5,008 research outputs found
Generation of rotationally dominated galaxies by mergers of pressure-supported progenitors
Through the analysis of a set of numerical simulations of major mergers
between initially non-rotating, pressure supported progenitor galaxies with a
range of central mass concentrations, we have shown that: (1) it is possible to
generate elliptical-like galaxies, with v/sigma > 1 outside one effective
radius, as a result of the conversion of orbital- into internal-angular
momentum; (2) the outer regions acquire part of the angular momentum first; (3)
both the baryonic and the dark matter components of the remnant galaxy acquire
part of the angular momentum, the relative fractions depend on the initial
concentration of the merging galaxies. For this conversion to occur the initial
baryonic component must be sufficiently dense and/or the encounter should take
place on a orbit with high angular momentum. Systems with these hybrid
properties have been recently observed through a combination of stellar
absorption lines and planetary nebulae for kinematic studies of early-type
galaxies. Our results are in qualitative agreement with such observations and
demonstrate that even mergers composed of non-rotating, pressure-supported
progenitor galaxies can produce early-type galaxies with significant rotation
at large radii.Comment: 5 pages, 6 figures, 2 tables. Accepted for publication in A&A Letter
A long-period Cepheid variable in the starburst cluster VdBH222
Galactic starburst clusters play a twin role in astrophysics, serving as laboratories for the study of stellar physics and also delineating the structure and recent star formation history of the Milky Way. In order to exploit these opportunities we have undertaken a multi-epoch spectroscopic survey of the red supergiant dominated young massive clusters thought to be present at both near and far ends of the Galactic Bar. Significant spectroscopic variability suggestive of radial pulsations was found for the yellow supergiant VdBH 222 #505. Follow-up photometric investigations revealed modulation with a period of ~23.325d; both timescale and pulsational profile are consistent with a Cepheid classification. As a consequence #505 may be recognised as one of the longest period Galactic cluster Cepheids identified to date and hence of considerable use in constraining the bright end of the period/luminosity relation at solar metallicities. In conjunction with extant photometry we infer a distance of ~6kpc for VdBH222 and an age of ~20Myr. This results in a moderate reduction in both integrated cluster mass (~2x10^4Msun) and the initial stellar masses of the evolved cluster members (~10Msun). As such, VdBH222 becomes an excellent test-bed for studying the properties of some of the lowest mass stars observed to undergo type-II supernovae. Moreover, the distance is in tension with a location of VdBH 222 at the far end of the Galactic Bar. Instead a birthsite in the near 3kpc arm is suggested; providing compelling evidence of extensive recent star formation in a region of the inner Milky Way which has hitherto been thought to be devoid of such activity
Microscopic analysis of shot-noise suppression in nondegenerate diffusive conductors
We present a theoretical investigation of shot-noise suppression due to
long-range Coulomb interaction in nondegenerate diffusive conductors.
Calculations make use of an ensemble Monte Carlo simulator self-consistently
coupled with a one-dimensional Poisson solver. We analyze the noise in a
lightly doped active region surrounded by two contacts acting as thermal
reservoirs. By taking the doping of the injecting contacts and the applied
voltage as variable parameters, the influence of elastic and inelastic
scattering in the active region is investigated. The transition from ballistic
to diffusive transport regimes under different contact injecting statistics is
analyzed and discussed. Provided significant space-charge effects take place
inside the active region, long-range Coulomb interaction is found to play an
essential role in suppressing the shot noise at . In the elastic
diffusive regime, momentum space dimensionality is found to modify the
suppression factor , which within numerical uncertainty takes values
respectively of about 1/3, 1/2 and 0.7 in the 3D, 2D and 1D cases. In the
inelastic diffusive regime, shot noise is suppressed to the thermal value.Comment: 11 pages, 13 figure
Component separation methods for the Planck mission
The Planck satellite will map the full sky at nine frequencies from 30 to 857
GHz. The CMB intensity and polarization that are its prime targets are
contaminated by foreground emission. The goal of this paper is to compare
proposed methods for separating CMB from foregrounds based on their different
spectral and spatial characteristics, and to separate the foregrounds into
components of different physical origin. A component separation challenge has
been organized, based on a set of realistically complex simulations of sky
emission. Several methods including those based on internal template
subtraction, maximum entropy method, parametric method, spatial and harmonic
cross correlation methods, and independent component analysis have been tested.
Different methods proved to be effective in cleaning the CMB maps from
foreground contamination, in reconstructing maps of diffuse Galactic emissions,
and in detecting point sources and thermal Sunyaev-Zeldovich signals. The power
spectrum of the residuals is, on the largest scales, four orders of magnitude
lower than that of the input Galaxy power spectrum at the foreground minimum.
The CMB power spectrum was accurately recovered up to the sixth acoustic peak.
The point source detection limit reaches 100 mJy, and about 2300 clusters are
detected via the thermal SZ effect on two thirds of the sky. We have found that
no single method performs best for all scientific objectives. We foresee that
the final component separation pipeline for Planck will involve a combination
of methods and iterations between processing steps targeted at different
objectives such as diffuse component separation, spectral estimation and
compact source extraction.Comment: Matches version accepted by A&A. A version with high resolution
figures is available at http://people.sissa.it/~leach/compsepcomp.pd
Investigating the trade-off between the effectiveness and efficiency of process modeling
Despite recent efforts to improve the quality of process models, we still observe a significant dissimilarity in quality between models. This paper focuses on the syntactic condition of process models, and how it is achieved. To this end, a dataset of 121 modeling sessions was investigated. By going through each of these sessions step by step, a separate ‘revision’ phase was identified for 81 of them. Next, by cutting the modeling process off at the start of the revision phase, a partial process model was exported for these modeling sessions. Finally, each partial model was compared with its corresponding final model, in terms of time, effort, and the number of syntactic errors made or solved, in search for a possible trade-off between the effectiveness and efficiency of process modeling. Based on the findings, we give a provisional explanation for the difference in syntactic quality of process models
Limits on dust emission from z~5 LBGs and their local environments
We present 1.2mm MAMBO-2 observations of a field which is over-dense in Lyman
Break Galaxies (LBGs) at z~5. The field includes seven
spectroscopically-confirmed LBGs contained within a narrow (z=4.95+/-0.08)
redshift range and an eighth at z=5.2. We do not detect any individual source
to a limit of 1.6 mJy/beam (2*rms). When stacking the flux from the positions
of all eight galaxies, we obtain a limit to the average 1.2 mm flux of these
sources of 0.6mJy/beam. This limit is consistent with FIR imaging in other
fields which are over-dense in UV-bright galaxies at z~5. Independently and
combined, these limits constrain the FIR luminosity (8-1000 micron) to a
typical z~5 LBG of LFIR<~3x10^11 Lsun, implying a dust mass of Mdust<~10^8 Msun
(both assuming a grey body at 30K). This LFIR limit is an order of magnitude
fainter than the LFIR of lower redshift sub-mm sources (z~1-3). We see no
emission from any other sources within the field at the above level. While this
is not unexpected given millimetre source counts, the clustered LBGs trace
significantly over-dense large scale structure in the field at z = 4.95. The
lack of any such detection in either this or the previous work, implies that
massive, obscured star-forming galaxies may not always trace the same
structures as over-densities of LBGs, at least on the length scale probed here.
We briefly discuss the implications of these results for future observations
with ALMA.Comment: 10 pages, 6 figures, MNRAS Accepte
Interpreting high [O III]/H β ratios with maturing starbursts
Star-forming galaxies at high redshift show ubiquitously high-ionization parameters, as measured by the ratio of optical emission lines. We demonstrate that local (z < 0.2) sources selected as Lyman break analogues also manifest high line ratios with a typical [O III]/Hβ=3.36+0.14−0.04 – comparable to all but the highest ratios seen in star-forming galaxies at z ∼ 2–4. We argue that the stellar population synthesis code BPASS can explain the high-ionization parameters required through the ageing of rapidly formed star populations, without invoking any AGN contribution. Binary stellar evolution pathways prolong the age interval over which a starburst is likely to show elevated line ratios, relative to those predicted by single stellar evolution codes. As a result, model galaxies at near-solar metallicities and with ages of up to ∼100 Myr after a starburst typically have a line ratio [O III]/Hβ ∼ 3, consistent with those seen in Lyman break galaxies and local sources with similar star formation densities. This emphasises the importance of including binary evolution pathways when simulating the nebular line emission of young or bursty stellar populations
Panchromatic Averaged Stellar Populations: PaasP
We study how the spectral fitting of galaxies, in terms of light fractions
derived in one spectral region translates into another region, by using results
from evolutionary synthesis models. In particular, we examine propagation
dependencies on Evolutionary Population Synthesis (EPS, {\sc grasil}, {\sc
galev}, Maraston and {\sc galaxev}) models, age, metallicity, and stellar
evolution tracks over the near-UV---near infrared (NUV---NIR, 3500\AA\ to
2.5\mc) spectral region. Our main results are: as expected, young (
400 Myr) stellar population fractions derived in the optical cannot be directly
compared to those derived in the NIR, and vice versa. In contrast, intermediate
to old age ( 500 Myr) fractions are similar over the whole spectral
region studied. The metallicity has a negligible effect on the propagation of
the stellar population fractions derived from NUV --- NIR. The same applies to
the different EPS models, but restricted to the range between 3800 \AA\ and
9000 \AA. However, a discrepancy between {\sc galev}/Maraston and {\sc
grasil}/{\sc galaxev} models occurs in the NIR. Also, the initial mass function
(IMF) is not important for the synthesis propagation. Compared to {\sc
starlight} synthesis results, our propagation predictions agree at 95%
confidence level in the optical, and 85% in the NIR. {\bf In summary,
spectral fitting} performed in a restricted spectral range should not be
directly propagated from the NIR to the UV/Optical, or vice versa. We provide
equations and an on-line form ({\bf Pa}nchromatic {\bf A}veraged {\bf S}tellar
{\bf P}opulation - \paasp) to be used for this purpose.Comment: 13 pages and 10 figures. Accepted by MNRA
QCD vacuum structure
Several issues related to the structure of the QCD vacuum are reviewed. We
concentrate mostly on results concerning instantons and center vortices.Comment: Lattice 2000 (Plenary), 8 pages, 2 figures; typos corrected,
hep-numbers of references updated, one figure replaced by black and whit
Black hole accretion and star formation as drivers of gas excitation and chemistry in Mrk231
We present a full high resolution SPIRE FTS spectrum of the nearby
ultraluminous infrared galaxy Mrk231. In total 25 lines are detected, including
CO J=5-4 through J=13-12, 7 rotational lines of H2O, 3 of OH+ and one line each
of H2O+, CH+, and HF. We find that the excitation of the CO rotational levels
up to J=8 can be accounted for by UV radiation from star formation. However,
the approximately flat luminosity distribution of the CO lines over the
rotational ladder above J=8 requires the presence of a separate source of
excitation for the highest CO lines. We explore X-ray heating by the accreting
supermassive black hole in Mrk231 as a source of excitation for these lines,
and find that it can reproduce the observed luminosities. We also consider a
model with dense gas in a strong UV radiation field to produce the highest CO
lines, but find that this model strongly overpredicts the hot dust mass in
Mrk231. Our favoured model consists of a star forming disk of radius 560 pc,
containing clumps of dense gas exposed to strong UV radiation, dominating the
emission of CO lines up to J=8. X-rays from the accreting supermassive black
hole in Mrk231 dominate the excitation and chemistry of the inner disk out to a
radius of 160 pc, consistent with the X-ray power of the AGN in Mrk231. The
extraordinary luminosity of the OH+ and H2O+ lines reveals the signature of
X-ray driven excitation and chemistry in this region.Comment: 5 pages, 2 figures, accepted for publication in Astronomy &
Astrophysics Special Issue on Herschel first result
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