204 research outputs found
Angular momentum, accretion and radial flows in chemodynamical models of spiral galaxies
Gas accretion and radial flows are key ingredients of the chemical evolution
of spiral galaxies. They are also tightly linked to each other (accretion
drives radial flows, due to angular momentum conservation) and should therefore
be modelled simultaneously. We summarise an algorithm that can be used to
consistently compute accretion profiles, radial flows and abundance gradients
under quite general conditions and we describe illustrative applications to the
Milky Way. We find that gas-phase abundance gradients strongly depend on the
angular momentum of the accreting material and, in the outer regions, they are
significantly affected by the choice of boundary conditions.Comment: 4 pages, 2 figures. Proceedings of the 592 WE-Heraeus Seminar. To
appear in Astronomische Nachricthen, special issue "Reconstructing the Milky
Way's history: spectroscopic surveys, asteroseismology and chemodynamical
models", Guest Editors C. Chiappini, J. Montalban and M. Steffe
Super-Eddington growth of the first black holes
The assembly of the first super massive black holes (SMBHs) at z ≳ 6 is still a subject of intense debate. If black holes (BHs) grow at their Eddington rate, they must start from ≳104 M⊙ seeds formed by the direct collapse of gas. Here, we explore the alternative scenario where ˜100 M⊙ BH remnants of the first stars grow at super-Eddington rate via radiatively inefficient slim accretion discs. We use an improved version of the cosmological, data-constrained semi-analytic model GAMETE/QSODUST, where we follow the evolution of nuclear BHs and gas cooling, disc and bulge formation of their host galaxies. Adopting SDSS J1148+5251 (J1148) at z = 6.4 as a prototype of luminous z ≳ 6 quasars, we find that ˜80 per cent of its SMBH mass is grown by super-Eddington accretion, which can be sustained down to z ˜ 10 in dense, gas-rich environments. The average BH mass at z ˜ 20 is MBH ≳ 104 M⊙, comparable to that of direct collapse BHs. At z = 6.4 the AGN-driven mass outflow rate is consistent with the observations and the BH-to-bulge mass ratio is compatible with the local scaling relation. However, the stellar mass in the central 2.5 kpc is closer to the value inferred from CO observations. Finally, ˜20 per cent of J1148 progenitors at z = 7.1 have BH luminosities and masses comparable to ULAS J1120+0641, suggesting that this quasar may be one of the progenitors of J1148
On the formation of the first quasars
Observations of the most luminous quasars at redshift z>6 reveal the
existence of numerous supermasssive black holes (>10^9 Msun) already in place
about twelve billion years ago. In addition, the interstellar medium of the
galaxies hosting these black holes are observed to be chemically mature
systems, with metallicities (Z>Zsun) and dust masses (>10^8 Msun) similar to
that of more evolved, local galaxies. The connection between the rapid growth
of the first supermassive black holes and the fast chemical evolution of the
host galaxy is one of the most puzzling issues for theoretical models. Here we
review state-of-the-art theoretical models that focus on this problem with
particular emphasis on the conditions that lead to the formation of quasar
seeds and their subsequent evolution at z>6
The density distribution of accreting cosmic filaments as shaped by Kelvin-Helmholtz instability
Cosmic filaments play a crucial role in galaxy evolution transporting gas
from the intergalactic medium into galaxies. However, little is known about the
efficiency of this process and whether the gas is accreted in a homogenous or
clumpy way. Recent observations suggest the presence of broad gas density
distributions in the circumgalactic medium which could be related to the
accretion of filaments. By means of high-resolution hydrodynamical simulations,
we explore here the evolution of cold accreting filaments flowing through the
hot circumgalactic medium (CGM) of high-z galaxies. In particular, we examine
the nonlinear effects of Kelvin-Helmholtz instability (KHI) on the development
of broad gas density distributions and on the formation of cold, dense clumps.
We explore a large parameter space in filament and perturbation properties,
such as, filament Mach number, initial perturbation wavelength, and thickness
of the interface between the filament and the halo. We find that the time
averaged density distribution of the cold gas is qualitatively consistent with
a skewed log-normal probability distribution function (PDF) plus an additional
component in form of a high density tail for high Mach-numbers. Our results
suggest a tight correlation between the accreting velocity and the maximum
densities developing in the filament which is consistent with the variance-Mach
number relation for turbulence. Therefore, cosmological accretion could be a
viable mechanism to produce turbulence and broad gas density distributions
within the CGM.Comment: 12 pages, 14 figures, submitted to MNRAS on April 3rd 201
The sustainable growth of the first black holes
Super-Eddington accretion has been suggested as a possible formation pathway
of supermassive black holes (SMBHs) 800 Myr after the Big
Bang. However, stellar feedback from BH seed progenitors and winds from BH
accretion disks may decrease BH accretion rates. In this work, we study the
impact of these physical processes on the formation of quasar,
including new physical prescriptions in the cosmological, data-constrained
semi-analytic model GAMETE/QSOdust. We find that the feedback produced by the
first stellar progenitors on the surrounding does not play a relevant role in
preventing SMBHs formation. In order to grow the SMBHs, the
accreted gas must efficiently lose angular momentum. Moreover disk winds,
easily originated in super-Eddington accretion regime, can strongly reduce duty
cycles. This produces a decrease in the active fraction among the progenitors
of bright quasars, reducing the probability to observe them
The angular momentum-mass relation: a fundamental law from dwarf irregulars to massive spirals
In a CDM Universe, the specific stellar angular momentum ()
and stellar mass () of a galaxy are correlated as a consequence of the
scaling existing for dark matter haloes ().
The shape of this law is crucial to test galaxy formation models, which are
currently discrepant especially at the lowest masses, allowing to constrain
fundamental parameters, e.g. the retained fraction of angular momentum. In this
study, we accurately determine the empirical relation (Fall
relation) for 92 nearby spiral galaxies (from S0 to Irr) selected from the
Spitzer Photometry and Accurate Rotation Curves (SPARC) sample in the
unprecedented mass range . We
significantly improve all previous estimates of the Fall relation by
determining profiles homogeneously for all galaxies, using extended HI
rotation curves, and selecting only galaxies for which a robust could
be measured (converged radial profile). We find the relation to be
well described by a single, unbroken power-law
over the entire mass range, with and orthogonal intrinsic
scatter of dex. We finally discuss some implications for galaxy
formation models of this fundamental scaling law and, in particular, the fact
that it excludes models in which discs of all masses retain the same fraction
of the halo angular momentum.Comment: A&A Letters, accepte
Implications of a spatially resolved main sequence for the size evolution of star-forming galaxies
Two currently debated problems in galaxy evolution, the fundamentally local
or global nature of the main sequence of star formation and the evolution of
the mass-size relation of star forming galaxies (SFGs), are shown to be
intimately related to each other. As a preliminary step, a growth function
is defined, which quantifies the differential change in half-mass radius per
unit increase in stellar mass () due to star
formation. A general derivation shows that , meaning
that is proportional to the relative difference in specific star formation
rate between the outer and inner half of a galaxy, with a dimensionless
structural factor for which handy expressions are provided. As an application,
it is shown that galaxies obeying a fundamentally local main sequence also
obey, to a good approximation, , where is the slope
of the normalized local main sequence ()
and the Sersic index. An exact expression is also provided. Quantitatively,
a fundamentally local main sequence is consistent with SFGs growing along a
stationary mass-size relation, but inconsistent with the continuation at
of evolutionary laws derived at higher . This demonstrates that either the
main sequence is not fundamentally local, or the mass-size relation of SFGs has
converged to an equilibrium state some finite time in the past, or both.Comment: Accepted for publication in MNRA
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