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IR Compendium: Background: Extragalactic Science

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Overview

Because Spitzer will be extraordinarily sensitive to mid- and far-IR radiation, it will be able to detect the youngest and most luminous galaxies. Their radiation, nearly all of which is emitted in the mid-IR, comes from stars in the process of forming and from dust clouds.

In 1985, a previous satellite named IRAS found a class of luminous "starburst" galaxies undergoing runaway star formation. Much of this star formation is obscured by dust and is invisible in the UV or optical. However, IRAS was only sensitive to local galaxies going through such a phase. Spitzer will vastly improve the census of luminous starbursts across cosmic history. These galaxies pinpoint where approximately half the stars in the Universe were formed.

The deepest images taken by HST, Chandra, and Spitzer will be in the same patch of sky. Together, these coordinated panchromatic images will show us what galaxies looked like when they were first forming, when the Universe was <10% its current age.

Accretion disks around black holes can heat dust just like hot stars. Spitzer will detect the energy generated by obscured supermassive black holes in the centers of galaxies. Spitzer surveys cover areas surveyed by Chandra and XMM; comparison of the results will show which Spitzer sources are powered by black holes and will show which Chandra sources are surrounded by dust and not likely to have been detected at visible wavelengths.

Images and Plots

Note: This Compendium is a work in progess. We have used the best information available, including data from other missions, and will update these pages as soon as possible with the new information.

What Spitzer will do:
  • Detect galaxies at wavelengths where star formation dominates -- two imaging science instruments cover 3 to 180 microns with an approximately 5 arcminute field of view
  • Detect galaxies to great distances (eg. Spitzer is very sensitive)
What Spitzer won't do: measure shapes of distant galaxies; most distant galaxies imaged by Spitzer will be unresolved (i.e., a point source).

Adapted from Marcia Rieke's talk at the Seattle AAS, January 2003

Spitzer should settle the question of how much star formation was obscured by dust and not seen in the UV or even in the visible.

Current estimates of high redshift star formation rates all rely on UV light which can be easily scattered by dust -- nearly 10x range in star formation rates permitted by observations so far. (red arrow in plot)

Adapted from Marcia Rieke's talk at the Seattle AAS, January 2003


Click for full image.		 An example of a "J dropout" object from the Hubble Deep Field North. Spitzer will search for similar objects that have similar dropouts in longer wavelengths.

Simulated IRAC image of HDF, from the GOODS Legacy team. Two different theoretical calculations of the IRAC Point Spread Function (PSF) are indicated, one the "handbook" PSF and the other from STinyTim simulations.


Click for larger image		 Typical Lyman-break galaxy spectra vs. redshift, with IRAC GOODS Legacy team survey limits indicated. Plot courtesy M. Dickenson, GOODS Legacy team (and STScI)


Click for larger image		 An indication of how mass estimates for galaxies will dramatically improve when IRAC data are included. Model galaxy spectra are plotted with expected data points overplotted; inset plots are confidence levels in mass vs. age parameter space. Plot courtesy M. Dickenson, GOODS Legacy team (and STScI)


Click for larger image		 The IR separates (in the intensity domain) galaxies by their population type. This figure is the nearby starburst, N253, as seen in the near-IR (NIR) and as seen in the optical. It shows how the NIR penetrates the dusty arms, revealing the bar and the older stellar population ("backbone of galaxies"). The optical image is obscured and basically a flocculent morphology.

More technical information: Nearby starburst galaxy NGC 253 (Vsys =250 km/s). The upper panel shows the near-infrared mosaic of galaxy NGC 253, with 2MASS bands: J, H and Ks, RGB combined to form a full color image. The lower panel shows the galaxy as seen in the visual (blue to red) window. The 4-color photophaphic image was taken by amateur astronomer Tony Hallas using a 14.5" Cassegrain telescope. The field of view is ~20 arcmin.

Adapted from Jarrett et al., 2003, AJ, 125, 525.


Click for larger image		 Image of the whirlpool galaxy, M51, as seen by 2MASS (J and K bands), and the ISO mid-infrared band LW2. Note the complete separation of processes generating the IR light -- one is the stellar backbone of M51 (something rarely discussed or noticed, as this beautiful galaxy is laced with massive, luminous stars), and the other is the warm dust radiating from massive star formation regions (including HII and PDR sites). M51's companion (M51b) looks completely different in these measures -- the NIR reveals the bar and underlying structure of this galaxy that is pumping the grand design spiral of M51a, with little or no star formation activity (it has no MIR or FIR).

More technical information: The Whirpool Galaxy (M51, NGC 5194/95) as seen in the near-infrared and mid-infrared wavelengths. The 2MASS near-infrared images J (1.2 microns) and Ks (2.2 microns) are shown with the blue and green channels, respectively. The ISOCAM LW2 (6.5 microns) image is shown with the red channel. The 2MASS images were smoothed to a beam resolution of 7 arcsec, corresponding to the ISOCAM resolution. The image is rotated roughly 54 degrees east of north, corresponding to the original spacecraft orientation during the M51 observations. Stars have been removed from the ISOCAM image. The field of view is ~8.8x9.9 arcmin.

Adapted from Jarrett et al., 2003, AJ, 125, 525.


Click for larger image		 Plot indicating what can be learned by just looking at the NIR-to-MIR window - much is going on within this small window, something not fully appreciated by the IR spectral energy distributions (SED)s.

Adapted from submission by T. Jarrett, IPAC.

Click for full image Circles show positions of sub-millimeter (SCUBA) sources without optical counterparts. These sources are thought to have redshifts between 2 and 4 and may be dusty protogalaxies undergoing an initial burst of star formation. Spitzer will discover further examples and determine the redshifts of these sources.

From Mike Werner's speaker's bureau slides, August 2001.

ISO IR spectra of Seyfert galaxies. Figure 5 from Clavel et al., 2000, A&A, 357, 839: The spectra of the non-active SB galaxy NGC 701 (heavy line) and of the Sf2 galaxy NGC 5953 (dotted line). The flux of NGC 701 has been divided by a factor of two.

ISO IR spectra of Seyfert galaxies. Figures 7 and 8 (click image to download) from Clavel et al., 2000, A&A, 357, 839: average rest-frame spectrum of the ~20 best Sf1 and Sf2 galaxies.

ISO IR spectra of normal galaxies. Figure 3 (click image to download full version) from Lu et al, 2003, ApJ, 588, 199: Average rest-frame spectra derived from weighted averages of 40 galaxies. Squares are average obtained by normalizing to the J fluxes, thick solid curve is from normalizing by the integrated flux of AFE (7.7 um). Two elliptical galaxy spectra are also shown, NGC 3379 (thin solid) and NGC 4374 (dotted).

ISO IR spectra of normal galaxies. Figure 4 (click image to download full version) from Lu et al, 2003, ApJ, 588, 199: Comparison of the average spectra of the FIR-quiescent and FIR-active subsamples. Squares are FIR-active subsample, solid curve is average spectrum of the FIR-quiescent. The mean of the elliptical galaxy spectra is a dotted line. See text for different normalizations leading to (a) and (b).

Table of molecular hydrogen emission line fluxes in Seyferts from ISO SWS spectra found in Rigopoulou et al, 2002, A&A, 389, 374.

Average ISO spectrum of NGC 4945, from Spoon et al, 2000, A&A, 357, 898. (Click image to obtain full version.)

Mid-infrared high resolution ISO/SWS spectra of five extragalactic templates, from Sturm, et al, 2000, A&A, 358, 481. (Click image to obtain full version.) See paper for discussion; Figure 2 zooms in on figure above (fig 2a, 2-6.5 microns, fig 2b, 8.5-12.5 microns, fig 2c, 12-25 microns, fig 2d, 25-46 microns).

Simulated IRS low-resolution spectrum of a faint galaxy at z=3. The redshift can be determined from the broad spectral features, which are PAH's from small grains. The rest wavelengths are 7.7, 6.2, 8.6, 11.3, and 12.6 micron.

From Mike Werner's speaker's bureau slides, August 2001.

Simulated IRS high-resolution spectrum of a galaxy at z=1. Emission lines of neon in several ionization states are seen.

From Mike Werner's speaker's bureau slides, August 2001.

This plot is the 2.5 to 45 micron spectrum of the Circinus galaxy from ISO (SWS). This galaxy exhibits bright, narrow, high-excitation emission lines typical of an AGN surrounded by an obscuring torus. The red and blue arrows indicate the approximate wavelength coverage provided by IRS.

After Morrwood et al 1996; this copy from Mike Werner's speaker's bureau slides, August 2001.

A simulated IRS spectrum - single exposure of an extended source with an M82+Circinus-like spectrum from 10-19.5 microns, including cosmic rays.

From Mike Werner's speaker's bureau slides, August 2001.

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