21,927 research outputs found
Giant cold satellites from low-concentration haloes
The dwarf satellite galaxies of the Milky Way Crater II and Antlia II have
uncommonly low dynamical mass densities, due to their large size and low
velocity dispersion. Previous work have failed to identify formation scenarios
within the CDM framework and have invoked cored dark matter haloes,
processed by tides. I show that the tidal evolution of CDM NFW haloes
is richer than previously recognised: tidal heating causes the innermost
regions of haloes that fall short of the mass-concentration relation to expand
significantly, resulting in the formation of giant, kinematically cold
satellites like Crater II and Antlia II. Furthermore, while the satellite is
reaching apocenter, extra-tidal material can cause an even more inflated
appearance. When present, as likely for the larger Antlia II, nominally unbound
material can be recognised thanks to its somewhat hotter kinematics and
line-of-sight velocity gradient. Contrary to other formation scenarios, Crater
II and Antlia II may well have experienced very little mass loss, as in fact
hinted by their observed metallicity. If indeed a satellite of NGC1052, tidal
evolution of a low-concentration halo may similarly have led to the formation
of NGC1052-DF2.Comment: MNRAS Letters submitted, comments welcom
Deadly dark matter cusps vs faint and extended star clusters: Eridanus II and Andromeda XXV
The recent detection of two faint and extended star clusters in the central
regions of two Local Group dwarf galaxies, Eridanus II and Andromeda XXV,
raises the question of whether clusters with such low densities can survive the
tidal field of cold dark matter haloes with central density cusps. Using both
analytic arguments and a suite of collisionless N-body simulations, I show that
these clusters are extremely fragile and quickly disrupted in the presence of
central cusps with . Furthermore, the
scenario in which the clusters where originally more massive and sank to the
center of the halo requires extreme fine tuning and does not naturally
reproduce the observed systems. In turn, these clusters are long lived in cored
haloes, whose central regions are safe shelters for . The
only viable scenario for hosts that have preserved their primoridal cusp to the
present time is that the clusters formed at rest at the bottom of the
potential, which is easily tested by measurement of the clusters proper
velocity within the host. This offers means to readily probe the central
density profile of two dwarf galaxies as faint as
and , in which stellar feedback is unlikely to be
effective.Comment: accepted versio
The accreted stellar halo as a window on halo assembly in L* galaxies
Theory and observations agree that the accreted stellar halos (ASHs) of Milky
Way-like galaxies display significant scatter. I take advantage of this
stochasticity to invert the link between halo assembly history (HAH) and ASH,
using mock ASHs corresponding to 750 CDM HAHs, sharing a final virial
mass of . Hosts with poor/rich ASHs assemble
following orthogonal growth-patterns. Hosts with rich ASHs experience accretion
events (AEs) with high virial mass ratios (HVMRs, ) at
, in a phase of fast growth. This maximizes
the accreted stellar mass under the condition these satellites are disrupted by
. At similar times, hosts with poor ASHs grow slowly through minor
mergers, with only very recent HVMR AEs: this results in a globally more
abundant satellite population and in distinctive surviving massive satellites
(stellar mass ). Several properties of the Milky
Way are in agreement with the predictions of this framework for hosts with
poor, concentrated ASHs, including: i) the recent infall of Sagittarius and
Magellanic Clouds, ii) the likely higher-than-average concentration of its dark
halo, iii) the signatures of fast chemical enrichment of a sizable fraction of
its halo stellar populations.Comment: accepted version, minor change
The virial mass distribution of ultra-diffuse galaxies in clusters and groups
We use the observed abundances of ultra-diffuse galaxies (UDGs) in clusters
and groups and \Lambda CDM subhalo mass functions to put constraints on the
distribution {of present-day halo masses of satellite} UDGs. If all of the most
massive subhaloes in the cluster host a UDG, UDGs occupy all subhaloes with
\log M_{sub}/M_\odot\gtrsim11. For a model in which the efficiency of UDG
formation is higher around some characteristic halo mass, higher fractions of
massive UDGs require larger spreads in the UDG mass distribution. In a cluster
with a virial mass of 10^{15}M_\odot, the 90% upper limit for the fraction of
UDGs with \log M_{sub}/M_\odot>12 is 7%, occupying 70% of all cluster subhaloes
above the same mass. To reproduce the observed abundances, however, the mass
distribution of satellite UDGs has to be broad, with >30% having \log
M_{sub}/M_\odot<10.9. This strongly supports that UDGs are part of a continuous
distribution in which a majority are hosted by low mass haloes. The abundance
of satellite UDGs may fall short of the linear relation with the cluster/group
mass M_{host} in low-mass hosts, \log M_{host}/M_\odot\sim 12. Characterising
these deviations -- or the lack thereof -- will allow for stringent constraints
on the UDG virial mass distribution.Comment: matches accepted versio
ACRIM total solar irradiance satellite composite validation versus TSI proxy models
The satellite total solar irradiance (TSI) database provides a valuable
record for investigating models of solar variation used to interpret climate
changes. The 35-year ACRIM TSI satellite composite was updated using
corrections to ACRIMSAT/ACRIM3 results derived from recent testing at the
Laboratory for Atmospheric and Space Physics/Total solar irradiance Radiometer
Facility (LASP/TRF). The corrections lower the ACRIM3 scale by ~5000 ppm, in
close agreement with the scale of SORCE/TIM results (solar constant ~1361
W/m^2). Relative variations and trends are not changed. Differences between the
ACRIM and PMOD TSI composites, e.g. the decadal trending during solar cycles
21-22, are tested against a set of solar proxy models, including analysis of
Nimbus7/ERB and ERBS/ERBE results available to bridge the ACRIM Gap
(1989-1992). Our findings confirm: (1) The validity of the TSI peak in the
originally published ERB results in early 1979 during solar cycle 21; (2) The
correctness of originally published ACRIM1 results during the SMM spin mode
(1981-1984); (3) The upward trend of originally published ERB results during
the ACRIM Gap; (4) The occurrence of a significant upward TSI trend between the
minima of solar cycles 21 and 22 and (5) a decreasing trend during solar cycles
22-23. Our findings do not support: (1) The downward corrections to originally
published ERB and ACRIM1 results during solar cycle 21; (2) A step function
sensitivity change in ERB results at the end-of-September 1989; (3) the
validity of ERBE's downward trend during the ACRIM Gap or (4) the use of ERBE
results to bridge the ACRIM Gap. Our analysis provides a first order validation
of the ACRIM TSI composite approach and its 0.037%/decade upward trend during
solar cycles 21-22. Thus, solar forcing of climate change may be a
significantly larger factor than represented in the CMIP5 general circulation
climate models.Comment: 26 pages, 15 figure
Dynamics and Thermodynamics of a Novel Phase of NaAlH4
We characterize a novel orthorhombic phase (gamma) of NaAlH4, discovered
using first-principles molecular dynamics, and discuss its relevance to the
dehydrogenation mechanism. This phase is close in energy to the known
low-temperature structure and becomes the stabler phase above 320 K, thanks to
a larger vibrational entropy associated with AlH4 rotational modes. The
structural similarity of gamma-NaAlH4 to alpha-Na3AlH6 suggests it acts as a
key intermediate during hydrogen release. Findings are consistent with recent
experiments recording an unknown phase during dehydrogenation.Comment: 10 pages, 4 figures, 1 table + supplementary info; In press (Physical
Review Letters
Planetary harmonics in the historical Hungarian aurora record (1523-1960)
The historical Hungarian auroral record extends from 1523 to 1960 and is
longer than the sunspot record. Harmonic analysis reveals four major
multidecadal secular cycles forming an approximate harmonic set at periods of
42.85, 57.13, 85.7 and 171.4 years. These four frequencies are very close to
the four major heliospheric oscillations relative to the center of mass of the
solar system caused by Jupiter, Saturn, Uranus and Neptune. Similar frequencies
are found in solar radiation models based on long cosmogenic isotope records
(Steinhilber et al. 2012) and in long records of naked-eye sunspot observations
(Vaquero et al., 2002). Harmonic regression models are used to reconstruct and
forecast aurora and solar activity for the period 1956-2050. The model
predicts: (1) the multidecadal solar minimum in the 1970s that is also observed
in the sunspot record; (2) a solar maximum in 2000-2002 that is observed in the
ACRIM total solar irradiance satellite composite; (3) a prolonged solar minimum
centered in the 2030s. These findings support a hypothesis that the Sun, the
heliosphere and the terrestrial magnetosphere are partially modulated by
planetary gravitational and magnetic forces synchronized to planetary
oscillations, as also found in other recent publications (Scafetta, 2010,
2012a, 2012c, 2012d; Abreu et al., 2012; Tan & Cheng, 2012).Comment: 29 pages, 6 figures, 2 table
Multi-scale comparative spectral analysis of satellite total solar irradiance measurements from 2003 to 2013 reveals a planetary modulation of solar activity and its non-linear dependence on the 11-year solar cycle
Herein we adopt a multi-scale dynamical spectral analysis technique to
compare and study the dynamical evolution of the harmonic components of the
overlapping ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM total solar irradiance
(TSI) records during 2003.15 to 2013.16 in solar cycles 23 and 24. The three
TSI time series present highly correlated patterns. Significant power spectral
peaks are common to these records and are observed at the following periods:
0.070 year, 0.097 year, 0.20 year, 0.25 year, 0.30-0.34 year, 0.39 year. Less
certain spectral peaks occur at about 0.55 year, 0.60-0.65 year and 0.7-0.9
year. Four main frequency periods at 24.8 days (0.068 year), 27.3 days (0.075
year), at 34-35 days (0.093-0.096 year) and 36-38 days (0.099-0.104 year)
characterize the solar rotation cycle. The amplitude of these oscillations, in
particular of those with periods larger than 0.5 year, appears to be modulated
by the 11-year solar cycle. Similar harmonics have been found in other solar
indices. The observed periodicities are found highly coherent with the spring,
orbital and synodic periods of Mercury, Venus, Earth and Jupiter. We conclude
that solar activity is likely modulated by planetary gravitational and
electromagnetic forces acting on the sun. The strength of the sun's response to
planetary forcing depends non-linearly on the state of internal solar dynamics:
planetary-sun coupling effects are enhanced during solar activity maxima and
attenuated during minima.Comment: 18 pages, 10 figures, 3 table
Challenges and solutions for autism in academic geosciences
Researcher diversity promotes research excellence. But academia is widely perceived as inaccessible to those who work in non-stereotypical ways, and disabled researchers are consequently chronically under-represented within higher education. The barriers that academia presents to the inclusion and success of disabled individuals must therefore be understood and removed in order to enhance researcher diversity and improve the quality and quantity of research. Autism is a disability that is particularly under-represented within higher education, despite many autistic individuals having attributes that are conducive to research excellence. With a focus on geosciences, we use the experiences of an autistic PhD student to evaluate why academia can be inaccessible, and propose simple strategies that can reduce and remove barriers to academic success. We suggest that minor changes to communication, the academic environment and better disability awareness can make significant differences to the inclusion of disabled researchers, particularly those with autism. These changes would also benefit the wider scientific community and promote research and teaching excellence
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