3. SPATIALLY EXTENDED STARBURSTS

Interaction-induced starbursts tend to be spatially extended (~ 10-20 kpc) for most of their duration. Only relatively late in a merger do they become strongly concentrated.

For example, any good HST, Spitzer, or Chandra image of NGC 4038/39 shows that enhanced star formation extends over a projected area of ~ 8 × 11 kpc (Fig. 2). In the optical, H and blue images are best at showing the extended nature of the starburst. In the infrared, a Spitzer/IRAC image at 8 µm emphasizes the warm dust associated with star formation throughout the two disks, glowing especially bright in the optically obscured disk contact ("Overlap") region (Wang et al. 2004). And in X-rays, a deep Chandra image displays not only two disks filled with superbubbles of hot gas (typical diameter ~ 1.5 kpc, T 5 × 10⁶K, M 10^5-6 M), but also two giant, 10 kpc-size loops extending to the south (Fig. 2, middle panel). Their nature remains unclear (wind-blown?, or tidal ejecta?). These various images illustrate that early in a merger the extended starburst heats the ISM in a chaotic manner, rather than leading to well-directed bipolar superwinds.

Figure 2. Extended starburst in NGC 4038/39, as imaged by (left) HST (~ 8 × 11 kpc field of view) and (middle) Chandra. (Right) Metallicity map for hot ISM, showing spotty chemical enrichment (from Whitmore et al. 1999; Fabbiano et al. 2004; and Baldi et al. 2005).

An interesting consequence of the extended starburst in NGC 4038/39 is the spotty chemical enrichment of the hot ISM, observed for the first time in any merger galaxy (Fabbiano et al. 2004; Baldi et al. 2005). The high S / N ratio of the Chandra emission-line spectra permits the determination of individual Fe, Ne, Mg, and Si abundances in ~ 20 regions. Figure 2 (3rd panel, = color Fig. 3 in Fabbiano et al.) shows a metallicity map, with various shades of gray marking individual elements. The -elements are enhanced by up to 20-25 × solar and follow an enhancement pattern distinctly different from Fe, as one would expect if they were recently produced by SNe II. A question for future study is how such spotty chemical enrichment may affect stars still to form.

Another important consequence of the large spatial extent of merger-induced starbursts is that newly-formed stars decouple from the inward-trending gas continuously and at many different radii. This process differs sharply from the widely held misconception that such starbursts occur mainly in the central kiloparsec, where they are being fueled by infalling gas. As a result of this extended star formation, radial age gradients in merger remnants are weak (e.g., Schweizer 1998). This is also the likely reason why in ellipticals age gradients are near zero, and mean metallicity gradients are only ~ 40% per decade in radius (Davies et al. 1993; Trager et al. 2000; Mehlert et al. 2003).

The strongest evidence linking extended starbursts to merger remnants and ellipticals is the wide radial distribution of the resultant star clusters. In both remnants (Fig. 3) and Es, second-generation metal-rich globular clusters track the underlying light distribution of their host galaxies with surprising accuracy. It is true that they tend to be more centrally distributed than metal-poor globulars, but only by little. Typically, half of them lie within R_eff 3-5 kpc from the center. This is consistent with some additional gaseous dissipation, but completely inconsistent with nuclear-only ( 1 kpc) starbursts. Hence wide-flung globular-cluster systems are signatures of ancient extended starbursts.

Figure 3. Radial distributions of second-generation globular clusters (data points) and background V-light (lines) in the merger remnants NGC 3921 (Schweizer et al. 1996) and NGC 7252 (Miller et al. 1997). The far-flung cluster distributions are remnant signatures of extended starbursts.