3,729 research outputs found
Primordial gas heating by dark matter and structure formation
Dark matter (DM) decays and annihilations might heat and partially reionize
the Universe at high redshift. Although this effect is not important for the
cosmic reionization, the gas heating due to DM particles might affect the
structure formation. In particular, the critical halo mass for collapse is
increased up to a factor of ~2. Also the fraction of gas which collapses inside
the smallest halos is substantially reduced with respect to the cosmological
value. These effects imply that DM decays and annihilations might delay the
formation of the first structures and reduce the total star mass in the
smallest halos.Comment: 4 pages, 4 figures, to appear in the "Memorie della SAIt",
proceedings of the "LI congresso della Societa' Astronomica Italiana",
Firenze, April 17-20 200
Signatures of planets and protoplanets in the Galactic center: a clue to understand the G2 cloud?
Several hundred young stars lie in the innermost parsec of our Galaxy. The
super-massive black hole (SMBH) might capture planets orbiting these stars, and
bring them onto nearly radial orbits. The same fate might occur to planetary
embryos (PEs), i.e. protoplanets born from gravitational instabilities in
protoplanetary disks. In this paper, we investigate the emission properties of
rogue planets and PEs in the Galactic center. In particular, we study the
effects of photoevaporation, caused by the ultraviolet background. Rogue
planets can hardly be detected by current or forthcoming facilities, unless
they are tidally disrupted and accrete onto the SMBH. In contrast,
photoevaporation of PEs (especially if the PE is being tidally stripped) might
lead to a recombination rate as high as ~10^45 s^-1, corresponding to a
Brackett-gamma luminosity ~10^31 erg s^-1, very similar to the observed
luminosity of the dusty object G2. We critically discuss the possibility that
G2 is a rogue PE, and the major uncertainties of this model.Comment: 10 pages, 5 figures, 1 table, ApJ, accepte
Broad [OIII] in the globular cluster RZ 2109: X-ray ionized nova ejecta?
We study the possibility that the very broad (~1500 km/s) and luminous
(L_5007 ~ 1.4e37 erg/s) [OIII] line emission observed in the globular cluster
RZ 2109 might be explained with the photoionization of nova ejecta by the
bright (L_X ~ 4e39 erg/s) X-ray source hosted in the same globular cluster. We
find that such scenario is plausible and explains most of the features of the
RZ 2109 spectrum (line luminosity, absence of H emission lines, peculiar
asymmetry of the line profile); on the other hand, it requires the nova ejecta
to be relatively massive (>~ 0.5e-3 Msun}), and the nova to be located at a
distance <~ 0.1 pc from the X-ray source. We also predict the time evolution of
the RZ 2109 line emission, so that future observations can be used to test this
scenario.Comment: 11 pages, 2 figures, 6 tables; accepted for publication on MNRA
Radiation from the first forming stars
The evolution of radiation emitted during the dynamical collapse of
metal-free protostellar clouds is investigated within a spherically symmetric
hydrodynamical scheme that includes the transfer of radiation and the chemistry
of the primordial gas. The cloud centre collapses on a time scale of about
10^5-10^6 years, thanks to line cooling from molecular hydrogen (H2). For most
of the collapse time, when the evolution proceeds self-similarly, the
luminosity slowly rises up to about 10^36 erg/s and is essentially due to H2 IR
line emission. Later, continuum IR radiation provides an additional
contribution, which is mostly due to the accretion of an infalling envelope
upon a small hydrostatic protostellar core which develops in the centre. We
follow the beginning of the accretion phase, when the enormous accretion rate
(~ 0.1 Msun/yr) produces a very high continuum luminosity of about 10^36 erg/s.
Despite the high luminosities, the radiation field is unable to affect the gas
dynamics during the collapse and the first phases of accretion, because the
opacity of the infalling gas is too small; this is quite different from
present-day star formation. We also find that the protostellar evolution is
similar among clouds with different initial configurations, including those
resulting from 3D cosmological simulations of primordial objects; in
particular, the shape of the molecular spectra is quite universal. Finally, we
briefly discuss the detectability of this pristine cosmic star formation
activity.Comment: 39 pages, 12 figures; revised version with major changes (including
title) to appear in MNRA
The association of economic and cultural capital with the NEET rate: differential geographical and temporal patterns
AbstractUsing data from 103 Italian provinces, we investigated the relationship between local/regional development, and NEET. We constructed an indicator of cultural capital and another of economic capital and we studied their relation with the NEET rate. Covariance Structure Analysis with Generalized Least Squares estimation was employed, considering a three time-points retrospective model. Results indicate a consistent protective effect of the economic capital on the NEET rate, both in the short run (2 years) and in the medium run (10 years). However, this effect has been obtained in the Central provinces (at 2 and 10 years) and Southern provinces (at 10 years), but not in the Northern provinces. A mediation analysis indicated that, historically, the cultural capital may partly mediate the effect of the economic capital. We did not detect a significant direct effect of the cultural capital on the NEET rate, which is strongly mediated by the action of the economic capital. Together, these results denote that the economic capital is a strong predictor of NEET, but not in very competitive economic areas
Constraining DM through 21 cm observations
Beyond reionization epoch cosmic hydrogen is neutral and can be directly
observed through its 21 cm line signal. If dark matter (DM) decays or
annihilates the corresponding energy input affects the hydrogen kinetic
temperature and ionized fraction, and contributes to the Ly_alpha background.
The changes induced by these processes on the 21 cm signal can then be used to
constrain the proposed DM candidates, among which we select the three most
popular ones: (i) 25-keV decaying sterile neutrinos, (ii) 10-MeV decaying light
dark matter (LDM) and (iii) 10-MeV annihilating LDM. Although we find that the
DM effects are considerably smaller than found by previous studies (due to a
more physical description of the energy transfer from DM to the gas), we
conclude that combined observations of the 21 cm background and of its gradient
should be able to put constrains at least on LDM candidates. In fact, LDM
decays (annihilations) induce differential brightness temperature variations
with respect to the non decaying/annihilating DM case up to Delta_delta T_b=8
(22) mK at about 50 (15) MHz. In principle this signal could be detected both
by current single dish radio telescopes and future facilities as LOFAR;
however, this assumes that ionospheric, interference and foreground issues can
be properly taken care of.Comment: 9 pages, submitted to MNRA
The population of ULXs in the spiral galaxy NGC 2276
We present results for X-ray point sources in the Sc galaxy NGC 2276,
obtained by analyzing Chandra data. The galaxy is known to be very active in
many wavelengths, possibly due to gravitational interaction with the central
elliptical of the group, NGC 2300. However, previous XMM-Newton observations
resulted in the detection of only one bright ULX and extended hot gas emission.
We present here the X-ray population in NGC 2276 which comprises 17 sources. We
found that 6 of them are new ULX sources in this spiral galaxy resolved for the
first time by Chandra. We constructed the Luminosity Function that can be
interpreted as mainly due of High Mass X-ray binaries, and estimate the Star
Formation rate (SFR) to be SFR ~ 5-10 M_sun/yr.Comment: 4 pages, 4 figures, Proceedings of the meeting 'Ultra-Luminous X-ray
sources and Middle Weight Black Holes', ESAC, Madrid, Spain, May 201
- …