Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 29 July 2018 (MN LATEX style file v2.2) Detection of a radial velocity gradient in the extended local disc with RAVE A. Siebert1⋆, B. Famaey1, I. Minchev1, G.M. Seabroke2, J. Binney3, B. Burnett3, K.C. Freeman4, M. Williams5, O. Bienaym´e1, J. Bland-Hawthorn6, R. Campbell5, J.P. Fulbright7, B.K. Gibson8, G. Gilmore9, E.K. Grebel10, A. Helmi11, U. Munari12, J.F. Navarro13, Q.A. Parker14, W.A. Reid14, A. Siviero5,12, M. Steinmetz5, F. Watson15, R.F.G. Wyse7, T. Zwitter16,17 1Observatoire Astronomique, Universit´ede Strasbourg, CNRS, Strasbourg, France 2Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, RH5 6NT, UK 3Rudolf Peierls Centre for Theoretical Physics, Oxford, UK 4Australian National University, Canberra, Australia 5Astrophysikalishes Institut Potsdam , Potsdam, Germany 6Sydney Institute for Astronomy, University of Sydney, Sydney, Australia 7Johns Hopkins University, Baltimore, MD, USA 8University of Central Lancashire, Preston, UK 9Institute of Astronomy, Cambridge, UK 10Astronomisches Rechen-Institut, Zentrum f¨ur Astronomie der Universit¨at Heidelberg, Heidelberg, Germany 11Kapteyn Astronomical Institut, Groningen, the Netherlands 12Astronomical Observatory of Padova in Asiago, Asiago, Italy 13University of Victoria, Victoria, Canada 14Macquary University, Sydney, Australia 15Anglo-Australian Observatory, Sydney, Australia 16University of Ljubljana, Faculty of Mathematics and Physics, Ljubljana, Slovenia 17Center of excellence, SPACE-SI, Ljubljana, Slovenia Accepted . Received ; in original form 29 July 2018 ABSTRACT Using a sample of 213 713 stars from the Radial Velocity Experiment (RAVE) sur- vey, limited to a distance of 2 kpc from the Sun and to |z| < 1 kpc, we report the detection of a velocity gradient of disc stars in the fourth quadrant, directed radially from the Galactic centre. In the direction of the Galactic centre, we apply a simple method independent of stellar proper motions and of Galactic parameters to assess arXiv:1011.4092v1 [astro-ph.GA] 17 Nov 2010 the existence of this gradient in the RAVE data. This velocity gradient corresponds > −1 −1 to |K + C| ∼ 3 kms kpc , where K and C are the Oort constants measuring the local divergence and radial shear of the velocity field, respectively. In order to illus- trate the effect, assuming a zero radial velocity of the Local Standard of Rest we then reconstruct the two-dimensional Galactocentric velocity maps using two different sets of proper motions and photometric distances based either on isochrone fitting or on K-band magnitudes, and considering two sets of values for the Galactocentric radius of the Sun and local circular speed. Further observational confirmation of our finding with line-of-sight velocities of stars at low latitudes, together with further modelling, should help constrain the non-axisymmetric components of the Galactic potential, including the bar, the spiral arms and possibly the ellipticity of the dark halo. Key words: Stars: kinematics – Galaxy: fundamental parameters – Galaxy: kine- matics and dynamics. 1 INTRODUCTION Our Milky Way Galaxy is a unique laboratory in which ⋆ E-mail: [email protected] to study galactic structure and evolution from stellar kine- c 0000 RAS 2 A. Siebert et al. 1 matics. The story of the efforts to obtain such kinematic Here, we use line-of-sight velocity data (vlos) from the data nicely illustrates how theoretical progress and data RAVE survey to investigate whether the mean radial motion acquisition have to go hand in hand if one wants to gain of disc stars remains zero in the extended solar neighbour- insight into the structure and history of the Galaxy. Un- hood as compared to the one in the solar neighbourhood, til recently, most studies have been limited to the so- or whether large-scale radial streaming motions are present lar neighbourhood. The zeroth order approximation for in the vicinity of the Sun, which would translate in a ra- modelling these kinematic data assumes an axisymmet- dial velocity gradient in one or the other direction. In Sec. 2 ric model, in which the Local Standard of Rest (LSR) we present the sample used for our analysis while in Sec. 3 is on a perfectly circular orbit. However, the local veloc- we present the detection of a radial velocity gradient using ity field in the solar neighbourhood already displays pos- RAVE vlos, as well as the associated Oort constants and 2- sible signatures of the non-axisymmetry of the Galactic dimensional velocity field. Finally in Sec. 4 we discuss the potential in the form of stellar moving groups contain- implications of our results and present our conclusions. ing stars of very different ages and chemical compositions (Dehnen 1998; Famaey et al. 2005; Quillen & Minchev 2005; Bensby et al. 2007; Minchev et al. 2010). 2 THE RAVE SAMPLE In external galaxies, large-scale distortions of the (line- Our analysis is based on the RAVE catalogue (Steinmetz et al. 2006; Zwitter et al. 2008) which provides of-sight) isovelocity contours of the gas have long been recog- −1 nised in two-dimensional velocity maps (e.g. Bosma 1978), vlos to 2 kms and stellar atmospheric parameters and allowing a quantification of the strength of non-circular distances to 30% for a large number of bright stars in the streaming motions (e.g., Sellwood & Zanmar Sanchez southern hemisphere with 9 <I < 12 using moderate 2010). A prominent example is the galaxy M81 (e.g., resolution spectra (R∼7 500). RAVE selects its targets Adler & Wefstpfahl 1996), which has roughly the same randomly in the I-band interval, and so its properties are circular velocity as the Milky Way and in which the radial similar to a magnitude limited survey. The RAVE cata- streaming motions of the gas reach maxima of the order logue is cross-matched with astrometric (PPMX, UCAC2, of 50 km s−1. However, the stellar kinematics often turn Tycho-2) and photometric catalogues (2MASS, DENIS) to out to be slightly more regular and symmetric than the provide additional proper motions and magnitudes making gas kinematics (e.g. Pizzella et al. 2008). For instance, RAVE a suitable tool to study the Milky Way. Rix & Zaritsky (1995) estimate from 18 face-on spiral We use the latest version of the Zwitter et al. (2010) galaxies that the azimuthally averaged radial streaming catalogue, which contains four sets of distances in addition motions of disk stars should typically amount to a maxi- to the standard RAVE data (line-of-sight velocities and stel- mum of 7 km s−1 due to lopsided distortions and 6 km s−1 lar parameters). These four sets of distances are (i) the dis- due to intrinsic ellipticity. tance moduli computed using three sets of isochrones (in- cluding the Padova isochrones), and (ii) the distances of Breddels et al. (2010). In this paper we will use the dis- tance moduli based on the Padova isochrones but our re- In the Milky Way, non-axisymmetric motions are sults do not strongly depend on the set of isochrones used clearly visible in radio-frequency observations of gas flow (see also Sect.3.2 where all distances are multiplied by an in the inner Galaxy, implying that the amplitude of spiral arbitrary factor). This catalogue is cleaned from potentially structure could be larger by a factor ∼1.5 than its amplitude problematic fields using a list kindly provided by G. Matije- in the near-infrared luminosity density (Bissantz et al. vic (private communication) and we further select stars with 2003; Famaey & Binney 2005). On the other hand, radio |z| < 1 kpc to minimise contamination by halo stars. This interferometry used to determine the parallax of star- leaves us with a sample of 213 713 stars with known dis- forming regions in the Perseus spiral arm, together with tances, atmospheric parameters (log g, Teff , [m/H]), proper the measurement of their line-of-sight (los) velocities, show motions as well as vlos measurements that can be used to that these star-forming regions have peculiar non-circular compute the 3D space velocities. −1 motions amounting to ∼20 km s , assuming standard To verify that our analysis does not depend on the IAU Galactic parameters and a nearly-flat rotation curve method used to determine the distances or on the source (Xu et al. 2006; Binney 2006; Reid et al. 2009). However, of proper motions, we also use stars for which distances the amplitude of such non-circular motions strongly depends and proper motions are obtained independently. To do so, on the Galactic parameters adopted (McMillan & Binney we select a sub-sample of red clump candidates for which 2010), namely the Galactocentric radius of the Sun R0, the distance can be estimated using the K–magnitude alone the local circular speed vc0, and the reflex solar motion, as because of the narrow, almost Gaussian, luminosity func- well as the model for the rotation curve. Another example tion of the red clump. Red clump candidates are selected of such non-circular motion is the 3 kpc arm which has in the (log g, J − K) plane, as illustrated in Fig. 1. We −1 vlos = −80km s towards the Galactic centre (GC) and select the over-density using 0.5 < J − K < 0.735 and is known to have a counterpart on the far side of the GC (Dame & Thaddeus 2008). Its origin is associated to the 1 Galactic bar showing that non-circular motion of the similar Throughout this paper we will use the notation vlos or line-of- amplitude can be caused by various types of perturbations sight velocity to refer to the heliocentric radial velocity measured and not necessarily by spiral arms. by RAVE while radial velocity or VR means galactocentric radial velocity c 0000 RAS, MNRAS 000, 000–000 A radial velocity gradient in the Galactic disc 3 0 67.23 to bottom in each panel in Fig.