Transcript Formation of Disk Galaxies

Current Issues in
Disk Galaxy
Formation
van den Bosch, Burkert, Swaters (2001)
Abadi et al. (2003)
Swaters et al. (2003)
MMW Analytic Disk model
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Given the following parameters:
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Vc = Circular velocity at some time,
jd/md = Specific angular momentum of disk,
l = Spin parameter (from LSS tidal torques),
H(z) = Hubble constant
… we can determine both the structural
properties and evolution of the disk.
 RdVc, jd/md , l, 1/H(z)
All cosmology dependence is in H(z)!
Specific Angular Momentum
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Disk properties well reproduced in simple
model (e.g. MMW) IF jD/mD~1.
But… is this assumption justified?
Let’s simulate!
Abadi et al. (2003)
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Discussion…
Hydrodynamic Simulations
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Initial conditions: A small patch of the
Universe, in the linear regime, with dark
matter and gas particles.
Model gravitational force between
particles, additional pressure forces, shock
heating, and cooling for gas.
Advance particle positions + velocities,
lather, rinse, repeat.
What happens?
Simulate a Single Disk Galaxy
320 kpc/h
40 kpc/h
(last panel only)
Successful Photometric Disk Galaxy
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Thin gaseous disk,
bulge, halo.
SB profile has r1/4
bulge, exponential
disk.
Gas is more extended
than stars.
Looks a lot like
UGC615, an Sab…
even reproduces
colors!
SFR ~ 2 M/yr since
last merger at z~1.
But when AM comes into play,
life gets much worse.
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RC is FAR too centrally
concentrated.
Owes to large, overconcentrated bulge.
But this is a direct
result of early
hierarchical growth…
and in a relatively
quiescent halo!
Specific Angular Momentum
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Specific AM j of
stars/baryons <<
observed late-types.
Disk-only j agrees well…
if it only weren’t for that
pesky bulge!
DM halo has similar j to
observed disks; seems
like nature conserves j
even though sims don’t.
And this is for favorable
quiescent case; typical
case produces lower j.
Bottom line…
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Simulation produces disk with some basic
structural and photometric properties as
observed.
But… best-case scenario only produces an Sab,
no later.
Even then, the bulge component is too
concentrated and too large.
Specific AM is too low, because bulk of stars
form early in a stochastic, non-ordered mode.
What Can We Do?
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Suppress early bulge growth:
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Ignore it!!! Simulations are wrong, basic
model (e.g. MMW) is right.
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Feedback?
Preheat gas?
Modify dark matter (e.g. warm)?
Modify cosmological power spectrum?
Nature somehow conserves specific AM.
But even that turns out to not work so
great…
van den Bosch et al. (2001)
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Discussion…
LSB Galaxies
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Low Surface Brightness galaxies: What are they?
Late-type Spirals, mostly bulge-less, gas-rich.
Why LSBs?
Quiescent history, RC is dark matter dominated:
Ideal testbed for simple rotationally-supported
collapse.
How to measure rotation?
Ha (optical) rotation curves in inner region, HI
(21cm) rotation curve in outer region.
Angular Momentum Distributions
Thick: Data
(3 values of M/L)
Thin: Halos
(3 values of m)
Total AM of Disk (l)
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For reasonable M/L ratio, l of observed disks are
similar to that of DM halos.
So total AM is apparently conserved in collapse…
as assumed in MMW, but in contrast with
simulations that cannot reproduce this.
Punchline: Halo vs. Disk AM
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Normalized to
total AM: Disk
has similar
specific AM, but
much less total
AM because it
contains much
less than all
baryons.
Key Results
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Only a rather small fraction of baryons end
up in disk.
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Total AM of disk ≈ Total AM of all baryons.
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Challenge to standard cooling model
Challenge to simulations showing AM loss
The disk is missing both highest and
lowest AM baryons.
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Challenge to models that assume simple
rotationally-supported collapse
So… How do we get rid of all this
“unwanted” angular momentum?
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Need other physical processes!
Perhaps high-AM material lost in mergers,
or tidally stripped, or still left in halo?
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… Yet somehow it’s AM ends up in disk!
Perhaps low-AM material blown out from
center of galaxy?
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… but simple model of feedback where large
galaxies retain more baryons is opposite to
trend seen in data!
Wackier Notions…
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Perhaps dark matter is not collisionless, and
hence halo AM distribution is actually much
like that observed.
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… but still have trouble explaining shape of p(s)
Perhaps baryons decouple from halos early
on, acquire different AM.
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… no evidence for this in more recent work
(e.g. Sharma & Steinmetz 2006)
So the bottom line is…
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Disk formation is a LOT more complicated
than the MMW scenario.
We still don’t understand it!
More recent results (e.g. Governato et al.
2004, Robertson et al. 2005) have used
strong feedback to produce later-type
spirals. But still not clear that distribution
of AM is correct.
Maybe there’s a hint from a related but
slightly different angle…
Swaters et al. (2003)
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Discussion…
Universal Halo Profile
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CDM predicts universal halo profile (NFW).
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All halos well fit by
NFW (1997): a = 1.
Moore et al (1998): a ≈ 1.5
Others: Everything in between
Observations, at face value: a ≈ 0!
What’s going on?
LSBs: Dark Matter Dominated RC’s
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Slowly rising RC’s, no bulge… can probe
inner dark matter profile.
Density Profiles from Inverted RC
Stay out of Bars!
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Bars cause noncircular motions
that mimic lower a.
Non-barred sample
fairly consistent
with NFW.
Fair and Balanced View
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Observations, at face value, tend to prefer
low value of a.
But there are many systematics… and they
almost all tend to lower a!
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Misidentifying center of potential
Non-circular motions
Overly edge-on galaxies
Hence cannot rule out NFW (a=1).
Can rule out Moore profile (a=1.5).
If NFW is Right, What About
Concentrations?
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Most agree with
simulations, but
a few clearly
low: Mostly
barred galaxies!
Do bars cause
lower mass
concentration?
(Weinberg & Katz
2002)
Bottom Line
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Overall, no obvious disagreement with
NFW, either in inner slope or
concentration.
But debate is far from over! 2-D IFU data
also shows shallow mass profiles.
Theory debate not over either (paper on
astro-ph today claiming a totally new
functional form).
If NFW is right, then can’t solve disk AM
problem by changing DM distribution.
That’s It For Today!
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Some successes, many unresolved issues
in theory of disk galaxy formation.
Next time (next Wed): Marcia will tell you
all about elliptical galaxies.