A 3D Drizzle Algorithm for JWST and Practical Application to the MIRI Medium Resolution Spectrometer

Abstract

We describe an algorithm for application of the classic “drizzle” technique to produce 3D spectral cubes using data obtained from the slicer-type integral field unit (IFU) spectrometers on board the James Webb Space Telescope. This algorithm relies upon the computation of overlapping volume elements (composed of two spatial dimensions and one spectral dimension) between the 2D detector pixels and the 3D data cube voxels, and is greatly simplified by treating the spatial and spectral overlaps separately at the cost of just 0.03% in spectrophotometric fidelity. We provide a matrix-based formalism for the computation of spectral radiance, variance, and covariance from arbitrarily dithered data and comment on the performance of this algorithm for the Mid-Infrared Instrument’s Medium Resolution IFU Spectrometer. We derive a series of simplified scaling relations to account for covariance between cube spaxels in spectra extracted from such cubes, finding multiplicative factors ranging from 1.5–3 depending on the wavelength range and kind of data cubes produced. Finally, we discuss how undersampling produces periodic amplitude modulations in the extracted spectra in addition to those naturally produced by fringing within the instrument; reducing such undersampling artifacts below 1% requires a four-point dithering strategy and spectral extraction radii of 1.5 times the point-spread function FWHM or greater.

Discovery of a Collimated Jet from the Low-luminosity Protostar IRAS 16253−2429 in a Quiescent Accretion Phase with the JWST

“…We measure the FWHM of the slices adjacent to the protostar on either side and find the width of the jet close to the central protostar to be 0″.33 ± 0″03. This is comparable to the average point-spread function (PSF) FWHM of the MIRI MRS at 5.34 μm of 0″28 (Law et al 2023).…”

“…IRAS 16253−2429 was observed using all four channels of the MIRI MRS with wavelength coverage from 4.9 to 27.9 μm, with a spectral resolving power ranging from ∼1500 to 4000 (Jones et al 2023). 31 The spatial resolution of MIRI MRS ranges from ∼0″27 to 1″ (Law et al 2023). Similar to the NIRSpec observations, we employed a 2 × 2 mosaic pattern with a 10% overlap and a four-point dither for the MIRI observations.…”

“…Given the high signal-to-noise and high angular resolution at 5.34 μm (0″28; Law et al 2023), it is possible to measure the width of the jet as a function of distance from the protostar. The jet has a position angle of 23°(measured from north toward east).…”

JWST MIRI MRS Observations of T Cha: Discovery of a Spatially Resolved Disk Wind

“…However, we safely retain all the source emission in all four channels owing to which we obtain a spectrum for T Cha for the complete wavelength range of MIRI MRS (∼5-28 μm). After creating a pixel-by-pixel backgroundsubtracted IFU cube, we retrieve a 1D spectrum by summing all the pixels in an aperture of radius 2.5 ×FWHM, where FWHM = 0.033 × (λ/μm) + 0 106 (Law et al 2023). We perform the aperture correction after extracting the spectrum, with values retrieved from the aperture correction reference file in CRDS.…”

MIRI MRS Observations of β Pictoris. I. The Inner Dust, the Planet, and the Gas

“…The background subtraction from the dedicated background observations was done in the Spec3 pipeline along with the outlier rejection step. We built the spectral cubes using the drizzle algorithm in the pipeline (Law et al 2023). We left the pixel size to be the pipeline default, which is 0 13 for channel 1, 0 17 for channel 2, 0 20 for channel 3, and 0 35 for channel 4.…”