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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