1.5 Outline of this thesis
1.5.6 Chapter 7 – Conclusions, summary and future work
In this final chapter I summarise the major conclusions which arise from this work, and discuss future research which will be conducted to further these results.
The reduction of Spitzer-IRS
spectroscopic data
In the proceeding chapters, a large proportion of the analyses will be based on the iden- tification of AGNs using mid-IR spectroscopic data obtained by the NASA Spitzer Space Telescope (hereafter, Spitzer.) The Spitzer observatory was launched on 23rd August 2003 and is a 950 kg, cryogenically cooled 4 × 2 m spacecraft with a 0.85 m diameter mirror placed in a receding Earth-trailing heliocentric orbit.1 The observatory consists
of three primary science instruments: the infrared spectrograph (IRS), the multi-band imaging photometer (MIPS) and the infrared array camera (IRAC). In this chapter, I out- line the properties of the spectrograph instrument and the data-reduction processes used throughout this thesis.
2.1 The Spitzer infrared spectrograph
The cold assembly of the Spitzer infrared spectrograph primarily consists of four mid- IR spectrograph modules, providing low and moderate resolution spectroscopy at λ ∼ 5.2–38.0 µm, with each operating at tightly controlled temperatures of T ∼ 1.4–1.6 K. Specifically, the two long-slit modules provide low-resolution R ∼ 60–120 spectroscopy at λ ∼ 5.2–38.0 µm (short-low and long-low), whilst the two cross-dispersed echelle modules provide higher resolution (R ∼ 600) spectroscopy at λ ∼ 9.9–37.2 µm (short- high and long-high). Each detector is a 128 × 128 pixel array CCD with varying plate
1Spitzer is now in its ‘warm-phase’ after its cryogen fluid expired on 15th May 2009 (towards the end of
the G05 proposed program presented in Chapter 6).
scales (1.8–5.1$$ pixel−1). The main properties of each module and associated dispersal
sub-slits are summarised in Table 2.1.
Observations are available for each module in two modes: (1) a spectral mapping mode which consists of a custom grid of slits around a central target position (see Fig. 2.1) and (2) a standard “point and shoot” nod-staring mode (see Fig. 2.2).
All Spitzer-IRS observations are pre-processed for all ‘Spitzer specific’ effects by the
Spitzer Science Center (SSC) prior to being made available to the observer. Specifically,
the raw IRS data consists of individual data collection events (DCEs) which are pro- duced on a per exposure basis. During the SSC pre-processing, these DCEs are flagged for obvious cosmic rays, and then gain, droop, dark and ramp corrected. The corrected frames are then collapsed to 2-D images and flat-fielded to produce final Basic Cali- brated Data (BCD) images which may then be additionally processed by the observer (see Fig. 2.3). For further information on the high- and low-resolution Spitzer-IRS pre- processing pipelines, see Chapter 7 of the Spitzer Observers Manual.2 Retrieval of all
BCDs for a specific Spitzer-IRS observation is carried-out through the SSC Legacy pro- gram,LEOPARD.
The majority of currently published Spitzer-IRS spectroscopy is produced using the contributed software package, the Spectroscopic Modeling Analysis and Reduction Tool (SMART; Higdon et al. 2004). It has been our experience that whilst theSMARTpackage
is sufficient for quick-look spectra of bright sources, in-general it does not produce opti- mal science-quality spectroscopy of most sources. In the following sections, we outline the custom pipeline processes which we have designed in-conjunction with available SSC packages, in order to produce science-quality Spitzer-IRS spectroscopy for low– and high-resolution IRS-staring observations (§§ 2.2 and 2.3.1) and high-resolution IRS map- ping observations (§ 2.3.2). The final resulting spectroscopy is then analysed in Chap- ters 3, 4 and 6 using the spectral-line fitting routines built intoSMART.
2The Spitzer-IRS Data Handbook and observer’s manual are available at
Table 2.1: IRS module and slit descriptions
Module Name Wavelength Abbreviation Detector Plate Scale R and Order Range (µm) Compound ($$pixel−1)
(1) (2) (3) (4) (5) (6) Short-low 2ndorder 5.2–8.7 SL2 Si:As 1.8 60–127
Short-low 1storder 7.4–14.5 SL1 Si:As 1.8 61–120
Long-low 2ndorder 14.0–21.3 LL2 Si:Sb 5.1 57–126
Long-low 1storder 19.5–38.0 LL1 Si:Sb 5.1 58–112
Short-high 9.9–19.6 SH Si:As 2.3 ∼ 600 Long-high 18.7–37.2 LH Si:Sb 4.5 ∼ 600
NOTES: (1) Formal module and sub-slit name. (2) Operating wavelength range includ-
ing bonus-segment for 2nd order low-resolution modules. (3) Module abbreviation. (4) Chemical compound of module detector. (5) Detector plate scale. (6) Resolving power.
Figure 2.1: ESO Digital Sky Server image (11$× 12$) of a typical nearby
spiral galaxy with Spitzer-IRS SL (3.6$$× 136.0$$) and LL (10.5$$× 360.0$$)
apertures overlaid in a 2×10 (SL) and 6×10 (LL) mapping-mode obser- vation. Essentially, a mapping mode observation consists of a grid-style configuration of IRS slit positions arranged around a central target po- sition. Slits are nodded as in a standard IRS-staring observation; 1st
and 2nd order modules are shown in yellow and orange, respectively.
The object shown is the Sbc galaxy, NGC 4321 at D ≈ 15.2 Mpc with LIR≈ 2.5 × 1010L!. Additionally, we show peak-up imaging apertures
(red and blue overlays) used for photometric and positional calibration of the SL modules during SSC pre-processing.
Figure 2.2: Hubble Space Telescope (HST) WFPC2 (3-colour) optical image of the circumnuclear region of a typical galaxy in the D < 15 Mpc sam- ple (studied in Chapters 3 and 4), with Spitzer-IRS SH (4.7$$ × 11.3$$,
λ ∼ 9.9–19.6 µm) and LH (11.1$$ × 22.3$$, λ ∼ 18.7–37.2 µm) aper- tures overlaid in the two nod positions (staring-mode observations). The object shown is the Sbc galaxy, NGC 0278 at D ≈ 11.4 Mpc with LIR ≈ 1.1 × 1010L!. Linear size-scales of apertures are ≈ 0.26 × 0.63
kpc (SH) and ≈ 0.61× 1.23 kpc (LH). In highly resolved sources such as NGC 0278 the two nod positions will produce slightly differing spectra but the inner central region will be bright and thus will dominate.
Figure 2.3: An example of a mock Basic Calibrated Data (BCD) image for the SL Spitzer-IRS module (Left panel of Fig. 5.2 of Spitzer-IRS ob- server’s manual). Columns from left to right: SL1 (7.4–14.5 µm), SL2 and bonus segment (5.2–8.7 µm), and red (top) and blue (bottom) peak- up images. Dark areas around the dispersion regions are masked dur- ing the processing steps described in § 2.2. Additionally, single and group rogue ‘hot’ pixels are clearly evident in both SL1 and SL2 mod- ules, respectively which must be identified and masked.