Annu. Rev. Astron. Astrophys. 2000. 38: 761-814 Copyright © 2000 by Annual Reviews. All rights reserved

3.1.2. Distinguishing AGNs and Starbursts: ISO Diagnostic Diagrams

One of the most powerful applications of the ISO (mid-)IR spectroscopy is as a tool for distinguishing between AGN and star formation dominated sources. The right-hand insets of Figure 3 (from Sturm et al 1999b) show two examples: the starburst galaxy M82 and the nuclear region of the Seyfert 2 galaxy NGC 1068. The difference between the mid-IR spectra is striking, and agrees with earlier ground-based results (Roche et al 1991). M82 is characterized by strong, low-excitation, fine structure lines, prominent UIB features, and a weak 10 µm continuum (Sturm et al 1999b). High-excitation lines are absent or weak. In contrast, NGC 1068 displays a fainter but highly excited emission line spectrum ([OIV] / [NeII] and [NeV] / [NeII] 1; Lutz et al 2000 and weak or no UIB features, plus a strong mid-IR continuum (Genzel et al 1998, Sturm et al 1999b). The far-IR spectra of NGC 1068 and M82 (and Circinus) are more similar because star formation in the disks dominates the 40 µm SED in all three galaxies (Colbert et al 1999, Spinoglio et al 1999). The deep 10 µm dip in the mid-IR SED of M82 (and other galaxies) has often been interpreted as the result of absorption by silicate dust grains [_9.7µm ~ 2 or A(V) ~ 30]. Sturm et al (1999b) showed that the M82 spectrum can be fitted extremely well by the superposition of the (absorption-free) spectrum of the reflection nebula NGC 7023 (which exhibits strong UIB features) plus a VSG continuum at > 10 µm. No silicate absorption is required. Instead, the deep 10 µm dip is caused by the strong UIB emission on either side of the silicate feature.

Diagnostic diagrams can empirically characterize the excitation state of a source (Osterbrock 1989, Spinoglio & Malkan 1992, Voit 1992). Genzel et al (1998) use a combination of the 25.9 µm [OIV] to 12.8 µm [NeII] line flux ratio (or [OIV]/[SIII], or [NeV]/[NeII]) on one axis, and of the UIB strength ⁵ on the other axis. Figure 5a shows that this ISO diagnostic diagram clearly separates known star-forming galaxies from AGNs. The three AGN templates that are located fairly close to the starbursts in Figure 5b (CenA, NGC 7582, and Circinus; Mirabel et al 1999, Radovich et al 1999, Moorwood et al 1996) are known to contain circumnuclear starbursts; correction for the star formation activity would move these sources to the upper left (marked by arrows). The diagram is insensitive to dust extinction if the numerator and denominator in both ratios are affected by a similar amount of extinction. Lutz et al (1998b) found low-level [OIV] emission ([OIV] / [NeII] ~ 10^-2) in a number of starburst galaxies. The [OIV] emission in M82 is spatially extended and tracks galactic rotation. It cannot come from low-level AGN activity. Fast, ionizing shocks in galactic "super" winds are the most likely sources of this low-level, high-excitation gas (Lutz et al 1998b, Viegas et al 1999), but WR stars may play a role in some objects (Schaerer & Stasinska 1999).

Figure 5. (a, left) SWS diagnostic diagram (from Genzel et al 1998). The vertical axis measures the flux ratio of high excitation to low excitation mid-IR emission lines, and the horizontal axis measures the strength (i.e. feature to continuum ratio) of the 7.7 µm UIB/PAH feature. AGN templates are marked as rectangles with crosses, starburst templates as open triangles, and UILRGs as filled circles. A simple mixing curve from 0% to 100% AGN is shown with long dashes. (b, right) ISOCAM diagnostic diagram (from Laurent et al 2000), including 35 CVF spectra from a sample of galaxies of different characteristics. The vertical axis measures the ratio of 14.5 to 5.4 µm continuum, and the horizontal axis measures the strength of the UIB feature. Active starbursts are in the upper left, AGNs in the lower left, and PDRs in the lower right. The curves from lower right to upper left denote HII/starburst fractions of 25%, 50%, and 75%. The two curves from lower left to upper right denote AGN fractions of 75% and 50%.

Based on ISOCAM CVF spectra of the central part of M17 (an HII region), of the nuclear position of Cen A (an AGN; Mirabel et al 1999), and of the reflection nebula NGC 7023 (a PDR; D Cesarsky et al 1996a), Laurent et al (2000) proposed another diagnostic diagram. It is based on the 15 µm/6 µm continuum ratio on one axis and the UIB/6 µm continuum ratio on the other axis. Laurent et al (2000) demonstrated that this mid-IR diagnostic diagram also distinguishes well between AGNs and starbursts (Figure 5b).

One example of the value of the techniques proposed by Laurent et al (1999) is the study of Centaurus A, the closest radio galaxy. In the visible band, Cen A is a giant elliptical galaxy with a prominent central dust lane that was interpreted 45 years ago to be the result of a merger between the elliptical galaxy and a small gas-rich galaxy (Baade & Minkowski 1954). The central AGN is hidden in the optical band, and its presence can only be directly detected from the prominent, double-lobed, radio and X-ray jet system (Schreier et al 1998). The ISOCAM mid-IR observations penetrate the dust and reveal a strong and compact hot dust source associated with the central AGN, as well as a bisymmetric structure of circumnuclear dust extending in the plane of the dust lane (Figure 6, see color insert; Mirabel et al 1999). The mid-IR spectrum toward the nuclear position is characteristic of an AGN, as described in previous sections (Genzel et al 1998, Mirabel et al 1999, Laurent et al 2000, D Alexander et al 2000). In contrast, the off-nuclear spectra are characteristic of star-forming regions (Madden et al 1999a, Unger et al 2000). Mirabel et al interpreted the bisymmetric dust structure as a highly inclined (i = 72°) barred spiral. Its cold dust content is comparable to a small spiral galaxy. The gas bar may funnel material from kpc scales toward the circumnuclear environment (10² pc scale), from where it may be transported farther by a circumnuclear bar/disk inferred from near-IR polarization observations. As such, Cen A is a beautiful nearby example of how a merger can convert an (old) spheroid into a bulge+disk system.

Figure 6. (left): ISOCAM LW2 map (red) of Centaurus A, superimposed on a visible image and contours (blue) of the 20 cm radio continuum. The infrared data penetrate the dust lane and reveal the presence of a barred dust spiral centered on the AGN (Mirabel et al 1999). Figure 6b (right): IOSCAM LW3 map (red contours) of the Antennae galaxies (upper right inset shows the larger scales and the tails) superimposed on a V/I-band HST image (Mirabel et al 1998). The nuclei of NGC 4038 and NGC 4039 are top and bottom right, respectively. The interaction region is located below the center of the image and exhibits the strongest mid-IR emission.

⁵ The 7.7 µm (UIB) 'line to continuum' ratio or 'strength' is the ratio of the peak flux density in the UIB feature (continuum subtracted) to the underlying 7.7 µm continuum flux density. The latter is computed from a linear interpolation of the continuum between 5.9 µm and 11.2 µm. While the 11.2 µm continuum is often affected by silicate absorption and various emission features longward of 11 µm, 7.7 µm is close enough to the fairly clean 5.9 µm region that a simple linear extrapolation is justified as a simple and fairly robust estimate of the 7.7 µm continuum. Back.