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

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are needed to see this picture.

This process continuesnowadays 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 formationof our galaxy = set of individual mergers

However, the remnants of accreted satellitesblur 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 surveysProper motions can only be measured in a small volume (GAIA<20 kpc)

In contrast, radial velocities can be measured < 1 Mpc (DEIMOS @ Keck)

Mapping the sky via Radial velocity surveys: RAVE: m<16 => D< 1.6 kpc (M=5) 120 objects - field LAMOST: m<20 => D< 10 kpc (M=5) 4000 objects - field

Position + radial velocity = 4D info of stellar streams

to infer the orbit+mass of progenitorwe 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 LAMOSTPrevious 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. 2006(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, 5.60,-14.50)

•Sgr dwarf’s radial velocity: vrad = 171 km/s

•Orbital plane inclination: i=76o

Free parameters•tangential vel. (vtan) (eccentricity==rapo)

•halo axis-ratio (qh)

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 datainconsistent results !!

Law et al. 2005

Johnston et al. 2005

Example 2: The Monoceros stream

Again, M-Giant over-densities

Penarrubia et al. 2005

•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 > 220o

l < 110o

will break model degeneracy

Example 3: Field(s) of streams

In the next few years, the number of streams detected via photometricsurveys (SDSS I,II,III; Pan-STARRS) will dramatically increase.

Orphan

Sgr

Monoceros

VirgoPal 5

Kinematics will be crucial for their modelling….

Example 4: Halo clumpiness

According to CDM, there are ~104 subhaloes in the MW with M>107 Msol

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