Overview of Gas Flows: Observational

Ken Freeman ANU, Canberra

Gas in galaxies now: star-forming galaxies

Gurovich et al 2010

Sakai et al 2000HIPASS

M33 MWMV = -12.5

Star-forming galaxies with MB < -18 typically have more mass in (cold) gas than in stars.

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 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 baryon content of the Milky Way

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

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.

Paradigm for baryon acquisition

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

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.

NGC 891

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.

IGM median

log M* = 9.5 - 11.5

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 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 havecooled (or heated) so that the OVI is undetectable

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) Vc = 190 km s-1

Outer contour 3.1017 cm-2

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

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 QStabilized by DM ?(cf Schaye 2008)

Very low star formation efficiency in outer regions of HI disks Bigiel et al 2010b :

17 spirals 5 dwarfs

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.

(Black points are mean of HII & PN)

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,

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.

From Veilleux et al 2005

M82 wind: red is H

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 ….)

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.

UGC7321 Matthews et al (1999)

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

Brook et al 2011

Angular momentum distributions of gas and stars within rvir at z = 0Blown-out gas has low-J

HI

gas blown out

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

Disk is seen edge-on. Gas which feeds SF is near plane of the disk.Outflowing gas is perpendicular to plane.

All gas at z = 0.5 Gas at z = 0.5 which will form stars by z = 0

Outflowing gas

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 4from 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

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

T = 105-7

T > 107

104 K 108 K

Kang et al 2005(Z = 0)

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.

(Bottema 1995)

NGC 4013 is extreme example of edge-on warp - but system may have had a recent minor merger (Martinez-Delgado 2009). Is thewarp and minor merger related ?

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

NGC 4013

Polar ring galaxies have a ring or disk of gas in polar orbit around the central galaxy. NGC 4650A is the prototype

HI

Arnaboldi et al 1997

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.

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.

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 evolutionof 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

Chemical Tagging

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 clumpsIn 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