¶ Observations
HI-KIDS aims to estimate global and local properties of both the ionized gas (kinematics, chemical abundances of O/H and N/O, but also He/H, Fe/O, S/O, Ne/O, Ar/O when possible, ionization structure, Hα-based star-formation rate) and the stellar component (massive stars, stellar populations, star-formation history). HI-KIDS continues KOALA+AAOmega Integral Field Spectroscopy (IFS) observations in the λλ3650 – 9600 range.
The observations are unique due to the following factors:
- KOALA (Kilofibre Optical AAT Lenslet Array) has a large field of view (27.4” × 50.6” with 1.25” spatial sampling).
- Obtaining good signal to noise data of the diffuse stellar component of galaxies will require less hours using AAO’s 3.9m telescope in comparison to smaller telescopes (e.g. using WiFeS at the 2.3m ANU Telescope).
- KOALA has a large frequency range and can observe key emission lines especially in the blue spectrum ([O II] λ3727, Hγ, [O III] λ4363, [S III] λ9560) which are essential to constrain stellar population models. Much larger telescopes such as MUSE at the 8.2m VLT covers the λλ4600 – 9300 range and misses this data.
KOALA’s field of view is approximately the same as the 21cm H I line’s interferometric data which is already available. This data provides:
- Amount of gas available
- Average surface density within the galaxy (to address the Schmidt-Kennicutt law in dwarf systems)
- Large-scale distribution
- Kinematics, tracing the overall gravitational potential well, accretion and outflow of cold gas, and interactions with nearby companions
The Australia Telescope Compact Array (ATCA) H I velocity fields also allow us to determine the rotation curve and estimate the dark matter content as a function of radius.
Observations are carried out using KOALA in wide field mode (27.4” × 50.6” with 1.25” spatial sampling). The 580V grating in the AAOmega blue arm, centered on λ4550 (λλ3500 – 5600) and the 1000R grating in the red arm, centered on λ6880 (λλ6280 – 7450) are chosen to obtain the most optimal balance between sensitivity, spectral resolution and wavelength. This setup allows us to measure all the critical diagnostic lines: [O II] λ3727, Hγ, [O III] λ4363, He II λ4686, Hβ, [O III] λλ4959,5007, [O I] λ6300, [S III] λ6312, Hα, [N II] λ6583, He I λ6678, [S II] λλ6717,31, [O II] λλ7318,30, and observe the absorption features (including the D4000 absorption) that are crucial to get reliable results from stellar population fitting.
¶ AAT - KOALA + AAOmega
Hi-KIDS uses the KOALA + AAOmega instruments at the Anglo-Australian Telescope.
- AAOmega is a double beam spectrograph with selectable wavelength coverage and resolution
- KOALA (Kilofibre Optical AAT Lenslet Array) is a wide field, highly efficient integral field unit (IFU)
- KOALA is mounted at the f/8 Cassegrain focus and feeds the AAOmega spectrograph via a 31m fibre run
- It has 1000 hexagonal lenslets where each lenslet acts as spatial pixels (spaxels)
- Selectable field of view:
- 15.3” x 28.3”, with 0.7” spatial sampling
- 27.4” x 50.6”, with 1.25” spatial sampling (Hi-KIDS uses this one).
¶ Current status
- 72 galaxies observed (220 pointings)
- 50 nights so far
Full target list available here
¶ Processing data
Observations are reduced with two software packages outlined below.
¶ Using 2dFdr
The first step is extracting the Raw Stacked Spectra (RSS). This is achieved using the standard AAO software 2dFdr [ref] that reduces the RAW data obtained with AAOmega. 2dFdr considers these processes:
- Dark subtraction
- Long-slit flat correction
- Cosmic ray removal
- Tramline extraction (GAUSS, the OPTEX does not work well for KOALA data)
- Wavelength calibration
¶ Running PyKOALA
This software processes RSS data to create data cubes for science analysis:
- Throughput correction
- Correction for extinction
- Telluric absorption correction (red CCD)
- Sky subtraction
- Cubing (from hexagonal lenslets) and mosaicking
- Correcting for atmospheric differential refraction
- Absolute flux calibration (3-5 stars per night)
- Combine blue + red spectra
- Tools for plotting, mapping & analyse the data
