IAL 29: The Large-Scale Structure of the Universe

Don't Panic

Sections:
  1. Introduction
  2. Units, Distance Scales, and the Observable Universe
  3. Groups, Clusters, Superclusters, and Large-Scale Structure
  4. The Cosmological Principle
  5. Large-Scale Structure Formation and Evolution

  1. Introduction

    2. The large-scale structure of the universe is the structure formed by galaxies (including absolutely postively their dark matter halos) treated as the smallest units---but one has to go to smaller scales too to understand how galaxies act and evolve.

    The large-scale structure includes galaxies, galaxy groups (see the figure of Seyfert's Sextet above: local link / general link: seyfert_sextet.html; and below: local link / general link: seyfert_sextet.html), galaxy clusters, galaxy superclusters, galaxy filaments, galaxy walls, voids, and whole lot more.

    The study of the large-scale structure is considered part of cosmology, but it is at a level just below that of the observable universe as a whole---which we will study in IAL 30: Cosmology.

  2. Units, Distance Scales, and the Observable Universe

    3. 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 (with observable universe radius = 14.3 Gpc) 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_logarithmic_map.html).

    To further explicate the observable universe, consider the 2 artist's conceptions of it 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).

  3. Groups, Clusters, Superclusters, and Large-Scale Structure

    4. Galaxies are NOT the largest structures.

    To recapitulate, there is the large-scale structure (of the observable universe) which includes galaxies, galaxy groups (see the figure of Seyfert's Sextet above: local link / general link: seyfert_sextet.html), galaxy clusters, galaxy superclusters, galaxy filaments, galaxy walls, voids, and whole lot more.

    In this section, we will discuss all the aforesaid features from a largely observational point of view: i.e., what is there to see.

    In section Large-Scale Structure Formation and Evolution, we will discuss structure formation (i.e., the formation and evolution of the large scale structure of the observable universe).

    1. Gravitationally Bound or NOT:

      Galaxy groups and galaxy clusters are probably mostly gravitationally bound: i.e., they will NOT expand with the expansion of the universe. On the other hand, galaxy superclusters though gravitationally interacting are probably mostly NOT gravitationally bound. In most cases, most of their component galaxy groups and clusters and field galaxies will move apart forever with the expansion of the universe---provided the Λ-CDM model or some other forever expanding cosmological models is true.

      Note, velocity dispersion (σ) is a sort of average of the absolute values of the velocities of a set of astro-bodies (relative to their mutual center of mass) forming a gravitationally-bound system. The kinetic energy of the astro-bodies holds them up from collapse to the center of mass under their own self-gravity.

    2. Galaxy Groups:

      The structure just above galaxies themselves are galaxy groups.

      Characteristics of galaxy groups (see Wikipedia: Galaxy group: Characteristics):

      1. ⪅ 50 galaxies of Milky-Way luminosity (∼ 10**10 L_☉) or brighter (see Wikipedia: Galaxy group). There may be tens or hundreds of dwarf galaxies???---it is hard do a census of them since they are small and dim.
      2. Size scale ∼ 1--2 Mpc.
      3. Velocity dispersion (σ): ∼ 150 km/s. Recall, velocity dispersion (σ) is a sort of average of the absolute values of the velocities of a set of astro-bodies (relative to their mutual center of mass) forming a gravitationally-bound system.
      4. Mass ⪅ 10**14 M_☉ ≅ 100*(golden mass ≅ 10**12 M_☉) (Ci-165). This mass includes dark matter.
      5. Baryon fraction (i.e., the mass ratio of baryonic matter to baryonic matter plus dark matter) is in the range 8--13 % (Ci-165,174--175). The baryon fraction tends to increase with mass.
      6. Galaxy groups are the commonest grouping of galaxies in the local universe.
      7. ∼ 50 % of galaxies in the local universe are in galaxy groups.
      8. No one suggests that galaxy groups have a meaningful analogue to the brightest cluster galaxies (BCGs) that galaxy clusters have in general. Also, they do NOT have in all/most cases a meaningful analogue to Intracluster medium (ICM) that galaxy clusters have in general.????
      9. Galaxy groups are probably mostly gravitationally bound.???
      10. Some folks (like yours truly) just consider galaxy groups as VERY POOR galaxy clusters. It saves one from have to say galaxy groups and clusters all the time tediously.

      Our own Milky Way belongs to a galaxy group with the inspiring name of the Local Group.

      The Local Group is a very poor galaxy group: it has only 3 large galaxies.

      A map of the Local Group is shown in the figure below (local link / general link: local_group.html).

      Another well known galaxy group is the Hickson Compact Group HCG 87 which is shown in the figure below (local link / general link: galaxy_hcg_87.html).

    3. Galaxy Clusters:

      The structure just above galaxy group is galaxy cluster.

      The basic characteristics of galaxy clusters (see Wikipedia: Galaxy Cluster: Basic properties and Wikipedia: Galaxy Cluster: Composition):

      1. Mass (counting dark matter): ∼ 10**14--10**15 M_☉ (Ci-165). So 100 to 1000 times the golden mass ≅ 10**12 M_☉.
      2. Baryon fraction (i.e., the mass ratio of baryonic matter to baryonic matter plus dark matter) is in the range 12--16 % (Ci-165,174--175). The baryon fraction tends to increase with mass. Note, the observable universe has the overall universal baryon fraction 16 %, and so only the largest galaxy clusters approach the universal baryon fraction (Ci-27,174-175).
      3. Size scale: ∼ 1--5 Mpc.
      4. Velocity dispersion (σ): ∼ 1000 km/s.
        Recall, velocity dispersion (σ) is a sort of average of the absolute values of the velocities of a set of astro-bodies (relative to their mutual center of mass) forming a gravitationally-bound system.
      5. A brightest cluster galaxy (BCG) is definitionally just the brightest galaxy in a galaxy cluster. However, brightest cluster galaxies (BCGs) are usually giant ellipticals, D giant elliptical galaxies, or cD giant elliptical galaxies, and are usually close to the centers of mass their host galaxy clusters.

      6. There are 3 main components in galaxy clusters:
        1. ∼ 50--few 1000s galaxies. But is this counting all dwarf galaxies????. Probably NOT, but it is hard to find reference to say this.???? The galaxies (including their dark matter) contribute only ∼ 1 % of the galaxy cluster mass (see Wikipedia: Intracluster medium: Composition).
        2. Intracluster medium (ICM): This is a hot totally ionized gas (i.e., a hot extreme plasma) that permeates a galaxy cluster. The gas probably mostly has the primordial cosmic composition (which is also almost cosmic present intergalactic medium composition: fiducial values by mass fraction: 0.75 H, 0.25 He-4, 0.001 D, 0.0001 He-3, 10**(-9) Li-7), but is typically enriched in the range 1/3 to 1/2 of the solar composition (fiducial values by mass fraction: 0.73 H, 0.25 He-4, ∼ 0.02 metals) in metals (including iron (Fe, Z=26)) ejected from galaxies by supernovae (SNe) and AGN feedback (see Wikipedia: Intracluster medium: Composition). The ICM is ∼ 10 % of the galaxy cluster mass (see Wikipedia: Intracluster medium: Composition). The ICM has temperatures in the range 10**7 to 10**8 K (see Wikipedia: Intracluster medium). The high temperatures are due to AGN feedback and/or shock heating from galaxy mergers plus slow cooling (see Wikipedia: Intracluster medium: Heating).
        3. Dark matter: It contributes ∼ 84--88 % of a galaxy cluster mass (see Wikipedia: Intracluster medium: Composition; Cimatti 2020, p. 165,174).

      7. There are 3 types of galaxy clusters which are somewhat vaguely specified:
        1. Galaxy groups if you consider them as VERY POOR galaxy clusters. They contain ⪅ 50 galaxies of Milky-Way luminosity or brighter. Probably NOT counting dwarf galaxies????. See subsection Galaxy Groups above.
        2. POOR galaxy clusters: from galaxy group size to a few hundreds of galaxies. Probably NOT counting dwarf galaxies????.
        3. RICH galaxy clusters: from > POOR galaxy cluster population to thousands of galaxies. Probably NOT counting dwarf galaxies????.

        Galaxy clusters are also divided into REGULAR CLUSTERS which are roughly spherically symmetric with a concentration of galaxies in the center and IRREGULAR CLUSTERS which are irregular (FK-593).

        As usual, there is actually a continuum between the types of galaxy clusters.

        Also since the specifications are a bit vague, which category a galaxy cluster falls into will vary a bit with reference.

        For an example of a RICH galaxy cluster, see the Coma Cluster in the figure below (local link / general link: galaxy_cluster_coma.html).

      8. Probably, most galaxies are located in galaxy clusters (here counting galaxy groups as VERY POOR galaxy clusters???): see B. Ryden: Clusters and Superclusters.

        How many? Someone must have good statistics, but NOT yours truly. However, within 5 Mpc of the Milky Way ∼ 80 % of galaxies are in galaxy groups and clusters (see Wikipedia: Field galaxies).

        POOR galaxy clusters far outnumber rich ones (FK-592) and most galaxies are NOT in rich clusters????.

        Galaxies NOT in galaxy groups and clusters (i.e., isolated galaxies) are called field galaxies.

      9. In fact, the number of galaxies in a galaxy cluster or galaxy group is often hard to determine since there are many dwarf galaxies (e.g., dwarf elliptical galaxies) that are hard to find (FK-594). In the Coma Cluster (see figure above: local link / general link: galaxy_cluster_coma.html), there are thousands of dwarf galaxies which we are only made obvious by imaging in the infrared.

        Also galaxy groups and galaxy clusters have NO sharp edges.

        This last comment applies both to the galaxy distribution in galaxy clusters and the dark matter in galaxy clusters which makes most of their mass.

      10. The galaxy clusters appear to be mostly gravitationally bound systems (FK-594).

      11. The neareast RICH galaxy cluster is the Virgo Cluster.

        Now yours truly knows what you are thinking---"that's in Virgo"---well it straddles the line between the constellations Virgo and Coma Berenices.

        A sky map of constellation Virgo (with the Virgo Cluster shown) is shown in the figure below (local link / general link: iau_virgo.html).

      The relative locations of       Local Group and the Virgo Cluster are shown in the figure below (local link / general link: large_scale_structure_030_mpc.html).

      Images of the central region of the       Virgo Cluster are shown in the figure below (local link / general link: galaxy_cluster_virgo.html).

    4. Galaxy Superclusters:

      Galaxy clusters can themselves be part of superclusters that can contain tens of galaxy clusters and have a size scale of 50 Mpc (FK-594). Yours truly believes they are all irregular in shape.???

      Superclusters although gravitationally interacting appear to be mostly NOT gravitationally bound (FK-594). In most cases, most of their component galaxy groups and clusters and field galaxies will move apart forever with the expansion of the universe.

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

      The Local Group belongs to the Virgo Supercluster which itself is part of the Laniakea Supercluster.

      The Virgo Supercluster is centered on the Virgo Cluster (Wikipedia: Virgo Supercluster: Galaxy distribution) and on the celestial sphere is mostly in constellations Virgo and Coma Berenices, but reaches into constellations Ursa Major, Leo, Canes Venatici, and Crater (Google AI: How many constellations are covered by the Virgo supercluster?).

      The Virgo Supercluster, Laniakea Supercluster (sans name), and the local large-scale structure of the universe are shown in the figure below (local link / general link: large_scale_structure_z_0x035.html).

    5. The Large-Scale Structure of the Universe:

      Galaxies and larger groupings collectively are called the large-scale structure of the universe.

      The larger structures include galaxy groups, galaxy clusters, galaxy superclusters, galaxy filaments (string-like bands of galaxies and larger groupings), galaxy walls (walls of galaxies and larger groupings), and (cosmic) voids (low density of galaxies). Collectively, galaxies and these larger structures are often referred to as the cosmic web which is essentially a descriptive synonym for the actual existing (as opposed to a just hypothetical) large-scale structure of the observable universe.

      The voids are roughly spherical and have diameters of 10 to 100 Mpc (Wikipedia: voids). The voids are NOT empty, but just have a lower density of dark matter, intergalactic medium (IGM), and galaxies than elsewhere in the large-scale structure of the universe. In fact, they are the "gaps" in the cosmic web.

      The study of the large-scale structure is actually considered part of cosmology.

      The local large-scale structure or cosmic web is illustrated in four figures below:

      1. Figure 1 of the local large-scale structure:

      2. Figure 2 of the local large-scale structure (see the figure below (local link / general link: local_universe_ir.html):

      3. Figure 3 of the local large-scale structure:

      4. Figure 4 of the local large-scale structure (see the figure below (local link / general link: large_scale_structure_z_0x035.html).

  4. The Cosmological Principle

    1. Cosmological Principle Explicated:

      The cosmological principle is explicated in the figure below (local link / general link: cosmological_principle.html).

    2. No End of Greatness?:

      It was once wondered if there was NO limit to structure---NO End of Greatness. That one would keep finding structure on larger scales.

      To explicate, say galaxy superclusters were grouped into Type II superclusters which were grouped into type III superclusters which were ... and so on forever. There would be NO End of Greatness and the mean mass-energy of the universe would go to zero if there were no mass-energy between galaxies. This cosmological model was considered once and was called the hierachical world model (Bo-14--15,19).

      A hierachical world model could even be a fractal.

      A fractal is a thing that is or looks the same on all or many scales at least in some approximation. The branches and roots of trees are approximate fractals.

      For example, one can make a fractal by drawing an iteration of 3 branches from each branch.

      The animation in the figure below (local link / general link: fractal_koch_snowflake.html) illustrates the construction of a fractal.

      But the observable universe is NOT a fractal nor any other kind of hierachical world model so far as we can tell.

      Also, so far as we can tell, the observable universe obeys the cosmological principle: i.e., the assumption that on a large enough distance scale the observable universe is homogeneous (same in all places) and isotropic (same in all directions).

      But there is NO sharply-defined distance scale for the size of the largest structures. However, it is still useful to have a fiducial cosmological principle size scale and the Yadav scale = 370/h_70 Mpc (where reduced Hubble constant h_70=H_0/(70 km/s/Mpc)) can be adopted. It has some theoretical justification (Wikipedia: Cosmological principle: Violations of homogeneity).

      For further explication of the cosmological principle, see Cosmology file: cosmological_principle.html.

  5. Large-Scale Structure Formation and Evolution

    6. We discuss structure formation (i.e., the formation of the large scale structure of the observable universe or the cosmic web) in the insert below (local link / general link: large_scale_structure_formation_outline.html).