139 research outputs found
One-pot microwave-assisted green synthesis of amine-functionalized graphene quantum dots for high visible light photocatalytic application
Nowadays, graphene quantum dots (GQDs) have gained a huge interest in the field of visible-range photocatalysts because of their tunable band gap and stable photochemical properties. In this work, amine-functionalized GQDs (AGQDs) were successfully prepared by one-step microwave-assisted conversion of glucose, H2O2, and NH3 solution. The obtained quantum dots possess the high quality of graphene structure with the average size of 3.78 nm as well as exhibit a strong green fluorescence with a high quantum yield. Interestingly, the amine-functionalized dots perform outstanding visible-light absorption. To further investigate photocatalytic properties, a composite of AGQDs and TiO2 was then prepared by a simple mixing route. The hybrid material showed high catalytic activity of dye degradation under visible light irradiation, which indicates the key role of AGQDs in enhancing light absorption and induced electron–hole separation. The current study may open a new way for construction of effective visible light photocatalytic systems with a cost-effective, simple approach.Scopu
The spatial and age distribution of stellar populations in DDO 190
The spatial distribution of stellar populations, the star formation history,
and other properties of the dIrr galaxy DDO 190 have been analyzed using
color--magnitude diagrams (CMDs) of about 3900 resolved stars and the Ha fluxes
of HII regions. From the mean color index of the red giant branch, a mean
metallicity [Fe/H]=-2.0 is obtained. The I magnitude of the TRGB has been used
to estimate the distance. DDO 190 is 2.9+/-0.2 Mpc from the Milky Way, 2.1 Mpc
from the M 94 group (CnV-I), 2.4 Mpc from the M 81 group and 2.9 Mpc from the
barycenter of the Local Group, all indicating that it is an isolated, field
galaxy. The surface-brightness distribution of the galaxy is well fitted by
ellipses of ellipticity e=1-a/b=0.1 and P.A.=82deg. The radial star density
distribution follows an exponential law of scale length a=43."4, corresponding
to 611 pc. The Holmberg semi-major axis to mu_B=26.5 is estimated to be
r^B_(26.5)=3.'0. Stellar populations of different ages in DDO 190 show strong
spatial decoupling, the oldest population appearing much more extended than the
youngest. Stars younger than 0.1 Gyr occupy only the central 40'' (0.55 kpc);
stars younger than a few (~4) Gyr extend out to ~80'' (125 kpc), and for larger
galactocentric distances only older stars seem to be present. This behavior is
found in all the dIrr galaxies for which spatially extended studies have been
performed and could be related with the kinematical history of the galaxy.Comment: To be published in the AJ. 29 pages, 13 figure
The evolutionary status of the low-metallicity blue compact dwarf galaxy SBS 0940+544
We present the results of spectrophotometry and V,R,I, Halpha CCD photometry
of the blue compact dwarf (BCD) galaxy SBS 0940+544. Broad-band images taken
with the 2.1m KPNO and 1.23m Calar Alto telescopes reveal a compact
high-surface-brightness H II region with ongoing star formation, located at the
northwestern tip of the elongated low surface brightness (LSB) main body of the
BCD. High signal-to-noise 4.5m Multiple Mirror Telescope (MMT) and 10m Keck II
telescope long-slit spectroscopy of SBS 0940+544 is used to derive element
abundances of the ionized gas in the brightest H II region and to study the
stellar population in the host galaxy. The oxygen abundance in the brightest
region with strong emission lines is 12+log(O/H) = 7.46-7.50, or 1/29-1/26
solar, in agreement with earlier determinations and among the lowest for BCDs.
Hbeta and Halpha emission lines and Hdelta and Hgamma absorption lines are
detected in a large part of the main body. Three methods are used to put
constraints on the age of the stellar population at different positions along
the major axis. They are based on (a) the equivalent widths of the emission
lines, (b) the equivalent widths of the absorption lines and (c) the spectral
energy distributions (SED). Several scenarios of star formation have been
considered. The observed properties in the main body can be reproduced by a
continuous star formation process which started not earlier than 100 Myr ago,
if a small extinction is assumed. However, the observations can be reproduced
equally well by a stellar population forming continuously since 10 Gyr ago, if
the star formation rate has increased during the last 100 Myr in the main body
of SBS 0940+544 by at least a factor of five. In summary, we find no compelling
evidence which favors either a young or an old age of SBS 0940+544.Comment: 20 pages, 15 Postscript figures, accepted for publication in the
Astronomy and Astrophysic
Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies. I. SBS 1129+576
Spectroscopy and V,I CCD photometry of the dwarf irregular galaxy SBS
1129+576 are presented for the first time. The CCD images reveal a chain of
compact H II regions within the elongated low-surface-brightness (LSB)
component of the galaxy. Star formation takes place mainly in two
high-surface-brightness H II regions. The mean (V-I) colour of the LSB
component in the surface brightness interval between 23 and 26 mag/sq.arcsec is
relatively blue ~0.56+/-0.03 mag, as compared to the (V-I)~0.9-1.0 for the
majority of known dwarf irregular and blue compact dwarf (BCD) galaxies.
Spectroscopy shows that the galaxy is among the most metal-deficient galaxies
with an oxygen abundance 12+log(O/H)= 7.36+/-0.10 in the brightest H II region
and 7.48+/-0.12 in the second brightest H II region, or 1/36 and 1/28 of the
solar value, respectively. Hbeta and Halpha emission lines and Hdelta and
Hgamma absorption lines are detected in a large part of the LSB component. We
use two extinction-insensitive methods based on the equivalent widths of (1)
emission and (2) absorption Balmer lines to put constraints on the age of the
stellar populations in the galaxy. In addition, we use two extinction-dependent
methods based on (3) the spectral energy distribution (SED) and (4) the (V-I)
colour. The observed properties of the LSB component can be reproduced by a
stellar population forming continuously since 10 Gyr ago, provided that the
star formation rate has increased during the last 100 Myr by a factor of 6 to
50 and no extinction is present. However, the observational properties of the
LSB component can be reproduced equally well by continuous star formation which
started not earlier than 100 Myr ago and stopped at 5 Myr, if some extinction
is assumed.(Abridged)Comment: 17 pages, 12 figures, Accepted for publication in A&
A study of HI-selected galaxies in the Hercules cluster
The present study is aimed at a sample of 22 galaxies detected in the blind
VLA HI survey of the Hercules cluster by Dickey (1997), 18 of which were
selected on an HI line width smaller than 270 km/s and 4 others with only
tentative optical counterparts on the Palomar Sky Survey. Sensitive single-dish
HI line spectra were obtained for 20 of them, and for one (47-154) the VLA
detection was not confirmed. Optical surface photometry was obtained of 10
objects, for 8 of which optical spectroscopy was obtained as well. Based on
various selection criteria, two (ce-143 and ne-204) can be classified as
dwarfs. The objects of which optical observations were made show star formation
properties similar to those of published samples of actively star forming
galaxies, and approximately half of them have properties intermediate between
those of dwarf galaxies and low-luminosity disc galaxies. No optical redshifts
could be obtained for two of the galaxies (sw-103 and sw-194) and their
physical association with the HI clouds detected at their positions therefore
remains uncertain. Unique among the objects is the Tidal Dwarf Galaxy ce-061 in
a tail of the IC 1182 merger system.Comment: 20 pages, 19 figures, accepted for publication in A&
New southern blue compact dwarf galaxies in the 2dF Galaxy Redshift Survey
Aiming to find new extremely metal-deficient star-forming galaxies we
extracted from the Two-Degree Field Galaxy Redshift Survey (2dFGRS) 100K Data
Release 14 emission-line galaxies with relatively strong [OIII] 4363 emission.
Spectroscopic and photometric studies of this sample and, in addition, of 7
Tololo and 2 UM galaxies were performed on the basis of observations with the
ESO 3.6m telescope. All sample galaxies qualify with respect to their
photometric and spectroscopic properties as blue compact dwarf (BCD) galaxies.
Additionally, they show a good overlap with a comparison sample of 100
well-studied emission-line galaxies on the 12+log(O/H) vs. log(Ne/O), log(Ar/O)
and log(Fe/O) planes. From the analysis of the 2dFGRS subsample we report the
discovery of two new extremely metal-deficient BCDs with an oxygen abundance
12+log(O/H) < 7.6 and of another seven galaxies with 12+log(O/H) < 7.8.
Furthermore, we confirm previous oxygen abundance determinations for the BCDs
Tol 1304-353, Tol 2146-391, UM 559 and UM 570 to be 12+log(O/H) < 7.8.Comment: 26 pages, 65 figures, 5 tables, uses psfig.sty, Accepted for
publication in Astronomy and Astrophysics. The paper with high-resolution
figures can be downloaded at
http://www.uni-sw.gwdg.de/~papade/Publications/Papaderos2006_2dF.pd
An Aromatic Inventory of the Local Volume
Using infrared photometry from the Spitzer Space Telescope, we perform the
first inventory of aromatic feature emission (AFE, but also commonly referred
to as PAH emission) for a statistically complete sample of star-forming
galaxies in the local volume. The photometric methodology involved is
calibrated and demonstrated to recover the aromatic fraction of the IRAC 8
micron flux with a standard deviation of 6% for a training set of 40 SINGS
galaxies (ranging from stellar to dust dominated) with both suitable
mid-infrared Spitzer IRS spectra and equivalent photometry. A potential factor
of two improvement could be realized with suitable 5.5 and 10 micron
photometry, such as what may be provided in the future by JWST. The resulting
technique is then applied to mid-infrared photometry for the 258 galaxies from
the Local Volume Legacy (LVL) survey, a large sample dominated in number by
low-luminosity dwarf galaxies for which obtaining comparable mid-infrared
spectroscopy is not feasible. We find the total LVL luminosity due to five
strong aromatic features in the 8 micron complex to be 2.47E10 solar
luminosities with a mean volume density of 8.8E6 solar luminosities per cubic
Megaparsec. Twenty-four of the LVL galaxies, corresponding to a luminosity cut
at M = -18.22 in the B band, account for 90% of the aromatic luminosity. Using
oxygen abundances compiled from the literature for 129 of the 258 LVL galaxies,
we find a correlation between metallicity and the aromatic to total infrared
emission ratio but not the aromatic to total 8 micron dust emission ratio. A
possible explanation is that metallicity plays a role in the abundance of
aromatic molecules relative to the total dust content, but other factors such
as star formation and/or the local radiation field affect the excitation of
those molecules.Comment: ApJ in press; 29 pages, 14 figures, 3 tables; emulateapj forma
Deep multiband surface photometry on star forming galaxies: II. A volume limited sample of 21 emission lines galaxies
We present deep surface photometry of a volume--limited sample of 21 UM
emission line galaxies in broadband optical UBVRI and near infra-red (NIR) HKs
filters. The sample comprises 19 blue compact galaxies (BCGs) and two spirals.
For some targets the exposure times are the deepest to date. For the BCG UM462
we observe a previously undetected second disk component beyond a surface
brightness level of mu_B=26 mag arcsec^{-2}. This is a true low surface
brightness component with central surface brightness mu_0=24.1 mag arcsec^{-2}
and scale length h_r=1.5 kpc. All BCGs are dwarfs, with M_B>=-18, and very
compact, with an average scale length of h_r~1 kpc. We separate the burst and
host populations for each galaxy and compare them to stellar evolutionary
models with and without nebular emission contribution. We also measure the
A_{180} asymmetry in all filters and detect a shift from optical to NIR in the
average asymmetry of the sample. This shift seems to be correlated with the
morphological class of the BCGs. Using the color-asymmetry relation, we
identify five BCGs in the sample as mergers, which is confirmed by their
morphological class. Though clearly separated from normal galaxies in the
concentration-asymmetry parameter space, we find that it is not possible to
distinguish luminous starbursting BCGs from the merely star forming low
luminosity BCGs.Comment: 48 pages, 39 figures, submitte
Galactic rotation curves in brane world models
In the braneworld scenario the four dimensional effective Einstein equation
has extra source terms, which arise from the embedding of the 3-brane in the
bulk. These non-local effects, generated by the free gravitational field of the
bulk, may provide an explanation for the dynamics of the neutral hydrogen
clouds at large distances from the galactic center, which is usually explained
by postulating the existence of the dark matter. In the present paper we
consider the asymptotic behavior of the galactic rotation curves in the brane
world models, and we compare the theoretical results with observations of both
High Surface Brightness and Low Surface Brightness galaxies. For the chosen
sample of galaxies we determine first the baryonic parameters by fitting the
photometric data to the adopted galaxy model; then we test the hypothesis of
the Weyl fluid acting as dark matter on the chosen sample of spiral galaxies by
fitting the tangential velocity equation of the combined baryonic-Weyl model to
the rotation curves. We give an analytical expression for the rotational
velocity of a test particle on a stable circular orbit in the exterior region
to a galaxy, with Weyl fluid contributions included. The model parameter ranges
for which the test provides agreement (within 1 confidence
level) with observations on the velocity fields of the chosen galaxy sample are
then determined. There is a good agreement between the theoretical predictions
and observations, showing that extra-dimensional models can be effectively used
as a viable alternative to the standard dark matter paradigm.Comment: to be published in MNRAS, 17 pages, 31 figures, version including
corrections on the proo
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