The VST-OAC Survey will address many relevant topics. The OAC researchers and their collaborators are mainly interested in the following scientific goals:
Galaxies, groups and clusters
Several factors, have so far prevented systematic studies of the cluster properties. Among the main problems we can mention:
It needs to be stressed that the properties of
clusters in the nearby Universe (z<0.2) to be used as zero redshift
template for evolutionary studies are the target of the CRoNaRio
project.
Active Galactic Nuclei
It seems firmly established that AGNs occur much more frequently in galaxies which have a nearby "companion" galaxy or in interacting systems (Adams 1977, ApJS, 33, 19; Vorontsov-Velyaminov 1977, A&AS, 28,1). Much work has been done to verify this hypothesis (e.g., Dahari 1984, AJ, 89, 966; Fuentes-Williams and Stocke 1988, AJ, 96,1235; MacKenty 1989, ApJ, 343,125, Rafanelli et al. 1995, AJ, 109, 1546) at low (z<0.05) redshift.
The OAC--VST survey wil provide a large sample of optically selected objects with redshifts as high as 1. The corresponding XMM Survey will complement this data set with an X-ray selected sample since AGNs are strong X--ray emitters often dominating the X--ray emission of the host galaxy. In the 0.5-2 keV X--ray band there is also the possibility to reveal narrow-emission-line galaxies (NELGs) (Pearson et al. 1997, MNRAS, 288, 273).
These data sets will allow us to address questions related to the evolution of AGNs and to a possible evolutionary link between AGNs and Starburst (SB) galaxies. In particular:
Extimate of cosmological parameters through weak lensing
Gravitational lensing is a powerful
tool to investigate the universe, both on small and large scale, since
the
deviation of the light rays
due to a matter distribution is independent on the nature of the matter.
Observations of gravitational lensing on cosmological scale, and in particular of weak lensing, lead to an estimate of the parameters W and L (id est the matter density of the universe and the cosmological constant).
Theory shows that the density
(number counts) of background galaxies around a cluster is affected by
the
gravitational effects of the
cluster and undergoes two opposite effects: an increase in density due
to the fact that the gravitational amplification makes detectable more
objects, and a decrease, due to the deviation of light (T. Broadhurst,
1995) far away from the cluster center.
A measure of these two effects
on the number counts of background galaxies around clusters as a function
of the angular distance from the center of the cluster has been already
used for just one cluster to constrain the cosmological constant (Y. Mellier
and B. Fort, 1996; B. Fort, Y. Mellier and M. Dantel-Fort, 1997).
These effects are observable
only if the slope of the background galaxy counts differs from 0.4. In
the planned apparent magnitude range this condition is matched in both
the R and V bands (cf. J.P. Gardner et al. 1996; D.W. Hogg et al., 1997).
The proposed survey will cover
at least 50 nearby clusters having mass luminosity profiles derived from
the XMM X-ray data. These data will allow to better model the properties
of the lensing clusters and to better constrain the lensing phenomena,
thus leading to a higher accuracy in the estimate of the cosmological parameters.
Accrection driven X-ray sources
in the galactic halo
The galactic ridge hard X-ray emission is formed by a narrow and a broad component (Valinia and Marshall 1998). Its origin is still not well understood although its spectrum is mainly thermal. At higher galactic latitudes there is evidence for a patchy galactic halo (Sidher et al. 1999) which is formed by both AGN's and a population of yet unidentified discrete X-ray sources. For the slope of their spectra and their optical and the X ray luminosities these objects are likely intrinsically faint cataclysmic variables (CV's) with low accretion rates.
Identification programs of high galactic latitude RASS
(Rosat All Sky Survey) sources have revealed a substantial fraction of
stellar X-ray sources (Thomas et al. 1988): mainly coronal (softest and
lowest X/optical ratio) and white dwarfs both isolated and in binaries
(high X/optical ratio).The extension at 15keV provided by XMM and its low
limiting flux over the LSS field will provide an unprecdented opportunity
to identify the galactic halo population of dim "accretion driven" X-ray
sources. An estimated number of 1100 such objects is expected to
fall within the VST-OAC Survey field. The VST-OAC survey will allow
the identification of most of the optical conterparts of these objects,
while the XMM-OM will provide for the brighter ones also simultaneous UV
and B band fluxes to be linked to the VST photometry. All these
data will be used to select candidates for spectroscopic follow-up's and
XMM pointed observations.
Diffuse stellar light in clusters
The presence of a diffuse intracluster light filling
up the regions between galaxies in a given cluster has been
known since Zwicky 's discovery in 1951.
Further studies on the presence of diffuse light in
clusters were based on photographic technique, and were later on followed
by CCD observations in the nineties, but these studied ended with ambiguous
results. The best estimate for the contribution of the diffuse light
ranged from 10% to 50% of the total light emitted by cluster galaxies in
the central region of the cluster itself.
A new interest in these kind of studies was brought
by the discovery of intracluster planetary nebulae free flying in the intracluster
regions of Virgo and Fornax.
Several surveys in Virgo aiming at detecting
point like-emission objects in the [OIII] 5007 line found more than a hundred
PNe candidates in the Virgo core region.
VST + XMM
In a very recent spectroscopic run at the Anglo Australian
Telescope with the 2 degree field fiber spectrograph (2dF), Freeman et
al. 1999 (in preparation) obtained spectroscopic confirmation for a sample
of intracluster PNe and the surface brightness associated with the
spectroscopically confirmed sample is mB
~ 29.4 mag arcsec-2.
Furthermore, the evidence gathered so far indicates
a clumpy distribution for the Virgo intracluster population of stars, and
this population could contribute al least 15% of the total light in galaxies
in the Virgo cluster.
Deep surveys in broad R and z band will be crucial to quantify and map the presence of diffuse light in clusters from z-=0.015 to z=0.3, and their comparison with detailed x-ray map will put constraints on the determination of baryonic content of clusters and indicate the presence of baryonic segregation. Moreover, there is an ongoing debate on the efficiency of star formation in clusters and whether the hot x-ray gas is primordial in origin. A better understanding of the presence of the diffuse light and its distribution with respect to the hot gas will help to address these issues.
The VST, with its 1 square degree field of view, is
the best instrument to date for such a challenging project. The large area
and the high CCD efficiency will allow the most accurate subtraction of
the sky background in the cluster regions. In rich clusters like Coma,
the expected surface brightness of the diffuse component is R ~24.5 in
mag arcsec-2: such a surface brightness can be detected by VST
in 1 hrs with S/N = 10, and in a rich cluster at z=0.3 in 10 hrs with S/N
= 10.
In the proposed survey, up to 50 clusters with different
richness will be imaged, giving an exceptional sample for which diffuse
vs hot gas distribution can be studied.
X-ray emitting pre-main sequence
stars
In order to study the global processes of star formation,
one must identify and sample the complete population of low mass pre-main
sequence (PMS) stars. The local Galactic star formation regions (SFR) subtend
extremely large solid angles and therefore even high galactic latitude
fields are necessarily contaminated by a population of newly formed stars.
A highly significant excess of young stars in the
field over predictions from standard galactic models has indeed been noted
by many researchers (e.g. Guillout et al. 1996 A&A 316, 89).
Classical T Tauri stars (cTTS) tend to be easy to identify
either photometrically due to the variability or their prominent near-IR
continuum excesses, or spectroscopically through their strong emission
lines.
It needs to be stressed, however, that the vast majority
of the low mass PMS stars, id est the so called weak T Tauri stars (wTTS)
is not as easy to detect. Considerable progress has been accomplished both
in finding wTTS as optical counterparts of ROSAT all-sky survey (RASS)
X-ray sources (Alcala et al. 1995 A&AS 114,109; Alcala et al. 1996
A&AS 119,7; Krautter et al. 1997 A&AS 123,329) in star forming
regions and in performing follow-up studies (Alcala et al. 1997 A&A
319, 184; Alcala et al. 1998 A&A 330 1017; Wichmann et al. 1997 A&A
320, 185, Covino et al. 1997 A&A 328, 187; Alcala et al. 1999, submitted).
Our main goal is to measure the density and excess (if any) of young field stars in the selected VST-OAC survey field. The survey must be complemented with optical spectroscopic observations in order to detect the lithium l~6708 Å absorption line of the selected candidates.
The XMM X-ray survey will be very efficient in identifying X-ray emitting young stars. We stress, however, that X-ray surveys give us the traces where to look at, but the faint population of low mass field young stars must be sought using wide field deep photometry and spectroscopy.
XMM observations, in combination with wide-field broad-band
imaging photometry with the VST down to V~25, provide for the first time
a very efficient tool to search for a spatially complete sample of young
field stars and to study the global process of low-mass star formation
on a galactic scale.
Halo white dwarf Luminosity
fuction
The (disk) White Dwarf Luminosity Function (WDLF) of our galaxy presents a strong decrease at L/Lsun =~-4.0. WDs with lower luminosity are not observed for the very simple reason that they don't exist: this limit corresponds in fact to the faintest and oldest WDs in the galaxy, having roughly the same age as the galactic disk himself. Hence, comparing the observed with the theoretical WDLF, it is possible to derive the age of the galactic disk. Recent estimates give values between about 7 and 10 Gyrs (Liebert et al. 1988, ApJ 332, 891; Leggett et al. 1998 ApJ 497, 294).
The same method may be applied in principle to the Halo WDLF (HWDLF), but here the main problem is that the observed HWDLF is almost totally unknown: there are presently not more than a dozen of halo WD candidates, giving just some upper limits to the HWDLF (Isern et al. 1998, ApJ 503, 239 and ref. therein).The HWDLF contains crucial informations on the genesis of our galaxy:age of the galactic halo, IMF, stellar formation rate. Moreover the recent results of the MACHO+EROS experiments indicate that the microlensing events are mainly produced by halo objects with an average mass of ~0.5 Msun (Alcock et al. 1997, ApJ 486, 697), suggesting that part of the dark matter could be formed by halo WDs.
A multicolor deep imaging survey
will permit to discover several HWDs giving an enormous improvement of
our knowledge of the HWDLF. Comparing the UBVRI colors with a large grid
of He and H WDs
(Bergeron et al. 1995, PASP
107, 1047), it will be possible to select a sample of candidate WDs, which
will require spectroscopic follow-up to be confirmed. Considering limiting
magnitudes as those foreseen by the OAC-VST Survey and assuming standard
halo models (Isern et al. 1998), it should be possible to discover something
like 30 or more HWDs for each square degree (sd), i.e. a total number of
about 2000 HWDs considering the entire 8x8 sd field. To put an upper limit
to the space density of HWDs is the first goal of this project.
Star counts in the halo
The use of star counts to constrain global Galactic structure has proved to be an effective way to investigate the general properties of stellar populations in our Galaxy. The determination of the distribution at large distances above the galactic plane requires large samples and accurate photometry extending to faint magnitudes (V>22). Such data sets are still rare since deep starcounts are often restricted to small areas, whereas investigations on larger area like those performed by the EIS group are not deep enough to properly constrain the theoretical predictions.
As a first goal, we want to derive better and more severe constraints on the Initial Mass Function of metal poor halo stars. This is a key parameter in the still debated issue of the nature of the missing mass. Moreover, we want to constrain the possible occurrence of metallicity and/or age gradients throughout the galactic halo, deriving also information on the halo white dwarfs. As a whole, we expect to obtain a relevant progress in the current knowledge of the galactic structure and, in due turn, on the history of the formation and evolution of the galactic halos in spiral galaxies.
The area covered by the foreseen VTS-OAC survey is about 8x8=64 square degrees, i.e. 50 times the area ivestigated by EIS. This means that even at the same limiting magnitude we shall increase the statistical significance of star counts by a large factor.Furthermore the VTS-OAC survey are significantly deeper than the EIS ones, allowing us to study with an unprecedented accuracy the distribution of the metal-poor halo stars.
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