An introduction to Galaxies

The World of Galaxies Spirals barred unbarred Ellipticals Irregulars

Gas cooling Initial Density Field Baryonic Matter protogalaxy Standard CDM Cosmology Non-baryonic Dark Matter Halos Initial Mass Initial Mass Initial Density Initial Density Angular Momentum Angular Momentum Dark Matter halo Dark Matter halo Primordial Chemical Primordial Chemical Composition Composition First Galaxies Star Formation Clump Mergers Dark Matter Halos Condensation on to DM halos

Galaxy formation through consecutive mergers of gaseous halos or from the current picture of the universe itself. Super-computer simulation of evolution of young small galaxies into larger galaxies in pronounced clusters. See also colliding galaxies Sci. Am. Aug 90.

Galaxy Evolution Total Mass (t) Z(t) Star Formation Gas fraction Morphology What changes in a galaxy as a function of time What causes these changes in a galaxy in a galaxy Galaxy Interactions Tides and mergers Stellar Evolution Luminosity Total, L(λ ) Dust content

Fundamental questions Properties of proto-galaxy : Properties of proto-galaxy : initial mass, initial density, initial angular momentum, dark matter initial mass, initial density, initial angular momentum, dark matter Initial epoch of astration Initial epoch of astration Does a system with similar initial Does a system with similar initial characteristics evolve very differently characteristics evolve very differently in different environments? in different environments? How is Star Formation triggered, propagated and regulated in different types of galaxies/environments How is Star Formation triggered, propagated and regulated in different types of galaxies/environments How do the various different morphological types of galaxies arise? How do the various different morphological types of galaxies arise?

Star Formation : Defining the problem Parameters that define the characteristics of SF Efficiency of star formation (SFE) Efficiency of star formation (SFE) Rate of star formation (SFR) Rate of star formation (SFR) Initial mass function (IMF) Initial mass function (IMF) Fundamental Theory of Star Formation (??): Determination of the above parameters from physics and chemistry of the interstellar medium physics and chemistry of the interstellar medium galactic dynamics galactic dynamics Densities of molecular species Ionization Dust extinction (grain composition, size …) Magnetic field strength Dust and gas temperature Turbulent velocity field Angular momentum distribution Galactic Differential Rotation Spiral density waves Tidal torques Satellite accretion SFR (M o /yr) SFE (%/100Myr) SFR (M o /yr) SFE (%/100Myr) E formation ~300 30% Disk formation ~ 10 1% Star Bursts ~ % Log-normal with peak at ~0.1 M o Does it change with time/environment?

The role of galactic interactions increases with look-back time (or redshift) Colliding Galaxies at high z – HST deep field

Galactic Interactions in the Local Universe NGC 6745 Antennae NGC Cartwheel Evidence for triggering of Star Formation Evidence for triggering of Star Formation Morphological Disturbances Morphological Disturbances

Study of Galactic Evolution Observe galaxies at different redshifts Observe stellar populations of different ages in nearby galaxies «snapshots» «snapshots» (danger of peaking exotic objects need to map need to map redshift - lookback time redshift - lookback time yield statistical information yield statistical information on morphological and on morphological and size evolution size evolution detailed star formation detailed star formation history for individual galaxies history for individual galaxies of different current sizes and of different current sizes and morphologies morphologies lookback time known lookback time known yield very little information on yield very little information on morphological and size evolution morphological and size evolution

Topography ofthe Local Group of Galaxies Topography of the Local Group of Galaxies 35 galaxies Size 1.2 Mpc

Gallery of Local Group Galaxies: Spirals M33 M31 Milky Way Range of Masses: 10 Million to 1 trillion solar masses

Sites of Current Star Formation in the Milky Way

NGC 3603 : Massive star formation site in Milky Way Trifid Nebula: Star Formation site in Milky Way

Gallery of Local Group Galaxies: Irregulars LMC SMC NGC 6822 Most Irrs are gas rich Most show current star Most show current star formation at a significant level formation at a significant level Most dwarf Irrs are remote Most dwarf Irrs are remote objects objects IC 5152

Gallery of Local Group Galaxies: Dwarf ellipticals and spheroidals Dwarf Spheroidal Leo I Dwarf Elliptical (nucleated) NGC 205 The least massive members of the LG (5 orders of magnitude less massive than the MW) The least massive members of the LG (5 orders of magnitude less massive than the MW) Most of the LG galaxies belong to this category. Most of the LG galaxies belong to this category. Most are close to the giant spirals Most are close to the giant spirals Most contain little or no gas Most contain little or no gas Evidence of dark matter halos (?) Evidence of dark matter halos (?) The least massive members of the LG (5 orders of magnitude less massive than the MW) The least massive members of the LG (5 orders of magnitude less massive than the MW) Most of the LG galaxies belong to this category. Most of the LG galaxies belong to this category. Most are close to the giant spirals Most are close to the giant spirals Most contain little or no gas Most contain little or no gas Evidence of dark matter halos (?) Evidence of dark matter halos (?)

Sagittarius dwarf Irregular galaxy: an instance of galactic cannibalism The closest dwarf galaxy to the Milky Way (16 kpc) The closest dwarf galaxy to the Milky Way (16 kpc) Tidally distorted – merging into the MW Tidally distorted – merging into the MW Its survival up to now is consistent with a flat Its survival up to now is consistent with a flat rather than a centrally concentrated dark matter halo rather than a centrally concentrated dark matter halo The closest dwarf galaxy to the Milky Way (16 kpc) The closest dwarf galaxy to the Milky Way (16 kpc) Tidally distorted – merging into the MW Tidally distorted – merging into the MW Its survival up to now is consistent with a flat Its survival up to now is consistent with a flat rather than a centrally concentrated dark matter halo rather than a centrally concentrated dark matter halo

Morphological segregation in Local Group Most dwarf spheroidals/ ellipticals are located close to giant spiral Gas-loss ?

The luminosity function of the Local Group of Galaxies The Missing Dwarf Problem : too few by a factor of 10! (according to hierarchical CDM models) (according to hierarchical CDM models)

For each LG galaxy we want to derive a three dimensional picture showing the SFR and chemical enrichment as a function of time

Approximate age-indicators

The observed Hertzsprung-Russell Diagram of an old coeval stellar population Measure of stellar luminosity Measure of surface effective temperature MS turnoff AGE AGE Red Giant Branch Metallicity +age Metallicity +age RR-Lyrae variables Distance (+Z) Distance (+Z)

How the population synthesis modeling works Initial mass function Stellar evolution theory: isochrones Assumed SFR(t) (Assumed?) Chemical evolution Synthetic color- magnitude diagram +errors

How the population synthesis modeling works Models for different coeval populations populations Observed diagram: areas of number comparisons Final Adopted model

The effect of distance on the c-m diagram

The simpler systems: Star Formation History of dwarf spheroidal galaxy Leo I Adopted model color- magnitude diagram Observed Color magnitude diagram SFR(t)

Carina Dwarf Spheroidal What caused this uniquely erratic star formation activity in Carina? What caused this uniquely erratic star formation activity in Carina? Age (Gyr) Burst strength 1573

Star Formation History Boxes for 15 Local Group Dwarf Spheroidal Galaxies X-axis: t(Gyr) Y-Axis: SFR Z-Axis: [Fe/H]

Star Formation History Boxes for 15 Local Group Irregular Galaxies X-axis: t(Gyr) Y-Axis: SFR Z-Axis: [Fe/H] Distances from LG barycenter

Direct Evidence of Environmental Influence to Galactic Evolution in the Local Group Morphological type segregation Morphological type segregation Tidal tails and bridges found in Tidal tails and bridges found in - Magellanic System - Magellanic System - Saggitarius - Saggitarius - Carina dsph - Carina dsph Possible merger : Saggitarius Possible merger : Saggitarius Interactions – triggered Star Formation: Interactions – triggered Star Formation: - Magellanic Bridge - Magellanic Bridge - Burst of star cluster formation in LMC - Burst of star cluster formation in LMC 2-3 Gyr ago 2-3 Gyr ago Morphological type segregation Morphological type segregation Tidal tails and bridges found in Tidal tails and bridges found in - Magellanic System - Magellanic System - Saggitarius - Saggitarius - Carina dsph - Carina dsph Possible merger : Saggitarius Possible merger : Saggitarius Interactions – triggered Star Formation: Interactions – triggered Star Formation: - Magellanic Bridge - Magellanic Bridge - Burst of star cluster formation in LMC - Burst of star cluster formation in LMC 2-3 Gyr ago 2-3 Gyr ago

Dark Matter in Local Group Galaxies Galaxy Type DM M/L Type of evidence ( M o / L o ) ( M o / L o ) Spirals - disks yes 1-3 HI rotation curves Spirals - bulges yes stellar kinematics Spirals – halos yes >20 stellar kinematics (microlensing) (microlensing) Dwarfs yes 1-80 (?) stellar kinemtics survival of Sgr survival of Sgr ?problem with tides ?problem with tides Compact HVC yes 10-50(?) HI rotation curves

STAR FORMATION HISTORIES OF LG DWARFS SHOW HUGE VARIETY Stochasticity of SF in low mass galaxies (no central control) Stochasticity of SF in low mass galaxies (no central control) highly variable SFR highly variable SFR changes in gas content which can result from: changes in gas content which can result from: mass (and metal) loss via galactic winds mass (and metal) loss via galactic winds gas accretion or gas loss via galactic interactions gas accretion or gas loss via galactic interactions gas infall from outer regions of galaxy gas infall from outer regions of galaxy changes in internal dynamics (due to interactions, Bar formation) - mergers changes in internal dynamics (due to interactions, Bar formation) - mergers The problem of unknown orbits The problem of unknown orbits UV radiation from giant spiral can delay cooling and SF in close-by dwarf UV radiation from giant spiral can delay cooling and SF in close-by dwarf role of dark matter halos in modifying SF activity (helping retain escaping gas?) role of dark matter halos in modifying SF activity (helping retain escaping gas?) Possible factors

Galactic Chemical evolution: A function of galactic mass Total visible mass seems to be the decisive parameter that determines the overall chemical evolution of a galaxy

Epoch of highest Star Formation Rate: A function of galactic mass? Or of Environment? Total visible mass is an important parameter that determines the general characteristics of the Star Formation History of a galaxy!!

Closing remarks No two LG galaxies have identicalSF histories No two LG galaxies have identical SF histories The smaller a galaxy the more intermittent its SF history The smaller a galaxy the more intermittent its SF history Gas poor, low mass, old galaxies mostly near giant spirals Gas poor, low mass, old galaxies mostly near giant spirals Gas rich star forming galaxies mostly isolated Gas rich star forming galaxies mostly isolated Minor mergers and tidal interactions currently happening Minor mergers and tidal interactions currently happening Triggering of SF in tidal tails observed Triggering of SF in tidal tails observed total amount of current visible matter total amount of current visible matter seems to be an important parameter for seems to be an important parameter for the overall chemical evolution of a galaxy and the overall chemical evolution of a galaxy and the epoch when the SFR was highest in a particular galaxy the epoch when the SFR was highest in a particular galaxy Mass, interactions, dark matter content