Document version: 29 December 2005
2.0 MIPS GOODS-S observations and image data products
3.0 MIPS GOODS-S source list
4.0 IRAC GOODS-S epoch 1 v0.3 re-reductions and image data products
Tables:
Other information:
This document describes the third release (DR3) of data products from the Great Observatories Origins Deep Survey (GOODS) Spitzer Space Telescope Legacy Science program. This release includes three new sets of data products:
Many of the details of the observations and data reduction for these products are the same as those from earlier data releases. We therefore will not repeat most of this information here, but will direct the reader to the earlier documentation where appropriate.
The GOODS team is writing a paper which will describe the observations, data reductions, and data products (Dickinson et al., in preparation), and a paper describing MIPS catalogs (Chary et al., in preparation). Please reference these papers when using these data products in published research.
2.0 MIPS GOODS-S observations and image data products
Here, we provide a brief description of the GOODS-S MIPS 24 micron observations, data reduction, and the FITS image products provided with this data release. The GOODS-S MIPS 24 micron source list is described in section 3.0.
The GOODS Spitzer Legacy program observations cover two fields on the sky. One of these fields (GOODS-N) coincides with the historical Hubble Deep Field North (Williams et al. 1996), while the other (GOODS-S) coincides with the Chandra Deep Field South (Giacconi et al. 2001). These fields have extensive observations at virtually every wavelength accessible from major space- and ground-based observatories, including deep, multicolor data from the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (Giavalisco et al. 2004).
Generically, the GOODS fields consist of sky regions that are approximately 10 x 16.5 arcmin on the sky. The orientations of these fields were originally chosen to match scheduling constraints for Spitzer observing (primarily for IRAC, where we observed the field at two epochs angles separated by 6 months and 180 degrees rotation). Coordinates, position angles, and other important parameters for the MIPS observations of GOODS-S are summarized in Table 1.
The GOODS observations are divided into two separate observing programs, one for each field: program 169 for GOODS-N, and program 194 for GOODS-S. The IRAC and MIPS observations of a given field share the same program identification number.
2.2 MIPS observing strategy and AORs
The details of the MIPS observing strategy and Astronomical Observation Requests (AORs) for GOODS-S are essentially the same as for GOODS-N. Those are described in the documentation for GOODS DR1+, section 2.2, and we refer the reader to that discussion.
The observations for each field were broken into a series of Astronomical Observation Requests (AORs), each several hours long, that were designed to enable efficient scheduling. 24 AORs were used for the GOODS-S MIPS observations.
The MIPS observations were designed to overlay, as nearly as possible, the corresponding IRAC observations of the GOODS field. Observations were executed over the course of several days during MIPS campaign number 12. This followed shortly after the second epoch of IRAC imaging for GOODS-S, which was carried out in IRAC campaign 12. The telescope orientation rotates by approximately 1 degree per day, and hence so does the coverage of the GOODS field. Because MIPS campaign 12 followed IRAC campaign 12 by u several days, the telescope position angle had rotated by approximately 3 degrees from that used for the IRAC observations.
Table 1 lists relevant parameters for the GOODS-S MIPS observations, including the overall pointing center, the mean position angle (east of north) of the long axis of the field averaged over the duration of the MIPS observations, the range of of position angles over the span of the observations, and the start and end times for the complete series of observations. Table 2 gives a detailed list of the AOR labels, AORKEY identifying numbers, and start dates/times for each AOR.
| Parameter | GOODS-S |
|---|---|
| Spitzer program ID | 194 |
| RA (J2000) | 03:32:30.37 |
| Dec (J2000) | -27:48:16.8 |
| Mean position angle | -11.3 deg |
| Range of PAs | 2.7 deg |
| Start date/time | 2004-08-19 00:50:39 |
| End date/time | 2004-08-23 00:04:37 |
The reduction of the MIPS GOODS-S images followed virtually identical procedures to those used for the MIPS GOODS-N data set, which are described in detail in the documentation for GOODS DR1+, section 3.0. We refer the reader to that discussion for more information.
The GOODS team created data products using images processed by the Spitzer Science Center (SSC) Basic Calibrated Data (BCD) pipeline. The GOODS-S MIPS data delivered to the GOODS team were processed with pipeline version S11.0.2, and the current data products are based on that version.
This release of GOODS data products includes FITS images of the MIPS data for the GOODS-S field. Our understanding of MIPS instrument behavior and data processing is continuing to evolve, as are the software pipelines and the calibration of the instrument. This first release consists of "best-effort" data products available at this time, and will eventually be superseded by reprocessed versions in a future data release. The version number for these data products, based on GOODS internal nomenclature, is v0.30. (Note: There is no particular significance to the fact that the GOODS-S version number [0.30] is smaller than that for the GOODS-N MIPS data released in DR1+ [0.36].) The GOODS-S MIPS data were reduced later than those for GOODS-N, following virtually identical procedures.)
The MIPS data products for GOODS-S are essentially the same as those for GOODS-N. For convenience, we repeat the relevant information describing these data products here, updating details as needed to correspond to the GOODS-S data products.
File names for these GOODS data products include the following components, separated by underscores ("_"):
As an example, the GOODS-S MIPS channel 1 superdeep epoch 1 science image (version 0.30) is named "s_mips_1_s1_v0.30_sci.fits".
Note the GOODS IRAC program includes both "superdeep" and "ultradeep" images, and that each IRAC data set was obtained in two separate observing epochs. The MIPS observations correspond only to the "superdeep" IRAC data (there is no separate "ultradeep" MIPS program), and were taken in only a single epoch (after the first IRAC epoch). Therefore, the "s1" designation (component 4 of the filename as outlined above) is formally superfluous, but has been retained for symmetry with the naming convention used for the GOODS IRAC data products.
All GOODS imaging data products are generated using a common scheme for world coordinates and pixel projection, which we briefly describe here. The images are projected on a tangent plane, with a the tangent point (CRVAL1,2) selected to be near the center of each field (GOODS-N, GOODS-S). They are aligned with north up (+y) and east left (-x). The pixel scales for GOODS imaging data products from different telescopes and instruments are always chosen to be integer multiples of one another. For the MIPS GOODS images, this scale is 1".200/pixel, which is approximately (but not exactly) half the native MIPS pixel scale. (Other scales for GOODS public-release data sets include 0".60/pixel for the IRAC data, 0".15/pixel for the ESO/VLT ISAAC CDF-S data, and 0".03/pixel for the HST/ACS Treasury Program images.) The pixel position (CRPIX1,2) that corresponds to the tangent point (CRVAL1,2) is always set to be a half-integer value. In this way, GOODS imaging data products from different telescopes and instruments can always be mapped to one another by simple integer rebinning, if desired.
The pixel intensities for GOODS MIPS data products are given in units of DN per second, derived from the original SSC BCD products (which have units of MJy/sr) using the FLUXCONV BCD header keyword (see the documentation for GOODS DR1+, section 3.2). The MIPS Data Handbook, section 3.7, discusses the flux conversion factors for MIPS as derived by the SSC. GOODS images have been drizzled to remove geometric distortion and thus have a constant pixel solid angle over the field of view. From the SSC-derived FLUXCONV factors and the original pixel solid angle at image center (6.483 square arcsec), we derive the conversion from instrumental to flux density units. We summarize this information in Table 3, providing the flux densities in micro-Janskys and the AB magnitudes that correspond to a count rate of 1 DN/sec. This information is also recorded in the image headers in the keywords FLUXCONV and MAGZERO (see section 4.7). For reference, we also list the detector gain (electrons/DN) in each channel; note that the effective gain for the GOODS mosaics (which are normalized to DN/sec) varies with the exposure time as a function of position.
Note that no adjustment for source color has been applied to the SSC-derived flux calibration (see the MIPS Data Handbook, section 3.7.4, for a discussion).
| Channel | Wavelength | FLUXCONV uJy/(DN/s) |
MAGZERO AB for 1 DN/s |
Detector gain (e/DN) |
|---|---|---|---|---|
| 1 | 24 microns | 6.691 | 21.836 | 5 |
The exposure maps represent the MIPS integration time in seconds at each position on the sky in the co-added image mosaics, after rejection and masking of outlier pixels (e.g., cosmic rays, pixel defects, muxbleed, etc.). Fine-scale granularity from pixel to pixel in the exposure maps is a consequence of the process of drizzling the images onto a subsampled pixel grid using the point kernel, as described in the documentation for GOODS DR1+, section 3.4.
2.4.5 Weight (inverse variance) maps
The weight maps represent the inverse square of the RMS pixel-to-pixel noise (in DN/s) at the background level of the images. The construction of these maps is described in the documentation for GOODS DR1+, section 3.4. This represent the shot noise component due to the sky background and instrument noise only, and does not include Poisson noise from sources, nor any measure of photometric uncertainty due to source crowding or confusion.
We have constructed flag maps that may be useful when making object catalogs and analyzing the GOODS MIPS images. These maps identify regions where there are and are not data in a given channel, and where the exposure time is low (i.e., around the edges).
The flag images are bit maps, i.e., integers that represent the sum of bit values, each of which indicates a different flag conditions. Table 4 describes the flag values, where the "bit number" starts at 0, and the "flag value" is the equivalent integer value for that bit setting. Bits not described in the table below are currently unused for flag settings.
| Bit number | Flag value | Condition |
|---|---|---|
| 0 | 0 | >50% of the modal exposure time |
| 0 | 1 | <50% of the modal exposure time |
| 1 | 2 | <20% of the modal exposure time |
| 6 | 64 | No data (zero retained exposure time) |
These bit values will often appear in combination. For example, regions with < 20% of the modal exposure time (bit 1, flag value 2) also have < 50% of the modal exposure time (but 0, flag value 1). Therefore, those pixels will have flag values of 2 + 1 = 3. Regions with no data will have flag values 64 + 2 + 1 = 67. For reference, the "modal exposure time" used for setting the thresholds used in the flag maps is 37453 sec, or 10.404 hours.
Note that regions with Flag = 2 (i.e. < 20% of the typical exposure time) still have integration times up to 2 hours - hardly shallow by Spitzer/MIPS standards! However, those regions will have a fairly steep gradient in their exposure time and local noise amplitude.
The FITS headers of the GOODS data product images incorporate various useful and relevant information about the images.
As part of data release DR3, we are providing a list of sources for the MIPS 24 micron imaging of the GOODS-S field. This is exactly analogous to the GOODS-N 24 micron source list from DR1+. The source lists are subsets from more extensive catalogs which will be presented by Chary et al. (in preparation). They are limited to flux densities greater than 80 microJy, where the source extraction is highly complete and reliable, and which is well below the flux limit currently explored for IRS spectroscopy. The early source lists are intended to facilitate early investigations by members of the scientific community, and to support planning for follow-up observations. Although we believe the source lists are reliable and scientifically useful, they are nevertheless preliminary, and will be superseded by more extensively validated and documented catalogs in Chary et al.
The user is refered to the DR1+ documentation for the GOODS-N 24 micron source list for details about the source detection and extraction procedure. Only a brief summary is given here.
Source extraction was performed on version 0.30 of the GOODS-S MIPS 24 micron images, described in section 2.0 above. A SExtractor (Bertin & Arnouts 1996) catalog of IRAC sources in the field was used to define a set of prior positions for fitting MIPS 24 micron point sources, using an empirically defined point spread function. The use of IRAC priors permits improved deblending and extraction in crowded environments. Small offsets (< 0.6 arcsec) in the MIPS source position relative to the IRAC source centroid were allowed.
After subtracting the best non-negative point source fits for the IRAC sources, a second pass of source detection was performed to identify 24 micron sources without IRAC counterparts, particularly outside the region covered by the GOODS IRAC imaging. MIPS sources without IRAC counterparts are indicated in the source list by a value of 0 for the parameter IRAC_Tag.
Although most faint 24 micron sources in the GOODS images are unresolved, a few brighter, larger galaxies (mostly at low redshift) are evidently extended in the MIPS data. We identified 10 such objects by visual inspection, used SExtractor to measure isophotal flux densities, and have substituted those for the point source fit values. These objects are indicated in the source list by values of 1 in the Extended_Tag column.
The GOODS-S DR3 MIPS 24 micron source list is released in IPAC Table format, and is available from the Spitzer Archive and the "Popular Products" repository of Legacy Data Products. Table 5 describes the columns provided in this source list.
| Column Name | Data Type | Units | Description |
|---|---|---|---|
| SrcID | integer |
|
Source number |
| ra | real | degrees | J2000.0 right ascension of MIPS 24 micron source |
| dec | real | degrees | J2000.0 declination of MIPS 24 micron source |
| RA_IRAC | real | degrees | J2000.0 right ascension of prior IRAC source (-1 if no IRAC prior) |
| DEC_IRAC | real | degrees | J2000.0 declination of prior IRAC source (-1 if no IRAC prior) |
| S_24 | real | microJy | Flux density of MIPS 24 micron source |
| S_24_err | real | microJy | Uncertainty in derived 24 micron flux density |
| IRAC_Tag | integer |
|
Set to 1 if there is IRAC coverage |
| Extended_Tag | integer |
|
Set to 1 if source is extended at 24 microns |
4.0 IRAC GOODS-S epoch 1 v0.3 re-reductions and image data products
In DR3, we provide new and slightly improved reductions (v0.3) of the GOODS-S IRAC epoch 1 data. These were the first GOODS Spitzer observations to be executed, and the data products (v0.21) originally released in GOODS DR1 were the first "science grade" reductions that the GOODS team carried out. As we gained experience, we improved some of the details of our reduction procedures. Moreover, the older, v0.21 data products were based on BCD images processed by a fairly early version of the SSC pipeline, S9.1.0, compared to the S10 and S11 BCDs used for our other GOODS data products. The differences between the v0.21 and v0.30 data products were discussed in the documentation for GOODS DR1, section 3.7.
The new v0.30 reductions of the GOODS-S IRAC epoch 1 images are based on data processed by the SSC BCD pipeline S10.5.0, which is the same version used for the GOODS-S epoch 2 data released in DR2. They were reduced using procedures identical to those for GOODS-N IRAC epoch 1 v0.3, and for GOODS-S and GOODS-N IRAC epoch 2. In addition to the change in the BCD pipeline version, the main changes are (1) the internal astrometric registration and alignment to the external reference frame used a refined procedure and should be more accurate, and (2) the method for scaling the weight map to inverse variance is now consistent for all four sets of IRAC data products.
All header parameters, flux scaling factors, etc. are the same as for the previous IRAC data releases, and are described in the documentation for GOODS DR1, section 4.
For most purposes, the new v0.30 GOODS-S IRAC data products will be very similar to the previous v0.21 reductions. In a future data release (DR4 or DR5), we expect to replace all of these with uniform re-reductions based on S13 BCD data, and with improved corrections for bright source artifacts (e.g., muxbleed, mux-striping).