742 research outputs found
Characteristics of Young Children\u27s Longest Utterances
This paper reviews research on mean length of utterance (MLU) and longest length of utterance (LLU) as measures of child language development. Although there has been much research on MLU, there is some controversy as to its validity, especially across different data gathering procedures and clinical populations. In contrast, LLU has rarely been the focus of research, but the limited amount of information available indicates that it is a potentially useful way to analyze child speech. This study provides a detailed analysis of 1655 utterances from three children at ages 3;10, 4;1 and 4;4, and 2938 utterances from adults interacting with the children. Variables included MLU, LLU, Fluharty-2 scores, utterance length distributions, and utterance intention codes. LLU did not co-vary strongly with either MLU or Fluharty-2 scores. Distributions of utterance lengt! hs were highly skewed, with single-morpheme utterances occurring most frequently. LLU was highly dependent on language sample size, as shown in analyses of fixed-length samples. Longest utterances were nearly all topic expansions and responses to questions. Adults tended to use topic expansions and questions immediately before children’s longest utterances. Overall, the results suggest that information from LLU may supplement MLU and standardized testing. However, LLU was dependent on a small number of utterances and very susceptible to change according to language sample size. Advantages, disadvantages, and a possible solution are discussed
The evolution of obscured accretion
Our current understanding of the evolution of obscured accretion onto
supermassive black holes is reviewed. We consider the literature results on the
relation between the fraction of moderately obscured, Compton-thin AGN and
redshift, and discuss the biases which possibly affect the various
measurements. Then, we discuss a number of methods - from ultradeep X-ray
observations to the detection of high-ionization optical emission lines - to
select the population of the most heavily obscured, Compton-thick AGN, whose
cosmological evolution is basically unknown. The space density of heavily
obscured AGN measured through different techniques is discussed and compared
with the predictions by current synthesis models of the X-ray background.
Preliminary results from the first half of the 3 Ms XMM observation of the
Chandra Deep Field South (CDFS) are also presented. The prospects for
population studies of heavily obscured AGN with future planned or proposed
X-ray missions are finally discussed.Comment: 6 pages, 2 figures. Invited talk at the conference "X-ray Astronomy
2009: Present status, multiwavelength approach and future perspectives",
September 2009, Bologna. To appear in AIP Conf. Proc. (editors: A. Comastri,
M. Cappi, L. Angelini)
A deep X-ray observation of M82 with XMM-Newton
We report on the analysis of a deep (100 ks) observation of the starburst
galaxy M82 with the EPIC and RGS instruments on board the X-ray telescope
XMM-Newton. The broad-band (0.5-10 keV) emission is due to at least three
spectral components: i) continuum emission from point sources; ii) thermal
plasma emission from hot gas; iii) charge exchange emission from neutral metals
(Mg and Si). The plasma emission has a double-peaked differential emission
measure, with the peaks at ~0.5 keV and ~7 keV. Spatially resolved spectroscopy
has shown that the chemical absolute abundances are not uniformly distributed
in the outflow, but are larger in the outskirts and smaller close to the galaxy
centre. The abundance ratios also show spatial variations. The X-ray derived
Oxygen abundance is lower than that measured in the atmospheres of red
supergiant stars, leading to the hypothesis that a significant fraction of
Oxygen ions have already cooled off and no longer emit at energies > ~0.5 keV.Comment: Accepted for publication in MNRAS. 20 pages, 15 figures, LaTeX2
The high-redshift Universe with the International X-ray Observatory
We discuss some of the main open issues related to the light-up and evolution
of the first accreting sources powering high redshift luminous quasars. We
discuss the perspectives of future deep X-ray surveys with the International
X-ray Observatory and possible synergies with the Wide Field X-ray Telescope.Comment: 6 pages, 6 figures. Proceedings of "The Wide Field X-ray Telescope
Workshop", held in Bologna, Italy, Nov. 25-26 2009. To appear in Memorie
della Societ\`a Astronomica Italiana 2010 (arXiv:1010.5889
The XMM deep survey in the CDF-S. X. X-ray variability of bright sources
We aim to study the variability properties of bright hard X-ray selected
Active Galactic Nuclei (AGN) with redshift between 0.3 and 1.6 detected in the
Chandra Deep Field South (XMM-CDFS) by a long XMM observation. Taking advantage
of the good count statistics in the XMM CDFS we search for flux and spectral
variability using the hardness ratio techniques. We also investigated spectral
variability of different spectral components. The spectra were merged in six
epochs (defined as adjacent observations) and in high and low flux states to
understand whether the flux transitions are accompanied by spectral changes.
The flux variability is significant in all the sources investigated. The
hardness ratios in general are not as variable as the fluxes. Only one source
displays a variable HR, anti-correlated with the flux (source 337). The
spectral analysis in the available epochs confirms the steeper when brighter
trend consistent with Comptonisation models only in this source. Finding this
trend in one out of seven unabsorbed sources is consistent, within the
statistical limits, with the 15 % of unabsorbed AGN in previous deep surveys.
No significant variability in the column densities, nor in the Compton
reflection component, has been detected across the epochs considered. The high
and low states display in general different normalisations but consistent
spectral properties. X-ray flux fluctuations are ubiquitous in AGN. In general,
the significant flux variations are not associated with a spectral variability:
photon index and column densities are not significantly variable in nine out of
the ten AGN over long timescales (from 3 to 6.5 years). The photon index
variability is found only in one source (which is steeper when brighter) out of
seven unabsorbed AGN. These results are consistent with previous deep samples.Comment: 14 pages, 11 figures. Accepted in A&
X-ray properties of radio-selected star forming galaxies in the Chandra-COSMOS survey
X-ray surveys contain sizable numbers of star forming galaxies, beyond the
AGN which usually make the majority of detections. Many methods to separate the
two populations are used in the literature, based on X-ray and multiwavelength
properties. We aim at a detailed test of the classification schemes and to
study the X-ray properties of the resulting samples. We build on a sample of
galaxies selected at 1.4 GHz in the VLA-COSMOS survey, classified by Smolcic et
al. (2008) according to their optical colours and observed with Chandra. A
similarly selected control sample of AGN is also used for comparison. We review
some X-ray based classification criteria and check how they affect the sample
composition. The efficiency of the classification scheme devised by Smolcic et
al. (2008) is such that ~30% of composite/misclassified objects are expected
because of the higher X-ray brightness of AGN with respect to galaxies. The
latter fraction is actually 50% in the X-ray detected sources, while it is
expected to be much lower among X-ray undetected sources. Indeed, the analysis
of the stacked spectrum of undetected sources shows, consistently, strongly
different properties between the AGN and galaxy samples. X-ray based selection
criteria are then used to refine both samples. The radio/X-ray luminosity
correlation for star forming galaxies is found to hold with the same
X-ray/radio ratio valid for nearby galaxies. Some evolution of the ratio may be
possible for sources at high redshift or high luminosity, tough it is likely
explained by a bias arising from the radio selection. Finally, we discuss the
X-ray number counts of star forming galaxies from the VLA- and C-COSMOS surveys
according to different selection criteria, and compare them to the similar
determination from the Chandra Deep Fields. The classification scheme proposed
here may find application in future works and surveys.Comment: 15 pages, 7 figures, 3 table
Statistical properties of the combined emission of a population of discrete sources: astrophysical implications
We study the statistical properties of the combined emission of a population
of discrete sources (e.g. X-ray emission of a galaxy due to its X-ray binaries
population). Namely, we consider the dependence of their total luminosity
L_tot=SUM(L_k) and of fractional rms_tot of their variability on the number of
sources N or, equivalently, on the normalization of the luminosity function. We
show that due to small number statistics a regime exists, in which L_tot grows
non-linearly with N, in an apparent contradiction with the seemingly obvious
prediction =integral(dN/dL*L*dL) ~ N. In this non-linear regime, the
rms_tot decreases with N significantly more slowly than expected from the rms ~
1/sqrt(N) averaging law. For example, for a power law luminosity function with
a slope of a=3/2, in the non-linear regime, L_tot ~ N^2 and the rms_tot does
not depend at all on the number of sources N. Only in the limit of N>>1 do
these quantities behave as intuitively expected, L_tot ~ N and rms_tot ~
1/sqrt(N). We give exact solutions and derive convenient analytical
approximations for L_tot and rms_tot.
Using the total X-ray luminosity of a galaxy due to its X-ray binary
population as an example, we show that the Lx-SFR and Lx-M* relations predicted
from the respective ``universal'' luminosity functions of high and low mass
X-ray binaries are in a good agreement with observations. Although caused by
small number statistics the non-linear regime in these examples extends as far
as SFR<4-5 Msun/yr and log(M*/Msun)<10.0-10.5, respectively.Comment: MNRAS, accepted for publicatio
Beewatching: A project for monitoring bees through photos
Bees play a key role in natural and agro-ecosystems and their diversity is worldwide threatened by anthropogenic causes. Despite this, there is little awareness of the existence of the numerous species of wild bees, and the common name “bee” is very often exclusively associated with Apis mellifera. Our aim was to create a citizen science project in Italy with the following objectives: (a) raising awareness of the importance and diversity of bees, (b) obtaining data on the biology, ecology and distribution of Italian species, and (c) launching the monitoring of alien bees. The first step of the project was to create a website platform with a section containing informative datasheets of the wild bee families and of the most common bee genera present in Italy, a form to send reports of observed bees and an interactive map with all citizen’s reports. During the 2 years of the project 1086 reports were sent by 269 users, with 38 Apoidea genera reported on 190 plant genera; furthermore, 22 reports regarding the alien species Megachile sculpturalis arrived. The majority of bees (34 genera) were observed on spontaneous plants, including 115 genera native to Italy. Considering the increasing number of reports and data obtained in these first two years of the project, our objectives seem to be achieved. Future steps will be to outline the profile of beewatchers, to plan activities in a more targeted way, and also to start some sub-projects for conservation purposes
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