Galaxy structure and kinematics towards the NGP
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GAIA Spectroscopy,Science and Technology ASP Conference Series,Vol.XXX,2002U.Munari ed.Galaxy structure and kinematics towards the NGP Spagna Alessandro INAF-Osservatorio Astronomico di Torino,I-10025Pino Torinese,Italy Cacciari Carla INAF -Osservatorio Astronomico di Bologna,I-40127Bologna Drimmel Ronald INAF-Osservatorio Astronomico di Torino,I-10025Pino Torinese,Italy Kinman Thomas Kitt Peak National Observatory,NOAO,Tucson,AZ 85726-6732,USA Lattanzi Mario G.INAF-Osservatorio Astronomico di Torino,I-10025Pino Torinese,Italy Smart Richard L.INAF-Osservatorio Astronomico di Torino,I-10025Pino Torinese,Italy Abstract.We present a proper motion survey over about 200square degrees towards the NGP,based on the material used for the construction of the GSC-II,that we are using to study the vertical structure and kinematics of the Galaxy.In particular,we measured the rotation velocity of the halo up to 10kpc above the galactic plane traced by a sample of RR Lyr?and BHB giants for which radial velocities were used to recover the complete distribution of the spatial velocities.Finally,the impact of
astrometric and spectroscopic GAIA observation are discussed.
1.Introduction
It is generally accepted that the Galaxy is constituted by four discrete main components,the bulge ,the thin disk ,the thick disk and the halo ,which are characterized by distinctive stellar populations in terms of spatial distribution,kinematics properties,metallicity and age.A detailed knowledge of such galactic components is essential to achieve a complete description of the Milky Way,as well as of the various processes and evolutionary phases which occurred during the history of our Galaxy (and other galaxies too)and that are responsible for the existence and the properties of those components we observe today.
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Ground based surveys providing photometry,proper motions and/or spec-troscopic observations have been carried out in the past to study the structure and kinematics of Galactic populations,and these will continue and be extended in the next years thanks to availability of all-sky catalogs such as GSC-II and USNO-B,based on multi-epoch photographic surveys,as well as other current and future photometric and spectroscopic surveys(eg.2MASS,DENIS,SDSS, EIS,HES,HK,VST,etc.)that bene?t from the availability of dedicated scan-ning and large?eld cameras as well as large multi-?ber spectrographs(eg.2dF, 6dF,FLAMES).
Clearly,the GAIA mission will provide an enormous contribution to the understanding of the formation and evolution of the Galaxy,thanks to very ac-curate astrometric parameters complemented by photometric and spectroscopic measurements which will permit the direct determination of the6D phase space distribution and the chemical abundance of large and complete samples of stellar tracers belonging to the di?erent galactic components.
Here we describe a new project which combines astrometric,photometric and spectroscopic data in order to investigate the kinematics of the outer halo by means of velocities derived from proper motions and radial velocities for a sample of RR-Lyr?and BHB giants towards the NGP.
2.NGP survey:proper motions and radial velocities
At the moment we have surveyed about200square degrees towards the NGP, and produced positions,proper motions and photographic photometry for about 500000objects down to plate limits(R F<20.5).
Table1.Plate material
POSS-I E1950-195625μm E103a-E+red plexiglass
POSS-I O1950-195525μm O103a-O un?ltered
Quick V1982-198325μm V12IIaD+Wratten12
POSS-II J1988-199615μm B J IIIaJ+GG385
POSS-II F1989-199615μm R F IIIaF+RG610
POSS-II N1990-199815μm I N IV-N+RG9
Galaxy structrure and kinematics towards the NGP3
Figure1.Left panel.Formal errors of?tted proper motions as a
function of the magnitude(all stellar objects).Right panel.Vector
point diagram(VPD)of a sample of15QSO’s,with1σerror bars.
Weighted means are μαcosδ =?0.02±0.23mas/yr and μδ =
+0.33±0.28mas/yr,respectively.Both plots refer to the POSS-II
?eld no.442.
classi?cation,position,and magnitude for each object by means of astrometric and photometric calibrations which utilized the Tycho2(H?g et al.2000)and the GSPC-2(Bucciarelli et al.2001)as reference catalogs.Accuracies better than0.1-0.2arcsec in position and0.15-0.2mag in photographic magnitude are generally attained.Relative proper motions were derived by applying the pro-cedure described in Spagna et al.(1996)and afterwards transformed to the absolute reference frame forcing the extended extragalactic sources to have null tangential motion.As shown in Figure1,the typical precision(σμ～3mas/yr down to R F?18)has been estimated from the formal errors of the?tted proper motions,while the zero point accuracy of the absolute proper motions have been tested by checking the mean motion of a set of known QSO’s that give values smaller than1mas/yr on each component.
Radial velocities and chemical abundances of the sample of RR Lyr?and BHB giants were derived by means of spectroscopic observations carried out with the4m Mayall telescope at Kitt Peak and with the3.5m TNG on La Palma. The data were processed with standard procedures and routines(IRAF),and typical errors areσRV≤40km s?1andσ[Fe/H]～0.2dex.
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Figure2.Color magnitude diagram(left panel)and vector point di-
agram(right panel)for stars with17≤R F≤18.Both plots are based
on data from the POSS-II?eld no.442.
3.The vertical structure
The color magnitude diagram and the vector point diagram observed in one?eld of our NGP survey are shown in Figure2.The observed distributions are the result of the complex mixture of the stars belonging to the various populations which are present towards high galactic latitudes.In particular,these may include:
1.the?at and rapidly rotating old thin disk,
2.the extended thick disk,including its metal weak tail,
3.a?attened and slowly rotating inner halo,
4.a spheroidal non-rotating outer halo,
5.satellite debris and kinematics substructures.
Actually,the physical properties of these components are not completely established and various problems are still controversial.For instance:(a)the density scale factor,rotation and metallicity distribution of the thick disk;(b)the nature of the metal weak thick disk(MWTD)and its relation with the standard thick disk(satellite debris or initial phase of a dissipative formation?);(c)the halo velocity ellipsoid,the spatial distribution and axial ratio of the halo as a function of the distance,(d)the search of halo streams;(f)the determination of the luminosity and mass function of the faintest Pop.II stars(white dwarfs, late M dwarfs and subdwarfs).
Due to the lack of trigonometric parallaxes for our and similar surveys, accurate distances and tangential velocities cannot be directly determined.In such a case it is convenient to analyze these large data-sets by comparing the observations against ad hoc Galaxy models which describe both the density and
Galaxy structrure and kinematics towards the NGP5
Figure3.Starcounts derived from plate XP444(histogram)and com-
pared with the distribution predicted by the Galaxy model(thick solid
line),which includes thin disk(solid line),thick disk(dashed line)and
halo(dotted line)components.
kinematics,as for instance the Besan?c on model(Robin&Oblak1987),the IAS Galaxy model(Bachall,Casertano&Ratnatunga1987),or the models developed
by Mendez&van Altena(1996)and Chen(1997).To this regard,in Figure3
starcounts are compared against the Mendez’s Galaxy model which has been extended to the photographic B J and R F magnitudes by Spagna(2001).
Alternatively,using tracers with known brightness,it is possible to derive distance and space velocity by means of photometry,proper motions and spec-
troscopic radial velocities.This approach has been adopted to investigate the
kinematics of the outer halo traced by a set of31RR Lyr?and65BHB giants, as will be discussed in the following section.These objects are distributed be-
tween2～ the M V as a function of metallicity or of Fourier components,as follows: M V=0.23[Fe/H]+0.92(1) M V=?1.876log P?1.158A1+0.821A3+0.448(2) where Eq.1is from Cacciari(2002)and Chaboyer(1999),while Eq.2is based on the relation from Kov′a cs&Walker(2001)where P is the period(in days), 6Spagna et al. A1and A3are the Fourier amplitudes(mag)of the fundamental and second harmonic components,respectively.The zero point has been calibrated by Kin- man(2002)with respect to the absolute magnitude(M V=0.61+0.10 ?0.11)of RR Lyr derived from the HST/FGS parallax measured by Benedict et al.(2002). Finally,the extinction E(B–V)has been estimated from the maps of Schlegel et al.(1998). 4.Halo rotation A retrograde rotation of the outer halo has been suggested by Majewski(1992) who measured a mean velocity V =?275km s?1,which corresponds to a galactocentric retrograde velocity v rot??55km s?1adopting V LSR=220 km s?1,from the analysis of a pure sample of halo subdwarfs at Z>5.5kpc towards the NGP.As shown in Table2,this parameter is still controversial,in fact Carney(1999),after correcting the kinematics bias of his kinematically-selected subdwarf sample,found a net prograde rotation of about V =?196 km s?1.On the contrary,Chiba&Beers(2000)measured a prograde rotating inner halo,v rot?20-60km s?1,up to about1kpc,with a decreasing vertical gradient of dV/d|Z|=?52±6km s?1kpc?1,while they did not detect any signi?cant rotation above Z～1.2kpc for very low abundance stars(?2.4≤[Fe/H]≤?1.9),where contamination of thick disk stars should be negligible.In addition,their halo sample at larger distances(212stars with4 Table2.Recent measurements of the halo rotation. Inner Halo RR Lyr?162(|Z|<2kpc)?210±12*Layden et al.(1996) RR Lyr?84(|Z|<2kpc)?219±10*Martin&Morrison(1998) RR Lyr?147(|Z|<2kpc)?217±13*Gould&Popowski(1998) Subdwarfs(|Z|<1kpc)?(160÷200)**Chiba&Beers(2000) (*)Heliocentric velocities.(Wrt.the LSR in the other cases.) (**)As a function of distance with a gradient dV/d|Z|=?52±6km s?1kpc?1 (?2.4<[Fe/H] (***)Preliminary results based on18RR Lyr?stars and35BHB giants). Galaxy structrure and kinematics towards the NGP7 However,recently Gilmore et al.(2002),who carried out a spectroscopic survey at intermediate galactic latitudes of about2000F/G stars,revealed a signi?cant excess of retrograde halo stars in their faintest magnitude bin(18< V<19.5)corresponding to a vertical distance|Z|≈5kpc. The fact that the velocities of halo stars do not match an exact gaussian distribution is well known(see eg.Martin&Morrison1998).How much this depends on the properties of the whole population or on the e?ects of kinematic substructures,such as satellite debris of ancient accretion events(eg.Helmi et al.1999),remains to be established. As shown in Table2,a preliminary analysis of our sample of RR Lyr?and BHB giants(Kinman et al.2002)seems to support a retrograde rotation of the outer halo.In fact,we measured a heliocentric velocity V =?285±17km s?1, which corresponds to v rot??60km s?1adopting a solar motion with respect to the LSR,V⊙=+5.25±0.62km s?1,from Dehnen&Binney(1998)and assuming V LSR=220km s?1.This result is con?rmed also by the separate analysis of the RR Lyr?and BHB giants,which both provide a retrograde rotation. How much this value may be a?ected by a velocity bias can be estimated by the level of systematic errors on proper motions which in practice give the main contribution to the U and V galactic components along this line of sight towards b≈90?.As reported in Sect.2,we found systematic errors?μ<1 mas/yr,from which velocity bias up to20-30km s?1can be expected for such stars located at Z≈5-6kpc,on average.Clearly,this is a critical point which need further analysis. 5.Kinematics simulation A Montecarlo simulation has been developed in order to compare the GAIA capability to recover the halo kinematics with respect to the velocity precision that can be attained by the current and future ground based surveys.For sim-plicity we considered an uniform spatial distribution with Pop.II-like kinematics, (σU,σV,σW)=(150,100,100)km s?1and an observer rotating with a velocity of220km s?1at a distance of8kpc from the galactic center.Implicitly we assumed to measure bright tracers,such as RR Lyr?,BH B and red giants,hav-ing apparent magnitude V～16mag at a distance of about d～10kpc.The following cases were tested: ?Case A(GAIA),withσπ=10μas,σμ=10μas yr?1(per component) andσVr=10km s?1,from astrometric and spectroscopic observations;?Case B(ground-based surveys),withσm?M=0.2mag,σμ=1mas yr?1 (per component)andσVr=10km s?1,from astrometric,photometric and spectroscopic observations. Note that the two cases di?er essentially in a factor100on the proper motion accuracy.In fact,at～10kpc the distance accuracy is the same for both cases. This points out the fact that,although radial velocities and distance moduli derived from ground based spectro-photometric surveys can attain similar ac-curacy,GAIA astrometry will uniquely provide accurate and reliable tangential velocities up to large distances. 8Spagna et al. Figure4shows the results of Montecarlo simulations with5000stars towards b=90?.Top panels present the velocity errors(i.e.the di?erences between the observed velocity and the true value)for Case A,and those at the bottom for Case B.As expected,errors increase linearly with distance,and for GAIA the rms of?U and?V residuals vary from～10km s?1at5kpc to20-30km s?1 at10kpc.For Case B the errors on U and V are about a factor2-3larger,while σ?W=σVr≡10km s?1in both the cases.Note that in Case A the error on the tangential velocity is in practice dominated by the distance uncertainty,σπ, while both the errors on proper motions and photometric parallaxes contribute to the velocity errors of Case B.Similar results are derived in other directions. These velocity errors should be compared with the typical motion of the halo stars(100-200km s?1)for which GAIA will provide individual3D spatial velocities with a signi?cant signal-to-noise,σv/v,up to d≈10-15kpc,a dis-tance where ground based surveys are not able to measure reliable tangential velocities.GAIA will measure direct distances and velocities for large samples of halo tracers,selected in situ without kinematics nor metallicity bias,from which it will possible to determine accurately the halo velocity ellipsoid and its orientation.In particular,the GAIAμ-arcsec level accuracy will be essential in √ order to take advantage of the Galaxy structrure and kinematics towards the NGP9 Acknowledgments.The GSC II is a joint project of the Space Telescope Science Institute and the Osservatorio Astronomico di Torino.Space Telescope Science Institute is operated by AURA for NASA under contract NAS5-26555. Partial?nancial support to this research comes from the Italian CNAA and the Italian Ministry of Research(MIUR)through the COFIN-2001program. 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