############################## ### CEERS Data Release 0.7 ### ############################## Date Released: 3 October 2023 Here we provide the CEERS team's reductions of CEERS NIRSpec MSA observations. In December 2022 (CEERS observing epoch 2), NIRSpec was the prime instrument for six pointings (4, 5, 7, 8, 9 and 10) with NIRCam imaging in coordinated parallel mode. The December PRISM observations in pointings 9 and 10 were impacted by bright stray light from an MSA short, and the affected data are not included in this release. Rescheduled PRISM observations were executed in February, using different coordinate centers and position angles than the original pointings 9 and 10. The February PRISM pointings are numbered 11 and 12. These data products will be available soon at MAST as High Level Science Products via DOI 10.17909/z7p0-8481. ############ ### Summary Instrument: NIRSpec Mode: Multi-Object Spectroscopy Calibration Pipeline Used: jwst v1.8.5 CRDS Context pmap: 1029 Target: NIRSpec pointings 4, 5, 7, 8, 9, 10, 11, 12 Dispersers/Filters: G140M/F100LP, G235M/F170LP and G395M/F290LP (R~1000); PRISM/CLEAR (R~100) Readout: NRSIRS2 Observation specification: 14 groups, 1 integration Dithers: 3 shutter slitlet nod pattern Contact: Pablo Arrabal Haro (parrabalh@gmail.com) Mark Dickinson (mark.dickinson@noirlab.edu) ################################# ### CEERS NIRSpec observations CEERS NIRSpec Multi-Shutter Assembly (MSA) observations were mostly executed in December 2022 ("CEERS epoch 2"). Prism observations of two fields (NIRSpec9 and NIRSpec10) were severely impacted by an MSA short, which caused bright stray light illumination over a significant portion of the field of view. Observations of fields 9 and 10 with the medium resolution gratings were not affected by the short. We have not included the affected prism data in DR0.7, although some useful information can be recovered for some objects. The failed observations were rescheduled in February 2023 ("CEERS epoch 3") with a different telescope roll angle and at different pointing centers, which we designate as NIRSpec11 and NIRSpec12. The locations of the NIRSpec fields are shown in the observation layout figures available at the CEERS website (https://ceers.github.io/obs.html#layout_figures). CEERS NIRSpec observations use the NIRSpec medium resolution gratings (MR or Mgrat) and the low-resolution prism. The NIRSpec pointings observed with each disperser are: G140M+F100LP Pointings 4, 5, 7, 8, 9, 10 G235M+F170LP Pointings 4, 5, 7, 8, 9, 10 G395M+F290LP Pointings 4, 5, 7, 8, 9, 10 PRISM+CLEAR Pointings 4, 5, 7, 8, 11, 12 For a given NIRSpec pointing, the observations with the MR gratings and the prism use different MSA configurations, observing different targets, although some objects were observed with both modes. The MR targets were observed with all three grating+filter combinations. The MSA plans used 3-shutter slitlets, and the observations were executed using the standard 3-shutter nodding pattern. In some cases, additional objects from the MSA catalog fell into open shutters during one or more nods; these objects may not have received the full exposure time that the planned targets received. (We provide a file slits_double_objects.dat with a list of these cases; see discussion under Auxiliary Files.) For objects observed in all three nod positions, the effective on-target integration time is 3064.667s. The effective exposure time is recorded in the header keyword EFFEXPTM. The pipeline generates another keyword, XPOSURE, but we have noted that this value is incorrect for some objects, particularly for spectra that are split across both NIRSpec detectors (identifiable by DETECTOR = MULTIPLE in the headers). We recommend considering only the EFFEXPTM values. STScI reports that this issue has been fixed for newer versions of the Calibration Pipeline. ##################################### ### Data reduction and calibration The NIRSpec data were reduced using the JWST Calibration Pipeline version 1.8.5 (Bushouse et al. 2022, DOI:10.5281/zenodo.7429939). The spectra were reduced using CRDS context jwst_1029.pmap. Most processing used standard pipeline parameters. Here we give some relevant specifics, including deviations from standard processing: Nodded background subtraction was employed (pipeline step = 'background'). Modified 'jump' parameters for an improved "snowball" correction. - det1.jump.expand_large_events = True - det1.jump.after_jump_flag_dn1 = 0 - det1.jump.after_jump_flag_time1 = 0 - det1.jump.after_jump_flag_dn2 = 0 - det1.jump.after_jump_flag_time2 = 0 - det1.jump.min_sat_area = 15.0 - det1.jump.min_jump_area = 15.0 - det1.jump.expand_factor = 2.0 The pipeline was instructed that all targets should be treated as point sources (SRCTYPE = 'POINT'). This has two main consequences: - Each 1D spectrum (x1d) is extracted from the 2D spectral data file (s2d) over a specified range of pixels in the cross-dispersion (y-axis) direction (boxcar extraction) - The extracted 1D (x1d) spectra have units of flux density (Jy), rather than surface brightness. Extraction apertures (from s2d to x1d) for each object were determined by interactive visual inspection and were specified explicitly for pipeline step 'extract_1d'. Most faint galaxies are compact, and the extraction apertures adopted for nearly all objects have heights ranging from 3 to 6 pixels (scale = 0.10 arcsec/pixel), with a median value of 4 pixels. The default pipeline pathloss (aka slit loss) correction was employed. That calibration is based primarily on pre-flight modeling, and assumes that the targets are point sources. For sources that are significantly extended, this correction will likely be incomplete. Flux calibration uses the default reference files for the adopted CRDS context. These are a mix of ground and in-flight calibrations. Comparison between spectroscopic and photometric fluxes (when available) is recommended if accurate flux calibration and slit loss correction is required. ######################### ### Directory Contents Spectra for each pointing are stored in separate directories, with subdirectories for each disperser. The directory names and filenames follow the format: [pointing]/[disperser]/hlsp_ceers_jwst_nirspec_[pointing]-[msa-id]_[disperser]_v0.7_[ext].fits where: - [pointing] is one of 'nirspec4', 'nirspec5', 'nirspec7', 'nirspec8', 'nirspec9', 'nirspec10', 'nirspec11' or 'nirspec12' - [disperser] is one of 'g140m', 'g235m', 'g395m', or 'prism' - [msa-id] is a 6 digit identification number for the source - [ext] is one of: - 's2d': rectified 2D spectrum in units of MJy/px - 'x1d': extracted 1D spectrum using a boxcar extraction in a custom aperture. Wavelength in microns, flux density in Jy - 'x1d-masked': same as x1d but with artifacts masked. Masked pixels are set to NaN. Wavelength in microns, flux density in Jy Combined MR 3-grating spectra: Additionally, data products combining data from all three MR gratings into a single spectrum are available in the [pointing]/comb-mgrat directories: [pointing]/comb-mgrat/hlsp_ceers_jwst_nirspec_[pointing]-[msa-id]_comb-mgrat_v0.7_[ext].fits where [ext] is one of 's2d' and 'x1d-masked' For the combined 3-grating spectra, data from the individual MR gratings are resampled to a common wavelength vector in the overlapping regions, adopting the wavelength sampling of the longer wavelength grating. Flux values in the overlap regions are the average of the individual grating values weighted by the flux errors, excluding pixels affected by masked artifacts. For the combined 3-grating x1d spectra, only the WAVELENGTH, FLUX, and FLUX_ERROR values are combined in the overlap regions. Other columns from the x1d FITS data tables (e.g., FLUX_VAR_POISSON) have values taken from the original spectra in the non-overlap regions, and generally NaN values in the overlap regions. Please note that most of the header information in the combined 3-grating products is copied from the headers of the G395M data, and does not attempt to present characteristics of the combined 3-grating data. For the combined 3-grating s2d files, most of the FITS extensions have valid information for the combined data, except for HDRTAB and ASDF. Under rare circumstances, the 2D spectra (s2d files) for a given object in the three MR gratings can have different sizes in the cross-dispersion (spatial) direction. In some cases, the difference in sizes is of order one pixel, but in some cases one grating may have a y-dimension that is roughly 10 pixels shorter than the other two gratings. We do not yet fully understand the reason for this, but in any case the source spectrum is generally well centered in the s2d files for all three gratings, so the spectra can be successfully extracted and combined. We have taken into account the different sizes of the s2d files when we combine the three gratings to produce the comb-mgrat 2D products. However, in a few cases, there can be a shift of one pixel in the spatial direction for a portion of the comb-mgrat 2D spectrum. In such cases, this shift does not affect the combined 3-grating (comb-mgrat) 1D spectrum, as that is not extracted from the combined 2D spectrum. Instead, the 3-grating 1D spectrum is produced from a combination of the separate 1D spectra for the three MR gratings, each extracted using its individual custom extraction aperture, each of the same size and correctly positioned. Auxiliary files: - CEERS_NIRSpec_MSA_master_yield_v0.7.csv - The MSA yield file provides the MSA ID (6 digit unique number across all pointings) and RA,Dec for each source. It also specfies the pointing and disperser coverage for each source - slits_double_object.dat - A list of cases where a multi-shutter MSA "slit" includes two objects from the MSA planning catalog. Central_target gives the MSA ID for the target for which the slit was intended, and Companion gives the MSA ID for the additional target that also falls on open MSA shutters. Note that it is sometimes the case that the Companion was not observed in all three nod positions, and therefore has reduced effective exposure time. - [pointing]/*_Mgrat_custaper.csv - table of custom aperture extraction windows used for MR grating extractions for each source - [pointing]/*_prism_custaper.csv - table of custom aperture extraction windows used for PRISM extractions for each source - [pointing]/[disperser]/plots_2D+1D - plots showing the 1D and 2D spectra for all sources and dispersers #################### ### File structure Rectified 2D spectra (*_s2d.fits) are multi-extension fits files with 10 extensions: 0. PRIMARY header 1. SCI - 2D data array (MJy/pixel) 2. ERR - 2D array of uncertainties, given as standard deviation constructed as the sum in quadrature of the resampled variance maps 3. WHT - 2D weight image giving the relative weight of the output pixels 4. CON - 2D context image, encoding info about which input exposures contribute to each output pixel 5. VAR_POISSON - 2D variance array based on Poisson noise only 6. VAR_RNOISE - 2D variance array based on read noise only 7. VAR_FLAT - 2D variance array based on uncertainty in the flat-field 8. HDRTAB - table containing metadata (FITS keyword values) for all the input exposures 9. ASDF - metadata for the JWST data model Extracted 1D spectra (*_x1d.fits, *_x1d-masked.fits) are multi-extension fits files with 3 extensions: 0. PRIMARY header 1. EXTRACT1D - table (one row per extracted spectral element in the dispersion direction) with the following columns: - WAVELENGTH (um) - FLUX (Jy) - FLUX_ERROR (Jy) - FLUX_VAR_POISSON (Jy^2) - FLUX_VAR_RNOISE (Jy^2) - FLUX_VAR_FLAT (Jy^2) # Note that all surface brightness columns (SURF_BRIGHT, SB_*) are set # to zero because sources are treated as point sources - SURF_BRIGHT (MJy/sr) - SB_ERROR (MJy/sr) - SB_VAR_POISSON ((MJy/sr)^2) - SB_VAR_RNOISE ((MJy/sr)^2) - SB_VAR_FLAT ((MJy/sr)^2) - DQ (DQ flags) - BACKGROUND (MJy/sr) - BKGD_ERROR (MJy/sr) - BKGD_VAR_POISSON ((MJy/sr)^2) - BKGD_VAR_RNOISE ((MJy/sr)^2) - BKGD_VAR_FLAT ((MJy/sr)^2) - NPIXELS - number of pixels included in source extraction region 2. ASDF - metadata for the JWST data model