > 5
170 h VST time over the 1st and 2nd years.
STEP
Year 1: Obs./data red.
Year 2: Obs/data red./expl.
Year 3: Obs/data red./expl.
Year 4: data red./expl.
Core obs: first 2 years!
FTEtot = 5
Hα-r’ Survey Summary:
65 Squared degrees imaged in Hα-r’ at r’21.5 mag with S/N > 10
25 h VST time over the 3rd year.

14. Conclusions
STEP is a project perfectly suited for the VST characteristics and without extreme requirements: grey/bright time, medium seeing.
STEP is a forefront scientific project which will provide a quantum leap in our understanding of galaxy evolution.
Synergies/complementarities with important international projects: a number of HST/VLT investigations and the approved public survey
Observing and data reductions strategies are already planned.
The data we shall release will be used for several stellar and galactic studies for many years to come.