* Astronomy

Fossil records of cosmic reionization in galactic stellar halos
Authors: Kenji Bekki, Masashi Chiba

Galactic stellar halos have long been considered to contain fossil information on early dynamical and chemical evolution of galaxies. We propose that the surface brightness distributions of old stellar halos contain the influence of reionization on early formation histories of galaxies. By assuming that reionisation significantly suppresses star formation in small subgalactic clumps virialised after reionisation redshift (zreion), we first numerically investigate how structural and kinematical properties of stellar halos formed from merging of these subgalactic clumps depend on zreion. We then discuss what observable properties of galactic stellar halos offer us the fossil records of reionisation influence on hierarchical formation of halos based on the current results of numerical simulations. We particularly
suggest that both the half-light radius of stellar halos and the slope of their surface brightness profile contain useful information on when star formation in subgalactic clumps were significantly influenced by reionisation. By using the simulated surface brightness distributions of galactic stellar halos for models with different zreion, we also discuss how wide-field imaging studies of extragalactic halos will help us to elucidate the epoch of cosmic reionisation.


The timing of the universe's reionisation left its mark on the Milky Way's stellar halo, say astronomers in Australia and Japan. The sooner this reionisation occurred, the more concentrated our galaxy's stellar halo should be.

For most of its early life, the universe was not ionised: It had cooled off from the Big Bang, and its hydrogen and helium had their full complement of electrons. Then, the first stars and quasars began to shine, ripping electrons from the hydrogen and helium. However, the exact timing of this reionisation is unknown.
It may have occurred as early as redshift 30, corresponding to a time just 100 million years after the Big Bang.
Or, it may have occurred as late as redshift 6, when the universe was 900 million years old.

Now, Kenji Bekki of the University of New South Wales in Sydney, Australia, and Masashi Chiba of Tohoku University in Sendai, Japan, have proposed a way to determine when reionisation occurred: Observe how concentrated or spread out a galaxy's stellar halo is.
Their work will be published in a future issue of Astrophysical Journal Letters.




If the universe reionised at redshift 15, it should have created compact stellar halos (left). But if the universe reionised later, at redshift 6, stellar halos should be more diffuse (right). The Milky Way's actual stellar halo matches the left model, suggesting the universe reionised around 260 million years after the Big Bang.
Reionisation affects the observed µB distribution of stellar halos. To derive these µB distributions, we divide the 60kpc × 60kpc halo region of a simulation into 50 × 50 meshes and thereby estimate µB for each mesh according to the photometric model described in the main text. We also smooth out the 2D distributions by using a Gaussian kernel with the smoothing length of 6 kpc.

The stellar halo is the Milky Way's oldest star population. Its brightest members are iron-poor globular star clusters. However, individual halo stars — such as Kapteyn's Star in Pictor, and Groombridge 1830 in Ursa Major — far outnumber cluster stars.
Although the stellar halo envelops the Milky Way's disk, most halo stars and clusters reside closer to the galaxy's centre than Sun does.

According to Bekki and Chiba, this concentration is a direct result of the epoch when the universe reionised itself. They conducted computer simulations of a developing galaxy as it came together in bits and pieces.
The sooner reionisation occurred; the sooner the galaxy's fragments ceased star formation. That's because an ionized universe allows extreme ultraviolet radiation to pass through, which destroys the cold molecular gas that gives birth to new stars.

Stars formed first in the densest of these clumps of gas and dust. If reionisation occurred early, then stars formed in only the densest clumps before star formation ceased; these clumps then merged to form a stellar halo. Smaller clumps joined the galaxy later; they added material to the outer part of the stellar halo.

Stars from these smaller clumps, if present, would have smeared out the stellar halo. But if reionisation occurred early, these latecomers would be starless and wouldn't alter the stellar halo's tight concentration.

The astronomers say the Milky Way's actual stellar halo matches their model in which reionisation occurred at redshift 15.
This corresponds to a time 260 million years after the Big Bang. In particular, this model yields a stellar halo whose density varies as R-3.5, where R is the distance from the Milky Way's centre.
In words, the equation says the number of halo stars per volume near the Sun is only 8.8 percent what it is at half the Sun's distance from the galactic centre. They note this relation holds throughout the Milky Way's stellar halo.

Bekki and Chiba suggest astronomers use large ground-based telescopes, such as Subaru in Hawaii, to observe the stellar halos of other giant spiral galaxies. These observations should further constrain the epoch of reionisation.

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