For this sample, rising rotation curves are the general rule. Thus the smallest…exhibit the same lack of a Keplerian velocity decrease at large R as do the high-luminosity spirals. This form for the rotation curves implies that the mass is not centrally condensed, but that significant mass is located at large R. The integral mass is increasing at least as fast as R.
The mass is not converging to a limiting mass at the edge of the optical image. The conclusion is inescapable that non-luminous matter exists beyond the optical galaxy. If we could observe beyond the optical image, especially for the smaller galaxies, would the velocities continue to rise, following the curve defined by those galaxies whose luminous matter extends to much larger nuclear distances? Is the luminous matter only a minor component of the total galaxy mass? The companions ceased to be invisible in , when Alvan G.
Then, in , Zwicky concluded that the velocity dispersions in rich clusters of galaxies required 10 to times more mass to keep them bound than could be accounted for by the luminous galaxies themselves. The low-mass torch, upheld for a time by Burbidge and Woltjer , has recently been refueled by Valtonen Freeman reported in ], suggesting that the light edge is also the mass edge.
These arguments include a the flaring of the Galactic H I disk ; b detailed deconvolutions of both the rotation curve 36, and surface photometry into several Galactic components, including nucleus, bulge, thin Population I disk, thick old Population I disk, Population II spheroid and dark halo; and c the need for a spheroidal or hot component not necessarily dark to stabilize bar modes in a thing cold disk , , Unfortunately, the only discriminations so far suggested are likely to be believed only by those who already believed a particular answer anyway.
A signature for bound but noninteracting systems is badly needed! True stars, which derive most of their energy from nuclear reactions, extend down to 0. Clearly it cannot be clustered or clumped, but it can mimic dark matter in clusters in some ways Primordial black holes could also dominate OMEGA , , and the extent to which they might cluster depends upon their unknown velocities.
Because PBHs must form very early, if at all, they do not count as baryonic matter in the context of nucleosynthesis and are not conspicuous in any other way either. Their main virtue is that of producing the required effect over a range of distance scales.
This has been attempted through the assumption of massive particles that decay to relativistic ones Section 6. Existing or proposed observations can constrain, or possibly provide evidence for, the presence of several possible kinds of DM. Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars.
They have quasar-like nuclei very luminous, distant and bright sources of electromagnetic radiation with very high surface brightnesses but unlike quasars, their host galaxies are clearly detectable. Seen in visible light, most Seyfert galaxies look like normal spiral galaxies, but when studied under other wavelengths, the luminosity of their cores is equivalent to the luminosity of whole galaxies the size of the Milky Way. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared to be similar to stars, rather than extended sources similar to galaxies.
Their luminosity can be times greater than that of the Milky Way. LIRGs are more abundant than starburst galaxies, Seyfert galaxies and quasi-stellar objects at comparable total luminosity. Infrared galaxies emit more energy in the infrared than at all other wavelengths combined.
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Galaxies have magnetic fields of their own. Magnetic fields provide the transport of angular momentum required for the collapse of gas clouds and hence the formation of new stars. For comparison, the Earth's magnetic field has an average strength of about 0. Galactic formation and evolution is an active area of research in astrophysics. Current cosmological models of the early Universe are based on the Big Bang theory.
About , years after this event, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral non-ionized and readily absorbed light, and no stars had yet formed. As a result, this period has been called the " dark ages ". It was from density fluctuations or anisotropic irregularities in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos.
Evidence for the early appearance of galaxies was found in , when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6. In December , astronomers reported that UDFj is the most distant object known and has a redshift value of The object, estimated to have existed around " million years"  after the Big Bang which was about The existence of such early protogalaxies suggests that they must have grown in the so-called "dark ages".
The light from EGS-zs has taken 13 billion years to reach Earth, and is now 30 billion light-years away, because of the expansion of the universe during 13 billion years. The detailed process by which early galaxies formed is an open question in astrophysics. Theories can be divided into two categories: top-down and bottom-up.
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In top-down correlations such as the Eggen—Lynden-Bell—Sandage [ELS] model , protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years. Once protogalaxies began to form and contract, the first halo stars called Population III stars appeared within them. These were composed almost entirely of hydrogen and helium, and may have been massive.
If so, these huge stars would have quickly consumed their supply of fuel and became supernovae , releasing heavy elements into the interstellar medium. Such stars are likely to have existed in the very early universe i. Within a billion years of a galaxy's formation, key structures begin to appear. Globular clusters , the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form.
The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added. During the following two billion years, the accumulated matter settles into a galactic disc. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets. The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology.
However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Although the Milky Way has never collided with a galaxy as large as Andromeda before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing. Such large-scale interactions are rare.
As time passes, mergers of two systems of equal size become less common.
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Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked approximately ten billion years ago. Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms.
At the end of the stellar age, galaxies will be composed of compact objects : brown dwarfs , white dwarfs that are cooling or cold " black dwarfs " , neutron stars , and black holes. Eventually, as a result of gravitational relaxation , all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions. Deep sky surveys show that galaxies are often found in groups and clusters. Solitary galaxies that have not significantly interacted with another galaxy of comparable mass during the past billion years are relatively scarce.
Isolated galaxies [note 2] can produce stars at a higher rate than normal, as their gas is not being stripped by other nearby galaxies.wytotozu.tk
Messier 74 – the NGC 628 Spiral Galaxy
On the largest scale, the Universe is continually expanding, resulting in an average increase in the separation between individual galaxies see Hubble's law. Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early in the Universe, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This on-going merger process as well as an influx of infalling gas heats the inter-galactic gas within a cluster to very high temperatures, reaching 30— megakelvins.
Most galaxies in the Universe are gravitationally bound to a number of other galaxies. These form a fractal -like hierarchical distribution of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster, and these formations contain a majority of the galaxies as well as most of the baryonic mass in the Universe.
If there is insufficient kinetic energy , however, the group may evolve into a smaller number of galaxies through mergers. Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity. Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the supercluster scale , galaxies are arranged into sheets and filaments surrounding vast empty voids.
The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf companions of these two galaxies. The peak radiation of most stars lies in the visible spectrum , so the observation of the stars that form galaxies has been a major component of optical astronomy. It is also a favorable portion of the spectrum for observing ionized H II regions , and for examining the distribution of dusty arms.
The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared , which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail.