Hector Galaxy Survey: Data Processing, Quality Control and Early Science

TL;DR

Hector addresses the challenge of delivering large, high-quality optical IFS data from a dual-instrument, multi-arm system to study galaxy evolution at $z<0.1$. The paper presents a robust end-to-end data-reduction framework built on the SAMI pipeline with major enhancements: a 2D arc-wavelength calibration, a chromatic distortion model, and a drizzle-like cubing approach; it also integrates Spector data and implements comprehensive flux calibration (primary and secondary) and telluric corrections. Key results include RMS improvements in wavelength solutions by factors of $1.2$–$3.4$, a distortion model that constrains residuals to $\

Abstract

The Hector Galaxy Survey is a new optical integral field spectroscopy (IFS) survey currently using the AAT to observe up to 15,000 galaxies at low redshift ($z < 0.1$). The Hector instrument employs 21 optical fibre bundles feeding into two double-beam spectrographs to enable wide-field multi-object IFS observations of galaxies. To efficiently process the survey data, we adopt the data reduction pipeline developed for the SAMI Galaxy Survey, with significant updates to accommodate Hector's dual-spectrograph system. These enhancements address key differences in spectral resolution and other instrumental characteristics relative to SAMI, and are specifically optimised for Hector's unique configuration. We introduce a two-dimensional arc fitting approach that reduces the RMS velocity scatter by a factor of 1.2--3.4 compared to fitting arc lines independently for each fibre. The pipeline also incorporates detailed modelling of chromatic optical distortion in the wide-field corrector, to account for wavelength-dependent spatial shifts across the focal plane. We assess data quality through a series of validation tests, including wavelength solution accuracy, spectral resolution, throughput characterisation, astrometric precision, sky subtraction residuals, and flux calibration stability (4\% systematic offset when compared to Legacy Survey fluxes). We demonstrate that Hector delivers high-fidelity, science-ready datasets, supporting robust measurements of galaxy kinematics, stellar populations, and emission-line properties, and provide examples. Additionally, we address systematic uncertainties identified during the data processing and propose future improvements to enhance the precision and reliability of upcoming data releases. This work establishes a robust data reduction framework for Hector, delivering high-quality data products that support a broad range of extragalactic studies.

Figures (24)

• Figure 1: The result of 2D wavelength calibration for an example arc frame from CCD3 (frame 19, 28 Oct 2024). (a) Histogram of residuals from the model fit, defined as (measured arc line wavelength) -- (model wavelength). The dotted lines mark $\pm0.1$ pixels on the detector. (b) Residuals across the detector. (c) Residuals as a function of detector $x$ pixel (i.e. the vertical collapse of panel (b)). Small red points are individual line measurements, various coloured connected points are locally averaged residuals in a 10$\times$10 grid across the detector. (d) Small red points are residuals as a function of $y$ detector pixel (i.e. the horizontal collapse of panel (b)). Coloured points are average residuals, as for (c).
• Figure 2: Comparison of twilight sky velocity residuals from 1D (blue) and 2D (red) arc fitting. We show CCDs 1 through 4 in panels (a), (b), (c) and (d) respectively. The broader distribution for 1D fitting in CCD4 is in part due to the difficulty of fitting the high order distortion on single fibres (see text for details). The standard deviations shown in the legends are before correcting for statistical velocity measurement uncertainty. Vertical dotted lines indicate the velocity corresponding to 0.1 pixels at 4800 Å (for CCD1 and CCD3) and 6800 Å (for CCD2 and CCD4).
• Figure 3: Flux density derived from dome flat (top) and twilight flat (bottom) frames, extracted from the fibre in the middle of each CCD. The flux density is normalised for gain, photon energy, collecting area, and spectral resolution. The dark and light blue spectra originate from the blue arms of AAOmega and Spector, respectively, while the red and orange spectra are from their red arms. The flat spectra, converted to a flux density scale, illustrate the shape of the flat-field frames and only indirectly reflect relative throughput. For absolute throughput measurements for Hector, refer to Section \ref{['sec:thput']}.
• Figure 4: Stellar observations illustrating the effects of chromatic variations in distortion (CVD) are shown as a function of wavelength and position across the Hector plate, presented in the coordinate system used by the Hector robot. Black-filled circles mark the stellar centroid at a reference wavelength of 6000 Å, while coloured points trace the shift in the centroids of stellar observations across wavelength, shifting from redder to bluer wavelengths (red-to-blue filled-in circles) relative to the centroid at the reference wavelength. For clarity, the centroid shifts due to CVD effects are exaggerated by a factor of 20; the maximum shift is $\sim$120 $\mu m$ (1.17 times the fibre core diameter). For several hexabundles, we also illustrate the hexabundle orientation and cable direction (see §\ref{['subsec:wcs']} for discussion on the orientation of hexabundles and associated corrections). Grey lines connect the physical centres of each hexabundle to the centre of the Hector plate.
• Figure 5: Modelling the Chromatic Variation in Distortion across the Hector plate. (a) Distortion as a function of position along the Plate y-coordinate across the Hector plate, from left-to-right, as shown in Figure \ref{['fig:cvd_hector_plate']}. Also, as in Figure \ref{['fig:cvd_hector_plate']}, the colour gradient from blue to red represents measured centroid offsets as a function of wavelength. The modelled distortion at wavelengths of 3730 Å and 7330 Å is shown as solid blue and red lines, respectively. (b) Residuals between the model and observed distortions at 3800, 5000, and 7200 Å, demonstrating that the model effectively reproduces the measured distortions across the Hector plate to approximately within $\pm 10 \mu m$. (c) RMS of the residuals as a function of radius on the Hector plate, with colours indicating increasing wavelength from blue to red. (d) RMS of the residuals as a function of wavelength, illustrating that RMS progressively becomes larger towards bluer wavelengths