Spiral Arms and Bars


    In this insert (Galaxies file: spiral_arms_bars.html), we explicate the spiral arms and bars of spiral galaxies.

    Features:

    1. For a representative spiral galaxy, see the image of M83 in the figure below (local link / general link: galaxy_spiral_m83.html).


    2. The spiral arms of spiral galaxies are quite beautiful.

      Besides their beautiful and often fragmentary spiral shape, the spiral arms have intricate coloration in ENHANCED true color: a complex mixture of blue (due to OB stars (AKA hot young blue stars), pink (due to H II regions caused mainly by illumination by the OB stars), and dark, brown star forming regions often orgainzed in dust lanes lacing the spiral arms.

      The OB stars and associated H II regions are always close to and often surrounded by star forming regions because the OB stars CANNOT wander far from their star forming region of origin in their lifetimes of ⪅ 30 Myr (see Wikipedia: Stellar evolution for representative lifetimes of stars). Recall orbital periods for stars around galaxy centers of mass are of order 200 Myr.

      Dust lanes often appear on the trailing edge of spiral arms and the OB stars and associated H II regions on the leading edge (Sh-274; FK-569).

      Note spiral arms are bright because of the OB stars and associated H II regions are very luminous. The density of stars is about the same throughout galaxy disks???. The galactic bulges are typically the brightest part of spiral galaxies because of high concentration of relatively old yellow stars (i.e., old Population I stars and Population II stars).

    3. The spiral arms of spiral galaxies CANNOT simply be a fixed set of stars and gas. See the explication in the figure below (local link / general link: spiral_arm_winding_problem.html).


    4. There are, in fact, two limiting cases of spiral arms formed by two different processes: flocculent spiral arms and grand design spiral arms.

      It seems that the processes probably overlap, and so there is probably NO absolute separation between the two kinds of spiral arms (see Wikipedia: SSPSF model (stochastic self-propagating star formation model)).

      The probable overlap of processes also means that the classification of spiral galaxies is probably just an approximation:

      1. flocculent spiral galaxies: ∼ 30 % of galaxies.
      2. multi-arm spiral galaxies: ∼ 60 % of galaxies. Since these are sometimes lumped with flocculent spiral galaxies, it is clear that the flocculent spiral arms are really the dominant kind of spiral arms.
      3. grand design spiral galaxies: ∼ 10 % of galaxies. It is clear that grand design spiral galaxies though often thought of as being what spiral galaxies ought to be are actually a minority species. Flocculent spiral arms dominate as aforesaid.
      4. For the relative abundances of the spiral galaxies by spiral arm classification, see Wikipedia: Flocculent spiral galaxy.

      Below we discuss how the spiral arms form in flocculent spiral galaxies and grand design spiral galaxies treating the formation processes as distinct as a simplification.

    5. Flocculent Spiral Galaxies and Self-Propagating Star Formation:

      The word FLOCCULENT means woolly or fleecy.

      Flocculent spiral galaxies have poorly defined, fuzzy arms.

      The figure below (local link / general link: galaxy_triangulum_noao.html) shows an example of a flocculent spiral galaxy.


      The mechanism that creates
      flocculent spiral arms is believed to be stochastic self-propagating star formation (SSPSF). Recall, we have discussed SELF-PROPAGATING star formation in IAL 21: Star Formation: The Evolution of Star Formation Regions.

      In brief, this is the story of flocculent spiral arms:

      1. There is a large molecular cloud in which star formation is triggered in one part.

      2. Massive OB stars (AKA hot young blue stars) form. Their radiation pressure and stellar winds and the ejecta from their supernova explosions compress nearby regions of the molecular cloud.

      3. This compression triggers a new generation of star formation adjacent to the original one, where star formation is tending to turn off because the cloud material is being pushed away the same pressure forces that triggers the adjacent star formation.

      4. Thus star formation self-propagates through the large molecular cloud.

      5. But the molecular cloud is so large that it is differentially revolving around the galaxy center of mass. In most galaxies the orbital speed (measured in, e.g., km/s) is fairly constant, but this means that the angular velocity (measured in, e.g., in degrees per megayear (Myr)) falls off with radius from the galaxy center of mass.

      6. Thus, as one goes outward in radius, the molecular cloud material is increasingly trailing.

      7. The molecular cloud is getting wound up and is forming spiral arms.

      8. But the WINDING-UP PROBLEM NEVER arises, because the molecular cloud disperses in a complex way after a few tens of megayears and the orbital periods of the material is of order hundreds of megayears.

      9. Because star formation regions are NOT created by organizing spiral density waves, these star formation regions ARE messy, the spiral arms created are messy: i.e., flocculent spiral arms.

      For an simulation of stochastic self-propagating star formation (SSPSF), see the animation in the figure below (local link / general link: stochastic_self_propagating_star_formation.html).


    6. Grand Design Spiral Galaxies and Spiral Density Waves:

      The grand design spiral galaxies have well defined arms.

      The figure below (local link / general link: galaxy_whirlpool.html) shows an example of a grand design spiral galaxy.


      The stars and dust clouds in these
      spiral arms are orbiting more rapidly than the arms which are NOT fixed structures, but spiral density waves that move more slowly than the stars and clouds that pass through them.

      The spiral density waves are somewhat like traffic jams on a highway at the location of a moving work crew. The jam moves forward slowly as cars jam up there, but eventually the cars pass through and speed on ahead.

      The spiral density waves are in fact regions of compression of gas and the accompanying star formation regions.

        Question: The strong star formation in the arms which leads to OB stars (AKA hot young blue stars) and H II regions is:

        1. just a coincidence.
        2. a coincidence, but NOT just a coincidence.
        3. triggered by the compression of the gas.











        Answer 3 is right.

      The OB stars (AKA hot young blue stars) and the H II regions run ahead of the compression, because they are moving faster than the compression wave.

      But because OB stars (AKA hot young blue stars) and star forming regions live only of order 30 Myr and the orbital periods in galaxies are typically of order hundreds of megayears, the OB stars (AKA hot young blue stars), and the H II regions never get far from their star forming regions in the spiral arms before they become die So they stay in the spiral arms for all or most of their lives.

      One might say the spiral density waves are like waves rippling around in a bucket.

      But that analogy CANNOT be pressed too far since water waves are formed by pressure force and downward gravity.

      In spiral galaxies, the gravity is strong between matter components and NOT downward. Pressure forces play a role too.

      So the actual spiral density wave mechanism is complex (Sh-275ff) and NOT yet fully worked out (FK-570). But vaguely the waves are self-propagating gas compressions and gravitational perturbations.

      Because of the viscosity of the gas, spiral density waves should lose energy (kinetic energy and gravitational potential energy) to heat energy that eventually is probably mostly radiated away as light energy.

      So something must pump energy into the spiral density waves or else they would fade after maybe a gigayear or so (CK-392).

      There are 2 proposed mechanisms, both of which may operate:

      1. The BAR of barred spirals creates an asymmetric gravitational field that somehow generates the spiral density waves.

        Of course, the energy for the waves must then come ultimately from the rotational kinetic energy and/or gravitational potential energy of the galaxy matter, and so some spiraling in toward the center of some matter must happen.

        Since barred spirals are in fact only of order half of all spirals (CK-393; FK-583: but the two authors disagree somewhat), there must be a second mechanism.

      2. Interactions between galaxies may also generate spiral density waves at the expense of kinetic energy and/or gravitational potential energy one supposes.

      In addition to the 2 proposed mechanisms, there is the fact that spiral density waves in many cases CANNOT be separated from the mechanism of SSPSF model (stochastic self-propagating star formation) discussed above in feature Flocculent Spiral Galaxies and Self-Propagating Star Formation.

      It seems to be that spiral arms form from a varying combination of spiral density waves and the SSPSF model mechanism. So spiral arm formation is complex---but that's what makes spiral arms so beautiful.

    7. Disappearance of Spiral Arms:

      To last spiral arms need interstellar medium (ISM) and, for grand design spiral arms, excitation by galaxy interactions (i.e., strong gravitational perturbations between galaxies).?????

      What if the ISM gets depleted somehow or there are insufficient excitations?

      The spiral arms can disappear and a spiral galaxy turns into a lenticular (SO or SB0) galaxy.

      The time scale for disapperance is probably of order a gigayear.???

    8. Galactic Bars:

      What causes the bars of barred spirals?

      The bars are thought to be some sort of spiral density waves themselves which FORM and DECAY on the time scale of gigayears (see Wikipedia: Barred spiral galaxy: Bars).

      But they are spiral density waves NOT in galactic disks, but in galactic bulges.

      The bars are probably excited by galaxy interactions (i.e., strong gravitational perturbations between galaxies).?????

      And that's enough explication of bars for us.

    9. Keywords: Spiral arms and bars (i.e., Keywords: Spiral arms and bars): barred spiral galaxies (barred spiral galaxy bar, bar), flocculent spiral galaxy, grand design spiral galaxies, multi-arm spiral galaxy, (flocculent multi-arm spiral galaxy), spiral arm, spiral density waves, spiral galaxies, SSPSF model (stochastic self-propagating star formation) (SSPSF model), winding problem, etc.

    Local file: local link: spiral_arms_bars.html.
    File: Galaxies file: spiral_arms_bars.html.