Download The difficulty of forming large disk galaxies with small bulges

Overview of Gas Flows:
Observational
Ken Freeman
ANU, Canberra
Gas in galaxies now: star-forming galaxies
MV = -12.5
M33
MW
HIPASS
Sakai et al 2000
Star-forming galaxies with MB < -18 typically have
more mass in (cold) gas than in stars.
Gurovich et al 2010
At the low mass end, the baryonic TF law shows that gas-rich galaxies
have baryons in relationship to the mass of their DM halos as
measured by the circular velocity.
McGaugh 2011
At the very low mass end, the various dSph galaxies have roughly
similar DM masses within ~ 300 pc spherical radius but a wide
range of stellar masses going down to < 1000 M in some ultrafaint
systems: M/L ratios > 1000, very low visible baryon content
The dSph are now devoid of gas, although some have had an
extended star formation history. They show a stellar mass metallicity relation similar to the gas-rich dwarf irregulars. The
outcome of the SFH and chemical evolution in dIrr and dSph
galaxies seems similar, although dIrr are still forming stars and the
dSph are not.
Did the faintest dSph lose most of their baryons via feedback or
stripping, or did they never acquire their share ? Does the baryon
acquisition become stochastic at some level in the DM hierarchy ?
dIrr and dSph objects share the same relation, though their
baryon contents are now very different.
Mateo 2008
The baryon content of the Milky Way
The universal baryon / DM = 0.17
In the Milky Way, the ratio (stellar mass) / (dark matter mass) = 0.03
A small fraction of the Galactic baryons is in cold gas in the disk.
With current star formation rate, believed to have been roughly constant
for several Gyr, the disk will use up its cold gas in a few Gyr.
We are missing about 3.1011 M of baryons: this represents about 80% of
the Galaxy’s share of the baryons. Were they lost, or never acquired ?
The rest of the baryons may be in the hot halo of the Galaxy:
a potential reservoir for feeding ongoing star formation.
The presence of the Galactic hot halo is consistent with non-detection
in HI of the dwarf galaxies within R = 270 kpc (eg Grcevich & Putman
2009). Their gas is stripped by ram pressure + internal heating
(Nichols & Bland-Hawthorn 2011). The gas-rich dIrr galaxies lie at
R > 270 kpc. Density of halo > 2-3.10-4 cm-3 out to R at least 70 kpc.
(Gas contribution to hot halo from stripped dwarfs is small)
HVC morphologies show head-tail
morphologies, suggesting interaction
with halo medium which extends out to
at least distance of Magellanic Stream
(Putman et al 2011)
All of the missing baryons may not lie in the hot halo.
eg Anderson & Bregman (2010): dispersion measure of LMC
pulsars, lack of X-ray emission from large spirals and lack of
OVII absorption by halos suggests that halos have only a fraction
(~ 10%) of the missing baryons. The rest may have been expelled
during galaxy formation or were never acquired by the DM halo.
Supported by Anderson & Bregman (2011)
detection of X-ray emission from the
(morphologically) disturbed giant spiral NGC
1961 (Vc > 300 km s-1). Galaxy has cold gas
content (HI + molecular) of about 8.1010 M.
Its total halo mass ~ 1-3.1011 M which is
only a fraction of its share of the missing
baryons. The cooling rate is ~ 0.4 M yr-1
which is much less than the gas consumption
rate (~ 3 M yr-1).
Combes et al 2009
Paradigm for baryon acquisition
Most of the baryonic mass in
galaxies with halo masses
< 2-3. 1011 M is acquired
through filamentary cold-mode
accretion of gas that was never
shock-heated to its virial
temperature (eg Birnboim &
Dekel 2003, Keres et al 2005,
2009).
Atmospheres of hot virialized
gas develop in halos above
2-3.1011 M but cold accretion
persists (especially at z > 2)
and is the main driver of the
cosmic SFH.
Keres et al 2009
How to get the hot gas from the halo into the disk to fuel star formation
(and how to get it out of the disk into the halo)
High Velocity Clouds: long believed to be gas fuel source for the disk. Their
formation is not completely understood yet. Their metallicities are ~ 0.2 Z
(eg Sembach 2004)
Condensation of HVCs from hot halo via thermal instability (Maller & Bullock
2004). Difficult to make this work in linear theory - would be mostly
suppressed by buoyancy and thermal conduction (Binney et al 2009).
Simulations of non-linear perturbations (overdensities 10-20) can generate
cool clouds from hot halo if cooling time is shorter than the dynamical time.
Otherwise clouds are disrupted by KH & RT instabilities (Joung et al 2012)
Only larger clouds (~ 105 M) can survive the trip from halo to disk. Smaller
clouds lose their HI over ~ 10 kpc of travel and may become part of warm
ionized Galactic disk.
Keres & Hernquist (2009) simulations: at high z, gas is
accreted from IGM in cold filamentary infall. At later times,
densities are lower and filamentary flows are disrupted in
inner regions of massive halos. Disrupted filaments are still
able to supply cold gas to MW-sized galaxies. Cooling and
RT instabilities produce cold (< 104 K) clouds.
Again, process needs seeding with moderate overdensities
from the filamentary gas.
Interaction of hot halo gas with infalling satellites : is this a
significant source of fuel ?
Cosmological hydro simulations provide a guide: simulations (Fernandez et al
2012) of a MW-sized galaxy shows that the amount of HI present in the halo
since z = 0.3 is roughly constant, at about 108 M. The HI accretion rate on to
the disk is about 0.2 M yr-1. The satellites are losing gas (about 0.06 M yr -1)
so other sources are needed to fuel the current star formation rate.
Most of the cold gas in the halo comes from filamentary flows - much of it does
not make it directly to disk but some is able to cool and form clouds. Gas
stripped from satellites provides a fraction of the cold halo gas.
Kauffmann et al 2012 agree: from study of isolated central galaxies (SDSS),
accretion rate of gas from satellites is too small to feed the current star formation
rate in the primary galaxies. SFR in primary galaxies correlates with the amount
of gas in satellites - maybe the satellites are tracing a larger reservoir of ionized
gas, that is accreting on to the primaries.
Entraining of hot halo material by galactic fountain: SN-driven accretion
NGC 891
Fraternali et al (2001 ff) : very
deep HI observations found halo
HI in NGC 2403, 891, 4559, 6946
lagging rotation of the disk.
In NGC 891, halo HI is detected
to z = 22 kpc from the galactic
plane.
Lag in rotation relative to disk
suggests that fountain gas loses
angular momentum to the hot
halo.
Binney, Fraternali, Marinacci et al (2006 ff) propose that SN-driven
fountain gas entrains hot halo material which returns to the disk.
Star formation sustains itself from the hot halo, with accretion rate
~ 2 M yr-1 in the Milky Way. (Deposition rate is smaller than the
overall rate of interchange of mass between disk and halo).
Cool clouds are ejected from the disk into halo : KH instability strips
gas from the clouds, and coronal gas condenses in their wake and
returns to the disk. Fountain gas loses angular momentum to the halo
and spins up the halo, which still lags the cold disk by ~ 100 km s-1.
Metals in the halos
If the halos are at least partly from gas ejected from star formation sites
in the disk, then they would also be a reservoir of metals. eg Tumlinson et
al (2011) found UV OVI absorption in 27/30 galaxies with sSFR > 10-11 yr -1
and only 4/12 passive galaxies with sSFR < 10-11 along sightlines with
projected radii up to 150 kpc.
log M* = 9.5 - 11.5
IGM median
The blue cloud / red sequence dichotomy is closely reflected in the chemical
properties of the gaseous halo out to 150 kpc
The halo O-mass and total gas mass is comparable with that of the ISM.
In passive galaxies, the gas may have been stripped, re-accreted or have
cooled (or heated) so that the OVI is undetectable
The gaseous halo may be a basic component of star forming galaxies that is
removed or transformed when star formation is quenched. cf the Fraternali &
Binney 2008 mechanism of halo gas entrainment: star formation drives the
entrainment which then feeds the star formation.
What is mode of quenching in this picture ? Mergers ?
The O was likely produced by star formation in the disk and ejected into the
CGM. Only 0.3 Gyr of SF at the present rate would be enough to produce it if
it were all ejected. It could have accumulated over many Gyr, or be leftover
from early starbursts. An estimate of [O/Fe] (or other Fe-peak element)
In the gaseous halo could give some constraint on when enrichment occurred.
If the mass-metallicity relation of galaxies is associated with ejection and inflow
of metals, (eg Davé et al 2011) rather than by rate of SF (eg Tassis et al 2008),
then ejected material in lower-mass galaxies may be locked in the CGM and
may return later to affect their chemical evolution.
The ejected material may also feed the extended non-star-forming HI envelopes
seen in many isolated disk galaxies and raise the abundances of these
envelopes to their observed level (> 0.2 Z - eg Werk et al 2010, 2011)
The extended HI envelopes of disk galaxies
Examples
NGC 2903 Irwin et al (2009)
Outer contour 3.1017 cm-2
Vc = 190 km s-1
NGC 2915 Meurer et al 1996
MB = -16
Vc = 80 km/s
Abundance Z ~ 0.4 Z in outer
disk HII regions (Werk et al
2010)
NGC 5055 Battaglia et al (2006) weak warp, weakly
lopsided: inner HI dominated by stellar disk, outer HI by DM.
Some SF in outer HI. Vc = 206 km/s.
M83 outer HI (Bigiel et al 2010a)
Some star formation, but depletion time ~ 100 Gyr in outer regions:
outer HI is available as reservoir for star formation in inner regions.
Is this reservoir utilized via radial flows ?
HI
UV, HI
M83 outer HI Bigiel et al 2010a
Long HI depletion time in outer disk
Very low star formation efficiency in outer regions of HI disks
Bigiel et al 2010b :
17 spirals
5 dwarfs
SF efficiency drops for lower
HI column density.
Depletion time ~ 100 Gyr in
outer regions.
In outer regions, SF
efficiency not strongly
correlated with local Q
Stabilized by DM ?
(cf Schaye 2008)
Radial Flows and chemical evolution
Expect slow (~ 1 km/s) radial inflow in disk as consequence of infall of matter with angular
momentum < that of local circular velocity (eg Lacey & Fall 1985). Could also be driven by
bar, spirals, viscosity. Not possible to detect observationally (Wong et al 2004: NGC 5055;
Elson et al 2011: NGC 2915): intrinsic asymmetries and warps in HI disks mask radial flows
< 5-10 km/s.
Spitoni & Matteucci (2011) looked at chemical
evolution of disk with inflow. Goal is to reproduce
an exponential stellar disk and this observed
abundance gradient in the gas. Succeed with infall
velocity increasing with radius and radially
dependent infall rate decay time (~ 8 Gyr at sun).
Not unique: other ways to get there without radial
flow.
Related study by Schoenrich & Binney (2009)
invoked radial gas flow and radial stellar
migration and stellar heating prescription.
Reproduced abundance gradient, solar n’hood
stellar MDF, bimodal alpha-Fe relation from
migration rather than SFH,
(Black points are mean of HII & PN)
Starburst- and AGN-driven winds
Picture of metal ejection from star-forming disk, enriches halo, returns gas to disk (maybe
with extra entrained gas from halo). Believed to be essential part of understanding the
mass-metallicity relation enrichment of IGM …
Look now briefly at recent observational work on winds driven by starbursts and AGNs at
low-z (Sharp & Bland-Hawthorn 2010). The material seen in the winds is not the winds
themselves which are believed to be hot - we see the cooler material that is entrained in the
wind.
M82 wind: red is H
From Veilleux et al 2005
Sharp and Bland-Hawthorn (2010): IFU observations in several diagnostic
lines of winds in some starburst and AGNs. See large-scale wind cone of
enriched material with velocities of 100-200 km/s, accelerating to increasing
heights above plane, originating from the region of active star formation.
In starbursts, winds are shock-ionized. UV photon flux dies away rapidly on
10 Myr timescales, followed by rapid rise in mechanical energy input driven
by SN and leading to shock ionization. UV evaporation from dense clouds
is followed by mechanical heating.
AGN timescales are longer, permitting photoionization of wind.
The difficulty of forming large disk galaxies with small bulges
led to current ideas on importance of suppressing early star
formation, removing baryons from forming galaxy via massive winds
and returning them slowly (Binney Gerhard Silk 2001, Governato,
Gibson ….)
UGC7321 Matthews et al (1999)
The mass loss avoids loss of baryonic angular momentum to the
assembling lumpy DM halo which leads to disks that are too small.
It also suppresses the steep DM cusps which are ubiquitous in
CDM simulations but are not observed in galaxies.
In the current simulations, galaxies start with more low-J
material than they end up with. The surplus is ejected as
wind. The halo absorbs some of the angular momentum and
expands. Disk forms later from higher-J material which falls in:
this suppresses the formation of large bulges which always
occurred in earlier simulations.
The angular momentum content of disks (the M-J relation) is a
critical constraint on formation mechanism (eg Fall 1983).
How much of this is true - what are the details. Simulations
are now showing these effects (eg Brook et al 2011) in context
of earlier discussion of winds, hot halo.
Brook et al (2011) on the formation of bulgeless galaxies. High resolution
cosmological SPH simulations, enough resolution to resolve star forming
regions. Adopt a high threshold density for SF: increases energy released
into gas affected by SN feedback.
In summary
• low-J material accreted and rapidly expelled early. Later
accreted material has higher-J and forms disk
• reservoir of high-J material exists beyond star forming
region
• outflows occur perpendicular to disk.
• Material that builds disk is accreted from outer regions
near the galactic plane
• mergers which cause gas to lose angular momentum trigger
starbursts which expel much of the low-J gas. (May also
expel gas which has lost angular momentum through secular
processes)
Loss of low-J material suppresses bulge formation
gas blown out
HI
Angular momentum distributions of gas and stars within rvir at z = 0
Blown-out gas has low-J
Brook et al 2011
Simulation:
B-band surface brightness
and HI contours (1019 to
1022 cm-2) at z ~ 1.2.
Extended HI relative to
star forming region.
mag arcsec-2
Brook et al 2011
All gas at z = 0.5
Gas at z = 0.5 which
will form stars by z = 0
Outflowing gas
Disk is seen edge-on. Gas which feeds SF is near plane of the disk.
Outflowing gas is perpendicular to plane.
Brook et al 2011
Maccio et al 2012 show how the heating of the cusp via the
outflow can erase the central cusp in the DM distribution, as many
have suggested.
Low feedback
High feedback
Flows on larger scales
Galaxy clusters
Nonthermal ring-shaped radio
structures in outer regions of A3376
may trace shocks from large scale
cosmological flows into the cluster.
Bagchi et al (2006) suggest cosmic
ray acceleration via magnetic shocks
Bagchi et al 2006
Largescale infall of galaxies into filaments
Contours of two-point correlation function as function of transverse () and
radial () separation. See finger-of-God elongation in radial direction and
the Kaiser flattening in transverse direction at large separation due to
coherent largescale infall
Peacock et al 2001
Ly- absorption kinematics of cosmic web filaments at z = 2 to 4
from adjacent QSO sightlines (Rauch et al 2005). Velocity shead
shows that most clouds are expanding faster than the Hubble flow filaments may be stretching and draining into more massive nodes.
Rauch et al 2005
Gas on large scales
T < 105
eg Kang et al (2005): simulations of infall
of gas into potential wells of the evolving
hierarchical structure of universe heats
gas - heating via shocks.
The hot gas (T ~ 107) is around clusters
and groups. WHIM (T ~ 105 to 107) is
mostly in filamentary structures. The
cooler gas (T < 105) is in sheet-like
structures and contains a significant
fraction of the missing baryons at low z.
Figures show shock structure at z = 0
T = 105-7
T > 107
104 K
108 K
(Z = 0)
Kang et al 2005
Galactic Warps and Gas Flows
Misalignments of inner and outer HI disks in disk
galaxies are very common. Almost all galaxies with
extended HI disks show kinematic or structural warps
Early ideas about warps involved waves and modes in disks. After DM was
discovered, external torques, misaligned angular momenta of gas components, gas and
DM, and gas infall became more prominent. The very flat stellar disk and the warped
outer HI disk may indicate different episodes of gas acquisition
Rules about warp geometry (eg Briggs 1990): the inner HI disks are flat and
aligned with the stellar disk out to the edge of the stellar disk. Then the warp begins - out
to about R26.5 the line of nodes (LON) of the warp does not rotate with increasing radius.
Beyond R26.5 the LON mostly advances in the leading sense.
See v.d.Kruit & KCF ARAA 2011 for more on structure of warps
Ideas about origin of warps
• misalignment of stellar disk and the DM (Dekel & Shlosman 1983)
• outer disk torqued by misaligned cosmic infall (eg Ostriker & Binney 1989
Shen & Sellwood 2006)
• tidal interactions and minor mergers (eg Martinez-Delgado et al 2009)
. • misalignment of inner disk and outer hot halo: halo torques infalling cold
clouds and aligns their angular momentum with that of the hot halo (Roskar
et al 2010)
Are warps long-lived ? This would help to distinguish whether they
are associated with misaligned dark matter or misaligned inflow or
hot halo.
About 65% of edge-on stellar disks also show weak warps which are
sometimes asymmetric in their onset (eg Saha et al 2009, Guijarro et al
2010). The stellar warps may be tidal in origin, or come from the
response of the stellar disk to the outer gas warp, or be part of the
(infall-generated) warp.
If the latter turns out to be correct, then it may be possible to use the
stars to do chemical studies on the infallen material.
NGC 4013 is extreme example of edge-on warp - but system may
have had a recent minor merger (Martinez-Delgado 2009). Is the
warp and minor merger related ?
(Bottema 1995)
NGC 4013
Binney (1992 ARAA on warps): “But one’s best guess must be that warps will in the
end prove to be valuable probes of cosmic infall and galaxy formation.”
Much work already done already in this direction - important to understand warps they provide potential opportunity to constrain epoch and overall properties of infall
Martinez-Delgado et al 2009
Polar ring galaxies have a ring or disk of gas in polar orbit around
the central galaxy. NGC 4650A is the prototype
HI
Are they a useful probe into gas flows and properties of hot halos ?
Maccio et al 2006 find a system very like NGC 4650A in a cosmological hydro simulation:
forms from cold gas infalling along a Mpc-scale filament. Polar disk lives for a few Gyr.
Arnaboldi et al 1997
The clump cluster galaxies
High-z star-forming galaxies are often disk galaxies
with massive kpc-sized star-bursting clumps.
The clump masses are ~ 109 M with star formation
rates ~ 20 M yr -1.
Clumps are seen in regions of the disk where Q < 1
(Genzel et al 2011). The disks believed to form
from cold streams (Dekel et al 2009): streams keep
the rotating disks intact, though turbulent and
broken into giant star-forming clumps. (Jeans
masses are large because disks are turbulent from
cold flows).
Genzel et al 2011
Observations (supported by simulations of giant clump formation) show that the individual
clumps are in equilibrium and are mostly rotationally supported (V/ ~ 2) with partial
turbulent support.
Broad emission-line wings indicate strong wind-driven mass loss. From observations and
simulations (eg Genel et al 2012), clump lifetimes are ~ 0.3 Gyr.
After a short (< 0.3 Gyr) period of star formation, the clumps are likely to
disperse and may form the galactic stellar thick disk (eg Bournaud et al 2009).
Thick disks are very common in disk galaxies: they are old (> 8 Gyr), have
metallicities ~ Z/5 and are -enhanced. They typically contribute about 10% of
the stellar mass in the more massive disk galaxies (ie about 5.109 M). Their
formation is not understood. If thick disks do form from dispersed clumps,
then they would have formed from a relatively small number of clumps.
If these clumps were chemically homogenous (e.g. Bland-Hawthorn et al 2010),
then it will be fairly easy to identify the debris of this small number of clumps in
the Milky Way from their distribution in chemical C-space.
Chemical Tagging
Use the detailed chemical abundances of stars to tag or
associate them to common ancient star-forming aggregates with
similar abundance patterns (eg Freeman & Bland-Hawthorn
ARAA 2002)
The detailed abundance pattern reflects the chemical evolution
of the gas from which the aggregate formed.
Chemical studies of the old disk stars in the Galaxy can help to identify
disk stars that are the debris of common dispersed star-forming aggregates
and also those which came in from outside in disrupting satellites
We can think of a chemical space of abundances of elements Na, Mg, Al,
Ca, Mn, Fe, Cu, Zr, Ba, Eu … for example (25 measurable elements
with HERMES). The dimensionality of this space is about 8 (Ting et al
2011). Most disk stars inhabit a sub-region of this space.
Stars from chemically homogeneous aggregates will lie in tight clumps
In C-space.
Stars which came in from satellites are different enough to stand out
from the rest of the disk stars in chemical space.
With this chemical tagging approach, we may be able
to detect or put observational limits on
the satellite accretion history of the galactic disk
For chemical tagging to work in reconstructing the star
formation history of the disk ….
• stars form in large aggregates
• aggregates are chemically homogenous
• Need sufficient spread in abundances from aggregate to
aggregate so that chemical signatures can be distinguished with
accuracy achievable (~ 0.05 dex differentially)
These conditions appear to be true:
see e.g. G. de Silva et al (2009)
Wylie de Boer et al (2010) have already used the chemical peculiarities
of  Centauri to identify field stars that were probably stripped from it
or its parent dwarf galaxy when it was accreted by the Milky Way.
Galactic Archaeology with HERMES
HERMES is a multi-object high resolution spectrometer on the AAT. It
will acquire spectra of about 370 stars at a time.
In 2013 we will begin a survey to measure abundances of about 25
elements in about a million stars with V < 14 in the southern sky
(resolution 30,000, SN = 100 per resolution element)
Simulations (Bland-Hawthorn and KCF 2004) show that this survey
can recover the debris of about 30,000 individual star formation
sites using chemical tagging techniques.
Recovery of debris of ~ 50 giant clumps should be easy.
THE END