102 research outputs found
NGC 4388: A Test Case for Relativistic Disk Reflection and Fe K Fluorescence Features
We present a new analysis of the Suzaku X-ray spectrum of the Compton-thin
Seyfert 2 galaxy NGC 4388. The spectrum above 2 keV can be described by a
remarkably simple and rather mundane model, consisting of a uniform, neutral
spherical distribution of matter, with a radial column density of cm, and an Fe abundance of
relative to solar. The model does not require any
phenomenological adjustments to self-consistently account for the low-energy
extinction, the Fe K and Fe K fluorescent emission lines, the Fe
K edge, and the Compton-scattered continuum from the obscuring material. The
spherical geometry is not a unique description, however, and the
self-consistent, solar abundance MYTORUS model, applied with toroidal and
non-toroidal geometries, gives equally good descriptions of the data. In all
cases, the key features of the spectrum are so tightly locked together that for
a wide range of parameters, a relativistic disk-reflection component
contributes no more than 2% to the net spectrum in the 2-20 keV band. We
show that the commonly invoked explanations for weak X-ray reflection features,
namely a truncated and/or very highly ionized disk, do not work for NGC 4388.
If relativistically-broadened Fe K lines and reflection are ubiquitous
in Seyfert 1 galaxies, they should also be ubiquitous in Compton-thin Seyfert 2
galaxies. The case of NGC 4388 shows the need for similar studies of more
Compton-thin AGN to ascertain whether this is true.Comment: MNRAS accepted. 21 pages, 7 figures, 4 tables; Appendix with
historical notes and 1 table. This version: Corrected minor typo and
affiliatio
Detection of broad 21-cm absorption at z = 0.656 in the complex sight-line towards 3C336
We report the detection of 21-cm absorption at z = 0.656 towards 1622+238
(3C336). The line is very broad with a Full-Width Half Maximum (FWHM) of 235
km/s, giving a velocity integrated optical depth of 2.2 km/s. The centroid of
the line is offset from that of the known damped Lyman-alpha absorption (DLA)
system by 50 km/s, and if the Lyman-alpha and 21-cm absorption are due to the
same gas, we derive a spin temperature of < 60 K, which would be the lowest yet
in a DLA. The wide profile, which is over four times wider than that of any
other DLA, supports the hypothesis that the hydrogen absorption is occurring
either in the disk of a large underluminous spiral or a group of dim
unidentified galaxies, associated with the single object which has been
optically identified at this redshift.Comment: 5 pages, 3 figures, accepted by MNRAS Letter
Cosmic Pathways for Compact Groups in the Milli-Millennium Simulation
We detected 10 compact galaxy groups (CGs) at in the semi-analytic
galaxy catalog of Guo et al. (2011) for the milli-Millennium Cosmological
Simulation (sCGs in mGuo2010a). We aimed to identify potential canonical
pathways for compact group evolution and thus illuminate the history of
observed nearby compact groups. By constructing merger trees for sCG
galaxies, we studied the cosmological evolution of key properties, and compared
them with Hickson CGs (HCGs). We found that, once sCG galaxies come
within 1 (0.5) Mpc of their most massive galaxy, they remain within that
distance until , suggesting sCG "birth redshifts". At stellar masses
of sCG most-massive galaxies are within . In several cases, especially in the two 4- and 5-member
systems, the amount of cold gas mass anti-correlates with stellar mass, which
in turn correlates with hot gas mass. We define the angular difference between
group members' 3D velocity vectors, , and note that
many of the groups are long-lived because their small values of
indicate a significant parallel component. For
triplets in particular, values range between
and so that galaxies are coming together along
roughly parallel paths, and pairwise separations do not show large pronounced
changes after close encounters. The best agreement between sCG and HCG physical
properties is for galaxy values, but HCG values are higher overall,
including for SFRs. Unlike HCGs, due to a tail at low SFR and , and a
lack of galaxies, only a few sCG galaxies
are on the star-forming main sequence.Comment: Style fixes to better match ApJ published version. Uses likeapj1.1
style files: 17 pages, 13 figures, 2 tables. LaTex style files available at
https://github.com/qtast/likeapj/releases/lates
Selection of ThAr lines for wavelength calibration of echelle spectra and implications for variations in the fine-structure constant
Echelle spectrographs currently provide some of the most precise and detailed spectra in astronomy, the interpretation of which sometimes depends on the wavelength calibration accuracy. In some applications, such as constraining cosmological variations in the fundamental constants from quasar absorption lines, the wavelength calibration is crucial. Here we detail an algorithm for selecting thorium-argon (ThAr) emission lines for wavelength calibration which incorporates the properties of both a new laboratory wavelength list and the spectrograph of interest. We apply the algorithm to the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (UVES) and demonstrate a factor of ≳3 improvement in the wavelength calibration residuals (i.e. random errors) alone. It is also found that UVES spectra calibrated using a previous, widely distributed line-list contain systematic ±30 -75 m s−1 distortions of the wavelength scale over both short and long wavelength ranges. These distortions have important implications for current UVES constraints on cosmological variations in the fine-structure constant. The induced systematic errors are most severe for Mg/Feii quasar absorbers in the redshift range 1.2 ≲zabs≲ 2.3, with individual absorbers studied by recent authors containing systematic errors up to four times larger than quoted statistical error
Research on differences in AIDS reports from Mainland China and Taiwan —From the Perspective of Health Communication
本文以两岸报纸对艾滋病相关议题的报道为研究对象,使用内容分析与文本分析相结合的方法对《人民日报》与《联合报》2006年至2012年期间的艾滋病报道进行对比剖析,探究这两家媒体在艾滋病议题的建构上有何异同,以及从传媒生态的角度探析其成因。 研究发现,两份报纸艾滋病相关议题报道具有多方面的共同点,即报道最多的年份均在2006年;“世界艾滋病日”前后报道比较集中;立足本土化;报道覆盖了艾滋病病毒的三大传播途径;消息来源以官方为主;涉及传播途径的报道不够科学。两份报纸艾滋病相关议题报道的不同点有:《联合报》艾滋病报道更趋“常态化”,报道数量相对稳定,而《人民日报》仍将艾滋病及其相关事件看作是社会的异...This research is designed to explore the differences between AIDS reports from mainland China and Taiwan media and to analyze the reason from the perspective of media ecology. Applying content analysis and text analysis, the study compares the AIDS reports from People’s Daily (PD) to those from United Daily News (UDN) during the period of 2006 to 2012. The results of this study propose the follow...学位:新闻与传播硕士院系专业:新闻传播学院_传播学学号:3192010115285
Selection of ThAr lines for wavelength calibration of echelle spectra and implications for variations in the fine-structure constant
Echelle spectrographs currently provide some of the most precise and detailed
spectra in astronomy, the interpretation of which sometimes depends on the
wavelength calibration accuracy. In some applications, such as constraining
cosmological variations in the fundamental constants from quasar absorption
lines, the wavelength calibration is crucial. Here we detail an algorithm for
selecting thorium-argon (ThAr) emission lines for wavelength calibration which
incorporates the properties of both a new laboratory wavelength list and the
spectrograph of interest. We apply the algorithm to the Very Large Telescope
Ultraviolet and Visual Echelle Spectrograph (UVES) and demonstrate a factor of
>3 improvement in the wavelength calibration residuals (i.e. random errors)
alone. It is also found that UVES spectra calibrated using a previous, widely
distributed line-list contain systematic +/-30-75 m/s distortions of the
wavelength scale over both short and long wavelength ranges. These distortions
have important implications for current UVES constraints on cosmological
variations in the fine-structure constant. The induced systematic errors are
most severe for Mg/FeII quasar absorbers in the redshift range 1.2 < z < 2.3,
with individual absorbers studied by recent authors containing systematic
errors up to 4 times larger than quoted statistical errors.Comment: 10 pages, 11 figures (16 EPS files). Accepted by MNRAS. Supplementary
material, including electronic versions of all input and output ThAr atlases
and colour versions of Fig. 3(right) available at
http://www.ast.cam.ac.uk/~mim/pub.htm
Probing variations in fundamental constants with radio and optical quasar absorption-line observations
Nine quasar absorption spectra at 21-cm and UV rest-wavelengths are used to
estimate possible variations in x=alpha^2 g_p mu, (alpha is the fine structure
constant, g_p the proton g-factor and mu=me/mp the electron-to-proton mass
ratio). We find ^weighted_total(=Dxxwt)=(0.63+-0.99) 10^-5 over
0.23~<z_abs~<2.35 (2.7 to 10.5 Gyr, look-back time, t_lb). A linear best fit
against t_lb, tied to Delta x/x=0 at z=0, gives (dot x)/x=(-0.6+-1.2) 10^-15
/yr. Our large sample demonstrates that intrinsic line-of-sight velocity
differences between the 21-cm and UV absorption redshifts, (on average
Delta_vlos~6km/s), with random sign and magnitude in each absorption system,
limit our precision. Combining our Delta x/x measurement with absorption-line
constraints on alpha-variation yields strong limits on the variation of mu. Our
most conservative estimate, obtained by assuming no variations in alpha or g_p
is Delta mu/mu(=Dmm)=Dxxwt. If we use only the four high-redshift absorbers in
our sample, we obtain Dmm=(0.58+-1.95) 10^-5, which agrees (2sigma) with
recent, more direct estimates from two absorption systems containing molecular
hydrogen, also at high redshift, and which have hinted at a possible
mu-variation, Dmm=(-2.0+-0.6) 10^-5. Our method of constraining Dmm is
completely independent from the molecular hydrogen observations. If we include
the low-redshift systems, our Dmm result differs significantly from the
high-redshift molecular hydrogen results. We detect a dipole variation in mu
across the sky, but this model is required by the data at only the 88 per cent
confidence level. Clearly, much larger samples of 21-cm and molecular hydrogen
absorbers are required to adequately resolve the issue of the variation of mu
and x.(Abridged)Comment: 19 pages, 11 figures. Accepted for publication in MNRAS. Replaced
with correct figure
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