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Evolution of galaxy morphology Lecture 1 Yogesh Wadadekar July 2014 ncralogo Galaxies Workhop 1 / 81 Galaxies are a very active field of research today! About 39% of papers published on Arxiv:astro-ph in 2013 were on Cosmology and Extragalactic astrophysics - 5510 papers in 2013 alone. Most of the content in my lectures reached its present form in the last 15 years. ncralogo Galaxies Workhop 2 / 81 My lectures will cover 1 A brief History of extragalactic research 2 Morphological Classification of galaxies 3 Quantitative morphology 4 Evolution of galaxy morphology primarily with time and environment ncralogo Galaxies Workhop 3 / 81 Study material I am putting in significant effort to make good slides for my talks. Use these as your primary reference material and remember to stop me and ask questions if you don’t understand something. Some of the slides will cite papers which you are encouraged to read to know more. A number of excellent online courses are available. I will use material from these as appropriate. I particularly like the Galaxies and Cosmology course by S. G. Djorgovski on Coursera. A similar course is also available on edX. For textbooks, I recommend Schneider’s Extragalactic Astronomy and Cosmology and Galactic Astronomy by Binney and Merrifield. ncralogo Galaxies Workhop 4 / 81 The big picture ncralogo Galaxies Workhop 5 / 81 Three ubiqitous basic structures in the Universe Atoms Stars Galaxies ncralogo Galaxies Workhop 6 / 81 Kant’s “Island Universes” In 1755 he wrote a book called General Natural History & Theory of the Heavens in which he said: 1 MW like a huge solar system, rotating; origin from rotating cloud. 2 stars far from disk plane on different orbits 3 disks (like MW) project to ellipses 4 oval nebulae (seen by de Maupertius) = “Island Universe” ncralogo Galaxies Workhop 7 / 81 Major milestones since Kant 1755: Kant’s Island Universes 1912: Slipher discovered that nebulae are rotating 1920: Shapley-Curtis debate “Are Spiral Nebulae Island Universes” - 30 minute presentations by each - Shapley “won” the debate, although Curtis was right. 1925: Hubble discovered Cepheids in the Andromeda galaxy Extragalactic astrophysics < 100 years old! ncralogo Galaxies Workhop 8 / 81 What are galaxies? gravitationally bound agglomerations of stars, dust, gas, dark matter. Mass ratio Gas:Stars:Dark Matter - 1:10:100 they are the basic building blocks of the Universe on large scales they show a broad range in their physical properties Understanding of galaxy formation and evolution is one of the main outstanding problems in modern cosmology there are ∼ 1011 galaxies in the observable universe typical total mass of 108 − 1012 M ncralogo Galaxies Workhop 9 / 81 Counting and cataloging galaxies In late 1700s,Messier made a catalog of 109 nebulae so that comet hunters wouldn’t mistake them for comets! About 40 of these were galaxies. NGC New General Catalogue (Dreyer 1888) had 7840 objects, of which about 50% were galaxies. In the 20th century, many catalogs were produced UGC, RC3 etc. Nowadays we have automated surveys, e.g. Sloan Digital Sky Survey, with tens to hundreds of millions of galaxies ncralogo Galaxies Workhop 10 / 81 First step census, second step classification third step understanding are common steps in any empirical science. Hubble proposed a scheme for classifying galaxies (the ”tuning fork” diagram) in his 1936 book, The Realm of the Nebulae. This scheme survives in its essence to the present day. ellipticals, lenticulars, spirals and irregulars are the main types. A better approach may be to look at the properties of subsystems within galaxies (e.g., disks, spheroids, halos, etc.), and deduce their origins and evolution ncralogo Galaxies Workhop 11 / 81 Why is galaxy classification useful? Classes bring order to diversity of galaxy forms Span/include majority of galaxies Unambiguous & easily identified criteria Relate to important physical properties → provide insight into internal processes, formation, & evolution ncralogo Galaxies Workhop 12 / 81 Why classify galaxies in the optical band? Historically, optical photometry was the method used to observe galaxies. Hence Hubble’s classification is most widely used. Today, many other criteria such as color indices, spectroscopic parameters (based on emission or absorption lines), the broad-band spectral energy distribution (galaxies with/without radio- and/or X-ray emission) are also used to group similar galaxies together. ncralogo Galaxies Workhop 13 / 81 Hubble tuning fork diagram ncralogo Galaxies Workhop 14 / 81 Elliptical Galaxies About 20% of field galaxies are Ellipticals, but most E’s are in clusters There are a number of different subtypes:E’s (normal ellipticals), cD’s (massive bright ellipticals at the centers of galaxy clusters), dE’s (dwarf ellipticals) dSph’s (dwarf spheroidals) Smooth and almost featureless. no spiral arms or dust lanes. Generally lacking in cool gas, and hence few young blue stars Elliptical galaxies are denoted En where:b/a = 1 − n/10 i.e. an E4 galaxy has an axis ratio of b/a = 0.6, and E0’s have circular isophotes. ncralogo Galaxies Workhop 15 / 81 Elliptical ncralogo Galaxies Workhop 16 / 81 Lenticular are characterised by the presence of a central bulge and disk and the absence of spiral arms i.e. little or no ongoing star formation intermediate in many of their properties between ellipticals and spirals. ncralogo Galaxies Workhop 17 / 81 Lenticular ncralogo Galaxies Workhop 18 / 81 Spiral Named for their bright spiral arms, which are prominent due either to bright O and B stars (evidence for recent star formation), or due to dust lanes. ncralogo Galaxies Workhop 19 / 81 Grand Design Spiral ncralogo What is the diameter of the Milky Way Galaxy? Galaxies Workhop 20 / 81 M/L ratio for stars For main sequence stars, we have L ∝ M 3.5 , giving : (M/L) ∝ M −2.5 ∝ L−0.71 showing, as one expects, later spectral types have higher M/L. eg K stars : M ∼ 0.5M → M/L ∼ 10; A stars : M ∼ 2.0M → M/L ∼ 0.1 ncralogo Galaxies Workhop 21 / 81 Mass to light ratio M/L of a galaxy M/L defined in units of M /L For galaxies, M/L reflects the average M/L over the population Pop I (young) : massive stars dominate light; low mass stars dominate mass Pop II (old) : giants dominate light; M.S. stars dominate mass Typical galaxy (& solar neighborhood) has M/LV 6, M/LB ∼ 10 In general : M/L increases with age and metallicity Maximum range : 2 < M/LB < 20. What are Pop III stars? ncralogo Galaxies Workhop 22 / 81 Initial Mass Function IMF The IMF ξ(M) specifies the distribution in mass of a newly formed stellar population and it is frequently assumed to be a simple power law: ξ(M) = c M −(1+x) . In general, ξ(M) is assumed to extend from a lower (M1 ) to an upper cutoff (M2 ). ncralogo Galaxies Workhop 23 / 81 Initial Mass Function IMF Salpeter Scalo Miller & Scalo M1 0.10 0.10 0.18 0.42 0.62 1.18 3.50 0.10 1.00 2.00 10.0 M2 125. 0.18 0.42 0.62 1.18 3.50 125. 1.00 2.00 10.0 125. x 1.35 -2.60 0.01 1.75 1.08 2.50 1.63 0.25 1.00 1.30 2.30 ncralogo Galaxies Workhop 24 / 81 Question For a Salpeter IMF, what is the relative number of K and A stars in a cluster that has just formed? ncralogo Galaxies Workhop 25 / 81 M100 nucleus ncralogo Galaxies Workhop 26 / 81 Barred galaxies Half of all disk galaxies - Milky Way included - show a central bar which contains up to 1/3 of the total light Bars are a form of dynamical instability in differentially rotating stellar disks S0 galaxies also have bars – a bar can persist in the absence of gas Bar patterns are not static, they rotate with a pattern speed, but unlike spiral arms they are not density waves. Stars in the bar stay in the bar The asymmetric gravitational forces of a disk allow gas to lose angular momentum (via shocks) compressing the gas along the edge of the bar. The gas loses energy (dissipation) and moves closer to the center of the galaxy ncralogo Galaxies Workhop 27 / 81 Barred Spiral ncralogo Galaxies Workhop 28 / 81 Lopsided galaxy - unstable disks ncralogo Galaxies Workhop 29 / 81 Irregular ncralogo Galaxies Workhop 30 / 81 Large Megallanic Cloud ncralogo Galaxies Workhop 31 / 81 Nomenclature: Early and late type Objects along the sequence are often referred to as being either an early-type or a late-type. Ellipticals and S0 galaxies are collectively called and early-type and spirals are called late-type. Within spirals, an Sa galaxy is called an early-type spiral, and an Sc galaxy a late-type spiral. This nomenclature is not a statement of the evolutionary stage of the objects but is merely a nomenclature of purely historical origin. ncralogo Galaxies Workhop 32 / 81 Galaxy classification affected by projection Morphological classification is at least partially affected by projection effects. If, for instance, the spatial shape of an elliptical galaxy is a triaxial ellipsoid, then the observed ellipticity will depend on its orientation with respect to the line-of-sight. Also, it will be difficult to identify a bar in a spiral that is observed from its side (“edge-on”). Similarly a weak disk in an “face-on S0” is hard to spot. ncralogo Galaxies Workhop 33 / 81 ∼30% of galaxies at z ∼ 1 are peculiar First, distant galaxies. ncralogo Dwarf galaxies are also not included. Galaxies Workhop 34 / 81 Dwarf galaxies Low-luminosity: 106 − 1010 L , low-mass: 107 − 1010 M , small in size, ∼few kpc, dark matter dominated Often low surface brightness, so they are hard to find! More than one family of objects: Gas-poor, passive (dE and dSph) Gas-rich, star forming Why are dwarf galaxies important? Majority of galaxies are dwarfs! Dwarf galaxies may be remnants of galaxy formation process: “proto-dwarf” gas clouds came together to form larger galaxies (hierarchical formation) Dwarf galaxies are currently being cannibalized by larger galaxies Dwarf galaxies are relatively simple systems, not merger products: in some sense, “pristine” low metallicity galaxies ncralogo Galaxies Workhop 35 / 81 I Zwicky 18 ncralogo Galaxies Workhop 36 / 81 Questions If you go look at the night sky most of the stars look white or blue with a few red ones which are all red giants. But the IMF tells us that most stars should be red looking M-dwarfs or G and K type dwarfs? Why are these common stars extremely uncommon in the night sky? ncralogo Galaxies Workhop 37 / 81 Mergers can alter morphology ncralogo Galaxies Workhop 38 / 81 The milky way - Andromeda galaxy merger Galaxies in the process of transformation, generally from disks to ellipticals In late stages of a merger, the 2 galaxies are no longer distinguishable. What does the merger product look like? Show movie ncralogo Galaxies Workhop 39 / 81 Other limitations of the Hubble classification Based on photographic images in the blue emphasises star formation (not mass distribution) High z galaxies sample the rest frame UV. Requires reasonably good spatial resolution across the galaxy ( 20 elements) progressively more difficult for cz > 8000 km/s from ground. To summarise, three kinds of galaxies don’t fit into the Hubble scheme: (1) Disturbed or interacting galaxies (2) Galaxies at high-z and (3) Low Surface Brightness (LSB) galaxies - almost always dwarf galaxies. ncralogo Galaxies Workhop 40 / 81 Modifications to the Hubble Sequence e.g. by van den Bergh 1976 ncralogo Galaxies Workhop 41 / 81 Kormendy’s version of the Hubble sequence ncralogo Galaxies Workhop 42 / 81 Why do we need quantitative morphological classification? Modern CCD imaging surveys generate vast numbers of galaxy images There is a need for fast, objective, robust classification. ncralogo Galaxies Workhop 43 / 81 Quantitative morphology: bulge-disk decomposition ncralogo Wadadekar et al. (1999) Galaxies Workhop 44 / 81 Connection between star formation history and morphology ncralogo Galaxy morphology, star formation history, stellar mass Galaxies and projected Workhop 45 / 81 Quantitative morphology: bulge-disk decomposition ncralogo Wadadekar et al. (1999) Galaxies Workhop 46 / 81 χ2 minimisation χ2 = X (Ii − I0 )2 σi2 where Ii , I0 and σi are the observed flux, model flux and the error in Ii respectively. The error depends on the signal to noise ratio of the data. Exactly how this is to be computed should be covered in your Astronomical Techniques I course. ncralogo Galaxies Workhop 47 / 81 Analytics model for the disk Exponential Disk: Idisk (x, y ) = Is e−rdisk /rs , q rdisk = x 2 + y 2 /(1 − ed )2 . ed = 1 − cos(i), rdisk is the galactocentric radius [putting it more accurately, the length of the semi-major axis of the (elliptical!) isophote], rd the length scale of the disk and IS refers to intensity at the centre. ncralogo Galaxies Workhop 48 / 81 Inner and outer disk - double exponentials It is possible to fit two different disks (inner and outer disks) that share the same position angle and ellipticity, but have different central brightness and length scales. This ability tends to become very useful since galaxies with double exponential disks are being now frequently found with ever deeper and finer images. ncralogo Galaxies Workhop 49 / 81 Analytic model for the bulge Sérsic Bulge: 1/n Ibulge (x, y ) = Ie e−bn [(rbulge /re ) −1] , q rbulge = x 2 + y 2 /(1 − eb )2 , Where e refers to effective values and n is the Sérsic index. For n=4 it becomes the de Vaucouleurs function; for n=1 an exponential, and when n=0.5, a Gaussian! For values in the range 1-4, approximately, it describes bulges in late-type spiral galaxies (or pseudo-bulges) to bulges in early-type spirals (or classical bulges) and elliptical galaxies. It is easy to realize that the larger the value of n the more concentrated is the light (and mass!) of the bulge (or elliptical) in the center. The effective radius of a galaxy is the one that contains half of the total light emitted by the galaxy. The numerical constants bn is chosen so that the brightness at the effective radius is the effective brightness,ncralogo and depends only on n. Galaxies Workhop 50 / 81 The Sérsic profile ncralogo Galaxies Workhop 51 / 81 Other equivalent forms of the Sérsic function Ibulge (x, y ) = I0 10−cn [(rbulge /re ) 1/n ] 1/n −1] Ibulge (x, y ) = Ie 10−dn [(rbulge /re ) ncralogo Galaxies Workhop 52 / 81