IAL 28: Galaxies

Don't Panic


  1. Introduction
  2. Units, Distance Scales, and the Observable Universe
  3. Types of Galaxies
  4. Elliptical (E) Galaxies
  5. Lenticular (SO and SBO) Galaxies
  6. Spiral (S and SB) Galaxies
  7. Unbarred Spiral (S) Galaxies
  8. Barred Spiral (SB) Galaxies
  9. Irregular (Irr) Galaxies
  10. Spiral Arms and Bars
  11. Galaxy Rotation Curves
  12. Galaxy Quenching
  13. Galaxies Without End?

  1. Introduction

  2. In this lecture, we take up the whole subject of galaxies.

    It's big, it's complex, it's a morass.

    We try to find a path through without sinking into the mire.

    1. Preview:

      First let's look at the galaxies in Seyfert's Sextet in the figure above (local link / general link: seyfert_sextet.html) and the figure below (local link / general link: seyfert_sextet.html) to get a preview.

      Let's have a look at the man himself. See the figure below (
      local link / general link: carl_seyfert.html)

    2. A General Remark:

      A general remark, NOT to be forgotten, is that stars, dust, gas, and, we believe, dark matter essentially orbit the centers of mass of galaxies.

      The orbits are, however, constantly being perturbed by interactions with other stars, dust, gas, and, perhaps, dark matter, and so are non-repeating and somewhat chaotic.

        Question: How do we know observationally about the motion of matter in galaxies?

        After all it is very slow on the human time scale. Typically, orbital periods are of order 100 million years.

        1. We don't know by observation. We only infer that there must be motion or the galaxies would collapse perhaps into SUPER supermassive black holes.
        2. There is no motion really and the foregoing is hogwash.
        3. Line spectra from the matter show the Doppler effect from which the velocities of the matter can be calculated.

        Answer 3 is right. However, answer 1 would be partially right too without the first sentence.

        Of course, only matter with line spectra show the Doppler effect.

        So for dark matter and other matter without line spectra, the motion is only inferred.

      Note also the disk orbital motion is differential---disks do NOT rotate like rigid bodies.

  3. Units, Distance scales, and the Observable Universe

  4. We need to orient ourselves a bit in the observable universe.

    There are some units and distance scales that it is convenient to note or recapitulate for a start. See the listing in the insert below (local link / general link: astronomical_distances_larger.html).

    Now what we can see of the
    observable universe (whose radius is given in the table above) is a sphere centered on us.

    To explicate, see the artist's conception of the observable universe in the figure below (local link / general link: cosmos_artist_conception.html).

    To further explicate the observable universe, consider the 2 diagrams in the figure below (local link / general link: observable_universe_cartoon.html).

    What is the
    observable universe made of? See the figure below (local link / general link: pie_chart_cosmic_energy.html).

  5. Types of Galaxies

    1. Introduction:

      Edwin Hubble (1889--1953) beginning in the 1920s developed the first galaxy morphological classification (see Wikipedia: Hubble sequence; No-508--510). It is is still in use today

      Hubble's classification scheme is called the Hubble sequence---Hubble probably didn't call it that himself---there's Hubble this and Hubble that nowadays. See figure below (local link / general link: alien_hubble.html).

      The main galaxy types in the
      Hubble sequence are spiral (S), barred spiral (SB), elliptical (E), lenticular (S0 and SB0), and irregular galaxy (Irr). These galaxy types are divided into subtypes which we discuss below in subsection The Hubble Sequence Explicated.

      If one says spirals without qualification, one often means both spirals and barred spirals, unless one doesn't---context must decide.

      Ellipticals and lenticulars are often called early-type galaxies and spirals, late-type galaxies.

      This is astro-jargon has NO physical meaning: early-type galaxies are NOT early and late-type galaxies are NOT late. Hubble himself emphasized that the Hubble sequence originated as a purely empirical galaxy morphological classification without theoretical understanding or bias (see Wikipedia: Hubble seqence: Physical significance).

      Note spiral galaxies are only be called spiral nebulae nowadays when speaking of them in a historical context---they were called the spiral nebulae before people knew they were other galaxies and for some time thereafter. Hubble himself entitled his book on extragalactic astronomy The Realm of the Nebulae (Edwin Hubble, 1936).

      Also note Hubble announced the discovery of the expansion of the universe in 1929 (see Wikipedia: Edwin Hubble: Redshift increases with distance; No-523). It was the observational discovery. The expansion of the universe had been theoretically predicted from general relativity earlier in the 1920s by Alexander Alexandrovich Friedmann (1888--1925) and Georges Lemaitre (1894--1966).

      Einstein completed general relativity by 1915 (see Wikipedia: History of general relativity; St. Andrews Mathematics Archives: Einstein biography).

    2. The Hubble Sequence Explicated:

      The Hubble sequence is best illustrated by a Hubble tuning-fork diagram such as the one shown in the figure below (local link / general link: galaxy_hubble_sequence.html).

      Table: Some Galaxy Properties for the Local Universe below gives some of the properties of galaxies classified by the Hubble sequence types for the local universe (which is also the modern universe as you recall from IAL 26: The Discovery of Galaxies).

      Table: Some Galaxy Properties for the Local Universe
      Property S and SB galaxies E galaxies Irr galaxies
      Mass 10**9 -- 4*10**11 10**5 -- 10**13 10**8 -- 3*10**10 (M_sun) baryonic matter only all matter baryonic matter only baryonic matter about 1/6 to 1/30 of all matter Luminosity 10**8 -- 2*10**10 3*10**5 --10**11 10**7 -- 10**9 (L_☉) Diameter 5 -- 250 1 -- 200 1 -- 10 (kpc) Stellar arms: young Pop I; Pop II and mostly Pop I population bulge and disk: old Pop I old Pop I and Pop II halo: Pop II Percentage 77 20 3 observed NOT counting numbers of ordinary many dwarf spirals and barred ellipticals? spirals are comparable. Color blue/pink/dark yellow blue/pink mostly? depends yellow in bulge mainly star ages Gas and Dust thick in arms very little generally rich and throughout disk obvious matter but probably significant very hot, nearly invisible hot gas seen in X-ray band

      References: CK-393, Dekel et al. (2019) (for the baryonic to total matter ratio), FK-565,582,583,585, Wikipedia: Elliptical galaxy: Sizes and shapes.
      1. Some of the data above is from old references, but it's accurate enough for qualitative thinking about galaxies. It could also be considered representative data.
      2. CK-393 says that the number ratio of ordinary spirals to barred spirals is 2:1, but FK-583 says 1:2. The web provides no tie breaker.
      3. Neither FK-582 nor CK-393 indicate where lenticulars are included for their tables: are they included as spirals or as ellipticals?

    3. The de Vaucouleurs System:

      The Hubble sequence was extended by Gerard de Vaucouleurs (1918--1995) in the de Vaucouleurs system as we call it now.

      The figure below (local link / general link: galaxy_vaucouleurs.html) illustrates the de Vaucouleurs system

  6. Elliptical (E) Galaxies

  7. Elliptical galaxies have, of course, no spiral arms and are roughly ellipsoidal in shape.

    They are a bland boring white/yellow in true color. NOT nearly as interesting to look at as spiral galaxies.

    1. The Nature of Ellipticals:

      Ellipticals have a tremendous range in mass from ∼ 10**5 M_☉ (smallest dwarf ellipticals) to 10**13 M_☉ (the largest giant ellipticals which are called cD elliptical galaxies).

      The stars in ellipticals orbit in all random orientations (FK-585).

        Question: If orbits in ellipticals have random orientations, then their net angular momentum should be:

        1. relatively small compared to spirals.
        2. huge.
        3. infinite.

        Answer 1 is right.

        Each star in an elliptical has its own angular momentum that keeps it from falling to the center. But the summing the random angular momentum components, there is a great deal of cancellation and the net angular momentum will be relatively small compared to spirals where the disk stars rotate in one direction.

        Angular momentum is actually a vector quantity, but we will say no more on that topic.

      Ellipticals generally have little obvious interstellar medium (ISM) (interstellar gas and dust) and are mostly composed of Population II stars and old Population I stars. So their stars are typically older than about 5 Gyr and there are very few younger stars.

      Nowadays, we know that at least large-mass ellipticals probably have significant ionized gas (i.e., hydrogen ion (H**(+) gas), but it is kept very hot by outflows from the central supermassive black hole (which in turn is fed by inflows from the intergalactic medium (IGM)), and is so nearly invisible, except in the X-ray band. The fact that the gas is very hot gives it high pressure, and so keeps it from undergoing gravitational collapse leading to star formation or the outflows push out the cold gas. The overall process can be called AGN feedback (see Wikipedia: Galaxy quenching).

      What of low-mass ellipticals (which in many cases are dwarf ellipticals)? Interaction with their environments seems to remove low-temperature gas by processes that could be galaxy ram-pressure stripping, galaxy strangulation, or galaxy virial shock heating (see Wikipedia: Galaxy quenching).

      It also seems to yours truly's that once a galaxy has lost its interstellar dust (by e.g., galaxy ram-pressure stripping or "evaporation" by electromagnetic radiation (EMR)), it is hard to get star formation going again since star formation occurs in molecular clouds that form in thick nebulae of interstellar dust. To oversimplify the hypothesis is that lack of interstellar dust leads to lack of star formation that leads to lack of interstellar dust that comes from mainly from post-main-sequence stars (which become very spread out in cosmic time because the slow stellar evolution of low-mass stars) and supernovae.

    2. Quenched Galaxies:

      So ellipticals usually have very little star formation. They are, in modern jargon, usually quenched galaxies or red sequence galaxies.


      1. Why red sequence galaxies? Quenched galaxies are red in a photometry sense (i.e., large B-V color index). Seen in true color they are white hot; (see Temperature of a "White Hot" Object, Carine Fang, 2001; Wikipedia: Thermal radiation: Subjective color to the eye of a black body thermal radiator; Wikipedia: Red heat).

      2. why are ellipticals photometrically red? Because ellipticals are quenched galaxies, and so have NO or very few few young stars, and so they have NO or very few blue stars which are OB stars and A stars.

        The OB stars and A stars have masses greater than about 1.4 M_☉ (see Wikipedia: A-type main-sequence star), and thus have lifetimes shorter than about 3 Gyr (see Wikipedia: File:Representative lifetimes of stars as a function of their masses).

      3. Why is there galaxy quenching? See section Galaxy Quenching below.

      The galaxy group in the figure below (local link / general link: galaxy_hcg_87.html) shows unquenched spiral galaxies and a quenched elliptical galaxy.

    3. The Appearance of Ellipticals:

      First, the morphology: the subtypes of ellipticals range from E0 (most round on the sky) to E7 (to most elongated on the sky).

      These subtypes are NOT completely intrinsic characterizations of the true 3-dimensional shapes of the ellipticals. The subtypes also depend on the on the orientation of the ellipticals on the sky.

      This is unlike the situation with spirals (the term here includes barred spirals) (see below), where we usually can determine the subtype independent of orientation on the sky (see below).

      Second the color.

      Without many blue stars and much interstellar medium (ISM), all that is left are yellow and red stars to give color to ellipticals.

      Thus, ellipticals look bland yellow in color.

      On most images, they are over-exposed bright near their centers where the stars are dense and then decrease in brightness as you move away from the center and the density of stars falls.

      The figure below (local link / general link: galaxy_cluster_abell_s740.html) shows a typical elliptical located in a rich galaxy cluster.

      The figure below (
      local link / general link: m87_virgo.html) shows another elliptical: M87 (NGC 4486) in the Virgo Cluster in Virgo.

      M87 (NGC 4486) in its region of the Virgo Cluster is shown in the figure below (local link / general link: galaxy_cluster_virgo.html).

      The locations of
      M87 in constellation Virgo and the Virgo Cluster in constellations Virgo and Coma Berenices are shown in the figure below (local link / general link: iau_virgo.html).

    4. Dwarf Ellipticals:

      There are a lot of them, but they are small and NOT so obvious as large ellipticals.

      The smallest dwarf ellipticals are comparable to globular clusters in mass, but are less compact I think????.

      Recall the distinction between dwarf ellipticals and globular clusters. Since they are galaxies, dwarf ellipticals underwent generations of star formation before galaxy quenching. Since they are star clusters, globular clusters formed their stars all in one episode or nearly of star formation.

      The dwarf ellipticals are quite transparent due their few stars (less than a few times 10**6) and lack of obvious interstellar medium (i.e., dust and gas): you can see right through them (FK-584). This is provided that their point spread functions don't overlap in an image.

        A point spread function (PSF) if the response of an imaging system to a point source. Frequently, a PSF is also used to mean the finite blob in an image which is recorded for an unresolved point object. The blob being the result of the PSF.

      See the figure below (local link / general link: galaxy_fornax_dwarf.html) of a nearby dwarf spheriodal galaxy (dSph) (which is much like a dwarf elliptical).

  8. Lenticular (SO and SBO) Galaxies

  9. The word LENTICULAR means lens-shaped.

    The SO (unbarred) and SBO (barred) lenticular galaxies have a bulge and a disk, but NO or ILL-DEFINED spiral arms (FK-584). They can have significant interstellar dust in their galactic disks.

    Lenticular galaxies are sort of middle case between ellipticals and spirals.

    They are usually galaxy quenched.

    An example lenticular galaxy is shown in the figure below (local link / general link: galaxy_lenticular_ngc_2787.html).

    Lenticular galaxy formation is uncertain.

    There are several theories all of which may be right in some cases since there may be multiple formation channels which may also overlap.

    Yours truly thinks the two most likely theories currently are:

    1. The faded spirals theory: Lenticulars are faded spirals that have exhausted or partially exhausted their interstellar gas that feeds star formation which in turn highlights the spiral arms of spirals.

      But why isn't the gas replaced by inflows of intergalactic medium (IGM)? Maybe the lenticulars are just in a low density region of IGM. Or maybe the lenticulars have fallen into galaxy clusters and had their interstellar gas stripped by galactic ram pressure stripping.

      Another possibility is that there may be a lack of galaxy interaction to excite spiral density waves.

    2. The merger theory: The merger of two spirals or maybe a spiral and an elliptical does NOT completely randomize the star orbits, and so a galactic disk survives. However, the mass of the merged-galaxy dark halos exceeds the golden mass 10**12 M_☉ above galaxy quenching since to follow always (see Dekel et al. (2019)).

      More extreme galaxy merger are probably the main channel for creating ellipticals other than dwarf ellipticals???.

  10. Spiral (S and SB) Galaxies

  11. Spiral galaxies, of course, have spiral arms and they are NOT galaxy quenched---though there might be some exceptions or near exceptions.

    Typically, spirals typically 2 major ones ??? plus maybe smaller or fragmentary ones: e.g., the Milky Way: see figure below (local link / general link: milky_way_map.html).

    There may also be isolated arm segments: e.g., the Orion arm segment in the
    Milky Way which the Sun is just outside of (FK-563).

    The spiral arms rotate in the direction you would think: i.e., the spiral arms are trailing with the ends pointing opposite to the direction of orbital rotation.

    There is one exception: NGC 4622 has leading spiral arms. See the figure below (local link / general link: galaxy_spiral_ngc_4622.html).

    We know the direction of rotation of the arms, stars, and gas from spectroscopy of the
    spirals and the Doppler effect. See IAL 7: Spectra for more on the Doppler effect.

    The stars, gas, and dust in the disk rotate in the same sense as the arms, but more rapidly in the GRAND-DESIGN ARMS, but NOT in FLOCCULENT ARMS. We discuss the nature of spiral arms further in the section Spiral Arms below.

    Spirals probably mostly have luminous halos and dark matter halos like the Milky Way.

    The extents of the luminous halos are harder to see for remote galaxies, of course.

    Galaxy rotation curves (see the section Galaxy Rotation Curves below) indicate massive dark matter halos just as they do for the Milky Way (FK-600).

    The closest spiral to the Milky Way (NOT counting itself, of course) is the Andromeda Galaxy (M31). See the collage figure below (local link / general link: m31_002_noao_moon.html).

  12. Unbarred Spiral (S) Galaxies

  13. Now what of unbarred spiral galaxies?

    The Hubble types are:

    1. Sa: large bulges; tightly wound, broad arms.
    2. Sb: moderate bulges, moderately wound, moderately well-defined arms.
    3. Sc: small bulges, loosely wound, narrow arms.

      References: CK-388; FK-583; Wikipedia: Spiral galaxy.

    See the Hubble tuning fork diagram in the figure below (local link / general link: galaxy_hubble_sequence.html).

    Note that
    spirals can be at any orientation to the Earth observer from face-on to edge-on.

    The arms of edge-on spirals are hard to see, but the shape of the bulge (which is correlated with the arms) permits the Hubble type to be distinguished.

    Except, it is sometimes hard to tell if there is a bar or NOT.

    In the figure below (local link / general link: galaxy_sombrero.html) is a famous example of an Sa spiral galaxy.

    The figure below (
    local link / general link: galaxy_whirlpool.html) shows a famous Sc spiral galaxy.

  14. Barred Spiral (SB) Galaxies

  15. Now what of barred spiral galaxies?

    The Hubble types are:

    1. SBa: large bulges; tightly wound, thin arms.
    2. SBb: moderate bulges, moderately wound, arms.
    3. SBc: small bulges, loosely wound, loosely lumpy arms.

      References: CK-392; FK-583; Wikipedia: Barred spiral galaxy.

    See the Hubble tuning fork diagram in the figure below (local link / general link: galaxy_hubble_sequence.html).

    Note the
    barred spirals (like ordinary spirals) can be at any orientation to the Earth observer from face-on to edge-on.

    The arms of edge-on barred spirals are hard to see, but the shape of the bulge (which is correlated with the arms) permits the spiral type to be distinguished.

    The figure below (local link / general link: galaxy_spiral_m83.html) shows an example of SABc (i.e., intermediate spiral galaxy which is a barred spiral with a small bar) seen face-on.

    Milky Way is a barred spiral: an SBb or SBc or somewhere in between (see Milky Way: Composition and structure).

    It's hard to classify Milky Way exactly because we don't have a good overall view---"can't see the forest for the trees."

    A real image of a galaxy that resembles the Milky Way (i.e., the Milky Way twin) is shown in the figure below (local link / general link: milky_way_ngc_6744.html).

  16. Irregular (Irr) Galaxies

  17. As their name suggests IRREGULARS are rather random in shape.

    Actually, there are two subtypes: Irr I's with some of ordered structure and Irr II's which are badly disordered and may often have resulted from violent collisions with other galaxies (FK-585--586).

    Irregulars are generally smaller than the large spirals and ellipticals.

    They are typically rich in dust and gas, and have young and old stars and STAR FORMATION. The Large Magellanic Cloud (LMC) is the nearest (at about 50 kps) and most famous irregular: an Irr I (FK-500, 585). See figure below (local link / general link: galaxy_lmc.html).

  18. Spiral Arms and Bars

  19. Spiral arms and bars are explicated in the figure below (local link / general link: spiral_arms_bars.html).

    What happens to the energy of the photons when they cosmological redshift if we have conservation of energy?

    Yours truly is embarrased to admit that energy conservation as a simple scalar quantity is NOT maintained in contexts of general relativity with expanding space. The Einstein field equations embody energy conservation in a general relativity sense and that is all one can say (see Sean Carroll, 2003, Spacetime and Geometry: An Introduction to General Relativity, p. 120).

    The photon energy just vanishes with the cosmological redshift.

    We take up the expansion of the universe in IAL 30: Cosmology: The Expansion of the Universe.

  20. Galaxy Rotation Curves

  21. Galaxy rotation curves are plots of orbital velocity of objects in a galaxy versus radial distance from the galaxy center.

    What the rotation curves should be for the observed visible baryonic matter (mainly stars) can be predicted using classical physics (i.e., Newtonian physics and Newton's law of universal gravitation).

    But the predictions are almost always too low.

    The rotation curves show that there is several times more dark matter than visible baryonic matter (mainly stars). The ratio of visible baryonic matter (mainly stars) to dark matter in galaxies is quite various it seems and ranges from ∼ 1/30 to ∼ 1/10.????

    The animation in the figue below (local link / general link: galaxy_rotation.html) shows how a spiral galaxy rotates without and with dark matter.

    The figure below (
    local link / general link: galaxy_rotation_cartoon.html) gives a cartoon galaxy rotation curve plot.

    The motions of
    galaxy clusters and galaxy superclusters also show evidence for dark matter.

    The first evidence of dark matter was found by Fritz Zwicky (1898--1974) in 1933 from studying the motions of the galaxy cluster the Coma Cluster (see English and Spanish Translation of Zwicky's (1933) The Redshift of Extragalactic Nebulae (Die Rotverschiebung von extragalaktischen Nebeln)). Zwicky coined the term for dark matter for dark matter. More precisely he said dunkle materie, but that translates from German to dark matter.

    However, it wasn't until circa 1980 that dark matter became a widely accepted theory.

    What is dark matter?

    Some of it is hot intergalactic hydrogen and helium gas which is usually called intergalactic medium (IGM).

    Some of it is probably unseen neutron stars and black holes.

    But these contributions must be minor if Big Bang cosmology is correct---and Big Bang cosmology is a very solidly established theory these days.

    Big Bang cosmology dictates how much ordinary matter (i.e., matter made up principally of protons, neutrons, and electrons) there is. (Black holes whatever they are now, were ordinary matter before becoming what they are.)

    And the dark matter weighs in at several times this amount.

    So it is believed that dark matter is some sort of exotic particle that we have NOT detected---except through its gravitational effects.

    It is very weakly interacting, except through its gravity.

    The dark matter paticles are thought to be clumped into clouds that are gravitational potential wells.

    The biggest potential wells are where galaxies, galaxy cluster, and galaxy superclusters formed.

    These structures are, in fact, largely held together by their dark matter.

    What is the dark matter particle?

    There are many theoretical candidates and there are experimental searches for them but at present we've had to no luck.

    A second idea about dark matter is that it is NOT an exotic particle, but that it is primordial black holes (PBHs). PBHs formed in the Big Bang, but before Big Bang nucleosynthesis (BBN, cosmic time ∼ 10--1200 s ≅ 0.17--20 m)---and so they do NOT spoil the good results of BBN. For some explication of primordial black holes (PBHs), see the figure below (local link / general link: black_hole_primordial.html).

    There is an alternative theory to
    dark matter: MOND (MOdified Newtonian Dynamics)---see the explication of MOND, the Devil, Dracula, and the worst of all possible worlds in the figure below (local link / general link: gravity_mond.html).

  22. Galaxy Quenching

  23. Why is there galaxy quenching---the turning off of star formation?

    For a slightly long answer, see the figure below (local link / general link: galaxy_quenching.html) for more on galaxy quenching.

  24. Galaxies Without End?

  25. Are there galaxies without end?

    Who knows? There are a lot of them.

    In the observable universe there are estimated to be at least ∼ 2*10**12 galaxies (see Wikipedia: Observable universe). But since galaxies get more abundant as one goes down in mass coordinate, where is the cut-off? Also, we don't know how far galaxies extend beyond the observable universe. Well beyond certainly since we see no boundary effects in the observable universe, but to infinity? For an example of the plethora of galaxies, see the Hubble Ultra-Deep Field (HUDF, 2003--2004) in the figure below (local link / general link: hubble_ultra_deep_field.html).