Aspects of the Galaxy Population/Evolution that are Hard to

Aspects of the Galaxy Population/Evolution that
are Hard to Understand
… where would it help to understand DM better?
… which are promising clues to constraining DM properties?
Hans-Walter Rix
MPIA, Heidelberg
Status Quo
Let’s presume we accept that
–! we know !DM ‘well enough’
–! it is predominately cold-ish
–! most of it is NOT in macroscopic constitutents
•! Not in 10 4-6 M0 black holes (e.g. Rix and Lake, 93)
•! Not in MACHOS (e.g. Alcock et al 2000)
–! galaxies are baryon concentrations within DM halos
then,
–! which current ‘problems’ in galaxy formation point
towards clues of DM properties?
–! which future (classical) observations seem promising?
Some Important Open Issues
in Understanding Galaxy Formation
•! Can we explain their immense regularity ab initio?
–! Why is MBlack Hole and MGalaxy so tightly related?
–! Why do massive galaxies have so few young stars?
•! What sets the star-formation history of a galaxy?
–! ‘Fuel supply & merging history’ or ‘feed-back’ ?
–! Baryons arrive as cold flows, cooling hot gas, bound sub-units?
•! How to make and preserve thin galaxy disks?
•! Is there a lower mass limit to galaxy formation?
–! Is the universe full of little ‘empty’ DM halos?
How surprising is the correlation between
BH mass and galaxy mass?
•!
•!
•!
•!
Haehnelt & Kauffmann 2000
Peng 2007
Hirschmann et al 2010
Janhke & Maccio 2010
•!
E.g. combination of
–!
–!
Merging-induced central-limit theorem
Uncorrelated accretion and SF episodes
that are drawn from the general ‘fuelsupply climate’
suffice to explain present-day
relation
Some Important Open Issues
in Understanding Galaxy Formation
•! Can we explain their immense regularity ab initio?
–! Why is MBlack Hole and MGalaxy so tightly related?
–! Why do massive galaxies have so few young stars?
•! What sets the star-formation history of a galaxy?
–! ‘Fuel supply’ or ‘feed-back’ ?
–! Baryons arrive as cold flows, cooling hot gas, bound sub-units?
•! How to make and preserve thin galaxy disks?
•! Is there a lower mass limit to galaxy formation?
–! Is the universe full of little ‘empty’ DM halos?
–
Courtesy A. Maccio
Stellar Population Map of M31’s Outskirts
(PAndas, McConnachie et al 2010)
Martinez-Delgado et al 2010
Thin and pure galaxy disks
•! How can galaxy disks form as large as observed?
–! very high-resolution simulations with feed-back are
getting there…
•! How can they stay so thin?
–! Ostriker & Toth 93; e.g. Katzantzidis et al 08, Moster et al 10
•! How can there be galaxies that have no bulge?
–! Kormendy 2007; Springel & Hernquist 2005; Kautsch 2006/9, Jun et al
2009, Hopkins et al 2010
–! the role of cold gas in suppressing bulge formation?
How well can disk-dominated galaxies handle
mergers of satellite (halos)?
Initial
•! Major mergers almost
always destroy disk
Post-merger
No gas
Post-merger
20% gas
•! Significant gas component
(>20%) in disk and in
satellite greatly aid the
presence of a post-merger
cold disk
Disk thickening
no, 20%, 40% gas
from Moster et al 2010
What do we know about bulge-less galaxies?
E.g. Kautsch 2006, 2009; Weinzirl et al 2009
MW
15% of disk galaxies (<200 km/s) are bulgeless
Modelling the Statistics of Bulge-less Galaxies
(e.g. Hokins et al 09, Weinzirl et al 09)
•! Madded-to-bulge ~ M*,disk + M*,Satellite
•! Bulge-formation/angular momentum
loss is suppressed in the presence of
high gas fractions in the disks, by
(1-fgas)
•! No bulge !" no major merger
sinze z~2 (unless very gas rich)
•! Such conditions are more easily
fulfilled for galaxies with M<1010M0
Can the number of observed satellites be reconciled
quantitatively with the number of DM sub-halos?
Simulation Dark Matter
Globular Clusters
106
Observed satellite galaxies
dSph
(M*~few
% of MDM)
LSun
105 LSun
104 LSun
103 LSun
Martin, de Jong, Rix 2008
Via Lactea Simulations 2007
SDSS
discoveries
2005-2008
Part I of the Reconciliation: Proper Volume Comparison
Koposov et al 2008,2009
Vmax for ~50% of ‘SDSS satellites’
Part II of the Reconciliation: There are physically plausible mechanism
to render star-formation in small halos extremely inefficient
Via Lactea Simulations
Is there a ‘common mass scale’ for the
faintest galaxies?
Strigari et al 08
Dark matter
Milky Way
(DM)
~ M(stars)
M(<300pc)~const. doesn’t mean Mvirial~const !
In what (sub-)halos are the satellite
satellite galaxies expected to live?
Strigari et al 08
Luminosity "
M(<300pc) ~ 107 is a natural outcome
if DM halos have NFW density profiles
and the predicted density/concentration
scaling
There is no characteristic halo mass,
in which these tiny galaxies reside!
MDM,tot(now) vs. L(stars)
Luminosity "
MDM,tot(zsatellite) vs. L(stars)
before tidal stripping
severe halo stripping common
What may the dark matter mass profiles look like in these small halos?
Walker et al 2009
Ensemble properties
consistent/suggestive of
NFW-like behaviour
On-sky position
Proper
Proper motions
Distances
Radial velocitiess
What to fly-bys of sub-halos do to kinematically
cold streams?
What do fly-byes of sub-halos do to
kinematically cold streams?
from Carlberg 2010
Is that already seen in the GD1 stream? Koposov, Rix & Hogg 2010
Dark Matter and Galaxy Structure
•! Many current problems/issues in galaxy formation are
not straightfowardly linked to the nature of DM
•! The most tangible issues revolve around small scale
structure of DM
–!
–!
–!
–!
Sub-structure battering of disks
Number of sub-halos vs number of satellite galaxies
Density scaling/profile of small halos
Direct observational evidence for un-occupied small halos