* Astronomy

Blobrana · L

RE: Galaxies

Blobrana · L

When did galaxies settle down?

Astronomers have long sought to understand exactly how the universe evolved from its earliest history to the cosmos we see around us in the present day. In particular, the way that galaxies form and develop is still a matter for debate. Now a group of researchers have used the collective efforts of the hundreds of thousands of people that volunteer for the Galaxy Zoo project to shed some light on this problem. They find that galaxies may have settled into their current form some two billion years earlier than previously thought.
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Blobrana · L

Title: Where Do Galaxies End?
Author: J. Michael Shull

Our current view of galaxies considers them as systems of stars and gas embedded in extended halos of dark matter, much of it formed by the infall of smaller systems at earlier times. The true extent of a galaxy remains poorly determined, with the virial radius (R_vir) providing a characteristic separation between collapsed structures in dynamical equilibrium and external infalling matter. Other physical estimates of the extent of gravitational influence include the gravitational radius, gas accretion radius, and "galactopause" arising from outflows that stall at 100-200 kpc over a range of outflow parameters and confining gas pressures. Physical criteria are proposed to define bound structures, including a more realistic definition of R_vir (M*, M_h, z_a) for stellar mass M* and halo mass M_h, half of which formed at "assembly redshifts" z_a = 0.7-1.3. We estimate the extent of bound gas and dark matter around L* galaxies to be ~200 kpc. The new virial radii, with mean R_vir ~ 200 kpc, are 40-50% smaller than values estimated in recent HST/COS detections of H I and O VI absorbers around galaxies. In the new formalism, the Milky Way stellar mass, log M* = 10.7 +/- 0.1, would correspond to R_vir = 153 (+25,-16) kpc for half-mass halo assembly at z_a = 1.06 +/- 0.03. The frequency per unit redshift of low-redshift O VI absorption lines in QSO spectra suggests absorber sizes ~150 kpc when related to intervening 0.1L* galaxies. This formalism is intended to clarify semantic differences arising from observations of extended gas in galactic halos, circumgalactic medium (CGM), and filaments of the intergalactic medium (IGM). Astronomers should refer to bound gas in the galactic halo or CGM, and unbound gas at the CGM-IGM interface, on its way into the IGM.

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Blobrana · L

UMass scientists explore the age of galaxies with help of NASA and Hubble Space Telescope

Two University of Massachusetts astronomers in a team of scientists have discovered that a lot of galaxies are more mature than they look.

Blobrana · L

A team of astronomers from Cardiff University and the University of Washington Seattle reports in the journal 'Nature' this week that they have discovered that galaxies are much simpler than anybody thought.
Galaxies are enormous whirlpools of stars, and the Sun is but one star in our own Milky Way galaxy which contains a hundred billion others. Galaxies must be older than the stars which have formed inside them and as some stars are at least ninety per cent as old as the Universe, galaxies must have formed near the very beginning, close to the initial Big Bang. The problem with that idea is that the Big Bang was so hot and violent that it would have torn infant galaxies to pieces. So astrophysicists dreamed up the ingenious idea of 'Cold Dark Matter,' some mysterious stuff that would dominate the ordinary material in galaxies and yet be immune to the heat of the Big Bang. What we see of a galaxy today is supposedly the inclusion of ordinary material buried amongst ten times as much of that strange but invisible Cold Dark Matter. And over the past thirty years some astronomers have built elaborate computer models of how galaxies evolve out of this hypothetical mixture of dark and light.

Source: Cardiff University