Lecture 02: • The Milky Way • Galaxy classification • Overview of observational facts Further reading: SG ch.1, MBW ch.2 The Milky Way Rsun ∼8 kpc Disk scale heights z ∼ 15 pc ∼ sun hH 150 pc V ∼ 250 km/s hmol ∼ 60 pc hthin ∼ 200 pc hthick ∼ 1 kpc SG, fig. 1.8 Useful coordinate systems • Equatorial • Galactic • Galactocentric • «Local Standard of Rest» (LSR) Fig. 1.9, 1.10 in SG, ch. 1 We are ‘upside down’ in the MW! Image credit: S. Mikelbank MW angular momentum Central black hole • Measure orbital parameters of stars close to the center of the MW from time-domain distance/velocity measurements • Infer the mass of the center mass: 6 consistent with 4⋅10 MA • No significant concentration of stellar objects in the small volume constrained through orbital calculations • «evidence» of a supermassive compact object in the center of the MW • Supported by sobservation of strong radio emission from the very same position (SagA) • See also SG, problem 2.6 Measuring galactic rotation Differential rotation discovered through proper motions (Oort, 1927) Measuring galactic rotation Radial motions may further probe rotation across the MW = sin Using푉푟 the푉푉푉푉 sine훼 theorem− 푉0 , 푙 = sin 푉 푉0 푉푟 푅0 푙 − Differential rotation 푅 푅0 discovered through proper motions (Oort, 1927) Measuring galactic rotation = sin 푉 푉0 푉푟 푅0 푙 − 푅 푅0 • Evidence of spiral/bar structure • Measure of rotation curve at R<R0 through the tangent SG, fig. 2.20 point method «From the outside» What do we aim to learn from galaxies? • How and why they form and evolve • The role of their components (stars, gas, dust, DM) in their history and how they are related (i.e. link observed properties to physics) • To what extent galaxies are biased tracers of matter distribution, since we basically observe only baryons in an underlying distribution of (mostly dark) matter. • the origin of extreme regularity of various parameters of the galaxy population • The link between galaxy formation and evolution to cosmology Galaxies and LSSs (clusters, superclusters) are unique astrophysical laboratories to investigate the processes related to the cycle of baryons in the universe. Galaxy classification Hubble Tuning Fork diagram (Hubble 1936) Spiral Galaxies •Disk + spiral arms + bulge (usually) •Subtype a b c defined by 3 criteria: •Bulge/disk luminosity ratio •Sa: B/D>1 Sc: B/D<0.2 •Spiral pitch angle •Sa: tightly wound arms Sc: loosely wound arms •Degree of resolution into knots, HII regions, etc. Barred Spiral Galaxies •Contain a linear feature of nearly uniform brightness centered on nucleus •Subclasses follow those of spirals with subtypes a b and c Elliptical Galaxies •Smooth structure and symmetric, elliptical contours •Subtype E0 - E7 defined by flattening •En where n = 10(a-b)/a where a and b are the projected major and minor axes (doesn’t tell what the 3-D shape is) Lenticulars or S0 Galaxies •Smooth, central brightness concentration (bulge similar to E) surrounded by a large region of less steeply declining brightness (similar to a disk) •No spiral arm structure •Originally thought to be transition objects between Sa and E but typical S0 is 1-2 mags fainter than typical Sa, E (van den Bergh 1998) Irregular Galaxies NGC 4485-Irr II M82-Irr II Irr I •No morphological symmetry •Lots of young, blue stars and interstellar material •Smaller than most spirals and elliptical galaxies •Two major subtypes: •Irr I: spiral-like but without defined arms, show bright knots with O,B stars •Irr II: asymmetrical with dust lanes and gas filaments (e.g. M82) - explosive General trends within Hubble sequence from E to Sc: • Decreasing LBulge/LDisk • Decreasing stellar age • Increasing fractional gas content • Increasing ongoing star formation Limitations of the Hubble Classification Scheme 1. Only includes massive galaxies (doesnt include dwarf spheroidals, dwarf irregulars, blue compact dwarfs) 2. Three different parameters for classifying spirals is unsatisfactory because the parameters are not perfectly correlated. 3. Bars are not all-or-nothing. There is a continuum of bar strengths. de Vaucouleurs’ Revised Hubble Classification System (de Vaucouleurs 1958, Handbuch der Phys. 53, 275) (de Vaucouleurs2 1964, Reference Catalog of Bright Galaxies) Basic idea: retain Hubble system, but add lots of optional bells and whistles •Mixed types: E/S0, Sab, Sbc •Mixed barred/normal: SA (unbarred), SB (barred), SAB (in between) •Inner rings: S(s) (arms out of ring), S(r) (arms in ring), S(rs) •Outer rings: (R) S •Extended spiral, irr types: Sm (between spiral and Irr), Im (magellanic), Sd (extreme Sc), Sdm (between Sd and Im) •“t-types” scale Added in later editions of the Reference Catalog (de Vaucouleurs2, Corwin 1976) E0 → S0 → Sa → Sb → Sc → Im -5 -1 1 3 5 10 (t-type) Schematic Diagram of Revised Hubble Classification E E+ S0- S0 S0+ Sa Sb Sc Sd Sm Im Cross section of diagram No Bar Ring Spiral shaped shaped Bar Schematic Diagram of Revised Hubble Classification Cross section of diagram No Bar Ring Spiral shaped shaped Limitations: •E --- Im is not a linear sequence of one parameter •Rings and bars are not independent •Does not take into consideration mass or other important parameters. All based on optical surface brightness morphology. Bar “ ” Luminosity Classification or DDO System who was at David Dunlop Observatory in Ontario, “ ” van den Bergh (1960) - Canada - hence the DDO In spirals and irregular galaxies, some properties correlate with galaxy mass rather than type. For spirals, the key parameter is arm development (i.e. arm length, continuity and width relative to size) Sc I - long, well-developed arms Sc III - short, stubby arms Sc IV - dwarf, spiral galaxy -faint hint of spiral structure Revised DDO - van den Bergh (1976): Placed disk galaxies into 3 parallel classes based on luminosity: Gas-rich, anemics and lenticulars Anemics have weak and diffuse spiral arms and low level of ongoing SF Parameters which change systematically from Lenticular to Gas-rich •Mean stellar age •Gas fraction •Recent SF Van den Berg speculates that all disk galaxies are born as gas-rich spirals and gradually evolve to anemic ’ and finally S0 s. Yerkes System (Morgan 1958) Utilizes fact that there is a strong correlation between the nuclear light concentration (how big the bulge is) and its integrated spectrum. Type is based on this one parameter - integrated spectral type. •E, S0 K-type spectrum •S F-K stars dominate •Irr A stars dominate Nomenclature: g S 2 Spectral type (dominant stars) Hubble type flattening (I.e. bulge/disk) E - elliptical 10(a-b) a, af, f, fg, g, gk, k D - S0 a S - spiral B - barred I - Irregular R -rotationally symmetric but no S or E structure Galaxy classes not addressed in these classification systems Dwarf Ellipticals - dE - Dwarf Spheroidals - dSph - Dwarf Spirals – dS - Dwarf Irregulars – dI - Morphological Distributions The range and frequency of different morphological types is sensitive to the sample studied. Some key results: •The Local Group is the only sample that includes a significant number of very faint galaxies. Of the 35 galaxies now considered members of the Local Group, only the 3 brightest (M31, MW and M33) are spirals, the remainder are equally divided between irregular and dwarf elliptical /spheroidal galaxies (Hodge 1995). •Magnitude-limited sample of galaxies outside of clusters (in the “field”) are biased towards late-type (Sc) spirals. A typical field sample might consist of 80% S galaxies, 10% S0 galaxies, and 10% E galaxies. •Within rich clusters, the population of bright galaxies is dominated by early-type systems (Dressler 1980). An intermediate density cluster will have 40% S galaxies, 40% S0 galaxies, and 20% E galaxies. A high density cluster will have 10% S, 50% S0, and 40% E. Automated Classification Visual classification is inherently time consuming and different observers are unlikely to agree in ambiguous cases. This motivates the development of algorithms to automatically and impartially classify galaxy images - very important for large surveys like 2MASS and SDSS. Abraham et al. (1994, 1996): Concentration parameter C - fraction of light within ellipsoidal radius σ 0.3 x outer isophotal radius (1.5 above sky level). Asymmetry parameter A - fraction of light in features not symmetric wrt a 180 degree rotation Naim, Ratnatunga & Griffiths (1997) use 4 parameters: blobbiness, asymmetry, filling factor and elongation. Naim et al. (1995) used artificial neural nets to classify galaxies into the numerical T types. Achieved uncertainty of +/- 1.8 in T which is comparable to the dispersion between observers. For distant galaxies (greater than z=0.5), classification is difficult because of “ ” small angular size and apparent faintness of galaxies. HST field galaxies (z~1) classified by 2 humans (Ellis and van den Bergh) and A and C parameters of Abraham. For faint galaxies (I>21mag), C parameter alone is fairly good. For brighter galaxies, C is degenerate between E and S0. Pay attention to the band…. End of today’s lecture (additional slides follow) Overview of observational facts (additional slides – use them as a track for further reading) LOTS of them out there 50/sq. arcmin/0.5 mag @m=25.5, rising to 175@m=29 http://www.sdss.org From R. Mushotzky’s lecture When and where did they form? • Farthest object as of Nov. 15th, 2012: z=11 (about 400Myr after BB), observed thanks to lensing magnification from galaxy cluster MACS0647-JD • But things have changed somehow since then! Present galaxy pattern established at z=1 When and where did they form? • Baryons are (biased) tracers of the cosmic web: galaxies assemble in the deep gravitational wells produced by underlying dark matter distribution Snapshots from the Millennium Simulation: http://www.mpa-garching.mpg.de/galform/virgo/millennium/ When and where did they form? • Baryons are (biased) tracers of the cosmic web: galaxies assemble in the deep gravitational wells produced by underlying dark matter distribution Snapshots from the Millennium Simulation: http://www.mpa-garching.mpg.de/galform/virgo/millennium/ Galaxy regularity Present day galaxy population exhibits a remarkably small range of masses, sizes, ages of stellar pops., shapes, and all correlated.