![]() Shore, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 V.B.4 Shock Chemistry ![]() On the basis of our preferred biological hypothesis it could be argued that the absence of the 2175 Å feature indicates either no biological activity or a greatly diminished biology in starburst galaxies. Although one might try to argue that PAH carriers of the 2175 Å bump are destroyed in shocks and intense radiation associated with starburst activity, this would not be consistent with the well-defined presence of 3.3, 3.4, 6.2, 6.7, 8.6, 11.2, and 12.7 μm features in the infrared. The data on the extinction curves of starburst galaxies, however, show no sign of any significant 2175 Å ultraviolet absorption. The Spitzer telescope data (top frame) clearly show strong PAH features very similar in strengths and wavelengths to similar bands that occur in sources within the Milky Way. This is illustrated in the bottom frame of Fig. 1.4. The argument that these bands are independent of the 2175 Å dust feature is strengthened by observations of these bands in starburst galaxies, in which no 2175 Å band is seen ( Catzetti et al., 2000). They are present in galactic as well as extragalactic sources extending to redshifts of up to at least z = 2. The infrared data are mainly in the form of absorptions at 3.3, 3.4, 6.2, 6.7, 8.6, 11.2, and 12.7 μ m. In addition to data on the ubiquitous 2175 Å ultraviolet absorption band discussed in Section 4, infrared, microwave and radio observations have shown the ubiquitous presence of organic molecules including PAHs. Infrared, microwave and radio observations are used to detect the presence of such molecules and the current list of positive detections is likely to be constrained only by limitations of techniques. This, in our view, is extremely improbable and poses a problem for abiotic explanations of the data.ĭetections of interstellar organic molecules of ever-increasing complexity have continued in the last two decades following the deployment of newer and better instruments and telescopes. More recent studies of other comets have yielded generally similar results.Īny alternative nonbiological explanation of the points in Fig. 1.3 would involve abiotically formed organic molecules possessing functional groups that fortuitously matched biology. The solid curve is for nonirradiated bacteria the dashed curve is for X-ray irradiated bacteria. ![]() The midinfrared spectrum ( Fig. 1.3: right panel) of the comets dust coma following a major outburst on Ma( Wickramasinghe and Allen, 1986) showed unambiguous evidence of aromatic-aliphatic linkages (C–H stretching modes) that were uncannily consistent with desiccated E. coli ( Wickramasinghe et al., 1986). The discovery of organic dust in comets came with the last perihelion passage of Comet Halley in 1986 ( Wickramasinghe and Allen, 1986). Right panel: Emission by dust coma of Comet Halley observed by Dayal Wickramasinghe and David Allen on Ma(points) ( Wickramasinghe and Allen, 1986) compared with normalized fluxes for desiccated E. coli at an emission temperature of 320 K( Wickramasinghe et al., 1986). Left panel: Comparision of the normalized flux from GC-IRS7 obtained in 1980 ( Allen and Wickramasinghe, 1981) with the laboratory spectrum of dessicated Escherichia coli ( Hoyle et al., 1982a,b).
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