IFU observations of an extended source

IFU observations of an extended source

Having observed your standard, you will probably move on to your extended target. The image below shows a raw frame of an extended source (one of the bullets in Orion) through the HK grism. Even though this target is a bright source, in this 240sec exposure only OH sky lines and diffuse thermal emission (at longer wavelengths, to the left) are evident..

After sky subtraction the emission from the target itself becomes apparent. Following sky-subtraction, rearrangement of the slices (so that the slices are in the order in which they cover the field of view) and wavelength calibration (“scrunching”) the spectral image looks like this:

In the above example we now see emission lines from Bracket gamma (center-right), HeI, molecular hydrogen and, at shorter wavelengths (to the left) the H-band Bracket series. Note that the BrG is extended; it spans the 6″ width of each slice and appears in all 14 slices. That’s because the emission is extended across the whole IFU field. Conversely, the H2 emission peaks towards the centre of each slice, and is brightest in the middle few slices. That’s because the H2 is from a fairly compact bow-shock that was positioned in the centre of the IFU field-of-view (shown below).

A datacube will be created from the above scrunched image. Division by a standard star spectrum will take out the atmospheric absorption bands. Images in specific emission lines can then be extracted from this final, fully-reduced data cube by running the DR when a simple ascii text file is present in the reduced data directory. This “extract.images” file is described in the next section.

In our example, images in 1-0S(1) H2 emission and Brackett Gamma have been extracted. The distribution of emission across the field is markely different, the BrG being background diffuse emission associated with the Orion nebula, and the H2 being excited in the bullet. The axis scale below is in pixels, which aren’t square. Pixel sizes are 0.24″ in X (the 2-pixel slit width) and 0.12″ in Y; hence, the bullet appears stretched vertically.

Finally, note that in most cases the images are up-side down (we don’t want to make it too easy for you…).

IFU observations of a source using a single on-source position with offset to sky should be reduced using the EXTENDED_SOURCE recipe (as was used above). However, if you wish to map a larger field using small offsets, then the MAP_EXTENDED_SOURCE recipe should be used (a template sequence exists in the UKIRT-ot template library). Versions of both of these recipes are available for use without a standard star (not recommended).

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