Embed Size (px)
Citation preview
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
QuickTime™ and a decompressor
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
