The SAMI Galaxy Survey (science overview) is making an early data release (EDR) to the astronomical community. This release contains 107 galaxies observed in March and April, 2013. This early sample spans the full range in mass and redshift of the main sample and is a sub-sample of the GAMA Survey. Included in the release are datacubes for all 107 galaxies and a sample catalogue with basic parameters of each galaxy. The SAMI Galaxy Survey Team are committed to making further data releases as the project progresses. The early data release is described in detail by Allen et al. (2015). We provide links to the data and a description of the data formats below.
The early data release is described in these three papers:
- Allen et al. (2015)
- An overview of the EDR data.
- Bryant et al. (2015)
- A discussion of the parent sample and sample selection.
- Sharp et al. (2015)
- A description of the unusual data format and its processing requirements.
There are various ways to access the SAMI data cubes (also linked on the right of these pages):
Browse images of the galaxies making up the release and download individual cubes with the EDR galaxies browser.
Download the machine-readable catalogue, which includes links to the data, for use with tools such as Topcat.
Download any or all data, including calibration stars, using a wget script.
EDR Sample Catalogue
The full input catalogue of all SAMI targets within the GAMA regions is presented and described by Bryant et al. (2015). A subset of that catalogue listing only the galaxies contained in the EDR is described by Allen et al. (2015). The EDR catalogue page includes links to the ASCII format table and a detailed description of the columns.
EDR Data Cubes
The primary products in the EDR are the data cubes for each galaxy. These are taken from version 0.8 of the SAMI reduction pipeline (see Sharp et al. (2015) and Allen et al. (2015) for details). There are two cubes for each galaxy, one in the blue, and a second in the red. Note that the red and blue cubes do not overlap in wavelength coverage. Some basic characteristics of the data are given below:
- Blue cube covers ~3700-5700A with a resolution of $R\simeq 1750$ and a dispersion of 1.0 Angstrom per pixel.
- Red cube covers ~6300-7400A with a resolution of $R\simeq 4500$ and a dispersion of 0.57 Angstroms per pixel.
- The cubes are each $50\times 50$ spatial pixels by $2048$ spectral pixels with the spatial sampling at 0.5 arcsec.
- Data are flux calibrated and corrected for telluric absorption to within 6% in relative flux.
- The included world coordinate system is centred on catalogue coordinates.
- Primary HDU contains the data cube: $50\times50$ spatial elements, $2048$ spectral elements ($50\times50\times2048$).
- Variance cube ($50\times 50\times 2048$), weight-map cube ($50\times 50\times 2048$) and covariance hyper-cube ($50\times 50\times 5\times 5\times n$, where $n$ can vary) are included as extensions. See below for discussion of covariance.
In order to obtain secondary calibrations, one 'hexabundle' (IFU probe) is allocated to an F-type star in every field ever observed. This is also used to map the seeing disks at Siding Spring Observatory at the time of each survey field observation. These stars are not tabulated in the browser but they can be obtained through the wget script, where they have been listed in the bottom of the file. A short listing of the correspondence between target and calibrator identifiers is also provided in this stars catalogue.
Variance and Covariance
During the reduction, SAMI hexabundles are dithered and resampled onto a regular grid, which causes nearby output spaxels to have correlated noise in the spatial dimension. This spatial covariance is included in the 'COVAR' extension of the cube. (N.B. spectral covariance is small enough to be safely ignored, and is not included). Full details of how covariance is handled in SAMI data is included in Sharp et al. (2015).
The full covariance matrix for a SAMI cube is both very large and highly redundant. Therefore, we calculate and store the covariance only (i) at a subset of wavelength slices, and (ii) over small spatial scales only. Our testing shows that covariance is only significant over a ~2 spaxel radius, therefore, a $5 \times 5$ covariance sub-array is sufficient to record all of the covariance for a given pixel. The covariance varies significantly along the spectral direction only when the resampling is updated to account for the effects of chromatic differential atmospheric refraction. Therefore we densely sample only these wavelength slices, and sparsely sample slices in between.
The full $50\times 50 \times 5 \times 5 \times 2048$ covariance hyper-cube can be reconstruced from the included covariance data as described by Sharp et al. (2015). Briefly, missing values in the full hyper-cube should be replaced with the nearest measured value along the wavelength axis. Then, each of the 25 $50\times 50 \times 2048$ cubes (one per element of the $5 \times 5$ covariance sub-arrays) must be multiplied by the variance cube to recover the full covariance. Our testing has shown that this approach recovers $> 91\%$ of the covariance in the blue cubes, and $> 97\%$ in the red cubes.
We leave the choice of how to apply (or otherwise) this covariance information to subsequent analysis up to the user, but strongly encourage users to read the description in Sharp et al. (2015) in detail.
If using SAMI data products, please include this acknowledgement:
"The SAMI Galaxy Survey is based on observations made at the Anglo-Australian Telescope. The Sydney-AAO Multi-object Integral field spectrograph (SAMI) was developed jointly by the University of Sydney and the Australian Astronomical Observatory. The SAMI input catalogue is based on data taken from the Sloan Digital Sky Survey, the GAMA Survey and the VST ATLAS Survey. The SAMI Galaxy Survey is funded by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020, and other participating institutions. The SAMI Galaxy Survey website is http://sami-survey.org/ ."
Please cite the following papers:
- "The Sydney-AAO Multi-object Integral field spectrograph", Croom et al. (2012), MNRAS, 421, 872. Instrument description paper.
- "The SAMI Galaxy Survey: Target selection and instrument specification", Bryant et al. (2015). Details of survey target selection and revised instrument description for upgraded SAMI instrument.
- "The SAMI Galaxy Survey: Cubism and covariance, putting round pegs into square holes", Sharp et al. (2015). Data processing pipeline, including flux calibration and drizzle cubing.
- "The SAMI Galaxy Survey: Early Data Release", Allen et al. (2015). Description of early data release, including quality metrics and details of data products.
The SAMI Galaxy Survey team welcome feedback on this data release and the products contained within it. If you are actively using the data and/or find issues, please contact us on feedbacksami-survey [dot] org.