4,232 research outputs found
Herschel photometric observations of the low metallicity dwarf galaxy NGC 1705
We present Herschel SPIRE and PACS photometeric observations of the low metallicity (Z ~ 0.35 Z_â) nearby dwarf galaxy, NGC 1705, in six wavelength bands as part of the Dwarf Galaxy Survey guaranteed time Herschel key program. We confirm the presence of two dominant circumnuclear IR-bright regions surrounding the central super star cluster that had been previously noted at mid-IR wavelengths and in the sub-mm by LABOCA. On constructing a global spectral energy distribution using the SPIRE and PACS photometry, in conjunction with archival IR measurements, we note the presence of an excess at sub-mm wavelengths. This excess suggests the presence of a significant cold dust component within NGC 1705 and was modeled as an additional cold component in the SED. Although alternative explanations for the sub-mm excess beyond 350 ÎŒm, such as changes to the dust emissivity cannot be ruled out, the most likely explanation for the observed submillimetre excess is that of an additional cold dust component
The dust morphology of the elliptical Galaxy M86 with SPIRE
We present Herschel-SPIRE observations at 250â500âÎŒm of the giant elliptical galaxy Mâ86 and examine the distribution of the resolved cold dust emission and its relation with other galactic tracers. The SPIRE images reveal three dust components: emission from the central region; a dust lane extending north-south; and a bright emission feature 10âkpc to the south-east. We estimate that ~10^6âM_â of dust is spatially coincident with atomic and ionized hydrogen, originating from stripped material from the nearby spiral NGC 4438 due to recent tidal interactions with Mâ86. The gas-to-dust ratio of the cold gas component ranges from ~20â80. We discuss the different heating mechanisms for the dust features
Searches for Ultra-Compact Dwarf Galaxies in Galaxy Groups
We present the results of a search for ultra-compact dwarf galaxies (UCDs) in
six different galaxy groups: Dorado, NGC1400, NGC0681, NGC4038, NGC4697 and
NGC5084. We searched in the apparent magnitude range 17.5 < b_j < 20.5 (except
NGC5084: 19.2 < b_j < 21.0). We found 1 definite plus 2 possible UCD candidates
in the Dorado group and 2 possible UCD candidates in the NGC1400 group. No UCDs
were found in the other groups. We compared these results with predicted
luminosities of UCDs in the groups according to the hypothesis that UCDs are
globular clusters formed in galaxies. The theoretical predictions broadly agree
with the observational results, but deeper surveys are needed to fully test the
predictions.Comment: 8 pages, 2 figures, accepted for publication in MNRA
Cold Dust but Warm Gas in the Unusual Elliptical Galaxy NGC 4125
Data from the Herschel Space Observatory have revealed an unusual elliptical galaxy, NGC 4125, which has strong and extended submillimeter emission from cold dust but only very strict upper limits to its CO and Hi emission. Depending on the dust emissivity, the total dust mass is 2-5 x 10(6) M-circle dot. While the neutral gas-to-dust mass ratio is extremely low (= 10(4) K faster than the dust is evaporated. If galaxies like NGC 4125, where the far-infrared emission does not trace neutral gas in the usual manner, are common at higher redshift, this could have significant implications for our understanding of high redshift galaxies and galaxy evolution.Canadian Space AgencyNatural Sciences and Engineering Research Council of CanadaAgenzia Spaziale Italiana (ASI) I/005/11/0BMVIT (Austria)ESA-PRODEX (Belgium)CEA/CNES (France)DLR (Germany)ASI/INAF (Italy)CICYT/MCYT (Spain)CSA (Canada)NAOC (China)CEA, (France)CNES (France)CNRS (France)ASI (Italy)MCINN (Spain)SNSB (Sweden)STFC (UK)NASA (USA)National Aeronautics and Space AdministrationAstronom
Herschel spectral-mapping of the Helix Nebula (NGC 7293): Extended CO photodissociation and OH+ emission
The Helix Nebula (NGC 7293) is the closest planetary nebulae. Therefore, it
is an ideal template for photochemical studies at small spatial scales in
planetary nebulae. We aim to study the spatial distribution of the atomic and
the molecular gas, and the structure of the photodissociation region along the
western rims of the Helix Nebula as seen in the submillimeter range with
Herschel. We use 5 SPIRE FTS pointing observations to make atomic and molecular
spectral maps. We analyze the molecular gas by modeling the CO rotational lines
using a non-local thermodynamic equilibrium (non-LTE) radiative transfer model.
For the first time, we have detected extended OH+ emission in a planetary
nebula. The spectra towards the Helix Nebula also show CO emission lines (from
J= 4 to 8), [NII] at 1461 GHz from ionized gas, and [CI] (2-1), which together
with the OH+ lines, trace extended CO photodissociation regions along the rims.
The estimated OH+ column density is (1-10)x1e12 cm-2. The CH+ (1-0) line was
not detected at the sensitivity of our observations. Non-LTE models of the CO
excitation were used to constrain the average gas density (n(H2)=(1-5)x1e5
cm-3) and the gas temperature (Tk= 20-40 K). The SPIRE spectral-maps suggest
that CO arises from dense and shielded clumps in the western rims of the Helix
Nebula whereas OH+ and [CI] lines trace the diffuse gas and the UV and X-ray
illuminated clumps surface where molecules reform after CO photodissociation.
[NII] traces a more diffuse ionized gas component in the interclump medium.Comment: Accepted for publication in Astronomy and Astrophysic
Herschel SPIRE-FTS Observations of Excited CO and [CI] in the Antennae (NGC 4038/39): Warm and Cold Molecular Gas
We present Herschel SPIRE-FTS observations of the Antennae (NGC 4038/39), a
well studied, nearby ( Mpc) ongoing merger between two gas rich spiral
galaxies. We detect 5 CO transitions ( to ), both [CI]
transitions and the [NII] transition across the entire system, which
we supplement with ground based observations of the CO , and
transitions, and Herschel PACS observations of [CII] and [OI].
Using the CO and [CI] transitions, we perform both a LTE analysis of [CI], and
a non-LTE radiative transfer analysis of CO and [CI] using the radiative
transfer code RADEX along with a Bayesian likelihood analysis. We find that
there are two components to the molecular gas: a cold ( K)
and a warm ( K) component. By comparing the warm gas mass
to previously observed values, we determine a CO abundance in the warm gas of
. If the CO abundance is the same in the warm and
cold gas phases, this abundance corresponds to a CO luminosity-to-mass
conversion factor of $\alpha_{CO} \sim 7 \ M_{\odot}{pc^{-2} \ (K \ km \
s^{-1})^{-1}}_263\mu m\sim 0.01 L_{\odot}/M_{\odot}G_0\sim 1000$. Finally, we find
that a combination of turbulent heating, due to the ongoing merger, and
supernova and stellar winds are sufficient to heat the molecular gas.Comment: 50 pages, 15 figures, 8 tables, Accepted for publication in The
Astrophysical Journa
Herschel/HIFI observations of spectrally resolved methylidyne signatures toward the high-mass star-forming core NGC6334I
In contrast to extensively studied dense star-forming cores, little is known
about diffuse gas surrounding star-forming regions. We study molecular gas in
the high-mass star-forming region NGC6334I, which contains diffuse, quiescent
components that are inconspicuous in widely used molecular tracers such as CO.
We present Herschel/HIFI observations of CH toward NGC6334I observed as part of
the CHESS key program. HIFI resolves the hyperfine components of its J=3/2-1/2
transition, observed in both emission and absorption. The CH emission appears
close to the systemic velocity of NGC6334I, while its measured linewidth of 3
km/s is smaller than previously observed in dense gas tracers such as NH3 and
SiO. The CH abundance in the hot core is 7 10^-11, two to three orders of
magnitude lower than in diffuse clouds. While other studies find distinct
outflows in, e.g., CO and H2O toward NGC6334I, we do not detect outflow
signatures in CH. To explain the absorption signatures, at least two absorbing
components are needed at -3.0 and +6.5 km/s with N(CH)=7 10^13 and 3 10^13
cm^-2. Two additional absorbing clouds are found at +8.0 and 0.0 km/s, both
with N(CH)=2 10^13 cm^-2. Turbulent linewidths for the four absorption
components vary between 1.5 and 5.0 km/s in FWHM. We constrain physical
properties of our CH clouds by matching our CH absorbers with other absorption
signatures. In the hot core, molecules such as H2O and CO trace gas that is
heated and dynamically influenced by outflow activity, whereas CH traces more
quiescent material. The four CH absorbers have column densities and turbulent
properties consistent with diffuse clouds: two are located near NGC6334, and
two are unrelated foreground clouds. Local density and dynamical effects
influence the chemical composition of physical components of NGC6334, causing
some components to be seen in CH but not in other tracers, and vice versa.Comment: Accepted by A&A Letters; 5 pages, 1 figure; v2: minor textual and
typographical change
Why did British Electricity Prices Fall after 1998?
In an attempt to reduce high electricity prices in England and Wales the government has reduced concentration among generators and introduced New Electricity Trading Arrangements (NETA). Econometric analysis on monthly data from April 1996 to September 2002 implies support for two conflicting hypotheses. On a static view, increases in competition and the capacity margin were chiefly responsible for the fall in prices. If generators had been tacitly colluding before NETA, however, the impending change in market rules might have changed their behaviour a few months before the abolition of the Pool. That view implies that NETA reduced prices
Herschel imaging of the dust in the Helix Nebula (NGC 7293)
In our series of papers presenting the Herschel imaging of evolved planetary
nebulae, we present images of the dust distribution in the Helix nebula (NGC
7293). Images at 70, 160, 250, 350, and 500 micron were obtained with the PACS
and SPIRE instruments on board the Herschel satellite. The broadband maps show
the dust distribution over the main Helix nebula to be clumpy and predominantly
present in the barrel wall. We determined the spectral energy distribution of
the main nebula in a consistent way using Herschel, IRAS, and Planck flux
values. The emissivity index of 0.99 +/- 0.09, in combination with the carbon
rich molecular chemistry of the nebula, indicates that the dust consists mainly
of amorphous carbon. The dust excess emission from the central star disk is
detected at 70 micron and the flux measurement agree with previous measurement.
We present the temperature and dust column density maps. The total dust mass
across the Helix nebula (without its halo) is determined to be 0.0035 solar
mass at a distance of 216 pc. The temperature map shows dust temperatures
between 22 and 42 K, which is similar to the kinetic temperature of the
molecular gas, strengthening the fact that the dust and gas co-exist in high
density clumps. Archived images are used to compare the location of the dust
emission in the far infrared (Herschel) with the ionized (GALEX, Hbeta) and
molecular hydrogen component. The different emission components are consistent
with the Helix consisting of a thick walled barrel-like structure inclined to
the line of sight. The radiation field decreases rapidly through the barrel
wall.Comment: 8 pages, 9 figures, revised version A&A in pres
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