Evolution of the Ionized Universe
The epoch of reionization in the early universe and the subsequent evolution of ionized material is fundamentally related to
cosmological evolution. New work measures
the mean free path of ionizing photons at
redshifts z > 4.4 and shows strong evolution
in the ionized structures.
Gábor Worseck (Max-Planck-Institut für Astronomie and University of California Santa
Cruz) and collaborators base these results on
a multi-semester, multi-partner study that
obtained observations of quasars using both
Gemini Multi-Object Spectrographs (Figure
3). Quasars serve as bright background continuum sources and are therefore sensitive
to absorption by neutral hydrogen along
the line of sight. Specifically, they measured
the mean free path of photons at the Lyman
limit (λmfp912).
In this first large set of such high-redshift
quasars (numbering 163 targets), the team
divided the sample into redshift bins and
measured the evolution of λmfp912 over time.
They found λmfp912 ∝ (1 + z)-5.4, which is significantly steeper than that due to cosmic
expansion alone [∝ (1 + z)-3]. The team interprets these results as due to large-scale
structures of the universe. The density and
neutral fraction of these filaments decreases with time, accounting for the short distances Lyman continuum photons could
pass freely before absorption in the early
universe.
The smooth evolution observed supports
the expected result that the universe was
substantially ionized by z = 5.2, which is
the highest redshift examined in this study.
Thus, this work does not probe the epoch of
reionization directly. Approaching this period, however, it does set an important constraint on models of reionization, and this
work rules out several models of absorbing
systems in the early universe.
January 2015
The paper is published in Monthly Notices
of the Royal Astronomical Society (viewable
here) and reduced quasar spectra are available here.
The Origins of Massive Field Stars
in the Galactic Center
The existence of massive field stars in the
extreme environment of the Galactic center
raises fundamental questions about their
origin. The challenge is to form these shortlived stars that are not presently located in
sites of obvious recent stellar birth, such as
molecular clouds (which would offer the raw
materials) or clusters (which would show the
effects of concentrated star formation).
Figure 3.
Quasar spectra
averaged over the
redshift intervals [4.4,
4.7], [4.7, 5.0], and [5.0,
5.5] (top to bottom).
The onset of the
Lyman limit is marked
in each case (LL).
The model intrinsic
quasar emission is
overplotted in color.
New work by Hui Dong (National Optical
Astronomy Observatory) and collaborators
provides accurate kinematic measurements
of massive stars and their nearby gas regions. This work suggests some scenarios for
the origin of these massive stars, finding evidence for both ejection from clusters and in
situ formation in different examples.
The sample of eight stars was selected on
the basis of Paschen α emission, characteristic of massive stars. Observations using the
Gemini Near-Infrared Spectrograph (GNIRS)
and Near-infrared Integral Field Spectrom-
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