151 research outputs found
On The Complexity and Completeness of Static Constraints for Breaking Row and Column Symmetry
We consider a common type of symmetry where we have a matrix of decision
variables with interchangeable rows and columns. A simple and efficient method
to deal with such row and column symmetry is to post symmetry breaking
constraints like DOUBLELEX and SNAKELEX. We provide a number of positive and
negative results on posting such symmetry breaking constraints. On the positive
side, we prove that we can compute in polynomial time a unique representative
of an equivalence class in a matrix model with row and column symmetry if the
number of rows (or of columns) is bounded and in a number of other special
cases. On the negative side, we show that whilst DOUBLELEX and SNAKELEX are
often effective in practice, they can leave a large number of symmetric
solutions in the worst case. In addition, we prove that propagating DOUBLELEX
completely is NP-hard. Finally we consider how to break row, column and value
symmetry, correcting a result in the literature about the safeness of combining
different symmetry breaking constraints. We end with the first experimental
study on how much symmetry is left by DOUBLELEX and SNAKELEX on some benchmark
problems.Comment: To appear in the Proceedings of the 16th International Conference on
Principles and Practice of Constraint Programming (CP 2010
Young stellar populations in type II quasars: timing the onset of star formation and nuclear activity
Despite the emerging morphological evidence that luminous quasar-like active galactic nuclei
(AGN) are triggered in galaxy mergers, the natures of the triggering mergers and the order
of events in the triggering sequence remain uncertain. In this work, we present a detailed
study of the stellar populations of the host galaxies of 21 type II quasars, with the aim of
understanding the sequence of events between the onset of the merger, the triggering of the
associated starburst and the initiation of the quasar activity. To this end, we model highquality,
wide spectral coverage, intermediate-resolution optical spectra of the type II quasars.
We find that of the 21 objects, the higher order Balmer absorption lines, characteristic of
young stellar populations (YSPs), are directly detected in ∼62 per cent of the sample. We also
fit these spectra using a number of combinations of stellar and/or power-law components,
representative of viable formation histories, as well as including the possibility of scattered
AGN light. We find that ∼90 per cent of the type II quasar host galaxies require the inclusion of
a YSP to adequately model their spectra, whilst 71 per cent of the sample require the inclusion
of a YSP with age <100 Myr. Since the ages of the YSP in most type II quasar host galaxies
are comparable with the expected lifetimes of the AGN activity, these results provide strong
evidence that the quasars are triggered close to the peaks of the merger-induced starbursts
Estimating the Number of Solutions of Cardinality Constraints through range and roots Decompositions
International audienceThis paper introduces a systematic approach for estimating the number of solutions of cardinality constraints. A main difficulty of solutions counting on a specific constraint lies in the fact that it is, in general, at least as hard as developing the constraint and its propaga-tors, as it has been shown on alldifferent and gcc constraints. This paper introduces a probabilistic model to systematically estimate the number of solutions on a large family of cardinality constraints including alldifferent, nvalue, atmost, etc. Our approach is based on their decomposition into range and roots, and exhibits a general pattern to derive such estimates based on the edge density of the associated variable-value graph. Our theoretical result is finally implemented within the maxSD search heuristic, that aims at exploring first the area where there are likely more solutions
Investigating the impact of quasar-driven outflows on galaxies at redshift 0.3-0.4
We present a detailed study of the kinematics of 19 QSO2s in the range
0.3 10^{8.5}_{\odot}\lambda\r{A}_{OF}_{\odot}\pm_{OF}\pm_{\odot}^{-1}_{kin}\pm^{-1}\pm_{\odot}^{-1}\pm^{-1}$). We do not find any significant
correlation between the outflow properties and the previously mentioned galaxy
properties.
4 out of 5 QSO2s without a YS<100 Myr show highly disturbed kinematics,
whereas only 5 out of the 14 QSO2s with YSPs show similarly asymmetric [OIII]
profiles. This might be indicative of negative feedback. The lack of
correlation between the outflow properties and the presence of mergers in
different interaction stages might be due to their different dynamical
timescales. Lastly, the small radio luminosity range covered by our sample may
be impeding the detection of any correlation between radio emission and outflow
properties.Comment: 26 pages, 9 figures. Accepted for publication in A&
Galaxy interactions are the dominant trigger for local type 2 quasars
The triggering mechanism for the most luminous, quasar-like active galactic
nuclei (AGN) remains a source of debate, with some studies favouring triggering
via galaxy mergers, but others finding little evidence to support this
mechanism. Here, we present deep Isaac Newton Telescope/Wide Field Camera
imaging observations of a complete sample of 48 optically-selected type 2
quasars the QSOFEED sample (L10; ). Based on visual inspection by eight classifiers, we find clear evidence
that galaxy interactions are the dominant triggering mechanism for quasar
activity in the local universe, with 65 per cent of the type 2
quasar hosts showing morphological features consistent with galaxy mergers or
encounters, compared with only 22 per cent of a stellar-mass- and
redshift-matched comparison sample of non-AGN galaxies a 5
difference. The type 2 quasar hosts are a factor 3.0 more
likely to be morphologically disturbed than their matched non-AGN counterparts,
similar to our previous results for powerful 3CR radio AGN of comparable [OIII]
emission-line luminosity and redshift. In contrast to the idea that quasars are
triggered at the peaks of galaxy mergers as the two nuclei coalesce, and only
become visible post-coalescence, the majority of morphologically-disturbed type
2 quasar sources in our sample are observed in the pre-coalescence phase
(61 per cent). We argue that much of the apparent ambiguity that
surrounds observational results in this field is a result of differences in the
surface brightness depths of the observations, combined with the effects of
cosmological surface brightness dimming.Comment: Accepted for publication in MNRAS. 17 pages, 7 figure
Ionized outflows in luminous type 2 AGNs at z < 0.6: no evidence for significant impact on the host galaxies
We investigate the presence of extended ionized outflows in 18 luminous type 2 AGNs (11
quasars and 7 high-luminosity Seyfert 2s) at 0.3 <z< 0.6 based on VLT-FORS2 spectroscopy.
We infer typical lower limits on the radial sizes of the outflows Ro several × 100 pc and
upper limits Ro 1–2 kpc. Our results are inconsistent with related studies which suggest that
large scale (Ro ∼ several-15 kpc) are ubiquitous in QSO2. We study the possible causes of
discrepancy and propose that seeing smearing is the cause of the large inferred sizes. The
implications in our understanding of the feedback phenomenon are important since the mass
Mo (through the density), mass injection M˙ o and energy injection E˙ o rates of the outflows
become highly uncertain. One conclusion seems unavoidable: Mo, M˙ o and E˙ o are modest
or low compared with previous estimations. We obtain typically Mo (0.4–22) × 106 M
(median 1.1 × 106 M) assuming n = 1000 cm−3. These are ∼102–104 times lower than
values reported in the literature. Even under the most favourable assumptions, we obtain M˙ o
10 M yr−1 in general, 100–1000 times lower than claimed in related studies. Although the
uncertainties are large, it is probable that these are lower than typical star-forming rates. In
conclusion, no evidence is found supporting that typical outflows can affect the interstellar
medium of the host galaxies across spatial scales 1–2 kpc
Clear evidence for the early triggering of a luminous quasar-like active galactic nuclei in a major, gas-rich merger
We present deep, intermediate resolution, long-slit Gemini GMOS-S optical spectra of the
Sloan Digital Sky Survey (SDSS) type II quasar J002531−104022, which is a highly disturbed
system currently undergoing a major merger event. We use these data to model the ages and
reddenings of the stellar populations in three distinct spatial regions and find a remarkable
uniformity in the properties of the young stellar populations (YSPs) that dominate the optical
spectra. The YSPs are all found to have relatively young ages (tysp < 40 Myr), strongly
implying that the latest episode of star formation and quasar activity have been triggered
quasi-simultaneously. The lack of reddening deduced from both continuum modelling and the
measured Balmer decrements (E(B − V) < 0.3) suggests that starburst and/or active galactic
nuclei-induced outflows have already effectively removed a substantial proportion of the gas
and dust from the central region. These findings starkly contrast with model predictions which
suggest an offset of a few 100 Myr between the peak of merger-induced star formation and the
emergence of the optically visible quasar activity. Based on our stellar population fits, we also
show that the total stellar mass is in the range (4–17) × 1010 M, lower than typically found
for quasar host galaxies
Are luminous radio-loud active galactic nuclei triggered by galaxy interactions?
We present the results of a comparison between the optical morphologies of a
complete sample of 46 southern 2Jy radio galaxies at intermediate redshifts
(0.05<z<0.7) and those of two control samples of quiescent early-type galaxies:
55 ellipticals at redshifts z<0.01 from the Observations of Bright Ellipticals
at Yale (OBEY) survey, and 107 early-type galaxies at redshifts 0.2<z<0.7 in
the Extended Groth Strip (EGS). Based on these comparisons, we discuss the role
of galaxy interactions in the triggering of powerful radio galaxies (PRGs). We
find that a significant fraction of quiescent ellipticals at low and
intermediate redshifts show evidence for disturbed morphologies at relatively
high surface brightness levels, which are likely the result of past or on-going
galaxy interactions. However, the morphological features detected in the galaxy
hosts of the PRGs (e.g. tidal tails, shells, bridges, etc.) are up to 2
magnitudes brighter than those present in their quiescent counterparts. Indeed,
if we consider the same surface brightness limits, the fraction of disturbed
morphologies is considerably smaller in the quiescent population (53% at z<0.2
and 48% at 0.2<z<0.7) than in the PRGs (93% at z<0.2 and 95% at 0.2<z<0.7
considering strong-line radio galaxies only). This supports a scenario in which
PRGs represent a fleeting active phase of a subset of the elliptical galaxies
that have recently undergone mergers/interactions. However, we demonstrate that
only a small proportion (<20%) of disturbed early-type galaxies are capable of
hosting powerful radio sources.Comment: 20 pages, 7 figures, accepted for publication in MNRA
Galaxy and Mass Assembly (GAMA): merging galaxies and their properties
We derive the close pair fractions and volume merger rates for galaxies in the Galaxy and Mass Assembly (GAMA) survey with −23 < Mr < −17 (ΩM = 0.27, ΩΛ = 0.73, H0 = 100 km s−1 Mpc−1) at 0.01 < z < 0.22 (look-back time of <2 Gyr). The merger fraction is approximately 1.5 per cent Gyr−1 at all luminosities (assuming 50 per cent of pairs merge) and the volume merger rate is ≈3.5 × 10−4 Mpc−3 Gyr−1. We examine how the merger rate varies by luminosity and morphology. Dry mergers (between red/spheroidal galaxies) are found to be uncommon and to decrease with decreasing luminosity. Fainter mergers are wet, between blue/discy galaxies. Damp mergers (one of each type) follow the average of dry and wet mergers. In the brighter luminosity bin (−23 < Mr < −20), the merger rate evolution is flat, irrespective of colour or morphology, out to z ∼ 0.2. The makeup of the merging population does not appear to change over this redshift range. Galaxy growth by major mergers appears comparatively unimportant and dry mergers are unlikely to be significant in the buildup of the red sequence over the past 2 Gyr. We compare the colour, morphology, environmental density and degree of activity (BPT class, Baldwin, Phillips & Terlevich) of galaxies in pairs to those of more isolated objects in the same volume. Galaxies in close pairs tend to be both redder and slightly more spheroid dominated than the comparison sample. We suggest that this may be due to ‘harassment’ in multiple previous passes prior to the current close interaction. Galaxy pairs do not appear to prefer significantly denser environments. There is no evidence of an enhancement in the AGN fraction in pairs, compared to other galaxies in the same volume
Safety perspectives on presently considered drugs for the treatment of COVID‐19
Intense efforts are underway to evaluate potential therapeutic agents for the treatment of COVID‐19. In order to respond quickly to the crisis, the repurposing of existing drugs is the primary pharmacological strategy. Despite the urgent clinical need for these therapies, it is imperative to consider potential safety issues. This is important due to the harm–benefit ratios that may be encountered when treating COVID‐19, which can depend on the stage of the disease, when therapy is administered and underlying clinical factors in individual patients. Treatments are currently being trialled for a range of scenarios from prophylaxis (where benefit must greatly exceed risk) to severe life‐threatening disease (where a degree of potential risk may be tolerated if it is exceeded by the potential benefit). In this perspective, we have reviewed some of the most widely researched repurposed agents in order to identify potential safety considerations using existing information in the context of COVID‐19
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