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Normal galaxies (physical properties (related to Hubble classes (angular…

- - - - likely determined by formation history
    - - rate parts of galaxy rotates around centre
      - general compactness
      - low for elliptical and irregular
      - high for spirals and lenticulars
        
        for spirals and barred spirals, it generally increases through the types a-c
    - - 5-15% in spiral galaxies (barred/unbarred)
        
        increases progressively through types
        
        10% in Milky Way
      - 15-20% in irregular galaxies
      - elliptical and lenticular galaxies can have little gas
        
        gas that is present is hot
        
        lower rates of ongoing star formation
      - more cool gas=>
        
        Ongoing star formation
        
        open clusters
        
        HII regions
        
        Population I stars (vs Population II)
        
        high mass MS stars
  - - - spirals have narrow range of mass, luminosity, and diameter
      - masses of ellipticals have a large range
      - irregulars have an intermediate range of properties
  - - - elliptical class based on "apparent" shape
        
        ellipsoid: object that appears to be elliptical from all directions
        
        shape determined by lengths of 3 semi-axes
        
        spheroid: ellipsoid with two equal semi-axes.
        
        oblate spheroid: unequal axis is shorter than the others
        
        Once thought to be the shape of every elliptical galaxy
        
        2 more items...
        
        prolate spheroid: third axis longer than the other two
        
        triaxial ellipsoid: all three semi-axes unequal
        
        considering surface light contours, some elliptical galaxies must be triaxial
  - - - flux density (rate at which energy would reach a detector measuring 1 m\(^{2}\)) from an area of 1 arcsec\(^{2}\)
        
        SI units: \(\text{W m}^{−2}\text{ arcsec}^{−2}\)
        
        in practice, usually quoted in apparent magnitudes expressed in magnitudes arcsec\(^{2}\)
        
        often restricted to range of wavelengths like V-band
      - usually declines smoothly
        
        displayed using isophotal contours
        
        last one just marks where lowest level can be seen over instrumental noise (not edge of galaxy)
        
        Isophote: closed curve connecting points of equal surface brightness
    - - dark matter
      - 3 methods
        
        Rotation curves for spiral galaxies
        
        plot of \(v\) (rotational speed) against \(r\) (distance from galactic centre)
        
        rotation curve measured and compared with theoretical models predicted by models
        
        unsuitable for spirals with strongly pronounced bars
        
        gives only lower limit
        
        usually based on Doppler shift of the 21 cm line
        
        can be difficult for distant galaxies
        
        velocity dispersions for elliptical galaxies
        
        elliptical galaxies "star piles"
        
        stars settled into orbits around each other
        
        measure of the range of speeds along a line of sight
        
        expected to be proportional to \(M/R^{1/2} \) where M is mass of galaxy and R is a scale length related to its size.
        
        virial theorem
        
        statistical quantity
        
        elliptical galaxies where R is known and \(\Delta v\) can be estimated from Doppler-shift
- - - - "visible" shape and structure
        
        regularity/symmetry?
        
        light concentrated at centre?
        
        disc?
        
        spiral arms?
      - sometimes subjective
        
        "Galaxy Zoo"
        
        input from hundreds or thousands of people
    - - Elliptical
        
        elliptical outline
        
        generally featureless
        
        no arms, no central bulge
        
        light output highly concentrated in centre
        
        decreases steadily with increasing distance from centre
        
        8 types
        
        E0 for circular to E7 to most elongated
        
        number determined by relative size of semimajor axis (a) and semiminor axis (b)
        
        Number obtained by multiplying flattening factor \(f=(a-b)/a\) by 10
        
        assigned by visually observed shape
        
        "true shape" as 3D object not considered
        
        non-visual wavelengths not considered
        
        faint dwarf elliptical galaxies with masses around \(10^6 M_☉\) most common type of galaxy
        
        more than 60% of galaxies are elliptical
        
        cD galaxies
        
        classification no longer used
        
        ellipticals distorted by interacting with other galaxies
        
        1% atomic gas mass compared to stellar mass
        
        Population II stars
        
        mass range ~ \(10^{5} M_☉ \text{ to }10^{13} M_☉\)
        
        Luminosity range: a few times \(10^{5} L_☉ \text { to ~}10^{11} L_☉\)
        
        approximate diameter: \((0.01-5)d_{\text {MW}}\)
        
        low angular momentum per unit mass
      - Spiral
        
        two subclasses
        
        Spiral (S)
        
        Barred spiral (SB)
        
        Milky Way
        
        ~60% are barred
        
        Each subclass divided into Hubble types
        
        based on spiral arms unwinding and central bulge getting smaller
        
        a, b, c--progresively
        
        sometimes not unambiguous
        
        combination of nearest two (e.g. SBab)
        
        circular disc
        
        spiral arms
        
        central nuclear bulge
        
        light more evenly distributed across galaxy than elliptical
        
        <30% of galaxies are spiral
        
        mass of atomic gas around 5-15% that of stellar mass
        
        Populations I and II
        
        mass range ~ \(10^{9} M_☉ \text{ to a few times }10^{12} M_☉\)
        
        Luminosity range: ~\(10^{9} L_☉ \text { to ~}10^{11} L_☉\)
        
        approximate diameter: \((0.02-1.5)d_{\text {MW}}\)
        
        high angular momentum per unit mass
      - Lenticular
        
        lens shaped
        
        disc
        
        central bulge
        
        can be large in comparison with disc
        
        no arms
        
        subclasses
        
        barred: SB0
        
        ~60% are barred
        
        otherwise: S0
      - Irregular
        
        anything that won't fit into other classes
        
        little sign of regularity or symmetry
        
        may have vague signs of bars or arms
        
        Large and Small Magellanic Clouds
        
        <15% of galaxies are irregular
        
        mass range ~ \(10^{7} M_☉ \text{ to }10^{10} M_☉\)
        
        Luminosity range: a few times \(10^{7} L_☉ \text { to ~}10^{10} L_☉\)
        
        approximate diameter: \((0.05-0.25)d_{\text {MW}}\)
        
        low angular momentum per unit mass
      - ~1% do not fall into these classes
        
        Peculiar galaxies
        
        "p" added to end of classification
        
        jets
        
        interacting galaxies
        
        gravity effects