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Integral Field Spectroscopy
Sarah Brough (AAO)
Integral Field Spectroscopy
 What is an Integral Field Spectrograph?
 What can I study with an IFS?
 What kinds of IFS are there?
 How do I prepare IFS observations
 How do I reduce/view/analyse my data?
Integral Field Spectroscopy
 What is an Integral Field Spectrograph?
 What can I study with an IFS?
 What kinds of IFS are there?
 How do I prepare IFS observations
 How do I reduce/view/analyse my data?
What is an Integral Field Spectrograph?
 An IFS is an instrument that allows you to gather optical/NIR
spectra over a two-dimensional field-of-view. The final
product is a data-cube, with axes of RA, Dec and wavelength.
DEC
RA
Wavelength
Credit: Stephen Todd (ROE) and Douglas Pierce-Price (JAC)
Integral Field Spectroscopy
 What is an Integral Field Spectrograph?
 What can I study with an IFS?
 What kinds of IFS are there?
 How do I prepare IFS observations
 How do I reduce/view/analyse my data?
What can I study with an IFS?
 Any astronomical object where you require spatially
resolved spectroscopic information is ideally suited for study
with IFS.
Where shocks occur
Where star formation occurs
Where dust is
Rotation measures
Angular momentum
Where the oldest stars are
Where the most metal rich stars
and gas are
• Dynamical mass measurements
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Galaxy Winds:
Emission-line integrated flux maps
generated by single Gaussian fitting to
individual spectra
Line ratio maps
Classic ionization diagnostic diagrams,
starburst vs AGN excited vs shockexcitation – Sharp & Hawthorn (2010)
*Also Fogarty+(2012), Ho+(2014,
2016)*
Where Star Formation occurs:
Map of flux under Hα Emission
Line, i.e. star formation.
Brough et al. (2013)
*Also Schaefer et al. (in prep)*
Whether galaxies are disc- or merger-dominated?
WiggleZ Survey, z~1
Wisnioski et al. 2011
SINS survey, z~2
Förster Schreiber, et al. 2009
*Also Green et al. (2014) – DYNAMO, z~0.1*
Stellar Rotation
Stellar velocity across galaxy from cross-correlation to stellar
templates (Brough et al., 2011)
Rotating
Projected stellar angular momentum
lR = < R|V| > / < R sqrt(V2 + s2) >, is a proxy for the observed
projected stellar angular momentum per unit mass.
Early-type galaxies are separated into slow and fast rotators,
depending on their lRe and ellipticity
ATLAS3D Survey; Emsellem et al. (2011)
*Also Jimmy+2013, Fogarty et al. (2014, 2015), van de Sande et al. (in prep)
 Galaxy star formation histories:
McDermid et al. (2015).
* Also Scott et al. (in prep)
Where the oldest and most metal-rich stars are:
SAURON sample. Stellar ages, metallicities and alpha-enhancements from singlestellar population models. Radial profiles are averaged along isophotes
(Kuntchner et al. 2010). *Also Oliva-Altamirano et al. (2015)
Integral Field Spectroscopy
 What is an Integral Field Spectrograph?
 What can I study with an IFS?
 What kinds of IFS are there?
 How do I prepare IFS observations
 How do I reduce/view/analyse my data?
What kinds of IFS are there?
 An integral field spectrograph consists of two components:
the spectrograph and an integral field unit (IFU). The job of
the IFU is to divide the 2D spatial plane into a continuous
array. This division can be achieved in three ways:
Credit: M. Westmoquette, adapted from Allington-Smith et al. 1998
Lenslet array
 The input image is split up by a microlens array (MLA).
 Light from each element of your observed object is concentrated
into a small dot and dispersed by the spectrograph.
 Dots are small so that the spectra do not fall on top of each other,
allowing the input image to be sampled contiguously.
 The disadvantage is that the length of spectrum that can be
produced without overlapping is very small and the packing of the
CCD is not efficient.
 Examples: VLT FLAMES(GIRAFFE)-IFU, Keck OSIRIS, WHT SAURON
Fibres (with/without lenslets)
 A 2D bundle of optical fibres transfer the light to the slit of the spectrograph.
 Fibre flexibility allows the round/rectangular field-of-view to be reformatted into
one (or more) "slits", from where the light is directed to the spectrograph. The
spectra are obtained without wavelength shifts between them.
 Placing an array of contiguous lenslets in front of the fibre bundle to focus light
into the fibre improves these IFUs as lenslets can be packed contiguously.
Microlenses also slow the telescope focal beam so that FRD can be minimised.
 Examples, without lenslets: WHT Integral; WIYN SparsePak
 with lenslets: AAOmega KOALA/SAMI; Gemini GMOS-IFU; VLT VIMOS
Image-slicer
Datacube
 The input image is formed on a mirror that is segmented in thin horizontal
sections, sending each 'slice' in different directions. A second segmented
mirror is arranged to reformat the slices so that, instead of being above each
other they are now laid out end to end to form the slit of the spectrograph.
 FRD is avoided and the slicing arrangement gives contiguous coverage of the
field at potentially high spatial resolution.
 Examples: ANU 2.3m WiFeS; Gemini NIFS; VLT SINFONI/MUSE
Fabry-Pérots (tunable filters)
 Fabry-Pérot imaging spectrographs are different from
what is usually meant by integral field spectroscopy, but
the result is still a data-cube.
 Fabry-Pérots allow a large FoV (e.g. 5 arcmin) to be
surveyed at high spatial resolution (e.g. 0.3
arcmin/pixel) in a single exposure but only at a single
wavelength — the required data volume (x, y, λ) is built
up by scanning through the desired wavelength range.
 Time-inefficient and difficult to perform accurate flatfielding and flux calibration of the data.
Integral Field Spectroscopy
 What is an Integral Field Spectrograph?
 What can I study with an IFS?
 What kinds of IFS are there?
 How do I prepare IFS observations
 How do I reduce/view/analyse my data?
How do I prepare observations
with an IFS?
 What are you looking at? What FOV/spatial and spectral
resolution/wavelength coverage do you need?
(http://ifs.wikidot.com/instruments)
 Choose your IFS based on these (and the availability to you).
 Use ETC to calculate exposure times to get the S/N you need.
 Think about the calibration observations you will need (obviously
bias/flat/arc, probably twilight flats for throughput correction,
and spectro-photometric standard stars for flux calibration)
 Prepare finding charts: label your target, add the chart scale
(about 5'x5’) & indicate N&E, then print it out in b&w
 Think about vignetting and bad fibres and centering/dithering
your target to avoid these issues.
Vignetted
Footprint of VLT VIMOS-IFU in high-resolution mode -> 20*20 fibres * 4 quadrants
Integral Field Spectroscopy
 What is an Integral Field Spectrograph?
 What can I study with an IFS?
 What kinds of IFS are there?
 How do I prepare IFS observations
 How do I reduce/view/analyse my data?
How do I reduce these data?
Basic Reduction Steps (reduce each observation separately):
 FYI: Your data will probably arrive as Raw Stacked Spectra (RSS) NOT a cube
 Create master bias by averaging bias frames
 TRACE the spectra.
 Create master spectroscopic flat by tracing spectra in flat field
 Create master lamp by subtracting bias & normalising by the flat arc frames.
(Can wavelength calibrate from sky lines but not ideal)
 Subtract bias, and divide image by flat field (which also notes spectra positions)
 Extract 1D spectra from 2D image: Apply wavelength calibration and remove
cosmic rays.
 Can check wavelength calibration with sky lines – see how much peak varies in
wavelength from spectrum to spectrum and frame to frame.
These processes are easily done within 2dfdr for AAOmega data. For VLT VIMOS-IFU
data using vipgi it’s a hands-on point and click exercise including flagging bad fibres!
P3D is another open source software for these tasks.
IFS Specific Reduction:
 Some of these steps are possible within packages. I used IDL or python
 Take each observation (per quadrant if relevant and per observation):
 create a variance array for each spectrum (trust me, you want to know
about your noise)
 Inverse gain = X electrons/ADU
 Variance = √(e-)/gain = √(count*gain)/gain = √(count/gain) + ReadOutNoise
 scale for different fibre transmissions
 subtract sky
 Make into a cube
 Combine quadrants (if relevant) into 1 cube using known pixel mapping.
 If observations are dithered by integer spaxel offsets then combine spectra
by known RA, DEC offset and scale for the relative throughput and noise of
each observation. If dithered by part-spaxel shifts then you will need to
resample your spectra into the new cube (See Sharp et al. 2015).
Reduced Data:
Spectrum In central spaxel
Image is RA x Dec. Each spaxel is summed along the
wavelength direction
.
How do I view these data
 At most of these points you’re going to want to look at your
data, to see what it looks like.
 I used to plot parts of the spectrum from IDL or for later,
science plots and map values (e.g. intensity/velocity/velocity
dispersion) using SAURON plot_spaxels and plot_bins IDL code.
 PINGSoft is a series of IDL routines including basic tools to
visualize spatially and spectrally the IFS data, to extract
regions of interest by hand or within a given geometric
aperture, to integrate the spectra within a given region, to
read, edit and write RSS FITS files, and to perform simple
analyses to the IFS data
 I am currently moving to Python. This is probably better.
How do I analyse these data?
Depends on what you are interested in!
Example 1. Galaxy Stellar Kinematics
 http://www-astro.physics.ox.ac.uk/~mxc/software/:
 Voronoi 2d tesselation: bins pixels to up a defined signal-tonoise (which you’ll need to calculate across your cube)
 PPXF: which fits stellar templates to calculate velocities at each
spaxel. Emission lines can be excluded.
 SAURON PLOT_BIN: makes pretty colour plots of velocities
 I developed my own IDL script to calculate Lambda parameter.
 For SAMI analysis see van de Sande et al. (in prep).
Example 2. Galaxy Emission Lines
 LZIFU software fits emission lines (Ho et al.)
 Fit the emission lines you’re interested in at each
spaxel. You want to know their width, height, central
wavelength and flux.
 With these information you can plot intensity and
velocity maps of lines.
 You can also calculate emission line ratios and look at
dust and ratio maps.
 You can plot the line ratio intensities on IDD and
determine locations of shocks etc.
The FUTURE is HERE!
 SAMI (Sydney AAO MOS IFU)
 13 deployable “hexabundle” fibres – 61x1.6” fibres per
bundle and ~14” per bundle across 1 deg field
 http://sami-survey.org/