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Galaxies: Structure, formation and evolutionLecture 3

Yogesh Wadadekar

Jan-Feb 2018

IUCAA-NCRA Grad School 1 / 17

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M100 nucleus

IUCAA-NCRA Grad School 2 / 17

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Barred galaxies

Roughly half of all disk galaxies - Milky Way included - show acentral bar which contains up to 1/3 of the total light Bars are aform of dynamical instability in differentially rotating stellar disksS0 galaxies also have bars – a bar can persist in the absence ofgasBar patterns are not static, they rotate with a pattern speed, butunlike spiral arms they are not density waves. Stars in the bar stayin the barThe asymmetric gravitational forces of a disk allow gas to loseangular momentum (via shocks) compressing the gas along theedge of the bar. The gas loses energy (dissipation) and movescloser to the center of the galaxy.

IUCAA-NCRA Grad School 3 / 17

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Barred Spiral NGC 1300

IUCAA-NCRA Grad School 4 / 17

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Extra lectures this week!

on Wed 10 Jan and Fri 12 Jan at 2:30 p.m. This is addition to thelectures during the pre-lunch slot on both these days.There will be only 2 lectures next week on Monday and Wednesdaydue to NCRA Academic Days on 18-19 Jan.

IUCAA-NCRA Grad School 5 / 17

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Lopsided galaxy - unstable disks

IUCAA-NCRA Grad School 6 / 17

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Irregular

IUCAA-NCRA Grad School 7 / 17

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Large Megallanic Cloud

Wadadekar et al. (2006)IUCAA-NCRA Grad School 8 / 17

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Nomenclature: Early and late type

Objects along the sequence are often referred to as being either anearly-type or a late-type. Ellipticals and S0 galaxies are collectivelycalled and early-type and spirals are called late-type. Within spirals, anSa galaxy is called an early-type spiral, and an Sd galaxy a late-typespiral.This nomenclature is not a statement of the evolutionary stage of theobjects but is merely a nomenclature of purely historical origin.

IUCAA-NCRA Grad School 9 / 17

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Galaxy classification affected by projection

Morphological classification is at least partially affected by projectioneffects. If, for instance, the spatial shape of an elliptical galaxy is atriaxial ellipsoid, then the observed ellipticity ε will depend on itsorientation with respect to the line-of-sight. Also, it will be difficult toidentify 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.

IUCAA-NCRA Grad School 10 / 17

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∼30% of galaxies at z ∼ 1 are peculiar

de Mello et al. 2006IUCAA-NCRA Grad School 11 / 17

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Dwarf galaxies

Dwarf galaxies are also not included in the Hubble sequence.Low-luminosity: 106 − 1010 L�, low-mass: 107 − 1010 M�, small insize, ∼few kpc, dark matter dominatedOften 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 dIrr

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 galaxiesDwarf galaxies are relatively simple systems, not merger products:in some sense, “pristine” low metallicity galaxiesgood for near field cosmology, but can’t be detected atcosmological distances.

IUCAA-NCRA Grad School 12 / 17

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I Zwicky 18

IUCAA-NCRA Grad School 13 / 17

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Questions

If you go look at the night sky most of the stars look white or blue witha few red ones which are all red giants. But the IMF tells us that moststars should be red looking M-dwarfs or G and K type dwarfs? Whyare these common stars extremely uncommon in the night sky?

See Binney & Merrifield (1998) pp. 111-115

IUCAA-NCRA Grad School 14 / 17

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Questions

If you go look at the night sky most of the stars look white or blue witha few red ones which are all red giants. But the IMF tells us that moststars should be red looking M-dwarfs or G and K type dwarfs? Whyare these common stars extremely uncommon in the night sky?See Binney & Merrifield (1998) pp. 111-115

IUCAA-NCRA Grad School 14 / 17

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Mergers can alter morphology

IUCAA-NCRA Grad School 15 / 17

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Arp 273 - tidal distortions from interaction

IUCAA-NCRA Grad School 16 / 17

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The Milky Way - Andromeda galaxy merger

Galaxies in the process of transformation, generally from disks toellipticalsIn late stages of a merger, the 2 galaxies are no longer distinguishable.What does the merger product look like?Show movie

IUCAA-NCRA Grad School 17 / 17