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Physical Ingredients of Galactic Evolution Shustov B.M. Institute of Astronomy, Moscow Chinese Summer School “Modeling and Observing the Evolution in Galaxies” Chinese Astronomical Society Symposium 35 June 20—28, 2002 University of Science and Technology of China,Hefei and YunNan Observatory, Kunmin Plan of the lecture 1. Galaxies and their constituents 2. Why, what and how to model 3. Mathematical formulation of chemo-dynamical evolution of galaxies 4. Physical and chemical ingredients of the models 1. Galaxies and their constituents 1.1. Galaxies as dominant structures in the Universe – Galactic zoo – MW is a representative galaxy 1.2. Galactic features – Baryonic structures of MW – Dark matter – Sources of energy – Most important processes 1.1. Galaxies as dominant structures in the Universe Essentially everything of astronomical interest is either part of a galaxy, or from a galaxy, or otherwise relevant to the origin and evolution of galaxies. B.Tinsley, Fundamentals of Cosmic Physics, 1980 Classification scheme for galaxies, devised in its original form in 1925 by Edwin P. Hubble (1889–1953), and still widely used today. The Hubble classification recognizes four principal types of galaxy—elliptical, spiral, barred spiral and irregular—and arranges these in a sequence that is called the tuning-fork diagram (in fig. the classification includes additional types adopted from G.de de Vaucouleurs classification). EAA The Milky Way is most likely a barred spiral galaxy of Hubble type SBc. The Milky Way is most likely a barred spiral galaxy of Hubble type SBc. lgLB = 1.00lgM - 0.87, lgMg = 1.02lgM - 1.55, lgLFIR = 1.36lgM-5.45, lgMd= 1.47lgM-9.76, lgMH2 = 1.52lgM-7.67 Kostyunin1995 Sage 1992 Salucci&Persic (1999) computed the baryonic mass function, ψS(Mb)dlogMb, of disc galaxies using the luminosity functions and baryonic mass-to-light ratios reliable for this goal. On scales from 108Msolar to 1011Msolar this function is featureless, ψS~Mb-1/2. Outside this mass range ψS is a strong inverse function of Mb. The contributions to the baryon density from objects of different mass indicate that spirals have a characteristic mass scale at 2x1011Msolar, around which more than 50per cent of the total baryonic mass is concentrated. Salucci & Persic (1999) MW is a representative galaxy! Luminosity and mass functions of galaxies Salucci & Persic (1999) MW is a representative galaxy! 1.2. Galactic features Baryonic structures of MW • • • • • Halo, corona (up to 100 kpc) Thick disk (1 kpc) Thin disk (300 pc) Spiral arms (density waves) Substructures – Open star clusters and associations (about 1500 known) – Globular clusters (about 170 known) – Remnants of dwarf galaxies ? – Gaseous structures Main components of the MW Mass estimates • Central section of the Galaxy is (with separations from the center of <5 kpc) about 2×1011 M • Halo 4×1011M (Fich and Tremain 1991) • Corona (3–6)×1012M (Fich and Tremaine 1991) For the halo and the corona - substantial (up to 90% for corona) part of matter in exists as dark matter . Tracers of large scale structures • Stars – Stars with known distances (kinematics, counts) – IR (eg. NIR photometry, IR source counts) – Chemical abundances • Gas – HI 21 cm – HII – CO Structures and populations in the MW • Halo (H ~ 1kpc) - HEO, oldest Pop II, [Fe/H] < -1 • Bulge with a bar ~ 23.5 kpc (Eckart, Engmaier, EAA, 2002) - kinematic as for halo stars, -1< [Fe/H] < 1 • Center (here just to mention) • Thin disk (H < ~ 250 kpc) - circular orbits ~200 km/s at solar distance, flat rotation curve, [Fe/H] > -0.6 , concentration of molecular clouds (star forming regions) • Thick disk (~ 1kpc ) - intermediate kinematics, -1< [Fe/H] < -0.6 Substructures • Flows (stellar: Magellanic Flow, remnats of dwarf galaxies gaseous: galactic wind) • Open star clusters and associations (about 1500 known) • Globular clusters (about 150 known) • Gaseous structures: – diffuse clouds – molecular clouds – supershells and superbubbles The gas flow in the Galactic disk (Englmaier and Gerhard 1999). The location of the Sun is shown at x=−2.7 kpc and y=−7.5 kpc just outside the frame. The Orion arm is possibly a connection between the Aquila and Perseus arm in the vicinity of the Sun. The image is a result of hydrodynamical 2D modeling using 20 000 particles in a SPH code. - HII zones - GMC Structure of MW with mean metallicities of each component (from Matteucci 1991) Dark Matter • MaCHOs (microlensing experiments) • “Failed stars” — brown dwarfs, ultra-compact gas-dust globules (sub-mm source counts) • Small clouds of pure molecular hydrogen (no way to detect) • WIMPs and/or other exotic particles • New physics (MOND)? Sources of energy • Gravitation (governs all) • Nuclear energy • Kinetic energy • • • • Next three energetic ingredientsare dominant for the ISM Radiation field — stellar population Magnetic field (B from synchrotron radiation, polarization, mapping of linearly polarized spectral-line emission , B - Faraday rotation of the position angle of linearly polarized radiation if the density of interstellar electrons is known, dispersion measures of pulsars (B weighted by the electron density may be determined), Zeeman splitting) — dynamo? Cosmic rays (mostly protons) — SN accelerated Input of kinetic energy - from outflows, turbulence etc. Interstellar radiation field (from Black 1987) Magnetic field in MW The Galactic interstellar magnetic field has a spiral pattern, with reversals in field direction in interarm regions. Near the Sun Bt ~5– 10 μG, with a ratio of uniform to random components Bu/Br~ 0.3– 0.7. The field strength increases inward in the Galaxy, with milligauss fields in the Galactic center. The Galactic magnetic field is probably maintained by a Galactic dynamo, although the details are very unclear. Starlight polarization defining the direction of the Galactic magnetic field near the Sun (from R.Crutcher 2000 EAA) Equipartition in the ISM The different interacting components of the ISM discussed above have comparable energy densities (and pressures). For example, the energy density of the ISM radiation field from 91.2 nm to 8 μm is ~0.5 eV cm−3, the cosmic rays have ~1.5 eV cm−3, and B2/8π for a magnetic field with B=5×10−6 G is 0.6 eV cm−3. Gas motions associated with turbulence and waves in the ISM are estimated to have an energy density of ~1 eV cm−3. The different thermal phases - 0.3 eV cm−3. The similarity of these energy densities (and pressures) is probably not a coincidence but is suggestive of a strong interaction and resulting energy and pressure equipartition among these different components of the ISM. Most important processes Collapse of primeval galaxy Star formation Mergers with other galaxies Inflow and outflow Stellar evolution Stellar population Interstellar medium Stellar mass loss and death Colors, spectrum, etc. Remnants Tinsley (1980) A scenario of cosmic chemical evolution (from Pagel 1997) 2. Why, what and how to model 2.1. Main targets • Origin of the Galaxy, its past and future • Study of physical and chemical processes in a great lab – the Galaxy • Test for theories (e.g. for explosive nucleosynthesis in SN) • Life in the Galaxy (e.g Galactic Habitant Zone) GHZ Circumstellar Habitable Zone (CHZ) has generally been defined to be that region around a star where liquid water can exist on the surface of a terrestrial (i.e., Earth-like) planet for an extended period of time. Recently Gonzales and Brownlee (1991) proposed the concept of a "Galactic Habitable Zone" (GHZ). Analogous to the CHZ, the GHZ is that region in the Milky Way where an Earth-like planet can retain liquid water on its surface and provide a long-term habitat for animal-like aerobic life. Gonzales and Brownlee examined the dependence of the GHZ on Galactic chemical evolution. The single most important factor is likely the dependence of terrestrial planet mass on the metallicity of its birth cloud. A metallicity at least half that of the Sun is required to build a habitable terrestrial planet. GHZ The mass of a terrestrial planet has important consequences for interior heat loss, volatile inventory, and loss of atmosphere. A key issue is the production of planets that sustain plate tectonics, a critical recycling process that provides feedback to stabilize atmospheric temperatures on planets with oceans and atmospheres. Due to the more recent decline from the early intense star formation activity in the Milky Way, the concentration in the interstellar medium of the geophysically important radioisotopes 40K, 235,238U, and 232Th has been declining relative to Fe, an abundant element in the Earth. Also likely important are the relative abundances of Si and Mg to Fe, which affects the mass of the core relative to the mantle in a terrestrial planet. . All these elements and isotopes vary with time and location in the Milky Way, thus, planetary systems forming in other locations and times in the Milky Way with the same metallicity as the Sun will not necessarily form habitable Earth like planets. As a result of the radial Galactic metafficity gradient, the outer limit of the GHZ is set primarily by the minimum required metallicity to build large terrestrial planets. Regions of the Milky Way least likely to contain Earth-mass planets are the halo (including globular dusters), the thick disk. and the outer thin disk. The bulge should contain Earthmass planets, but stars in it have a mix of elements different from the Sun's. The existence of a luminosity-metallicity correlation among galaxies of all types means that many galaxies are too metal-poor to contain Earth-mass planets. Based on the observed luminosity function of nearby galaxies in the visual passband, Gonzales and Brownlee estimated that (1) the Milky Way is among the 1.3% most luminous (and hence most metal-rich) galaxies and (2) about 23% of stars in a typical ensemble of galaxies are more metal-rich than the average star in the Milky Way. 2.2. How to model - model classification By subject • Dynamic (plus stellar dynamics) • Chemical (chemo-dynamic) - primary topic of the lecture • Spectrophotometry • etc. By computational method • One zone • Multizone • HD — HD+SD — SPH By interaction with circumgalactic space • Closed • Open CGE - targets Chemical (chemo-dynamic) models deals with gas evolution in a galaxy including all processes Quite a lot of Chemical (chemo-dynamic) models of galactic evolution do exist. I will not comment on them here (this will be done in the next lecture) In principle to construct a model is not a great problem. But “devil is hidden in details”. Full mathematical and physical description of all relevant processes and object is still lacking. There are attempts to construct some comprehensive and software package for study of the galactic evolution. I just mention th Galaxy Evolution tool (GEtool) that is a software package currently being developed to selfconsistently model of chemical and spectral evolution of disk galaxies. It is important that GEtool will soon will be available to he community thriugh a web-paged interface that will enable users to predict observable properties of model galaxies such as colours, spectral gradients, Lick indices and elemental abundances. 3. Mathematical formulation of chemodynamical evolution of galaxies • Quite a lot of models do exist. To construct a model is not a great problem. But “devil is hidden in details”. • Full mathematical and physical description is still lacking. • We (Shustov Tutukov Wiebe, all from Institute of Astronomy, Moscow) elaborated a simple model capturing all the main features of the galactic evolution (it can be classified as a toy model, though it turned to be successful in solving some hard problems of CGE – I’ll touch this during the next lecture). “Thermodynamical” Approach dMg dt M (t ) R(t ) M out in dM i i i Z i (t ) Pi (t ) M out M in dt R(t) is the total gas ejection rate from stars of all masses and ages Pi(t) is the total ejection rate of an i-th element from all stars Gas and Metal Ejection M max R(t ) M M R ( M , Z ) (t t M )f ( M )dM M min M max P(t ) i M M R Pi ( M , Z )Zi (t t M ) Pi ( M , Z ) (t t M )f ( M )dM M min f (M) is the initial mass function IMF (t) is the star formation rate SFR tM is the lifetime of a star of (initial) mass M MR is the remnant mass Pi is the i-th element production in a star Dynamical features of toy model Idea: Self regulated disk thickness H. Input of mechanical energy from massive stars SN versus dissipation. dH/dt = H+ - HE pot GMM g H 2 M g dvt2 2dt vt H R R2 H 2GMM g H dH fM g2 M g vt2 2 R2 H dt R H 2t d 3GM H 4R 2 H td 3GM g ………. Firmani&Tutukov 1992 4. Physical and chemical ingredients of the models 4.1. Initial conditions 4.2. Star formation – Quick look – SFR: theory and observation – IMF: observations, theory – Philosophy of the IMF 4.3. Stellar evolution – Brief conventional picture of evolution of single and binary stars – Life times – Luminosities and ionizing fluxes – Masses of remnants – Stellar yields 4. Physical and chemical ingredients of the models (cont.) 4.4. Inflow and outflow – Accretion (+dynamic friction) – HVC and flows – Galactic wind – Expelling of dust – Dynamic stripping – Interacting galaxies 4.1. Initial conditions • Properties of protogalaxy (structure, temperature, etc.) • The primordial abundances from the Big Bang are typically assumed. There are models with quick pre-enrichment. This includes pre-galactic enrichment, or protogalactic processes, or preenrichment from other more evolved system. Population III Recent simulations of collapsing clouds in pre-galactic era predict that gigantic ~100 M stars formed. They lived fast and died so explosively that new telescopes can be able to see them as supernovae or gamma ray burst at the margin of the Universe. Probably some cycles of star formation could happen in the this epoch and lower long living mass star could form. Population III is a group name for these first stars In our Galaxy we never observed such stars, though this research direction seems to be promising “To make the leap to Population III some researches use silicon – not in stars. But in the chips of supercomputers” R.Irion 2000 Population III 4.2. Star formation General comments • Star formation is a a primary ingredient of galactic evolution • SF theory is still under construction • SF history - a challenging problem for new instruments globules supergiant star Sher 25 The ring structure is reminiscent of Supernova 1987a and Sher 25 itself may be only a few thousand years from its own devastating finale a cluster of bright hot blue stars whose strong winds and ultraviolet radiation have cleared away nearby material NGC 3603: Wolfgang, Grebel, You-Hua Chu, 1999 emission nebulae are similar to suspected proto-planetary disks (proplyds) encompassing stars in the Orion Nebula. Birthrate and Star Formation Rate B(m, t ) (t ) (m)dtdm f(m) - initial mass function IMF (t) - star formation rate SFR in fact this is a function of other physical prameters which are functions of t The Schmidt law a n gas Schmidt’s law (1959): star formation rate is proportional to some power of the volume gas density Star formation is a self-regulated process controlled by the energetic feedback from young stars that prevents new stars from being formed. Efficiency of the process is described by a. Other variants of SFR parametrization a k gas a k1 total Schmidt 1959 k2 gas Dopita&Ryder 1994 0.017gas gas Kennicutt 1989 ae Tosi 1998 t / t Comment: these are to fit observation. Lack of physical basis for such laws • Various modes of SF from observation – In disk galaxies sporadic SF – In gas rich dwarf galaxies bursts separated by long intervals of time – Large star bursts – interaction induced • Various modes of SF from theory – Stochastic SF – Self-regulated SF • Important physical factors: density, temperature, chemical composition, gravitational potential, rotation, spiral shocks, collisions, magnetic field etc. About the physical meaning of the Schmidt law Star formation is a self-regulated process controlled by the energetic feedback from young stars that prevents new stars from being formed. Sources which could supply significant negative feedback include stellar winds, ionizing UV, and supernova explosions. However, in normal case the heating by ionizing UV, is dominant. The UV production associated with a given amount of star formation is calculated, and the critical UV production rate above which the UV heating quenches star formation is estimated by Tutukov&Krugel (1980), Cox (1983). The physical meaning of the Smidt law Ionization balance in a volume V NLyc nOB = V n2 NLycnOB/V ~ OB tOB ~ OB = a 2 a = 5 107cm3g-1 s-1 “The simplest possible parameterization of the SFR is …Schmidt law…This at least has the merit of simplicity and of taking into account the necessity of having gas as the raw material from which stars are formed, and will be used widely here as it has been elsewhere, while always bearing in mind that the coefficient may vary with ambient conditions or stochastically. A number of the results of CGE theory are insensitive to the SFR, while others are affected by many other difficulties anyway.” B.Pagel 1997 Indicators of the SFR • Counts of luminous supergiants in nearby galaxies under the assumption that their number is proportional to the SFR. • The H flux from HII regions, which are ionized by young and hot stars, under the assumption that such flux is proportional to the SFR • The integrated UBV colors and spectra of galaxies (one can estimate the relative proportions of young and old stars and derive the ratio between the present time SFR and the average SFR in the past) • The frequency of type II SNe as well as the distribution of SN remnants and pulsars can be used as tracers of the SFR. These tracers have been used for deriving the SFR in the Galactic disk • The radio emission from HII regions can also be a tracer of the SFR • The UV continuum and the IR continuum (star forming regions are surrounded by dust) are also connected to the SFR • From the distribution of molecular clouds SFR calibration (an example) Hopkins et al.2001 LFIR ~ Mg Firmani, Tutukov 1994 SFR in solar vicinities 2 - 10 Mpc -2 Gyr-1 Timmes et al. 1995 Initial Mass Function (m) dN / dm (m) dN / d log m M max m (m)dm 1 M min (m) bm (1 x ) x = 1.35 (Salpeter 1955) Present day mass function and IMF IMF PDMF T (t )dt T t ms There are theories explaining Initial Mass Function though no unique answer • Fragmentation of protostellar clouds (PSO) • Accumulation of PSO fragments • Turbulence • Limiting mechanisms Computer simulation of supersonic turbulence in starforming molecular clouds Boldyrev et al 2001 clumps Mass spectrum of 59 pre-stellar fragments extracted from the 1.3mm Oph mosaic. The mass spectrum of clumps is more flat (Andre 1999) Initial Mass Function Mmin ~ 0.08 M Mmax ~ 100 M Questions on IMF • Is IMF constant in time? (Does IMF strongly depends on physical parameters: (t), T(t), did they change in star forming regions?) • Does IMF depend on chemical composition of parent gas? • Is Mmin and or Mmax “fixed”? • Do we really know the IMF for low mass end? IMF for massive stars (Massey and Meyer 2001 EAA) The dashed line is for a Salpeter exponent of ?2.35. The metallicities change by a factor of 4 between these three systems! x=1 Local Initial Mass Function ( normalised to a total number of stars ever born of 37 Mpc -2 (Kroupa et al. 1990, Scalo 1986) My philosophy of the IMF If there is no dominant physical process - white noise (x = 1). If some process is dominant - it is responsible for IMF slope. We are still not able to separate this process. 4.3. Stellar evolution SE is “responsible” for most important ingredients of any model of galactic evolution (except pure stellar dynamics) In any such a model one needs to have estimates of tM is the lifetime of a star of (initial) mass M MR is the remnant mass Pi is the i-th element production in a star SE gives also theoretical estimates of galactic luminosity (more general - photometric parameters of galaxies), Stars (with some exceptions) do not change the surface composition during their evolution. Thus, surface abundances reflect the interstellar medium (ISM, out of which the stars formed) at the time of their formation. Therefore, observations of the surface composition (via spectra) of stars over a variety of ages and metallicities give a clue to gas abundances throughout the evolution of our Galaxy. Sp Type Teff Luminosity What is a star? Radius Outline of Stellar Evolution HR diagram for early stellar evolution HR diagram for late stellar evolution Base information of the star evolution Initial star mass, M Life time, yr Evolution product Mass of product, M Radius of product, km 1 ~ 50 - 150 ~ 2 - 4106 black hole ~ 10 ~ 30 2 ~ 11 - 50 ~ 4 - 20106 neutron star ~ 1.4 ~ 10 3 ~ 0.8 - 11 ~ 0.2 - 100108 degenerate dwarf ~ 0.2 - 1.3 ~ 104 Рис 5.15 из Pagel Evolutionary tracks Life time t ms 1010 yrs ( M / M )3 Masses of remnants MR = 0.11m +0.45M m<6.8M MR = 1.5M m6.8M Iben&Tutukov 1984 Fig Hoek Stellar yields Theory of stellar evolution is an example of successful construction of might and necessary theoretical instrument for studying galactic evolution. For last decades the fundamentals of the theory were established though up to now serious problems remain: • Effects of mixing in stars and mass loss • Nuclear reaction rate (e.g. 12C(, )16O • Evolution of close binaries • Chemically dependent bifurcation points in the course of stellar evolution (e.g. formation of BH is sensitive to metallicity of massive star) • Stellar yields are corner stones of the CGE models. Abundances in the Solar System from solar spectra and metorite samples. The unit scaled to a Si-abundance of 106 (Grevesse&Sauval 1998) Origin of elements Big Bang Nucleosynthesis H, D, He, Li, Be, B Stellar Nucleosynthesis Main sequence stars Giant star cores Red giant outer shells He C, O, Mg, Si, etc. N, Na, Al Explosive Nucleosynthesis O, S, Si, iron peak elements (supernovae) and heavier Exotic Nucleosynthesis (spallation) Li, Be, B Hoek (1997) Yields in SN (Tsujimoto 1993) Mixing In most models the mixing is assumed to be instantaneous and chemical composition of gas in a given zone is considered to be uniform. This is probably true for evenly distributed late type stars – sources of some heavy elements but not for explosive events. For might starburst regions metals are partially mixed with the ISM and partially leak into the IGM. (High metallicity of distant quasars are explained by this?). However observation of galaxies with less massive starbursts demonstrate that products of bursts (even for blue compact galaxies) can retain in galaxies inside the supershells and will be dispersed over the IGM when Supershell will dissipate. Thus the mixing could take up to 100 Myr (Tenorio-Tagle 2000) Low metallicity stars as witnesses of SNII yields Recent promising trends in galactic evolution modeling might provide constraints on individual supernova models rather than only global properties of SNe II and SNe Ia. The reason for this possibility is the fact that there is no instantaneous mixing of ejecta with the interstellar medium, and therefore early phases of galactic evolution can present a connection between low metallicity star observations and a single supernova event. On average, each supernova pollutes a volume of the interstellar medium containing ~ 3-5104M (for references see e.g. Thielemann et al. 2002). Each volume of the interstellar medium containing ~ 3-5104M needs to be enriched by ~ 103 SNe in order to obtain solar metallicities. Comparision of observation of low metallicity objects (squares) with metallicities of SNII ejecta mixed with 5104M of primordial ISM (circles) and galactic evolution model with non insant mixing (dots for model stars) Argast et al. 2001 4.4. Inflow and outflow Important role of inflow of matter into galaxies is commonly recognized. Outflow also is becoming recognized as very important factor in CGE. Inflow • HVC and flows • Accretion (+dynamic friction) Outflow • Galactic wind • Expelling of dust • Dynamic stripping Outflow+inflow+redistributing gas • Interacting galaxies Gas Inflow in the Galaxy Wakker et al. (2000) Dynamic friction Dissipation zone Dust Expelling from Galaxy Shustov&Wiebe 1995 Dust loss rate dependence on a disk galaxy mass Wiebe 1995 Galactic Fountain Shustov 1989 Galaxy NGC 4388 Expels Huge Gas Cloud NGC 4388 is a member of the the Virgo Cluster. It is classified as an active galaxy. One hypothesis holds that the gas was stripped away as NGC 4388 made its way through the intergalactic medium of the Virgo Cluster. A competing hypothesis holds that the gas is all that remains of a smaller galaxy that was gravitationally deconstructed by the larger NGC 4388. (Suprime-Cam, Subaru Telescope, NAOJ, APOD) . Conclusion Plan of the lecture 1. Galaxies and their constituents 1.1. Galaxies as dominant structures in the Universe • Galactic zoo • MW is a representative galaxy 1.2. Galactic features • Baryonic structures of MW • Dark matter • Sources of energy • Most important processes 2. Why, what and how to model 2.1. Main targets 2.2. How to model - model classification 3. Mathematical formulation of chemo-dynamical evolution of galaxies 4. Physical and chemical ingredients of the models 4.1. Initial conditions 4.2. Star formation • Quick look • SFR: theory and observation • IMF: observations, theory • Philosophy of the IMF Plan of the lecture (cont.) 4.3. Stellar evolution • Brief conventional picture of evolution of single and binary stars • Life times • Luminosities and ionizing fluxes • Stellar yields • Masses of remnants 4.4. Inflow and outflow • HVC and flows • Accretion (+dynamic friction) • Galactic wind • Expelling of dust • Dynamic stripping • Interacting galaxies 5. Conclusion