23 research outputs found
The impact of metallicity and X-rays on star formation
Star formation is regulated through a variety of feedback processes. In this
study, we treat feedback by metal injection and a UV background as well as by
X-ray irradiation. Our aim is to investigate whether star formation is
significantly affected when the ISM of a proto-galaxxy enjoys different
metallicities and when a star forming cloud resides in the vicinity of a strong
X-ray source. We perform cosmological Enzo simulations with a detailed
treatment of non-zero metallicity chemistry and thermal balance. We also
perform FLASH simulations with embedded Lagrangian sink particles of a
collapsing molecular cloud near a massive, 10^{7} M\odot, black hole that
produces X-ray radiation. We find that a multi-phase ISM forms for metallicites
as small as 10^{-4} Solar at z = 6, with higher (10^{-2}Z\odot) metallicities
supporting a cold ( 10^{3} cm^{-3}) phase at higher (z =
20) redshift. A star formation recipe based on the presence of a cold dense
phase leads to a self-regulating mode in the presence of supernova and
radiation feedback. We also find that when there is strong X-ray feedback a
collapsing cloud fragments into larger clumps whereby fewer but more massive
protostellar cores are formed. This is a consequence of the higher Jeans mass
in the warm (50 K, due to ionization heating) molecular gas. Accretion
processes dominate the mass function and a near-flat, non-Salpeter IMF results.Comment: Proceedings IAU Symposium No. 270, 2010. 4 pages, 5 figure
Songlines from Direct Collapse Seed Black Holes: Effects of X-rays on Black Hole Growth and Stellar Populations
In the last decade, the growth of supermassive black holes (SMBHs) has been
intricately linked to galaxy formation and evolution and is a key ingredient in
the assembly of galaxies. To investigate the origin of SMBHs, we perform
cosmological simulations that target the direct collapse black hole (DCBH) seed
formation scenario in the presence of two different strong Lyman-Werner (LW)
background fields. These simulations include the X-ray irradiation from a
central massive black hole (MBH), self-shielding and stellar
feedback from metal-free and metal-enriched stars. We find in both simulations
that local X-ray feedback induces metal-free star formation Myr
after the MBH forms. The MBH accretion rate reaches a maximum of
yr in both simulations. However, the duty cycle differs
which is derived to be and for high and low LW cases,
respectively. The MBH in the high LW case grows only in 100 Myr
compared to in the low LW case. We find that the maximum accretion rate
is determined by the local gas thermodynamics whereas the duty cycle is
determined by the large scale gas dynamics and gas reservoir. We conclude that
radiative feedback from the central MBH plays an important role in star
formation in the nuclear regions and stifling initial MBH growth, relative to
the typical Eddington rate argument, and that initial MBH growth might be
affected by the local LW radiation field.Comment: 8 pages, 6 figures. Accepted for publication in ApJ, with minor
changes to submitted versio
Can the intergalactic medium cause a rapid drop in Lyman alpha emission at z>6?
The large cross-section of the Lyman alpha (Lya) line makes it a sensitive
probe of the ionization state of the intergalactic medium (IGM). Here we
present the most complete study to date of the IGM Lya opacity, and its
application to the redshift evolution of the 'Lya fraction', i.e. the fraction
of color-selected galaxies with a detectable Lya emission line. We use a tiered
approach, which combines large-scale semi-numeric simulations of reionization
with moderate-scale hydrodynamic simulations of the ionized IGM. This allows us
to simultaneously account for evolution in both: (i) the opacity from an
incomplete (patchy) reionization, parameterized by the filling factor of
ionized regions, Q_HII; and (ii) the opacity from self-shielded systems in the
ionized IGM, parameterized by the average photo-ionization rate inside HII
regions, \Gamma. In contrast to recent empirical models, attenuation from
patchy reionization has a unimodal distribution along different sightlines,
while attenuation from self-shielded systems is more bimodal. We quantify the
average IGM transmission in our (Q_HII, \Gamma) parameter space, which can
easily be used to interpret new data sets. Using current observations, we
predict that the Lya fraction cannot drop by more than a factor of ~2 with IGM
attenuation alone, even for HII filling factors as low as Q_HII>0.1. Larger
changes in the Lya fraction could result from a co-evolution with galaxy
properties. Marginalizing over \Gamma, we find that current observations
constrain Q_HII < 0.6 at z=7 [68% confidence level (C.L.)]. However, all of our
parameter space is consistent with observations at 95% C.L., highlighting the
need for larger observational samples at z >= 6.Comment: 12 pages, 10 figures, MNRAS submitte