144 research outputs found
21-cm signatures of residual HI inside cosmic HII regions during reionization
We investigate the impact of sinks of ionizing radiation on the
reionization-era 21-cm signal, focusing on 1-point statistics. We consider
sinks in both the intergalactic medium and inside galaxies. At a fixed filling
factor of HII regions, sinks will have two main effects on the 21-cm
morphology: (i) as inhomogeneous absorbers of ionizing photons they result in
smaller and more widespread cosmic HII patches; and (ii) as reservoirs of
neutral gas they contribute a non-zero 21-cm signal in otherwise ionized
regions. Both effects damp the contrast between neutral and ionized patches
during reionization, making detection of the epoch of reionization with 21-cm
interferometry more challenging. Here we systematically investigate these
effects using the latest semi-numerical simulations. We find that sinks
dramatically suppress the peak in the redshift evolution of the variance,
corresponding to the midpoint of reionization. As previously predicted,
skewness changes sign at midpoint, but the fluctuations in the residual HI
suppress a late-time rise. Furthermore, large levels of residual HI
dramatically alter the evolution of the variance, skewness and power spectrum
from that seen at lower levels. In general, the evolution of the large-scale
modes provides a better, cleaner, higher signal-to-noise probe of reionization.Comment: Minor edits to agree with MNRAS published versio
CO line emission from galaxies in the Epoch of Reionization
We study the CO line luminosity (), the shape of the CO Spectral
Line Energy Distribution (SLED), and the value of the CO-to-
conversion factor in galaxies in the Epoch of Reionization (EoR). To this aim,
we construct a model that simultaneously takes into account the radiative
transfer and the clumpy structure of giant molecular clouds (GMCs) where the CO
lines are excited. We then use it to post-process state-of-the-art zoomed, high
resolution (), cosmological simulation of a main-sequence
(, ) galaxy, "Alth{\ae}a", at . We find that the CO emission
traces the inner molecular disk () of Alth{\ae}a with
the peak of the CO surface brightness co-located with that of the [CII] 158 emission. Its is comparable
to that observed in local galaxies with similar stellar mass. The high
() gas surface density in
Alth{\ae}a, its large Mach number (\mach), and the warm kinetic
temperature () of GMCs yield a CO SLED peaked at the
CO(7-6) transition, i.e. at relatively high-, and a CO-to-
conversion factor lower than that of the Milky Way. The ALMA observing time
required to detect (resolve) at 5 the CO(7-6) line from galaxies
similar to Alth{\ae}a is h ( h).Comment: 16 pages, 14 figures, accepted for publication in MNRA
Nuclear rings are the inner edge of a gap around the Lindblad Resonance
Gaseous nuclear rings are large-scale coherent structures commonly found at the centres of barred galaxies. We propose that they are an accumulation of gas at the inner edge of an extensive gap that forms around the Inner Lindblad Resonance (ILR). The gap initially opens because the bar potential excites strong trailing waves near the ILR, which remove angular momentum from the gas disc and transport the gas inwards. The gap then widens because the bar potential continuously excites trailing waves at the inner edge of the gap, which remove further angular momentum, moving the edge further inwards until it stops at a distance of several wavelengths from the ILR. The gas accumulating at the inner edge of the gap forms the nuclear ring. The speed at which the gap edge moves and its final distance from the ILR strongly depend on the sound speed, explaining the puzzling dependence of the nuclear ring radius on the sound speed in simulations
A dynamical mechanism for the origin of nuclear rings
We develop a dynamical theory for the origin of nuclear rings in barred
galaxies. In analogy with the standard theory of accretion discs, our theory is
based on shear viscous forces among nested annuli of gas. However, the fact
that gas follows non circular orbits in an external barred potential has
profound consequences: it creates a region of reverse shear in which it is
energetically favourable to form a stable ring which does not spread despite
dissipation. Our theory allows us to approximately predict the size of the ring
given the underlying gravitational potential. The size of the ring is loosely
related to the location of the Inner Lindblad Resonance in the epicyclic
approximation, but the predicted location is more accurate and is also valid
for strongly barred potentials. By comparing analytical predictions with the
results of hydrodynamical simulations, we find that our theory provides a
viable mechanism for ring formation if the effective sound speed of the gas is
low (\cs\lesssim1\kms), but that nuclear spirals/shocks created by pressure
destroy the ring when the sound speed is high (\cs\simeq10\kms). We conclude
that whether this mechanism for ring formation is relevant for real galaxies
ultimately depends on the effective equation of state of the ISM. Promising
confirmation comes from simulations in which the ISM is modelled using
state-of-the-art cooling functions coupled to live chemical networks, but more
tests are needed regarding the role of turbulence driven by stellar feedback.
If the mechanism is relevant in real galaxies, it could provide a powerful tool
to constrain the gravitational potential, in particular the bar pattern speed.Comment: Accepted for publication in MNRA
- β¦