610 research outputs found
Detection of interstellar CH_3
Observations with the Short Wavelength Spectrometer (SWS) onboard the {\it
Infrared Space Observatory} (ISO) have led to the first detection of the methyl
radical in the interstellar medium. The branch at 16.5
m and the (0) line at 16.0 m have been unambiguously detected
toward the Galactic center SgrA. The analysis of the measured bands gives a
column density of (8.02.4) cm and an excitation
temperature of K. Gaseous at a similarly low excitation
temperature and are detected for the same line of sight. Using
constraints on the column density obtained from and
visual extinction, the inferred abundance is
. The chemically related
molecule is not detected, but the pure rotational lines of are seen
with the Long Wavelength Spectrometer (LWS). The absolute abundances and the
and ratios are inconsistent with published
pure gas-phase models of dense clouds. The data require a mix of diffuse and
translucent clouds with different densities and extinctions, and/or the
development of translucent models in which gas-grain chemistry, freeze-out and
reactions of with polycyclic aromatic hydrocarbons and solid
aliphatic material are included.Comment: 2 figures. ApJL, Accepte
The Opacity of Spiral Galaxy Disks VI: Extinction, stellar light and color
In this paper we explore the relation between dust extinction and stellar
light distribution in disks of spiral galaxies. Extinction influences our
dynamical and photometric perception of disks, since it can distort our
measurement of the contribution of the stellar component. To characterize the
total extinction by a foreground disk, Gonzalez et al. (1998) proposed the
``Synthetic Field Method'' (SFM), which uses the calibrated number of distant
galaxies seen through the foreground disk as a direct indication of extinction.
The method is described in Gonzalez et al. (1998) and Holwerda et al. (2005a).
To obtain good statistics, the method was applied to a set of HST/WFPC2 fields
Holwerda et al. (2005b) and radial extinction profiles were derived, based on
these counts. In the present paper, we explore the relation of opacity with
surface brightness or color from 2MASS images, as well as the relation between
the scalelengths for extinction and light in the I band. We find that there is
indeed a relation between the opacity (A_I) and the surface brightness,
particularly at the higher surface brightnesses. No strong relation between
near infrared (H-J, H-K) color and opacity is found. The scalelengths of the
extinction are uncertain for individual galaxies but seem to indicate that the
dust distribution is much more extended than the stellar light. The results
from the distant galaxy counts are also compared to the reddening derived from
the Cepheids light-curves Freedman et al. (2001). The extinction values are
consistent, provided the selection effect against Cepheids with higher values
of A_I is taken into account. The implications from these relations for disk
photometry, M/L conversion and galaxy dynamical modeling are briefly discussed.Comment: 9 pages, 2 tables, 10 figures, accepted by A&
The Role of the Euclid Archive System in the Processing of Euclid and External Data
Euclid is an ESA M2 mission which will create a 15,000 square degrees space-based survey: the Euclid Archive System (EAS) is a core element of the Science Ground Segment (SGS) of Euclid. The EAS follows a data-centric approach to data processing, whereby the Data Processing System (DPS) is responsible for the centralized metadata storage and the Distributed Storage System (DSS) supports the distributed storage of data files. The EAS-DPS implements the Euclid Common Data model and along with the EAS-DSS provides numerous services for Euclid Consortium users and SGS subsystems. In addition, the EAS-DPS assists in the preparation of Euclid data releases which are copied to the third EAS subsystem, the ESA developed Science Archive System (SAS) where they become available to the wider astronomical community. The EAS-DPS implements the object-oriented Euclid Common Data Model using a relational DBMS for the storage. The EAS-DPS supports the tracing of the lineage of any data item in the system, provides services for the data quality assessment and the data processing orchestration. The EAS-DSS is a distributed storage system which is based on a set of storage nodes located in each of the ten Science Data Centers of the Euclid SGS. The storage nodes supports a wide range of solutions from local disk, using a unix filesystem, to iRODS nodes or Grid storage elements. In this paper the architectural design of EAS-DPS and EAS-DSS are reviewed: the interaction between them and tests of the already implemented components are described
The Mid-Infrared Spectra of Normal Galaxies
The mid-infrared spectra (2.5 to 5 and 5.7 to 11.6 mu) obtained by ISO-PHOT
reveal the interstellar medium emission from galaxies powered by star formation
to be strongly dominated by the aromatic features at 6.2, 7.7, 8.6 and 11.3 mu.
Additional emission appears in-between the features, and an underlying
continuum is clearly evident at 3-5 mu. This continuum would contribute about a
third of the luminosity in the 3 to 13 mu range. The features together carry 5
to 30% of the 40-to-120 mu `FIR' luminosity. The relative fluxes in individual
features depend very weakly on galaxy parameters such as the far-infrared
colors, direct evidence that the emitting particles are not in thermal
equilibrium. The dip at 10 mu is unlikely to result from silicate absorption,
since its shape is invariant among galaxies. The continuum component has a f_nu
\~ nu^{0.65} shape between 3 and 5 mu and carries 1 to 4% of the FIR
luminosity; its extrapolation to longer wavelengths falls well below the
spectrum in the 6 to 12 mu range. This continuum component is almost certainly
of non-stellar origin, and is probably due to fluctuating grains without
aromatic features. The spectra reported here typify the integrated emission
from the interstellar medium of the majority of star-forming galaxies, and
could thus be used to obtain redshifts of highly extincted galaxies up to z=3
with SIRTF.Comment: 10 pages, 2 figures, uses AAS LaTeX; to appear in the Astrophysical
Journal Letter
The complete submillimetre spectrum of NGC 891
Submillimetre maps of NGC 891 have been obtained with the PRONAOS
balloon-borne telescope and with the ISOPHOT instrument on board the ISO
satellite. In this article, we also gather data from IRAS and SCUBA to present
the complete submillimetre spectrum of this nearby edge-on spiral galaxy. We
derive submillimetre emission profiles along the major axis. The modified
blackbody fits, assuming a single dust component, lead to temperatures of 19-24
K toward the centre and 18-20 K toward the edges, with possible variations of
the dust spectral index from 1.4 to 2. The two-component fits lead to a warm
component temperature of 29 K all along the galaxy with a cold component at 16
K. The interstellar medium masses derived by these two methods are quite
different: 4.6 x 10^9 Mo in the case of the one-component model and 12 x 10^9
Mo in the case of the two-component one. This two-component fit indicates that
the cold dust to warm dust ratio is 20 to 40, the highest values being in the
wings of this galaxy. Comparing to dust mass estimates, both estimations of the
ISM mass are consistent with a gas to dust mass ratio of 240, which is close to
the Milky Way value. Our results illustrate the importance of accurate
submillimetre spectra to derive masses of the interstellar medium in galaxies.Comment: 5 pages, 2 figures, accepted May 2003 in the MNRA
Towards an Understanding of the Mid-Infrared Surface Brightness of Normal Galaxies
We report a mid-infrared color and surface brightness analysis of IC 10, NGC
1313, and NGC 6946, three of the nearby galaxies studied under the Infrared
Space Observatory Key Project on Normal Galaxies. Images with < 9 arcsecond
(170 pc) resolution of these nearly face-on, late-type galaxies were obtained
using the LW2 (6.75 mu) and LW3 (15 mu) ISOCAM filters. Though their global
I_nu(6.75 mu)/I_nu(15 mu) flux ratios are similar and typical of normal
galaxies, they show distinct trends of this color ratio with mid-infrared
surface brightness. We find that I_nu(6.75 mu)/I_nu(15 mu) ~< 1 only occurs for
regions of intense heating activity where the continuum rises at 15 micron and
where PAH destruction can play an important role. The shape of the
color-surface brightness trend also appears to depend, to the second-order, on
the hardness of the ionizing radiation. We discuss these findings in the
context of a two-component model for the phases of the interstellar medium and
suggest that star formation intensity is largely responsible for the
mid-infrared surface brightness and colors within normal galaxies, whereas
differences in dust column density are the primary drivers of variations in the
mid-infrared surface brightness between the disks of normal galaxies.Comment: 19 pages, 6 figures, uses AAS LaTeX; to appear in the November
Astronomical Journa
Disk Galaxies in the Outer Local Supercluster: Optical CCD Surface Photometry and Distribution of Galaxy Disk Parameters
We report new B-band CCD surface photometry on a sample of 76 disk galaxies
brighter than B_T = 14.5 mag in the Uppsala General Catalogue of Galaxies,
which are confined within a volume located in the outer part of the Local
Supercluster. With our earlier published I-band CCD and high S/N-ratio 21cm HI
data (Lu et al. 1993), this paper completes our optical surface photometry
campaign on this galaxy sample. As an application of this data set, the B-band
photometry is used here to illustrate two selection effects which have been
somewhat overlooked in the literature, but which may be important in deriving
the distribution function of disk central surface brightness (CSB) of disk
galaxies from a diameter or/and flux limited sample: a Malmquist-type bias
against disk galaxies with small disk scale lengths (DSL) at a given CSB; and a
disk inclination dependent selection effect that may, for example, bias toward
inclined disks near the threshold of a diameter limited selection if disks are
not completely opaque in optical. Taking into consideration these selection
effects, we present a method of constructing a volume sampling function and a
way to interpret the derived distribution function of CSB and DSL. Application
of this method to our galaxy sample implies that if galaxy disks are optically
thin, CSB and DSL may well be correlated in the sense that, up to an
inclination-corrected limiting CSB of about 24.5 mag per square arcsec that is
adequately probed by our galaxy sample, the DSL distribution of galaxies with a
lower CSB may have a longer tail toward large values unless the distribution of
disk galaxies as a function of CSB rises rapidly toward faint values.Comment: 27 pages including 9 figures and 2 tables. To appear in the October
20, 1998 issue of the Astrophysical Journa
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