Download File 11 - School of Astronomy, IPM

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?
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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
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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
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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
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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
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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
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meSome
things
are
planned...
things are planned...
Some
things
are
planned...
Some
things
are planned..............
Some
things
are planned...
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..............others
just
happen
... others just happen
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Galaxy
The Scientific Legacy
Zoo Lagecy
- Zoo
of Galaxy
Zoo Universe
Chris Lintott
University of Oxford
Thank You