Transcript Freeman: Workshop Summary

Secular Evolution in Disk Galaxies
Summary: Ken Freeman
The main themes:
Formation of bulges by secular and non-secular processes
Secular processes : rings, orbits, inflow
Stellar population issues
Goals of Summary
Overview of points from talks
Some extra items
Identify some problems for discussion
Ringberg Workshop May 2004
The expression "secular evolution" .... what does it mean ?
I think it is intended to mean
"dynamical evolution that is slower than the
dynamical timescale of the disk"
but
I think it was used here by many to mean
"dynamical evolution that occurs after the disk has formed"
so I will use it in this sense
Appreciation of secular evolution in galaxies
has grown over last ~ 20 years
Its significance for properties of barred galaxies
was understood early
Accepting importance of secular processes for bulge
formation (including disk instabilities) is more recent
- not complete yet
JK&RK ARAA, and this workshop, will help a lot
Kormendy overview (based on ARAA article)
Secular evolution and the growth of pseudobulges.
• SE now becoming more important than hierarchical effects
• bars grow by outward transfer of J - orbits elongate,
pattern speed drops. Gas at low R goes in to center,
gas at intermediate R goes to inner ring, gas at large R
goes to outer ring near OLR
• dustlanes along bars identified as shocks - lead to
energy loss by gas and infall to center
• inner outer and nuclear rings usually star-forming
• circumnuclear star-forming rings should generate
pseudobulges on 1-3 Gyr timescales.
• pseudobulges as systems above the oblate rotator
curve in the (V/ - ) plane.
• exponential bulges
• bar destruction by growth of central mass.
lenses as defunct bars
Q: how do the smaller disky ellipticals fit into this picture ?
Athanassoula : N-body view of secular evolution
J-exchange drives bar growth: J goes into outer disk
and halo near resonances, or to interacting galaxy
The live halo is an important element in the growth and
evolution of the bar.
This realisation has really changed perception of role
of halo in dynamics of barred galaxies, from suppressing
bar growth (rigid halo) to enhancing bar growth (live halo)
N-body boxy bulges are bars
(or at least the inner ~60% of the bars)
Side-on bars are strongly B/P while
end-on bars appear as relatively round bulges.
Rotation is cylindrical in simulations, as in reality
These processes are a mixture of secular and rapid :
bar growth is slow,
bending instability is fast,
subsequent settling of the bar/bulge slow again.
Patsis : boxy isophotes in barred galaxies
Periodic orbit properties define the structure of products of
dynamical evolution
See some of the fine details of 3D periodic orbits reflected in
structure of unsharp-masked images of real peanut galaxies
away from the plane
Detailed structure of face-on SB's (eg boxy isophotes of
bar) relate to morphological properties of orbits
Debattista
Simulations of the buckling instability for anisotropic bar
and development of peanut bulge.
Bar survives the process.
Buckling driven by anisotropy, heats the disk vertically
to reduce the anisotropy
h4 provides potential diagnostic of face-on peanut, even
in axisymmetric systems.
Possible relation of buckling instability to observed double
exponential disks, often described as truncation, seen near OLR..
Not 1-1 : see truncations without bar and bars without truncation
Example of truncation in M33 : origin of truncation is
significant problem - is it an effect of secular evolution ?
The truncation
of
M33's disk
M33 is a pure
disk galaxy in
the Local Group
(Ferguson et al 2003)
Disk Truncation
surely the deepest surface brightness
profile ever measured for a pure
disk galaxy
M33 Surface Brightness Profile:
• i-band surface photometry out
to R ~ 35'
• profile extended to R ~ 60'
using star counts
sharp decrease in surface brightness
beyond 5 scalelengths..
V~31 mag arcsec -2
Ferguson 2003
Bureau
Observational and N-body overview of bar-driven
evolution in B/P bulges : ~40-50% of bulges are B/P
Structure of V, h3 gives a strong kinematic signature
of end-on bar where photometric signatures are absent
Good correspondence between observed stellar kinematics
in B/P bulges and the simulations.
These bars are not flat - by definition. See changes in
the vertical scale length with radius.
Often have thick bar with embedded central disk.
Unlike the Ferrars bars widely used for orbit studies
The typical B/P bulge structure has bow tie structure, and
includes a flat sort of pseudobulge with young population
Some evidence in NGC4526 for vertical extension of
young stars - maybe resonant heating near bar ends
Q: Is there such a thing as a flat bar ?
How important is resonant heating ?
Buta : morphology of barred galaxies
Rings usually blue - enhanced star formation relative to
background
Nuclear rings have range of shape
Secondary bars have no preferred orientation
Production of rings from sticky particle codes - predict
that SB(s) galaxies have weaker bars.
Observational indication that bar is actually stronger for the
SB(s) galaxies
Examples of galaxies that might have slow, medium and fast
pattern speeds (give various numbers and locations of resonances)
slow pattern speed <=> nuclear rings,
medium pattern speed <=> inner ring but no nuclear ring.
In systems with slow pattern speeds, we could expect up to
5 resonant features - maximum RB has seen so far in any one
galaxy is 4 !
Underlying the detailed morphology are fundamental
properties like
• the presence of secular evolution
• the presence of a live halo
Sparke - double and multiply-barred galaxies
~ 25% of 38 SB0, SBa galaxies are double/multiply barred
Secondary bars are randomly orientated - not bluer than
surroundings, so probably longlived
Look for orbits that reinforce the secondary bar in presence
of a time-periodic potential - eg in ordinary bars, x1 family
extends to CR and provides backbone of bar - what is analog
for secondary bars ?
Developed concept of stable invariant loops - require small
bar to be inside the ILR of the outer bar and well inside its
own CR
Regan - nuclear ring formation in SB galaxies
Hydro simulations of flow in fixed potential. The nuclear
ring is feature trapped between the ILRs, growing via
inflow associated with the near-radial shocks.
Rings form best when galaxy has a central mass concentration
and bar is not too strong.
Nuclear rings are associated with x2 orbits which extend over
region between IILR and OILR and are needed for offset shocks.
Rings start to form at the outer x2 orbit and gradually evolve
inwards as lower-J gas is accreted - associated inflow rate up
to ~ 0.5 M_sun yr -1. Inflow requires presence of nuclear rings
Gas prefers to associate with the more circular x2 orbit but
do see rarer x1 rings at location of largest non-looped x1 orbit..
Balcells - nuclear & global properties and secular evolution
HST NIR images for 19 SO-Sbc unbarred galaxies, SAURON
spectra, groundbased optical images
Sersic fits:  ~ r1/n : finds very few n=4 bulges. Mean is
n=1.7 plus point source (plus inner exp). Point source is
like 10-20 globular clusters. Low Sersic n => not much merger
growth for these bulges
30% have extended nuclear cpt with (0) as bright as
the Sersic component - inner bar/disk
Well defined scaling laws for most parameters with Lbulge,
but not much with type between SO and Sbc
Strong scaling laws with Lbulge => disk grew around existing
bulge, so bulges in place 10 Gyr ago
Q: how would the M31 bulge look in this analysis ?
Unambiguous evidence for r1/4 law - would this kind of
analysis have found it ?
M31 surface brightness
distribution
Pritchet & van den Bergh 1994
Carollo
Massive bulges have z > 1/2 zsun, ages 8-10 Gyr
Smaller bulges have ages 2-5 Gyr.
ie, many small bulges are younger than the more massive
spheroids, with ages like surrounding disk - see age spread
Deep HST images show presence of normal disk galaxies at
z=1, with disks and bulges in place, scale lengths not
much evolved since.
But bulges at z~0.5 are systematically bluer than ellipticals
Bars are now abundant up to z~1, despite earlier claims
that bars are present only for z < 0.7.
For z = 0.5 to 1, ( bulge age - disk age) < 2 Gyr for
intermediate to later type disks.
Conclude that bulges in Sb and later galaxies are products
of internal disk evolution
Bulge simulations
In fixed halo potential, bar buckling produces bulges
with V/ both above and below oblate rotator line.
In live halo plus SPH, outcome depends on gas fraction.
• Gas ~ 10% gives rounder smaller flatter bulges
• Gas ~ 50% with radiative cooling gives very clumpy
system with strong spiral structure (not bar). Very
fast evolution, generates an old bulge from disk.
Erwin - bars and secular evolution
Bar destruction via central mass growth - might expect
process like SBc -> SABb -> SAa
ie fewer bars at earlier types. However, bar fraction is
observed not to vary much with Hubble type. Little
room for bar suicide.
RC3 survey: SO-Sb bars have mean semilength 1.4h
Sc-Sd mean semilength is 0.6h
unlikely to be evolutionary phenomenon
Sample of 14 SB0 galaxies - about half lie above the
oblate rotator line.
Sample of disks, about half with type I profiles - most
show no truncation out to > 5h
Falcon-Barroso - SAURON results
See the complexity of inner regions : a range of central
features - kinematic twists, decoupled cores, counter-rotating
disks, pseudobulges [flat (r], non-axisymmetric objects in
more than 50% of examples, central stellar disks in 75%
N7332 - boxy edge-on bulge. Shows cylindrical rotation,
KDC, hint of central disk. Gas is irregular. Stellar
age homogeneous over the bar - near-solar, age 3-5 Gyr.
Minor axis slit spectra of 19 galaxies. Some show peaked
sigma(r) - classical bulges. Others have flat sigma(r) dominated by rotation, probably pseudobulges.
Barden - ACS survey of 30 x 30 arcmin field
Make Sersic fits - take galaxies with n < 2.5 to be disks
See mean surface brightness V increase by ~ 2 mag
from z = 0.2 to z >1 - good agreement with expectation from
theory. Much as expected from passive evolution.
Little change in surface density with z, suggests that galaxies
increase in size as they accrete gas - inside out disk formation
This was a report on how disks look at different epochs some discussion about whether this is evolutionary
Binney - secular evolution of galactic disks
Heating effect of transient spiral waves: stars exchange
angular momentum, mainly across CR - gives much
radial migration but with little associated heating. The
heating that does occur takes place near ILR. GMCs don't
heat but redistribute spiral arm heating from plane to z
Large spread in [Fe/H] near the sun is evidence that this
radial migration does occur
The spiral structure generates structure in UV plane
for Hipparcos stars - some of this structure is bar-driven
Q: Do we now undertand the mechanism for disk heating.
Disk heating is an important secular effect.
See that significant heating takes place near the sun for
stars with ages between 1 and 2 Gyr.
•
Binney-Sellwood heating requires the transient spiral arms to
have ILR near the solar radius - is this plausible ?
old disk
Velocity dispersions
of nearby F stars
important
thick
disk
Disk heating saturates at 2-3 Gyr
Freeman 1991; Edvardsson et al 1993; Quillen & Garnett 2000
Gerhard - clump and bulge formation in gas rich galaxies
Bulge formation options - mergers, secular evolution,
clump instability in gas-rich disk. Note how clumpy some
galaxies are at z ~ 1.5.
Models include gas and stars with interaction network, plus
dark matter.
Generates a few clumps per galaxy - dynamical friction and
spiral arm torques funnel clumps to center to form a bulge.
Fast bulge formation, as observed by Carollo. About 10% of
baryons end up in bulge. Can add more by secular processes.
Fragmentation efficiency depends on cooling rate - faster
cooling gives more clumps. Models with lower cooling rate
(hotter gas) give smooth distribution of star formation - no
clumps - till disk goes unstable.
Range of metallicities for clumps is -0.7 to -1.
Some stars have [Fe/H] > 0, [Mg/Fe] > 0
To produce pure disk galaxies, need to suppress clump
formation - hot gas, low cooling rate
Moore - harassment
Environmental effects can speed up secular evolution
Showed convincing model of Magellanic Stream :
hydro-tidal phenomenon as LMC orbits around MW
Effect of the cluster potential - smooth and lumpy components on evolution of disk galaxies.
Note the spiral structure seen in unsharp masked images of
some low-luminosity ellipticals.
Many stars are lost from disks - bending, bar instability,
leads to round objects.
Large variety in V/ -  distribution.
Rosolovsky - atomic -> molecular transition in LG galaxies
GMCs are significant in secular evolution of galaxies
In M33, MCO/Mvir for GMCs is independent of [O/H]
GMC populations in MW, LMC, M31, M33 look as
if they come from the same population
Mass function for M33 GMCs is steeper in galaxies with
higher Toomre Q.
Conversion of gas into GMCs may be main issue in star formation
GMCs in M33 lie on peaks of the HI distribution
Renzini : Stellar populations in the bulges of MW and M31
Zoccali MW bulge field at b = -6o :
Old stellar population, like bulge globular clusters.
No evidence for intermediate-age stars - the AGB star
density is as expected for metal rich globular clusters
M31 bulge has similar K-band LF to the MW - also old
Example of some bulges at z ~ 1.6 that are already
1 Gyr old, so formation z ~ 2.5
Q: the bulge stars are very old, but does this mean non-secular
formation of the bulge, before the disk ? Is the bulge structure
also so old ? How can we tell ?
A few slides
on
the structure of
the Milky Way bulge
The Galactic
Bar- Bulge
small exponential
bulge - typical of
later-type galaxies.
M31
Unlike the large r1/4
bulge of M31
Launhardt 2002
Pritchet & van den Bergh 1994
The galactic bulge is rotating, like most other bulges:
(Kuijken & Rich (2002) HST proper motions)
Beaulieu et al 2000
K giants from several sources
and planetary nebulae (+)
Velocity dispersion of bulge and
inner disk are fairly similar
- not easy to separate inner disk
and bulge kinematically
Bulge ends at |l| ~ 10o
Abundance
gradient in
the bulge
( kpc )
Inhomogeneous collection of photometric (
) and
spectroscopic ( ) mean abundances - evidence for
abundance gradient along minor axis of the bulge
Zoccali et al (2002)
Minniti et al 1995
Near the center of the bar/bulge is
a younger population,
on scale of about 100 pc :
the nuclear stellar disk
(M ~ 1.5 x 109 M_sun)
and nuclear stellar cluster
(~ 2 x 107 M_sun )
in central ~ 30 pc.
(Launhardt et al 2002)
~ 70% of the luminosity comes
from young main sequence stars.
Maraston, Thomas : Stellar population properties
of spheroids, including the MW bulge
Enhanced [Mg/Fe] ~ +0.3 needed to model the Fe-Mg
distribution in spheroids => short star formation timescale,
~ 1 Gyr.
[Mg/Fe] correlates with velocity dispersion : massive
systems form most rapidly
Need AGB stars to synthesize intermediate-age
populations. No evidence for intermediate age
population from M31 bulge SED
Bulges and ellipticals are similar in their stellar populations.
They follow an age -  (velocity dispersion) relation
Young apparent age at low  probably means rejuvenation :
underlying old population is present also.
No trend of properties with Hubble type, just with  :
ie mass drives the stellar properties, not T
Q: is there a definitive stellar population test of
concept that bulges form from disk instabilities ?
Are chemical and age gradients the key ?
Peletier : Secular evolution and stellar populations
Color maps do not show the peanut structure : the bulges
and inner disks appear very similar. Early-type bulges are
old (~ 10 Gyr) with small scatter (~ 2 Gyr).
Smallest bulges lie slightly off the fundamental
plane for the old systems.
Bulges of S0-Sb galaxies are like ellipticals in most
properties (at similar luminosity), except for smaller
Sersic index (1-2.5 vs 4).
Later-type bulges are different : star formation in rings,
shallow surface brightness profiles ... (cf Carollo surveys)
Courteau/MacArthur : Evidence for secular evolution
in non-barred galaxies
Exponential bulges in later-type spirals show
strong correlation of re,bulge with hdisk
Mean value of re,bulge / hdisk is 0.22, independent of
wavelength and Hubble type, consistent with secular
evolution models
Brodie : Constraints from star clusters on secular
evolution in spirals and lenticulars
Tight link between the red mode of cluster formation and
formation of the bulge - related to our problem in some
way, though cluster formation is still poorly understood
Faint fuzzies : old loosely bound clusters seen in annular
region of some SB0 galaxies. Apparently longlived, despite
apparent fragility. Survival suggests association with
low-e orbits. Formation in resonance rings would indicate
presence of bar at the time of formation, z > 2.
Bosma, D'Onghia, Burkert
Bulgeless disks - the other side of secular evolution
Why do we see bulgeless galaxies at all ?
Real problems understanding ...
(i) how such disks can form (the angular momentum
problem)
(ii) how they survive without going unstable and forming
bulges - role of the dark halo, anisotropy limits ...
Exciting field - serious problems - much work to do
in observations, dynamical theory, galaxy formation
theory ...
Two secular processes
that
did not
receive much attention ...
1. Effect of the potential of a slowly rotating triaxial
dark halo : these are common in LCDM simulations
2. Nucleation in spirals
Secular effects of rotating triaxial dark halo - ubiquitous
in LCDM simulations (Bekki & Freeman 2003)
eg spiral structure in far outer regions of gas-rich disk
galaxies, where Q, X too large for spiral structure
NGC 2915 - HI
out to 22 scalelengths
-see spiral structure
Bureau et al (1999)
Another example of far-outer spiral structure
NGC 6946 - Oosterloo et al
very deep WSRT HI image
and
another
example
NGC 5055
Oosterloo
et al
Nucleation in spirals - probably secular process
Nuclei mostly very central - why ?
Nuclei of spirals like M33 and NGC 7793 (Walcher) show
extended history of star formation. Looks like a
secular evolution process.
What is cause of nucleation ?
Nuclei of latetype spirals
Böker et al (2002)
survey: 59/77 latetype spirals (T > Sc)
have compact nuclei
very close to center,
as compact and
massive as globular
clusters.
Most of these late-type
spirals have no visible
bulge.
ngc1493
Nuclear clusters are mostly isolated - no spiral arms, dust lanes or
other indications of a dynamical center.
Offset of nuclei
from center of
disk isophotes
The nuclei lie
very close to
the galactic
centers
Böker et al 2002
Several authors make a point about the central location
of these nuclei - how do they know where the center is,
in the shallow central potential well of an exponential disk.
Exponential disk is not as shallow as some !
dF/dr does not  as r
Rotation curve slope dV/dr is singular at center
Density
M(r) ~
V(r) ~
dF/dr ~
Keplerian
const
r -1/2
r -2
S(r) ~ r -1
r
const
r -1
S(r) ~ e-r/h
r2
r 1/2
const
r = const
r3
r
r
The nucleus of the late-type spiral NGC 7793
Dynamical M/L is 2.2
Single burst M/L is 0.5
Mixed pop M/L is 2.8
Black is the observed UVES spectrum: R = 32,000
Red is a single-burst population, age 108 yr - not very good fit
Blue is a mixed-age fit - good fit
Walcher et al 2003
This shows
the fractional
mass
and
luminosity
in the mixed
components
The young component contributes 1.7% of the mass and its age is
0.8% of the age of the universe. Suggests that star formation in
this nucleus (NGC 7793) is an ongoing (secular) event, as in M33
Simon White's Questions & Issues
Are bulges built before or after disks ?
How do galaxies at intermediate redshift (as seen several
Gyr ago) map into today's galaxies - do we see
the bulge-forming events ?
How much of the light in the local universe comes from
bulges ?
Issues
Rundown of merging vs exhaustion of gas
Secular stellar dynamics vs starformation/gas driven evolution
Role of accretion/inflow in secular evolution
BH-bulge/pseudobulge relation and BH feeding.
Questions arising
Q: how do the smaller disky ellipticals (so similar to bulges
in many properties) fit into the secular evolution picture ?
Q: Is there such a thing as a flat bar ?
How important is resonant heating ?
Q: What would a Sersic fit (including a central point source)
give for the bulge of M31 ? M31 has an unambiguous
r1/4 bulge - would this kind of analysis have found it ?
Q: Disk heating is an important secular effect. Do we understand
the mechanism for disk heating in the solar neighborhood ?
Two related questions ...
Q: the MW bulge stars are very old, but does this imply
non-secular formation of the bulge, before the disk ?
Is the bulge structure also so old ?
How can we tell ?
Q: is there a definitive stellar population test of concept
that bulges form from disk instabilities ?
Are chemical and age gradients the key ?
Thanks to Andi, John & Ralf
for
a great workshop