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Galaxy Morphology Sudhanshu Barway South African Astronomical Observatory (SAAO) Cape Town [email protected] 23 August 2016 Plan of Talk • South African Astronomical Observatory (SAAO) • Galaxy Morphology • Galaxy Zoo South African Astronomical Observatory Modern History: dates from 1820 Royal Observatory Cape of Good Hope South African Astronomical Observatory Mission: to perform research in astronomy; to provide an international facility in Africa for such research; to educate and inform the community Total staff: 120 Scientists: Physics Astronomy Mathematics Engineers/Technicians Electronics Mechanical Optical Computer Science Educators HQ: Cape Town, Western Cape Observing Facilities: Sutherland, Northern Cape South African Astronomical Observatory Optical & NIR Observing Facility- • 1.9m telescope – still the largest (after SALT) SpCCD, HIPPO, GIRAFFE, STE4, STE3, UCT CCD, SHOC • 1.0m telescope - imaging STE4, STE3, UCT CCD, SHOC • 0.75m telescope – student training and new technologies (R&D) 1.0m robotic telescope (Installed) • 0.5m telescope– The MeerLICHT telescope for Radio-Optical Transients (under construction) • IRSF – 1.4m InfraRed telescope, Japanese and SA astronomers • Alan Cousin Telescope (ACT or APT) - (MP) Time allocation - week or weeks http://www.saao.ac.za/ South African Astronomical Observatory International telescopes - • BISON - Birmingham Solar Oscillation Network (UK) - Wide Angle Search for Planets (UK) - extra-solar • SuperWASP planet detection since 2005 (over a dozen discovered) (Gottingen, Germany) – extra-solar planet search, also • MONET for school projects (40%) • KELT - Kilodegree extremely Little Telescope (Vanderbilt, USA) • LCOGT (USA) – three 1.0meter telescopes 2013 and -2 (CAMK, Poland) – 0.5m robotic telescopes • SOLARIS-1 installed 2011 • KMTNet -1.6m Korean telescope - Part of a large project Global MASTER Robotic • MASTERS Network South African Astronomical Observatory SALT - Southern African Large Telescope http://www.salt.ac.za/ • SALT has fixed elevation angle (37°) • Rotate only about azimuth axis • 11 meter primary mirror • 91 hexagonal mirror segment of 1 meter diameter • Instruments - SALTICAM, RSS, HRS • a consortium of 13 partners • South Africa has ~30% time share Time allocation - seconds South African Astronomical Observatory Research Groups Stellar Astrophysics South Africa has had a long and rich history of Stellar astronomy. In 1833, the first ever measurement of the distance to a star (Alpha Centauri) was made. To continue tradition, group is working on- Transients e.g., novae and supernovae; interacting binaries e.g., cataclysmic variables, symbiotic and X-ray binaries; variable stars e.g., pulsating red giants, flare and R CrB stars; Exoplanets Planetary Astronomy Extragalactic Astronomy Cluster Astrophysics Instrumentation Galaxy Morphology Galaxy Morphology Discovering Galaxies • From the late 1700’s to1920 astronomers had noticed may spiral nebulae • It was not known that these nebulae were far away or nearby • Edwin Hubble (1924) • Discovered Cepheid variable stars in Andromeda Galaxy (M31) • Also in NGC6822 and M33 • Distances using Period-luminosity relation for Cepheid variables • Determine that M31 is a Galaxy - an “Island Universe” Galaxy Morphology Discovering Galaxies • In late 1700, Messier made a catalog of 109 nebulae so that comet hunters wouldn’t mistake them for comets! ~40 of these were galaxies • NGC New General Catalogue (Dreyer 1888) had 7840 objects, of which ~50% were galaxies • In the 20th century, many catalogs were produced RSA, UGC, RC3 etc. • Nowadays there are automated surveys, e.g. Sloan Digital Sky Survey (SDSS) with tens of hundreds of millions of galaxies Galaxy Zoo or Zoo Universe Galaxy Morphology What are galaxies? • • • • • • • Gravitationally bound system of stars, gas, dust, dark matter They are the basic building blocks of the Universe on the large scales There are ~1011 galaxies in observable universe Typical total mass of 108 - 1012 M⦿ Mass ratio => Gas:Stars:Dark Matter - 1:10:100 They show board range in their physical properties Understanding the galaxy formation and evolution is one of the main outstanding problems in modern cosmology. Galaxy Morphology Galaxy classification Hubble proposed a scheme for classifying galaxies (the “Tuning fork” diagram) in his 1936 book, The Realm of the Nebulae The 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 evolutions Galaxy Morphology Galaxy classification Galaxy Morphology Galaxy classification Why begin here? • Hubble classification serves as the basic language of the field. • The morphological sequence reflects a fundamental physical and evolutionary sequence, which offers important clues to galactic structure, formation and evolution. Galaxy Morphology Galaxy classification General trends within Hubble sequence E • Decreasing Bulge/Disk • Decreasing stellar age • Increasing fractional gas content • Increasing ongoing star formation Sc: Galaxy Morphology Galaxy classification Nomenclature: Early and Late type • Galaxies along the sequence often referred to as being either an early-type or a late type • Elliptical and Lenticular galaxies are collectively called an early-type and spirals are called late-type • Within spirals, an Sa galaxy is called an early-type spiral, an Sc a late-type spiral This nomenclature is not a statement of the evolutionary stage of objects but is merely a nomenclature of purely historical origin Galaxy Morphology Galaxy classification - Elliptical • Smooth and almost featureless; no spiral arms or dust lanes • Generally lacking in cool gas (or dust) hence few young blue stars; contain old stars (Pop II) • 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 have circular isophotes • Massive (~1013 M⦿) • 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 centres of galaxy clusters), dE’s (dwarf ellipticals), dSph’s (dwarf spheroidals) Galaxy Morphology Galaxy classification - Elliptical • M87 is Super giant galaxy. • Also known as M87, Virgo A or NGC • • • • 4486 Discovered in 1781 by French astronomer Charles Messier It is located about 16.4 million parsecs (53.5 million light-years) from Earth. M87 has no distinctive dust lanes and it has an almost featureless At the core there is a supermassive black hole. This object is a strong source of multiwavelength radiation, particularly radio waves. A jet of energetic plasma originates at the core and extends outward at least 1,500 parsecs (5,000 light-years). Galaxy Morphology Galaxy classification - Lenticular (S0) • Lens-like appearance when seen edge-on • S0 galaxies are a transition class • Properties intermediate to those of ellipticals and spirals • Often combine with ellipticals - early-type galaxies • Unlike elliptical and spirals, S0 galaxies has no subclass Galaxy Morphology From literature - Similar to Ellipticals • • • • • • • • • Arms absent defining characteristic large bulge red color few young stars minimal star formation little gas massive (~1013 M⦿) dense environments high central surface brightness ALL of these have at times been used to “identify” S0s, but reality is complex... Galaxy Morphology Bar Ring Dust Nuclear ring Bar + Ring Super Star Clusters Nuclear dust Nuclear Structures (from HST imaging) Galaxy Morphology Galaxy classification - Spirals • Display spiral arms/structures • Flattened systems with thin disk • Copious amount of gas and dust • Young (Pop I) and old (Pop II) stars • Divided into barred (SB) and unbarred (S) Spirals • Further subdivided into classes a, b, and c; e.g. SBb, Sc….. • a => large central bulge with tightly wounded spiral arms • c => small central bulge with loosely wounded spiral arms • Found mostly in Field • Less massive (~1011 M⦿) Galaxy Morphology Galaxy classification - Spirals The Whirlpool Galaxy (M51a or NGC 5194) is an interacting, grand-design spiral galaxy with a Seyfert 2 active galactic nucleus. Galaxy Morphology Galaxy classification - Irregular • By definition - irregular in shape • Lot of gas and dust • Mostly young (Pop I) stars • Found mostly in Field • mass => ~1010 M⦿ Galaxy Morphology Galaxy classification - Irregular Small Magellanic Cloud Large Magellanic Cloud Galaxy Morphology Galaxy classification - 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 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 Galaxy Morphology Galaxy classification - Dwarf Galaxies I Zwicky 18 is a dwarf irregular galaxy located about 59 million light years away. The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies. Galaxy Morphology Galaxy classification - 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 Galaxy Morphology Galaxy classification - Barred Galaxies NGC 1365, also known as the Great Barred Spiral Galaxy, is a huge barred spiral galaxy. It is over 200,000 light-years across and lies about 56 million light-years from Earth in the constellation of Fornax. It is a major member of the Fornax. Galaxy Morphology Limitation of Hubble Classifications • 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. Galaxy Morphology Limitation of Hubble Classifications • Galaxies in the process of transformation, generally from disks to ellipticals • In late stages of a merger, the 2 galaxies are no longer distinguishable. Mergers can alter Morphology Galaxy Morphology 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 • Irregular 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) Galaxy Morphology Modifications to Hubble Classifications Van den Bergh (1960) Luminosity Classification or “DDO System” 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 Galaxy Morphology Modifications to Hubble Classifications Van den Bergh (1976) Revised DDO — 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 Galaxy Morphology Modifications to Hubble Classifications Atlas 3D Survey (2011) Galaxy Morphology Modifications to Hubble Classifications Atlas 3D Survey (2011) In Hubble classification scheme — • Early type galaxies (built of old stars and little gas or dust) appear smooth and plain • Spiral galaxies show arms bursting with areas of active star formation, and disks rich in gas and dust. • Also while most stars in a spiral rotate in the same direction within its central plane, yielding a high net rotation • stars in early-type galaxies are generally assumed to move randomly in a rounder shape, canceling each other's angular momentum and giving such systems very little net rotation. Galaxy Morphology Modifications to Hubble Classifications Atlas3D comb diagram An alternative to Hubble's tuning fork — • It positions early-type galaxies along the handle in increasing order of their rotational speed, and spirals will occupy three teeth instead of two prongs. • The fast-rotating galaxies, placed at the junction of a tooth and the handle, are likely to have evolved from that family of spirals. Galaxy Morphology 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. Galaxy Morphology Automated Classification Abraham et al. (1994, 1996) — • Concentration parameter C - fraction of light within ellipsoidal radius 0.3 x outer isophotal radius (1.5σ 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 Hubble type T which is comparable to the dispersion between observers. Galaxy Morphology Automated Classification For distant galaxies (greater than z=0.5), classification is difficult because of small angular size and apparent faintness of galaxies. HST galaxies (z~1) classified by 2 experts (Ellis and van den Bergh) and also using A and C parameters of Abraham For faint galaxies, C parameter alone is fairly good. For brighter galaxies, C is degenerate between E and S0. Galaxy Morphology ...but we haven’t seen the end of visual classification! No matter how good the automated classifications become, the human eye is still better at determining patterns than neural networks (e.g. detecting spiral structure, smoothness) Modern CCD imaging surveys generate vast numbers of galaxy images There is a need for fast, objective, robust classification Galaxy Zoo is a project to employ volunteers to classify galaxies imaged in the Sloan Digital Sky Survey About 250,000 people have participated in this project to visually classify about 400,000 galaxies. Each galaxy receives over 20 classifications and the results are used together to determine the true classification. From July 2007... July 2008 •Over one hundred thousand participants •Nearly nine hundred thousand galaxies examined •Over thirty classifications per object •Equivalent to twenty-five years of full time effort From Feb 2009.......... •200,000 of the brightest of the SDSS galaxies •60 million classifications 43 44 45 46 meSome things are planned... things are planned... Some things are planned... Some things are planned.............. Some things are planned... 47 ..............others just happen ... others just happen 48 Galaxy The Scientific Legacy Zoo Lagecy - Zoo of Galaxy Zoo Universe Chris Lintott University of Oxford Thank You