Breaking tidal stream degeneracies with LAMOST Jorge Peñarrubia (IoA) Cambridge 2nd December 08

Local Group Cosmology Cosmological Paradigm predicts that galaxies form through mergers of smaller galaxies This process continues nowadays in the Milky Way…

Local Group Cosmology The Local Group is the only system where the kinematics of individual stars can be measured with high precision why is this important? We want to decompose the hierarchical formation of our galaxy = set of individual mergers However, the remnants of accreted satellites blur with time Photometric surveys can only reveal the most recent accretions

Full 3D Kinematics  Unbound particles escape through leading and trailing tails  Tails approx. follow the orbit of the progenitor system …. strong constraints on the present host potential progenitor’s orbit progenitor’s mass lost fraction progenitor’s luminosity Peñarrubia et al. (2005) for each accretion event cosmological merger tree

Radial velocity surveys Proper motions can only be measured in a small volume (GAIA<20 kpc) In contrast, radial velocities can be measured < 1 Mpc Keck) Mapping the sky via Radial velocity surveys:  RAVE: m D< 1.6 kpc (M=5) 120 objects - field  LAMOST: m D< 10 kpc (M=5) 4000 objects - field Position + radial velocity = 4D info of stellar streams to infer the orbit+mass of progenitor we need numerical modelling

Numerical modelling of tidal streams Owing to the large parameter space model degeneracies are unavoidable Free parameters Flattening (q) of the host potential Orbital apocentre Orbital inclination Orbital eccentricity Mass and concentration of the satellite’s DM halo Segregation of the satellite’s stellar component Satellite luminosity Accretion time (if progenitor is unknown) Present progenitor position + velocity (6 param.) Constraints: spectroscopic surveys will break fundamental model degeneracies position of stream pieces from photometric surveys

Ideal Targets for LAMOST Previous photometric surveys have revealed a large number of stream-like structures at D< 50 kpc Potential targets for LAMOST are:  Sagittarius stream  Monoceros stream  Virgo over-density  Hercules-Aquila over-density  Palomar 5 stream  …….. etc All located in the Northern Galactic Hemisphere !! (the South remains terra incognita) What could we learn if we had LAMOST data now??

Example 1: The Sgr stream and the shape of the Milky Way potential Belokurov et al (SDSS+2MASS) Sgr core

Example 1: The Sgr stream and the shape of the Milky Way potential Constraints: Sgr dwarf’s position: (D,l,b) = (25 kpc, 5.6 0, ) Sgr dwarf’s radial velocity: v rad = 171 km/s Orbital plane inclination: i=76 o Free parameters tangential vel. (v tan ) (eccentricity==rapo) halo axis-ratio (q h )

Example 1: The Sgr stream and the shape of the Milky Way potential Oblate halo models (0.85<q<0.95) match precession rate Prolate halo models (q>1) match radial velocities along the stream from 2-MASS (M-giants) : using the same data inconsistent results !! Law et al Johnston et al. 2005

Example 2: The Monoceros stream Again, M-Giant over-densities Penarrubia et al M-giants show a large dispersion on the sky They move on nearly circular orbits Do all over-densities belong to the Mon stream?

Example 2: The Monoceros stream Degenerated model : Prograde vs Retrograde orbits Radial velocities between l > 220 o l < 110 o will break model degeneracy

Example 3: Field(s) of streams In the next few years, the number of streams detected via photometric surveys (SDSS I,II,III; Pan-STARRS) will dramatically increase. Orphan Sgr Monoceros Virgo Pal 5 Kinematics will be crucial for their modelling….

Example 4: Halo clumpiness According to CDM, there are ~10 4 subhaloes in the MW with M>10 7 M sol Cold tidal streams (e.g from GCs with  ~ 1 km/s) may be heated by encounters with DM subhaloes kinematical surveys of GC streams could potentially constrain the number density of DM clumps Grillmair & Dionatos 2006