81 research outputs found
Statistical predictions for the first black holes
The recent observations of supermassive black holes (SMBHs) at high redshift
challenge our understanding of their formation and growth. There are different
proposed pathways to form black hole (BH) seeds, such as the remnants of the
first stars (chapter 4), gas-dynamical processes (chapter 5), direct collapse
(chapter 6), or stellar collisions in dense nuclear clusters (chapter 7). In
this chapter, we discuss the probability of forming supermassive black holes
(SMBHs) via these channels and determine the expected number density of the BH
seeds. We start with a brief discussion of the observational constraints on
SMBHs at low and high redshift that theoretical models have to reproduce (a
more detailed account is provided in chapter 12). We further present the most
popular formation channels of SMBHs, discuss under which conditions they can
reproduce the observations, and compare various estimates in the literature on
the expected number density of SMBHs. To account for the density of quasars at
requires very efficient gas accretion mechanisms or high BH seeds masses.
The bottleneck to obtain sufficiently high number densities of seed BHs with
masses M is the interplay between radiative and chemical
feedback, which constrains the conditions for primordial, isothermal gas
collapse.Comment: Preprint of the chapter "Statistical predictions for the first black
holes", to be published in the review volume "Formation of the First Black
Holes", Latif, M. and Schleicher, D. R. G., eds., World Scientific Publishing
Company, 2018, pp 161-175 [see
https://www.worldscientific.com/worldscibooks/10.1142/10652
Active Galactic Nuclei outflows in galaxy discs
Galactic outflows, driven by active galactic nuclei (AGN), play a crucial
role in galaxy formation and in the self-regulated growth of supermassive black
holes (BHs). AGN feedback couples to and affects gas, rather than stars, and in
many, if not most, gas-rich galaxies cold gas is rotationally supported and
settles in a disc. We present a 2D analytical model for AGN-driven outflows in
a gaseous disc and demonstrate the main improvements, compared to existing 1D
solutions. We find significant differences for the outflow dynamics and wind
efficiency. The outflow is energy-driven due to inefficient cooling up to a
certain AGN luminosity (erg/s in our fiducial model), above which
the outflow remains momentum-driven in the disc up to galactic scales. We
reproduce results of 3D simulations that gas is preferentially ejected
perpendicular to the disc and find that the fraction of ejected interstellar
medium is lower than in 1D models. The recovery time of gas in the disc,
defined as the freefall time from the radius to which the AGN pushes the ISM at
most, is remarkably short, of the order 1Myr. This indicates that AGN-driven
winds cannot suppress BH growth for long. Without the inclusion of supernova
feedback, we find a scaling of the black hole mass with the halo velocity
dispersion of .Comment: 22 pages, 22 figures, published in MNRA
Witnessing the birth of a supermassive protostar
The detection of quasars reveals the existence of supermassive
black holes of a few . One of the potential pathways to
explain their formation in the infant universe is the so-called direct collapse
model which provides massive seeds of . An isothermal
direct collapse mandates that halos should be of a primordial composition and
the formation of molecular hydrogen remains suppressed in the presence of a
strong Lyman Werner flux. In this study, we perform high resolution
cosmological simulations for two massive primordial halos employing a detailed
chemical model which includes cooling as well as realistic opacities
for both the bound-free emission and the Rayleigh scattering of
hydrogen atoms. We are able to resolve the collapse up to unprecedentedly high
densities of and to scales of about AU.
Our results show that the gas cools down to 5000 K in the presence
of cooling, and induces fragmentation at scales of about 8000 AU in
one of the two simulated halos, which may lead to the formation of a binary. In
addition, fragmentation also occurs on the AU scale in one of the halos but the
clumps are expected to merge on short time scales. Our results confirm that
cooling does not prevent the formation of a supermassive star and the
trapping of cooling radiation stabilises the collapse on small scales.Comment: Accpeted version, to appear in MNRAS, comments are still welcome and
high resolution version is available at
http://www2.iap.fr/users/latif/DCBH.pd
Constraining the primordial initial mass function with stellar archaeology
We present a new near-field cosmological probe of the initial mass function
(IMF) of the first stars. Specifically, we constrain the lower-mass limit of
the Population III (Pop III) IMF with the total number of stars in large,
unbiased surveys of the Milky Way. We model the early star formation history in
a Milky Way-like halo with a semi-analytic approach, based on Monte-Carlo
sampling of dark matter merger trees, combined with a treatment of the most
important feedback mechanisms. Assuming a logarithmically flat Pop III IMF and
varying its low mass limit, we derive the number of expected survivors of these
first stars, using them to estimate the probability to detect any such Pop III
fossil in stellar archaeological surveys. Following our analysis, the most
promising region to find possible Pop III survivors is the stellar halo of the
Milky Way, which is the best target for future surveys. We find that if no
genuine Pop III survivor is detected in a sample size of () halo stars with well-controlled selection effects, then we can
exclude the hypothesis that the primordial IMF extended down below at a confidence level of 68% (99%). With the sample size of the
Hamburg/ESO survey, we can tentatively exclude Pop III stars with masses below
with a confidence level of 95%, although this is subject to
significant uncertainties. To fully harness the potential of our approach,
future large surveys are needed that employ uniform, unbiased selection
strategies for high-resolution spectroscopic follow-up.Comment: 19 pages, 14 figures, published in MNRA
A Photon Burst Clears the Earliest Dusty Galaxies: Modelling Dust in High-redshift Galaxies from ALMA to JWST
The generation and evolution of dust in galaxies are important tracers for
star formation, and can characterize the rest-frame ultraviolet to infrared
emission from the galaxies. In particular understanding dust in high-redshift
galaxies are important for observational cosmology, as they would be necessary
to extract information on star formation in the early universe. We update the
public semi-analytical model A-SLOTH (Ancient Stars and Local Observables by
Tracing Halos) to model the evolution of dust, focusing on high-redshift
star-forming galaxies with stellar masses of --
observed by ALMA () and JWST (). We find that these
galaxies should qualitatively differ in their star formation properties; while
the samples in ALMA are explained by dust growth in normal star-forming
galaxies, the lack of dust in the samples by JWST requires dust ejection by
radiation pressure due to recent highly efficient star-formation within a few
10 Myr, with order 100 times higher efficiency than normal galaxies calibrated
by A-SLOTH. Depending on where the JWST galaxies locate on the luminosity
function, their bursty star formation histories inferred from our model can
have impacts for rates of star formation, supernova explosion, stellar
feedback, and detectability of dusty, mature galaxies in the very early
universe.Comment: 13 pages, 7 figures. Revised after MNRAS referee report. Comments
welcom
The Galaxy Assembly and Interaction Neural Networks (GAINN) for high-redshift JWST observations
We present the Galaxy Assembly and Interaction Neural Networks (GAINN), a
series of artificial neural networks for predicting the redshift, stellar mass,
halo mass, and mass-weighted age of simulated galaxies based on JWST
photometry. Our goal is to determine the best neural network for predicting
these variables at . The parameters of the optimal neural
network can then be used to estimate these variables for real, observed
galaxies. The inputs of the neural networks are JWST filter magnitudes of a
subset of five broadband filters (F150W, F200W, F277W, F356W, and F444W) and
two medium-band filters (F162M and F182M). We compare the performance of the
neural networks using different combinations of these filters, as well as
different activation functions and numbers of layers. The best neural network
predicted redshift with normalized root mean squared error NRMS =
, stellar mass with RMS = ,
halo mass with MSE = , and mass-weighted age with RMS
= . We also test the performance of GAINN on real
data from MACS0647-JD, an object observed by JWST. Predictions from GAINN for
the first projection of the object (JD1) have mean absolute errors , which is significantly smaller than with
template-fitting methods. We find that the optimal filter combination is F277W,
F356W, F162M, and F182M when considering both theoretical accuracy and
observational resources from JWST.Comment: 19 pages, 6 figures, submitted to Ap
How an improved implementation of H2 self-shielding influences the formation of massive stars and black holes
High redshift quasars at z>6 have masses up to ~ M. One of the
pathways to their formation includes direct collapse of gas, forming a
supermassive star, precursor of the black hole seed. The conditions for direct
collapse are more easily achievable in metal-free haloes, where atomic hydrogen
cooling operates and molecular hydrogen (H2) formation is inhibited by a strong
external UV flux. Above a certain value of UV flux (J_crit), the gas in a halo
collapses isothermally at ~ K and provides the conditions for
supermassive star formation. However, H2 can self-shield, reducing the effect
of photodissociation. So far, most numerical studies used the local Jeans
length to calculate the column densities for self-shielding. We implement an
improved method for the determination of column densities in 3D simulations and
analyse its effect on the value of J_crit. This new method captures the gas
geometry and velocity field and enables us to properly determine the
direction-dependent self-shielding factor of H2 against photodissociating
radiation. We find a value of J_crit that is a factor of two smaller than with
the Jeans approach (~2000 J_21 vs. ~4000 J_21). The main reason for this
difference is the strong directional dependence of the H2 column density. With
this lower value of J_crit, the number of haloes exposed to a flux >J_crit is
larger by more than an order of magnitude compared to previous studies. This
may translate into a similar enhancement in the predicted number density of
black hole seeds.Comment: 14 pages, 12 figures, published in MNRA
On the Detection of Supermassive Primordial Stars. II. Blue Supergiants
Supermassive primordial stars in hot, atomically-cooling haloes at
15 - 20 may have given birth to the first quasars in the universe. Most
simulations of these rapidly accreting stars suggest that they are red, cool
hypergiants, but more recent models indicate that some may have been bluer and
hotter, with surface temperatures of 20,000 - 40,000 K. These stars have
spectral features that are quite distinct from those of cooler stars and may
have different detection limits in the near infrared (NIR) today. Here, we
present spectra and AB magnitudes for hot, blue supermassive primordial stars
calculated with the TLUSTY and CLOUDY codes. We find that photometric
detections of these stars by the James Webb Space Telescope (JWST) will be
limited to 10 - 12, lower redshifts than those at which red stars
can be found, because of quenching by their accretion envelopes. With moderate
gravitational lensing, Euclid and the Wide-Field Infrared Space Telescope
(WFIRST) could detect blue supermassive stars out to similar redshifts in
wide-field surveys.Comment: 9 pages, 5 figures, accepted by MNRA
Predicting the locations of possible long-lived low-mass first stars: Importance of satellite dwarf galaxies
The search for metal-free stars has so far been unsuccessful, proving that if
there are surviving stars from the first generation, they are rare, they have
been polluted, or we have been looking in the wrong place. To predict the
likely location of Population~III (Pop~III) survivors, we semi-analytically
model early star formation in progenitors of Milky Way-like galaxies and their
environments. We base our model on merger trees from the high-resolution dark
matter only simulation suite \textit{Caterpillar}. Radiative and chemical
feedback are taken into account self-consistently, based on the spatial
distribution of the haloes. Our results are consistent with the non-detection
of Pop III survivors in the Milky Way today. We find that possible surviving
Population III stars are more common in Milky Way satellites than in the main
Galaxy. In particular, low mass Milky Way satellites contain a much larger
fraction of Pop~III stars than the Milky Way. Such nearby, low mass Milky Way
satellites are promising targets for future attempts to find Pop~III survivors,
especially for high-resolution, high signal-to-noise spectroscopic
observations. We provide the probabilities for finding a Pop~III survivor in
the red giant branch phase for all known Milky Way satellites to guide future
observations.Comment: 17 pages, 12 figures, 1 table, submitted to MNRA
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