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Proto-Cluster SSA22 (2019)

The SSA22 proto-cluster located at z = 3.1 was originally discovered in 1998. Its context within the cosmic structure is shown in Figure 03-05g. The time in Timeline (a) is measured from the Big Bang, while in Timeline (b) it flows in the opposite direction from the present. Both diagrams together show roughly the evolution sequence in a bottom up scenario from First Stars Most Distant Galaxy Most Distant Quasar Proto-cluster (SSA22) Supercluster with components in the form of Galactic Cluster (+ gaseous filaments), which can be broken down further into Galaxies (the basic cosmological units) represented in zoom in view in Figure 03-05g.
Cosmic Timeline Cosmic Timeline 2

Figure 03-05g Cosmic Timeline (a) [view large image]

Timeline (b)

Lyman-alpha Red-Shift The "drop-out technique" is an observational tool to search for high red-shift astronomical objects with z 3. It relies on the fact that the Lyman series in the hydrogen spectral lines is normally blocked by atmospheric absorption. However, for emission or absorption from high red-shift objects, these spectral lines are shifted into the atmospheric window which allows them to go through (see Figure 03-05h for the spectrum in rest and observer's frames, the range of wavelengths within the atmospheric window, and the corresponding range of red-shift z). The Lyman-alpha line at 1216 is usually the strongest component in the series, thus observations are mainly looking for such line. As an illustration of the technique, Figure 03-05h shows the "Lyman Break" and absorption lines from a galaxy at z = 3.2 for which the Lyman-alpha line has been shifted to 5107 (1 = 10-8 cm).

The various kind of astronomical objects in the SSA22 proto-cluster and beyond are shown in Figure 03-05i. A brief description for each one is in the followings :

Figure 03-05h Lyman-alpha Red-Shift [view large image]

High Red-Shift Objevts

Figure 03-05i High Red-Shift Objects [view large image]
  • LAE (Lyman-alpha Emitter) - It is a type of distant galaxy that emits Lyman-alpha radiation from neutral hydrogen. The Lyman-alpha line in most LAEs is thought to be caused by recombination of interstellar hydrogen that is ionized by an ongoing burst of star-formation. LAEs are typically low mass galaxies of 108 - 1010 solar masses. They are typically young galaxies that are 200 to 600 million years old, and have the highest specific star formation rate of any galaxies known. All of these properties indicate that the LAEs are younger than the LBGs and related to the progenitors of modern galaxies. Figure 03-05j shows the emission line of two LAEs at high red-shift.
  • Figure 03-05j LAE

  • LBG (Lyman Break Galaxy) - A LBG can be identified by looking for galaxies that are visible in longer wavelength filters, but are missing from lower wavelength filters. The UV end of the Lyman series at 912 (rest frame) shows up as a "Break" in the emission spectrum. They are identified as star-burst galaxies at high red-shifts, approximately in the range 2.5 < z < 8. Figure 03-05k shows the Lyman Break of a Galaxy at z = 7. There is a sharp drop at 1216 (rest frame) due to the strong absorption by intervening neutral hydrogen. The colorful labels denote the filters on-board the HST and Spitzer.
  • Figure 03-05k LBG

  • LAB (Lyman-alpha Blob) - Young galaxies with high rates of star formation were the source of the LAEs, then scattering by the gaseous blobs around them forms the LAB. They are some of the largest known individual objects in the Universe (Figure 03-05l). Some of these gaseous structures are more than 50 kpc across (size of Milky Way ~ 15 kpc). See the latest (2018) observation in "What is a Lyman-alpha Blob?".
  • Figure 03-05l LAB

  • LyC (Lyman-alpha Continuum) - LyCs are the photons emitted from stars at energies above the Lyman limit with wavelength shorter than 912 . Neutral hydrogen atom is ionized by absorbing such radiation. This is the same process for re-ionization of the universe
    Reionization Era LyC (Figure 03-05m). It seems that such process still lingering on into the stage of proto-cluster formation. The escape fraction of the LyC depends on the environment as shown by the simulations in "Understanding the Escape of LyC and Lya Photons from Turbulent Clouds". Figure 03-05n shows the escape fraction in time scale of million years for different mass of the central stars in a cloud of 106 Msun and size ~ 50 pc.

    Figure 03-05m Reionization Era [view large image]

    Figure 03-05n LyC

    The insert shows the appearance of the cloud (blue at temperature of 10K, red 106K, in 1, 3, 5 Myr time frames). As time progressed, the LyC ionized more neutral hydrogen atoms allowing the radiation to escape and the temperature getting higher.

  • NIR (Near Infrared Massive Red Galaxy) - The development of large telescopes with good image quality and large NIR detectors has made it possible to select faint high red-shift galaxies by infrared light. It is found that the Balmer break at 4000 (shifted to 12000 within the near infrared range of 8000 - 25000 ) can be used to select high-red-shift galaxies at z > 2. The simplest interpretation for the NIR sources is to link these red galaxies directly to LBGs. They may be the same kind of LBGs viewed along dusty lines of sight or during intermittent epochs of low star formation - a hypothesis to be confirmed by observation(s).
  • NIR Alternatively, the NIRs may have been LBGs at higher redshifts, and they may have become redder because of a decline in the star formation, an increase in age, and an increase of metallicity (and thereby dust). All three factors are expected to play a role in realistic galaxy evolution models. According to a paper on "NIR SPECTROSCOPIC OBSERVATION OF MASSIVE GALAXIES IN THE PROTO-CLUSTER AT z = 3.09", the reddest protocluster galaxies are massive galaxies with Mstar ~ 1011 Msun. showing quiescent star formation activities and plausibly dominated by old stellar populations. Most of these massive quiescent galaxies host moderately luminous AGNs detected by X-ray.

    Figure 03-05o High Z NIR
    [view large image]

    Figure 03-05o shows the Js-Ks red passband filter response to Balmer red-shift. The solid curves indicate single-age stellar populations with ages of 0.25, 0.5, and 1 Gyr. The Balmer break can be detected only for those with red-shift z > 2. The dotted and dashed curves indicate models with continuous star
    formation with ages and reddenings of 1 Gyr, and 100 Myr respectively. Many galaxies with continuous star formation will not reach the Js-Ks = 2.3 detection threshold, unless they are even older or have larger reddening. The dash-dotted curve indicates the color evolution of a single-burst population that formed at z = 5, and it also satisfies the color criterion above z = 2. The insert portrays a cross-section of the observational data of SSA22, the filled red pentagons show the galaxies confirmed at 3.04 < z < 3.12, while the open red pentagons for those at z < 3.04 or z > 3.12. The gray crosses are the targets with unidentified red-shifts. The green contours outline the surface density levels of LAEs at z = 3.09 while the radius of the circle corresponds to a physical scale of 1.5 Mpc (also see Figure 03-05i).

  • Other (more dusty, more massive) - such as :
  • Massive Filaments - Simulation shows that gas filaments led to the creation of galaxy clusters. This allowed galaxies to form in areas where filaments crossed. These galactic crossroads also created dense areas of matter. It is estimated that 60% of the hydrogen created during the Big Bang exists in these cosmic threads that help form a structured web. The long filaments can extend for more than a million parsecs. When astronomers looked at where the gigantic filaments crossed, they found the super-massive black holes acting as engines of galaxy formation. They also spied galaxies that were actively forming stars. The galaxies are fed by streams of cooling gas moving to these crossroads.
  • SSA22 Filaments It is only recently in 2019 that the Multi Unit Spectroscopic Explorer instrument on the European Southern Observatory's Very Large Telescope enabled the astronomers to detect faint Lyman alpha radiation in the form of filament wisps created by energized hydrogen gas from the galaxies in the SSA22 proto-cluster. The left side of Figure 03-05r is a map of the gas filaments in blue color; the schematic diagram on the right shows some of the objects such as SMGs, AGNs and SNRs (Supernova Remnants) within the filaments.

    Figure 03-05r
    SSA22 Filaments

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