8,920 research outputs found
Decoupled hot gas flows in elliptical galaxies
Get PDFWe present the results of a new set of hydrodynamical simulations of hot gas
flows in model elliptical galaxies with the following characteristics: the
spatial luminosity distribution approaches a power law form at small radii, in
accordance with the results of recent ground based observations, and with the
Hubble Space Telescope; the dark matter has a peaked profile too, as indicated
by high resolution numerical simulations of dissipationless collapse; the dark
to luminous mass ratio spans a large range of values, including low values
found by optical studies confined to within two effective radii; finally, the
type Ia supernova rate is that given by the latest estimates of optical
surveys, or zero, as suggested by the iron abundances recently measured in the
hot gas. We find that the resulting gas flows are strongly decoupled: an inflow
develops in the central region of the galaxies, while the external parts are
still degassing, i.e., the flows are mostly partial winds. This behavior can be
explained in terms of the local energy balance of the hot gas. A large spread
in the X-ray luminosity Lx at fixed optical luminosity Lb can be produced as in
previous simulations that used King models plus massive quasi-isothermal dark
halos, and higher supernova rates; the key factor causing large Lx variations
is now the size of the central inflow region. The highest Lx observed
correspond to global inflows. Finally, non negligible amounts of cold gas can
be produced by the partial winds; this could be an explanation for the possible
discovery of cold matter at the center of elliptical galaxies, an alternative
to the presence of a steady state cooling flow.Comment: 13 pages, plus 7 .ps figures, 2 .ps tables, LaTeX macro l-aa.sty, in
press on A&
Isothermal Bondi accretion in Jaffe and Hernquist galaxies with a central black hole: fully analytical solutions
Full text linkOne of the most active fields of research of modern-day astrophysics is that
of massive black hole formation and co-evolution with the host galaxy. In these
investigations, ranging from cosmological simulations, to semi-analytical
modeling, to observational studies, the Bondi solution for accretion on a
central point mass is widely adopted. In this work we generalize the classical
Bondi accretion theory to take into account the effects of the gravitational
potential of the host galaxy, and of radiation pressure in the optically thin
limit. Then, we present the fully analytical solution, in terms of the
Lambert-Euler -function, for isothermal accretion in Jaffe and Hernquist
galaxies with a central black hole. The flow structure is found to be sensitive
to the shape of the mass profile of the host galaxy. These results and the
formulae that are provided, mostly important the one for the critical accretion
parameter, allow for a direct evaluation of all flow properties, and are then
useful for the above mentioned studies. As an application, we examine the
departure from the true mass accretion rate of estimates obtained using the gas
properties at various distances from the black hole, under the hypothesis of
classical Bondi accretion. An overestimate is obtained from regions close to
the black hole, and an underestimate outside a few Bondi radii; the exact
position of the transition between the two kinds of departure depends on the
galaxy model.Comment: 18 pages, 6 figures, submitted to ApJ, comments welcom
Recommended from our members
Systemic risk determinants in the European banking industry during financial crises, 2006-2012
Get PDFThe recent financial turmoil has stimulated a rich debate in banking and financial literature on the identification of systemic risk determinants and devices to forecast and prevent crises. This paper explores the contribution of corporate variables to systemic risk using the CoVaR approach (Adrian and Brunnermeier, 2016). Using balanced panel data on 141 European banks from 24 countries, which were listed from 2006Q1 to 2012Q4, we investigated the impact of corporate variables during the three regimes that characterised the European banking sector-the subprime crisis (2007Q3-2008Q3), the European Great Financial Depression (2008Q4-2010Q2), and the sovereign debt crisis (2010Q3-2012Q4). Our results show that size did not play a significant role in spreading systemic risk, while maturity mismatch did. However, the nature and intensity of these two determinants varied across the three regimes
Star formation in early-type galaxies: the role of stellar winds and kinematics
Full text linkEarly-type galaxies (ETGs) host a hot ISM produced mainly by stellar winds,
and heated by Type Ia supernovae and the thermalization of stellar motions.
High resolution 2D hydrodynamical simulations showed that ordered rotation in
the stellar component results in the formation of a centrifugally supported
cold equatorial disc. In a recent numerical investigation we found that
subsequent generations of stars are formed in this cold disc; this process
consumes most of the cold gas, leaving at the present epoch cold masses
comparable to those observed. Most of the new stellar mass formed a few Gyrs
ago, and resides in a disc.Comment: 2 pages, 1 figure, to appear in proceedings of IAU Symposium 315,
"From Interstellar Clouds to Star-Forming Galaxies: Universal Processes?", P.
Jablonka, F. Van der Tak & P. Andre', ed
Spaceability and algebrability of sets of nowhere integrable functions
Full text linkWe show that the set of Lebesgue integrable functions in which are
nowhere essentially bounded is spaceable, improving a result from [F. J.
Garc\'{i}a-Pacheco, M. Mart\'{i}n, and J. B. Seoane-Sep\'ulveda.
\textit{Lineability, spaceability, and algebrability of certain subsets of
function spaces,} Taiwanese J. Math., \textbf{13} (2009), no. 4, 1257--1269],
and that it is strongly -algebrable. We prove strong
-algebrability and non-separable spaceability of the set of
functions of bounded variation which have a dense set of jump discontinuities.
Applications to sets of Lebesgue-nowhere-Riemann integrable and
Riemann-nowhere-Newton integrable functions are presented as corollaries. In
addition we prove that the set of Kurzweil integrable functions which are not
Lebesgue integrable is spaceable (in the Alexievicz norm) but not 1-algebrable.
We also show that there exists an infinite dimensional vector space of
differentiable functions such that each element of the -closure of
is a primitive to a Kurzweil integrable function, in connection to a
classic spaceability result from [V. I. Gurariy, \textit{Subspaces and bases in
spaces of continuous functions (Russian),} Dokl. Akad. Nauk SSSR, \textbf{167}
(1966), 971-973].Comment: Accepted for publication in 201
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