2,879 research outputs found
Acid/base-triggered switching of circularly polarized luminescence and electronic circular dichroism in organic and organometallic helicenes.
Electronic circular dichroism and circularly polarized luminescence acid/base switching activity has been demonstrated in helicene-bipyridine proligand 1 a and in its “rollover” cycloplatinated derivative 2 a. Whereas proligand 1 a displays a strong bathochromic shift (>160 nm) of the nonpolarized and circularly polarized luminescence upon protonation, complex 2 a displays slightly stronger emission. This strikingly different behavior between singlet emission in the organic helicene and triplet emission in the organometallic derivative has been rationalized by using quantum-chemical calculations. The very large bathochromic shift of the emission observed upon protonation of azahelicene-bipyridine 1 a has been attributed to the decrease in aromaticity (promoting a charge-transfer-type transition rather than a π–π* transition) as well as an increase in the HOMO–LUMO character of the transition and stabilization of the LUMO level upon protonation
Mid--IR emission of galaxies in the Virgo cluster: II. Integrated properties
We analyse the integrated properties of the Mid-IR emission of a complete,
optically selected sample of galaxies in the Virgo cluster observed with the
ISOCAM instrument on board the ISO satellite. The analysis shows that the
Mid-IR emission up to 15 mic of optically-selected, normal early-type galaxies
(E, S0 and S0a) is dominated by the Rayleigh-Jeans tail of the cold stellar
component. The Mid-IR emission of late-type galaxies is instead dominated by
the thermal emission from dust. The small dust grains emitting in the Mid-IR
have an excess of emission if compared to big grains emitting in the Far-IR.
While the Far-IR emission increases with the intensity of the interstellar
radiation field, their Mid-IR emission is non--linearly related to the UV
radiation field. The spectral energy distributions of the target galaxies
indicate that there is a linear relationship between the UV radiation field and
the Mid-IR emission of galaxies for low or intermediate activities of star
formation, while the emission from the hot dust seems to drop for strong UV
fields. The Mid-IR colour of late-type galaxies is not related to their
activity of star formation. The properties of the dust emission in the Mid-IR
seem more related to the mass than to the morphological type of the target
galaxy. Since the activity of star formation is anticorrelated to the mass of
galaxies, this reflects a relationship between the emission of dust in the
Mid-IR and the UV radiation field: galaxies with the lowest Mid-IR emission for
a given UV field are low mass, dwarf galaxies. These observational evidences
are easily explained if the carriers of the Unidentified Infrared Bands that
dominate the 6.75 mic emission are destroyed by the intense UV radiation field
of dwarf galaxies, although abundance effects can also play a role.Comment: 17 pages, Latex, 7 figures; to be published in Astronomy &
Astrophysics, Main Journal; Figure legend should be corrected in: 1 - 1a; 2 -
1b; 3 - 2; 4 - 3a; 5 - 3b; 6 - 3c; 7 - 3d; 8 - 3e; 9 - 4; 10 - 5; 11 - 6; 12
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Spectral Mapping Reconstruction of Extended Sources
Three dimensional spectroscopy of extended sources is typically performed
with dedicated integral field spectrographs. We describe a method of
reconstructing full spectral cubes, with two spatial and one spectral
dimension, from rastered spectral mapping observations employing a single slit
in a traditional slit spectrograph. When the background and image
characteristics are stable, as is often achieved in space, the use of
traditional long slits for integral field spectroscopy can substantially reduce
instrument complexity over dedicated integral field designs, without loss of
mapping efficiency -- particularly compelling when a long slit mode for single
unresolved source followup is separately required. We detail a custom
flux-conserving cube reconstruction algorithm, discuss issues of extended
source flux calibration, and describe CUBISM, a tool which implements these
methods for spectral maps obtained with ther Spitzer Space Telescope's Infrared
Spectrograph.Comment: 11 pages, 8 figures, accepted by PAS
New insight into the low-energy He spectrum
The spectrum of He was studied by means of the He(,)He
reaction at a lab energy of 25 MeV/n and small center of mass (c.m.) angles.
Energy and angular correlations were obtained for the He decay products by
complete kinematical reconstruction. The data do not show narrow states at
1.3 and 2.4 MeV reported before for He. The lowest resonant
state of He is found at about 2 MeV with a width of 2 MeV and is
identified as . The observed angular correlation pattern is uniquely
explained by the interference of the resonance with a virtual state
(limit on the scattering length is obtained as fm), and with
the resonance at energy MeV.Comment: 5 pages, 4 figures, 2 table
The Mid-Infrared Spectrum of Star-Forming Galaxies: Global Properties of PAH Emission
We present a sample of low-resolution 5-38um Spitzer IRS spectra of the inner
few square kiloparsecs of 59 nearby galaxies spanning a large range of star
formation properties. A robust method for decomposing mid-infrared galaxy
spectra is described, and used to explore the behavior of PAH emission and the
prevalence of silicate dust extinction. Evidence for silicate extinction is
found in ~1/8 of the sample, at strengths which indicate most normal galaxies
undergo A_V < ~3 magnitudes averaged over their centers. The contribution of
PAH emission to the total infrared power is found to peak near 10% and extend
up to ~20%, and is suppressed at metallicities Z < ~Z_sun/4, as well as in
low-luminosity AGN environments. Strong inter-band PAH feature strength
variations (2-5x) are observed, with the presence of a weak AGN and, to a
lesser degree, increasing metallicity shifting power to the longer wavelength
bands. A peculiar PAH emission spectrum with markedly diminished 5-8um features
arises among the sample solely in systems with relatively hard radiation fields
harboring low-luminosity AGN. The AGN may modify the emitting grain
distribution and provide the direct excitation source of the unusual PAH
emission, which cautions against using absolute PAH strength to estimate star
formation rates in systems harboring active nuclei. Alternatively, the low star
formation intensity often associated with weak AGN may affect the spectrum. The
effect of variations in the mid-infrared spectrum on broadband infrared surveys
is modeled, and points to more than a factor of two uncertainty in results
which assume a fixed PAH emission spectrum, for redshifts z=0-2.5.Comment: Accepted for publication in ApJ, 24 pages (abstract typo fixed,
reference added
Dissecting the origin of the submillimeter emission in nearby galaxies with Herschel and LABOCA
We model the infrared to submillimeter spectral energy distribution of 11
nearby galaxies of the KINGFISH sample using Spitzer and Herschel data and
compare model extrapolations at 870um (using different fitting techniques) with
LABOCA 870um observations. We investigate how the differences between
predictions and observations vary with model assumptions or environment. At
global scales, we find that modified blackbody models using realistic cold
emissivity indices (beta_c=2 or 1.5) are able to reproduce the 870um observed
emission within the uncertainties for most of the sample. Low values
(beta_c<1.3) would be required in NGC0337, NGC1512 and NGC7793. At local
scales, we observe a systematic 870um excess when using beta_=2.0. The
beta_c=1.5 or the Draine and Li (2007) models can reconcile predictions with
observations in part of the disks. Some of the remaining excesses occur towards
the centres and can be partly or fully accounted for by non-dust contributions
such as CO(3-2) or, to a lesser extent, free-free or synchrotron emission. In
three non-barred galaxies, the remaining excesses rather occur in the disk
outskirts. This could be a sign of a flattening of the submm slope (and
decrease of the effective emissivity index) with radius in these objects.Comment: 31 pages (including appendix), 7 figures, accepted for publication in
MNRA
Radio continuum and far-infrared emission from the galaxies in the Eridanus group
The Eridanus galaxies follow the well-known radio-FIR correlation. Majority
(70%) of these galaxies have their star formation rates below that of the Milky
Way. The galaxies having a significant excess of radio emission are identified
as low luminosity AGNs based on their radio morphologies obtained from the GMRT
observations. There are no powerful AGNs (L{20cm} > 10^{23} W Hz^{-1}) in the
group. The two most far-infrared and radio luminous galaxies in the group have
optical and HI morphologies suggestive of recent tidal interactions. The
Eridanus group also has two far-infrared luminous but radio-deficient galaxies.
It is believed that these galaxies are observed within a few Myr of the onset
of an intense star formation episode after being quiescent for at least a 100
Myr. The upper end of the radio luminosity distribution of the Eridanus
galaxies (L_{20cm} ~ 10^{22} W Hz^{-1}) is consistent with that of the field
galaxies, other groups, and late-type galaxies in nearby clusters.Comment: 16 pages; Accepted for publication in Journal of Astroph. & Astron.
March, 200
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