Chapter 2 - GALEX Science Surveys

Introduction   GALEX science surveys   Imaging Surveys   Spectroscopic surveys   Legacy Surveys   Guest Investigator Program   GALEX Observational Constraints and Safety

Introduction

GALEX is performing a variety of imaging and spectroscopic surveys of different depths and sky coverage. This chapter briefly summarizes the nature of each survey. Lists of fields that have been released publicly are available from the Multi-mission archive at the Space Telescope Science Institute (MAST).

GALEX was launched by a Pegasus-XL vehicle on April 28 2003 into a 690km altitude, 29 degree inclination, circular orbit with a 98.6 minute period. The GALEX instrument allows imaging and spectroscopic observations to be made in two ultraviolet bands, Far UV (FUV) 1350-1780Å and Near UV (NUV) 1770-2730Å. The instrument provides simultaneous co-aligned FUV and NUV images with spatial resolution 4.3 and 5.3 arcseconds respectively. The grism mode provides low resolution spectra (R FUV/NUV 200/90) of all objects in the field of view. Details of the performance of the instrument and detectors can be found in Morrissey et al. (2007) ApJS, 173, 682 and a summary in Chapter 1. The projected orbit lifetime is more then 25 years and the solar array power margin is expected to remain above 30% until 2012 with useful battery life through 2015. There are no consumables on board. The spacecraft is three axis stabilized with selective redundancy in the gyros and reaction wheels and no flight hardware failures have occurred.

The mission operations design is straightforward. Science data is only collected on the night side of the orbit with the instrument boresite pointing at the science target and performing a spiral dither motion of 1.5 arcminute diameter. This is done to reduce detector fatigue by bright stars. On the day side of each orbit the spacecraft is aligned with solar panels oriented toward the Sun and the instrument boresite pointed away from the Earth (see Figure 1).

Fig. 1 GALEX orbit attitude. Exposures are only taken on the night side of the orbit.

The length of the detector exposures on the night side ranges from 15 to 28 minutes. The voltage levels of the detectors are ramped to low values during the day side of each orbit to protect detectors from damage and the buildup of charged particles on the FUV detector window. The detectors are also held at low voltage during spacecraft passages through the South Atlantic Anomaly (SAA). When this occurs on the night side of an orbit the exposure times are reduced (see Figure 2), occasionally causing no exposure to be taken.

Fig. 2 Ground trace of a single GALEX orbit. The particle flux associated with the SAA is modeled (blue = low to red=high). The red line is the night side of the orbit, the day side is white. The high voltage supply on the detectors are not ramped until after exiting the SAA. The viewing cones of the two ground stations are also displayed. A contact with the Hawaii station is possible on this orbit.

GALEX typically downlinks 3.2GB of photon data and telemetry every day, comparable to the three great observatories combined (HST, Chandra, Spitzer). The data is transmitted from the ground stations, operated by Universal Space Networks, arriving at the Science Operations Center (SOC) at Caltech within 8 hours of each contact. Upon arrival, the raw telemetry files are automatically ingested by a sophisticated software pipeline that extracts housekeeping telemetry, which is trended to monitor the health of the spacecraft and science instrument. The pipeline processing of the raw time tagged photon event telemetry is described in Chapter 3.

GALEX Science Surveys

The science goals of the GALEX mission are achieved with a series of nested surveys. Observations of many survey types are made each day (14 to 15 eclipses), although only one survey type can be assigned to each eclipse. The baseline mission surveys were completed in the Fall of 2007 and are shown in the table below along with the number of tiles released at MAST in 2007 (GR2/3), 2008 (GR4), and 2010 (GR6).

GALEX Baseline Mission Surveys completed in Fall 2007
Survey Exposure Time (seconds)

Sky Coverage (deg2)

 Depth (mAB)

GR2/3

tiles

GR4

tiles
All-sky Imaging (AIS) 100 26,000 20.5 15721 28,000**
Medium Imaging (MIS) 1,500 1,000 23.5 1017 1615
Deep Imaging (DIS) 30,000 80 25.0 165 193
Nearby Galaxy (NGS) 1,500 300 28* 296 433
Medium Spectroscopic (MSS) 150,000 5 22 3 5**
*surface density (mag/sq arcsec)     **projected

A preliminary UV sky background map based on GALEX imaging observations from the DIS, MIS, and AIS is shown below. It represents over 10 million seconds of exposure time.

(Credit: Mark Seibert, OCIW)

Imaging Surveys

Deep Imaging Survey (DIS) - Exposure goal of 30,000s over 80 square degrees of sky. Targets are chosen to have extensive corollary data from other large surveys like COSMOS, DEEP and ELAIS.

Medium Imaging survey (MIS) - Single orbit exposures (1,500s) of 1000 square degrees in positions that match the Sloan Digital Sky Survey (SDSS) spectroscopic footprint. The MIS has been extended to cover the Two Degree Field Galaxy Redshift Survey (2dFGRS) and the AA-Omega (WiggleZ) project.

All-Sky Imaging Survey (AIS) - Exposure time of 100s over 26,000 square degrees of sky reaching a depth of mAB = 20-2 in both bands. The large sky coverage is achieved by observing a chain of up to 12 positions in a single eclipse. The pointing centers are chosen from a fixed grid of points on the sky that were chosen to avoid having many gaps between adjacent fields. Areas within 20 degrees of the Galactic plane have patchy coverage due to detector safety limits on the total UV sky brightness in the instrument field of view. GR4/5 contains a total of 28,269 individual pointing's.

Nearby Galaxy Survey (NGS) - This is a survey targeting nearby galaxies with a nominal exposure time of 1000s to 1500s. Note that some fields have significantly more exposure time. The core of the NGS targets the 71 galaxies included in the Spitzer Nearby Galaxies Survey (SINGS). However, the sample also includes a variety of nearby galaxies, typically included due to existing or planned data at other wavelengths. Thus, the NGS is not a statistically representative sample of the galaxies in the local universe. The GR4/5 release contains a total 458 pointing's.

Calibration Imaging (CAI) - GALEX has observed several white dwarf standards for the purposes of calibration. These are taken from the list in Bohlin (1996), AJ, 111, 1743. With the exception of the white dwarf LDS749B, all of these stars are saturated in the GALEX images and are not useful for calibration. The GALEX absolute calibration rests solely on observations of LDS749B although it is mildly saturated on some locations of the detector. (More information on the GALEX photometric calibration can be found in Chapter 1).

Spectroscopic Surveys

In addition to the imaging data, there is limited amount of spectroscopy available. The original plan for the spectroscopic surveys was similar to that for the imaging, with shallow exposures covering large areas and progressively deeper exposures targeting smaller solid angles. However, as the mission progressed, the decision was made to only continue with the Medium Spectroscopic Survey. Below are the three classes of spectroscopic data sets available in the GR2/GR3 releases.

Medium Spectroscopic Survey (MSS) - Spectroscopic observations with exposure goal of 150,000s covering 5 square degrees of fields observed as a part of the Deep Imaging Survey.

Calibration Spectroscopy (CAS) - GALEX has observed several white dwarf standards for the purposes of calibration. In contrast to the imaging, most of these standards are not saturated in spectroscopy.

Legacy Surveys

The GALEX mission is now extending the goals of the successful baseline mission surveys with a set of nested legacy surveys, formulated based on the experience operating the satellite, and on discoveries by GALEX and other observatories. They include a Time Domain Survey (TDS) that piggy-backs on the legacy surveys by exploiting the photon counting, repeated exposures, and the contrast sensitivity of the UV emission from most variable sources. GALEX is coordinating with the ground based Pan-STARRS Medium Deep Survey (PS-1) as part of the TDS and will soon provide variable object alerts to the community.

A brief summary of the GALEX legacy surveys is presented in the table below.

GALEX Legacy Surveys
Survey

Exposure Time

(seconds)

Sky Coverage

(deg2)
Galactic Cap Survey SDSS Galactic cap footprint 1,500 20,000
Legacy Deep Survey PS-1, M31, SDSS 30,000 100
Milky Way Survey SEGUE 1,500 5,000
Legacy Spectroscopy Project SDSS 150,000 20
Deep Galaxy Survey Nearby Galaxies 15,000 100
Ultra Deep imaging Survey   300,000 7

Guest Investigator Program

GALEX is the only SMEX to offer a Guest Investigator Program (GIP) and allocates 33% of observing time each year to peer reviewed proposals from the community. The SOC supports the GIP by performing technical reviews of the proposals, documentation for website (http://galexgi.gsfc.nasa.gov/) as well as the design and scheduling of GIP observations, processing of data, and regular delivery of GIP data to MAST. The variety of astronomical objects observed as part of the GIP has broadened the immediate science return of GALEX and to achieve this the mission planning system has expanded. New modes of operation and planning include:

  • Target of Opportunity observations, including comets, supernovae and variable stars.
  • Coordinated observations with ground and orbiting observatories (e.g. Deep impact encounter with Comet Tempel 1).
  • Observations timed at specific intervals to examine stellar variability.

For most GI proposals, there is a proprietary period of 6 months after an observation for data analysis. After this time GI data will appear in the public archive at MAST, and 788 GI imaging tiles are included in GR4/5. The data may have been processed with a different version of the GALEX pipeline than that which was originally given to the GI themselves if updated calibration is available. As this data is obtained for a variety of scientific projects, the sky coverage and exposure times are very heterogeneous.

For questions about the Guest Investigator Program, or technical questions about GALEX observations please contact galexhelp@galexgi.gsfc.nasa.gov.

GALEX Observational Constraints and Safety

The high-sensitivity GALEX detectors dictate strict brightness limits for target fields. The GALEX satellite operates in a largely autonomous way, with observing plans typically uploaded weekly. These properties constrain the observations GALEX is able to do. The major constraints are summarized below.

  • GALEX observes only during orbital night, when it is in the shadow of the earth. This constrains observations of most targets to an observing “season” of a few months or so duration per year.

  • GALEX does not observe near the sun, earth limb, moon, or bright planets. The large sun avoidance angle (85 degrees) combined with other avoidance zones that in practice most individual targets are observable for only a small fraction (~10%) of the year.

  • GALEX detectors will be saturated and potentially damaged by UV-bright stars that are commonly encountered on the sky (particularly in the Galactic plane and the Magellanic Clouds). Limits are currently, for point sources (assuming flat spectra):

    • FUV: 5,000cts/s    mAB =  9.5      Fλ = 7 x 10 -12 erg cm -2 s -1 Å -1 
    • NUV: 30,000cts/s  mAB = 8.9      Fλ = 6 x 10 -12 erg cm -2 s -1 Å -1

  • GALEX detectors will also be saturated and possibly damaged by overbright fields with a) too many UV-bright stars or b) high backgrounds. Potentially too-high backgrounds for the NUV detector usually arise from the blue end of solar spectrum in zodiacal light; for the FUV detector they are usually diffuse galactic emission. Total field upper limits are:

    • FUV: 15,000cts/s     Fλ =  2 x 10 -11 erg cm -2 s -1 Å -1 

    • NUV: 80,000cts/s    Fλ = 1.6 x 10 -11 erg cm -2 s -1 Å -1 

  • Target positions that have stars 5000 < NUV < 30000 cts/s in the field of view must be observed in "petal pattern" mode. In this mode each exposure is split into 12 pointing positions separated by 1.6 arcminutes arrayed in a circle of diameter 6 arcminutes. Stars that are estimated to produce > 5000 NUV cts/s can be no closer then 10 arcminutes from the center of the field of view. The global NUV count rate limit for petal pattern mode is raised to 80000 cts/s. However the FUV point source and global count rate limits remain the same as normal mode observations (5000 and 15000 cts/s respectively). Note that scheduling constraints only allow petal pattern mode targets to be assigned to eclipses where the SAA does not shorten the night side exposure. This will decrease the total number of eclipses during the GI cycle where petal pattern mode targets do not violate other observing constraints (e.g. boresite to Sun, Earth, Moon, etc.). The detailed construction of each petal pattern mode observation and the evaluation of the safety of such observations is made in phase 2 of the technical review of Guest investigator proposals.