235 research outputs found
Designing an autonomous environment for mission critical operation of the EUVE satellite
Since the launch of NASA's Extreme Ultraviolet Explorer (EUVE) satellite in 1992, there has only been a handful of occurrences that have warranted manual intervention in the EUVE Science Operations Center (ESOC). So, in an effort to reduce costs, the current environment is being redesigned to utilize a combination of off-the-shelf packages and recently developed artificial intelligence (AI) software to automate the monitoring of the science payload and ground systems. The successful implementation of systemic automation would allow the ESOC to evolve from a seven day/week, three shift operation, to a seven day/week one shift operation. First, it was necessary to identify all areas considered mission critical. These were defined as follows: (1) The telemetry stream must be monitored autonomously and anomalies identified. (2) Duty personnel must be automatically paged and informed of the occurrence of an anomaly. (3) The 'basic' state of the ground system must be assessed. (4) Monitors should check that the systems and processes needed to continue in a 'healthy' operational mode are working at all times. (5) Network loads should be monitored to ensure that they stay within established limits. (6) Connectivity to Goddard Space Flight Center (GSFC) systems should be monitored as well, not just for connectivity of the network itself but also for the ability to transfer files. (7) All necessary peripheral devices should be monitored. This would include the disks, routers, tape drives, printers, tape carousel, and power supplies. (8) System daemons such as the archival daemon, the Sybase server, the payload monitoring software, and any other necessary processes should be monitored to ensure that they are operational. (9) The monitoring system needs to be redundant so that the failure of a single machine will not paralyze the monitors. (10) Notification should be done by means of looking though a table of the pager numbers for current 'on call' personnel. The software should be capable of dialing out to notify, sending email, and producing error logs. (11) The system should have knowledge of when real-time passes and tape recorder dumps will occur and should know that these passes and data transmissions are successful. Once the design criteria were established, the design team split into two groups: one that addressed the tracking, commanding, and health and safety of the science payload and another group that addressed the ground systems and communications aspects of the overall system
The Star Formation Rate Function of the Local Universe
We have derived the bivariate luminosity function for the far ultraviolet
(1530Angstroms) and far infrared (60 microns). We used matched GALEX and IRAS
data, and redshifts from NED and PSC-z. We have derived a total star formation
luminosity function phi(L_{tot}), with L_{tot} = L_{FUV}+L_{FIR}. Using these,
we determined the cosmic ``star formation rate'' function and density for the
local universe. The total SFR function is fit very well by a log-normal
distribution over five decades of luminosity. We find that the bivariate
luminosity function phi(L_{FUV},L_{FIR}) shows a bimodal behavior, with L_{FIR}
tracking L_{FUV} for L_{TOT}< 10^10 L_sun, and L_{FUV} saturating at 10^10
L_sun, while L_{TOT} L_{FIR} for higher luminosities. We also calculate the SFR
density and compare it to other measurements.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http:/www.galex.caltech.edu/PUBLICATIONS/ after
November 22, 200
UV emission and Star Formation in Stephan's Quintet
we present the first GALEX UV images of the well known interacting group of
galaxies, Stephan's Quintet (SQ). We detect widespread UV emission throughout
the group. However, there is no consistent coincidence between UV structure and
emission in the optical, H\alpha, or HI. Excluding the foreground galaxy
NGC7320 (Sd), most of the UV emission is found in regions associated with the
two spiral members of the group, NGC7319 and NGC7318b, and the intragroup
medium starburst SQ-A. The extinction corrected UV data are analyzed to
investigate the overall star formation activity in SQ. It is found that the
total star formation rate (SFR) of SQ is 6.69+-0.65 M_\sun/yr. Among this,
1.34+-0.16 M_sun/yr is due to SQ-A. This is in excellent agreement with that
derived from extinction corrected H\alpha luminosity of SQ-A. The SFR in
regions related to NGC 7319 is 1.98+-0.58 M_\sun/yr, most of which(68%) is
contributed by the disk. The contribution from the 'young tail' is only 15%. In
the UV, the 'young tail' is more extended (~100 kpc) and shows a loop-like
structure, including the optical tail, the extragalactic HII regions recently
discovered in H\alpha, and other UV emission regions discovered for the first
time. The UV and optical colors of the 'old tail' are consistent with a single
stellar population of age t ~10^{8.5+-0.4} yrs. The UV emission associated with
NGC 7318b is found in a very large (~80 kpc) disk, with a net SFR of 3.37+-0.25
M_sun/yr. Several large UV emission regions are 30 -- 40 kpc away from the
nucleus of NGC7318b. Although both NGC7319 and NGC7318b show peculiar UV
morphology, their SFR is consistent with that of normal Sbc galaxies,
indicating that the strength of star formation activity is not enhenced by
interactions.Comment: This paper will be published as part of the Galaxy Evolution
Explorer(GALEX) Astrophysical Journal Letters Special Issue. Links to the
full set of papers will be available at
http:/www.galex.caltech.edu/PUBLICATIONS/ after November 22, 200
Recent star formation in nearby galaxies from GALEX imaging:M101 and M51
The GALEX (Galaxy Evolution Explorer) Nearby Galaxies Survey is providing
deep far-UV and near-UV imaging for a representative sample of galaxies in the
local universe. We present early results for M51 and M101, from GALEX UV
imaging and SDSS optical data in five bands. The multi-band photometry of
compact stellar complexes in M101 is compared to population synthesis models,
to derive ages, reddening, reddening-corrected luminosities and current/initial
masses. The GALEX UV photometry provides a complete census of young compact
complexes on a approximately 160pc scale. A galactocentric gradient of the
far-UV - near-UV color indicates younger stellar populations towards the outer
parts of the galaxy disks, the effect being more pronounced in M101 than in
M51.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Full paper available
from http://dolomiti.pha.jhu.edu . Links to full set of papers will be
available at http://www.galex.caltech.edu/PUBLICATIONS/ after November 22,
200
UV and FIR selected star-forming galaxies at z=0: differences and overlaps
We study two samples of local galaxies, one is UV (GALEX) selected and the
other FIR (IRAS) selected, to address the question whether UV and FIR surveys
see the two sides ('bright' and 'dark') of the star formation of the same
population of galaxies or two different populations of star forming galaxies.
No significant difference between the L () luminosity
functions of the UV and FIR samples is found. Also, after the correction for
the `Malmquist bias' (bias for flux limited samples), the FIR-to-UV ratio v.s.
L relations of the two samples are consistent with each other. In the
range of 9 \la \log(L_{tot}/L_\sun) \la 12, both can be approximated by a
simple linear relation of \log (L_{60}/L_{FUV})=\log(L_{tot}/L_\sun)-9.66.
These are consistent with the hypothesis that the two samples represent the
same population of star forming galaxies, and their well documented differences
in L and in FIR-to-UV ratio are due only to the selection effect. A
comparison between the UV luminosity functions shows marginal evidence for a
population of faint UV galaxies missing in the FIR selected sample. The
contribution from these 'FIR-quiet' galaxies to the overall UV population is
insignificant, given that the K-band luminosity functions (i.e. the stellar
mass functions) of the two samples do not show any significant difference.Comment: 21 pages, 7 figures. Accepted by Ap
GALEX Ultraviolet Photometry of Globular Clusters in M31
We present ultraviolet photometry for globular clusters (GCs) in M31 from 15
square deg of imaging using the Galaxy Evolution Explorer (GALEX). We detect
200 and 94 GCs with certainty in the near-ultraviolet (NUV; 1750 - 2750
Angstroms) and far-ultraviolet (FUV; 1350 - 1750 Angstroms) bandpasses,
respectively. Our rate of detection is about 50% in the NUV and 23% in the FUV,
to an approximate limiting V magnitude of 19. Out of six clusters with
[Fe/H]>-1 seen in the NUV, none is detected in the FUV bandpass. Furthermore,
we find no candidate metal-rich clusters with significant FUV flux, because of
the contribution of blue horizontal-branch (HB) stars, such as NGC 6388 and NGC
6441, which are metal-rich Galactic GCs with hot HB stars. We show that our
GALEX photometry follows the general color trends established in previous UV
studies of GCs in M31 and the Galaxy. Comparing our data with Galactic GCs in
the UV and with population synthesis models, we suggest that the age range of
M31 and Galactic halo GCs are similar.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http://www.galex.caltech.edu/PUBLICATIONS/ after
November 22, 200
The On-Orbit Performance of the Galaxy Evolution Explorer
We report the first year on-orbit performance results for the Galaxy
Evolution Explorer (GALEX), a NASA Small Explorer that is performing a survey
of the sky in two ultraviolet bands. The instrument comprises a 50 cm diameter
modified Ritchey-Chretien telescope with a 1.25 degree field of view,
selectable imaging and objective grism spectroscopic modes, and an innovative
optical system with a thin-film multilayer dichroic beam splitter that enables
simultaneous imaging by a pair of photon counting, microchannel plate, delay
line readout detectors. Initial measurements demonstrate that GALEX is
performing well, meeting its requirements for resolution, efficiency,
astrometry, bandpass definition and survey sensitivity.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issu
Number Counts of GALEX Sources in FUV (1530A) and NUV (2310A) Bands
Number Counts of galaxies in two GALEX bands (FUV: 1530A and NUV: 2310A, both
in AB magnitudes) are reported. They provide for the first time in the
literature homogeneously calibrated number counts of UV galaxies covering
continuously a very wide range of UV magnitude (14 -- 23.8). Both the FUV and
NUV counts are inconsistent with a non-evolution model, while they are in good
agreement with evolution models (essentially luminosity evolution) derived from
the high-z UV luminosity functions of Arnouts et al. (2004). It is found that
the contribution from galaxies detected by GALEX to the UV background is
0.68+-0.10 nW m-2 sr-1 at 1530A and 0.99+-0.15 nW m-2 sr-1 at 2310A. These are
66+-9% and 44+-6% of the total contributions of galaxies to the the UV
background at 1530A, respectively, as estimated using the evolution models. ...Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http:/www.galex.caltech.edu/PUBLICATIONS/ after
November 22, 200
Panoramic GALEX FUV and NUV imaging of M31 and M33
We present Galaxy Evolution Explorer (GALEX) far-UV and near-UV mosaic
observations covering the entirety of M31 and M33. For both targets, we measure
the decline of surface brightness (in FUV and NUV) and changes in FUV--NUV
color as a function of galactocentric radius. These UV radial profiles are
compared to the distribution of ionized gas traced by H-alpha emission. We find
that the extent of the UV emission, in both targets, is greater than the extent
of the observed HII regions and diffuse ionized gas. We determine the
ultraviolet diffuse fraction in M33 using our FUV observations and compare it
to the H-alpha diffuse fraction obtained from wide-field narrow-band imaging.
The FUV diffuse fraction appears to be remarkably constant near 0.65 over a
large range in galactocentric radius, with departures to higher values in
circumnuclear regions and, most notably, at the limit of the H-alpha disk. We
suggest that the increase in FUV diffuse fraction at large galactocentric radii
could indicate that a substantial portion of the diffuse emission beyond this
point is not generated in situ but rather scattered from dust, after
originating in the vicinity of the disk's outermost HII regions. Radial
variation of the H-alpha diffuse fraction was also measured. We found the
H-alpha diffuse fraction generally near 0.4 but rising toward the galaxy
center, up to 0.6. We made no attempt to correct our diffuse fraction
measurements for position-dependent extinction, so the quoted values are best
interpreted as upper limits given the plausibly higher extinction for stellar
clusters relative to their surroundings.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http://www.galex.caltech.edu/PUBLICATIONS/ after
November 22, 2004. Individual high-resolution figures can be found at
http://dolomiti.pha.jhu.edu/publgoto.htm
The UV Galaxy Luminosity Function in the Local Universe from GALEX Data
We present the results of a determination of the galaxy luminosity function
at ultraviolet wavelengths at redshifts of from GALEX data. We
determined the luminosity function in the GALEX FUV and NUV bands from a sample
of galaxies with UV magnitudes between 17 and 20 that are drawn from a total of
56.73 deg^2 of GALEX fields overlapping the b_j-selected 2dF Galaxy Redshift
Survey. The resulting luminosity functions are fainter than previous UV
estimates and result in total UV luminosity densities of 10^(25.55+/-0.12) ergs
s^-1 Hz^-1 Mpc^-3 and 10^(25.72+/-0.12) ergs s^-1 Hz^-1 Mpc^-3 at 1530 Ang. and
2310 Ang., respectively. This corresponds to a local star formation rate
density in agreement with previous estimates made with H-alpha-selected data
for reasonable assumptions about the UV extinction.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http://www.galex.caltech.edu/PUBLICATIONS/ after
November 22, 200
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