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Galaxies


Extremely Red Objects (ERO)

The Hercules Deep Field provides a detailed view of hundreds of distant galaxies. One particular object called Extremely Red Object (ERO, now renamed to "Hot DOG" - for Hot Dust-Obscured Galaxies) is marked with the yellow box as shown in Figure 05-06a. This type of galaxies is generally faint in the visible light, but can be very bright in the infrared. The six images below show how different the same object can appear from visible blue light (left-most image), to well into the infrared (far-right). This object appears to have achieved its extreme red color because the bulk of its star formation has been reddened with a thick layer of dust. This galaxy is believed to lie about 9 billion light years away, at a time when the universe was only a third of its present age. It is estimated that this galaxy has around 100 billion stars and may in fact be a very distant mirror -- an analog of our own Milky Way Galaxy in its formative years. Combining data over a period of 3 years obtained at UKIRT, astronomers in 2008 have produced an
ERO Infrared Galaxies Hot DOG image containing over 100,000 galaxies (Figure 05-06b). Many of the faint red objects in the background (against a relatively nearby spiral galaxy) are massive galaxies

Figure 05-06a ERO
[view large image]

Figure 05-06b Infrared Galaxies [view large image]

Figure 05-06c Hot DOG
[view large image]

at distances of over 10 billion light-years.

In 2012, news reported that the Wide-field Infrared Survey Explorer (WISE) designed for detecting infrared oddities over all sky, has found millions of supermassive black holes at distance of about 10 billion light years away. In addition, there are about 1000 dust-obscured galaxies with very high temperature dubbed "Hot DOG" (inside circles, Figure 05-06c). These are the same type of astronomical objects called ERO. Further observations are needed to determine the evolutionary sequence between the supermassive black hole and Hot DOG (see "Formation and Evolution of Galaxy").

Furthest Galaxies Furthest Cluster of Galaxies In August 2009 the rejuvenated Hubble Telescope took an infrared deep field image (Figure 05-07a). It shows many small galaxies with redshift of up to 8.5 corresponding to 13.1 billion light years from us or about 600 million years after the Big Bang. Their size and mass are about 1/20 and 1/100 of those of the Milky Way respectively. Although detected in the near infrared region of the spectrum, they are intrinsically blue (before the redshift) - meaning that they may be

Figure 05-07a Furthest Galaxies [view large image]

Figure 05-07b Furthest Cluster of Galaxies [view large image]

deficient in heavier elements, i.e., made with primordial matter, and as a result, quite free of the dust that reddens light through scattering. The discovery of these galaxies so near the
beginning of the "reionization epoch" (Figure 05-07c) with the seemingly insufficient radiation output raises the possibility that there were more efficient processes to ionize the neutral hydrogen in an even earlier epoch unobserved and unknown to us yet.
Reionization 2013 Version Hubble Ultra-Deep Field One possibility is that there have been a large population of unseen small galaxies working together to do the job. NASA is now pursuing a tactic which uses high-mass cluster of foreground galaxies as cosmic zoom lenses to see further (with the existing Hubble telescope). Meanwhile in Chile, ALMA will ioin the hunt for distant galaxies starting from summer 2013. The 6.5-meter JWST (to be launched in 2018) will image the faint, primitive

Figure 05-07c Reionization (2013 Version) [view large image]

Figure 05-07d HUDF [view large image]

bodies that Hubble can only glimpse (see Hubble Ultra-Deep Field, Figure 05-07d).

Figure 05-07b shows the X-ray image of the furthest galaxy cluster (appeared as blue diffuse gas) at redshift 1.9 corresponding to 3.7 billion years after the Big Bang. The delayed appearance of the cluster together with the smaller size of the early galaxies lend support to the "bottom up" theory (see below), which suggests smaller units merge and form larger ones.

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