Volume 542, June 2012
|Number of page(s)||12|
|Section||Interstellar and circumstellar matter|
|Published online||19 June 2012|
The 5 Herschel wavebands of M 16, from top to bottom, 70, 160, 250, 350 and 500 μm. Note also the extended PACS coverage compared with SPIRE (see Sect. 2). The observed level of cirrus noise in M 16 is ~ 20 mJy and 200 mJy at λ = 70 and 160 μm, respectively and 2, 1 and 1 Jy at the SPIRE 250, 350 and 500 μm bands. At shorter wavelengths only those warmer objects, such as protostars and HII regions are seen. At longer wavelengths Herschel detects cold, deeply embedded filaments and the progenitors of high-mass stars.
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To examine the dust temperature and column density structure in the entire M 16 region probability distribution functions (PDFs) were derived (Fig. B.1). Column density PDFs have been used by many authors to characterise cloud structure based on star formation activity (Kainulainen et al. 2009; Federrath et al. 2010). The column density in M 16 spans almost two orders of magnitude (5.0 × 1021–3.7 × 1023 cm-2), whilst the dust temperature ranges from 14.5 K to 23.5 K. The highest temperature in the maps is recorded directly west of the base of the Pillars of Creation and thus south of the NGC 6611 cluster.
PDFs of column density (top) and temperature (bottom) with the sub-regions as defined in Fig. 2. Only the high column density material that we are interested in > 10 mag is shown.
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In order to investigate the variation of PDF profiles within M 16, we segregated the complex into six sub-regions (see Fig. 2, left), according to the prominent structures or cloud features impacted by the NGC 6611 cluster. These sub-regions include: the main component of material, corresponding to the Eagle’s wings in optical images and the arch structure (hereafter referred to as the “M 16-N” sub-region; green in Fig. B.1), the Pillars of Creation (blue), Spire (cyan), the MYSO detected by Indebetouw et al. (magenta 2007), the eastern portion of the map (hereafter “M 16-E”; red), and the northern periphery (hereafter “M 16-NW”; yellow). The MYSO is treated as a separate sub-region due to its high column density tail, to prevent this MYSO from skewing the PDFs of other sub-regions. While the column density PDF of this MYSO should be treated with caution, its inclusion here allows comparison with the other sub-regions with respect to dust temperature.
The column density PDFs are reliable for the larger regions (e.g., M 16-N, M 16-E, M 16-NW) above AV > 10 mag7, whilst the smaller regions (e.g., Pillars of Creation, Spire and the MYSO) are reliable only above AV > 20 mag. We are only concerned with higher column density (>20 mag) here. The M 16-N, Spire and M 16-E sub-regions have similar column density slopes (around AV > 30 mag). The M 16-N and MYSO sub-region have high column density tails, similar to that seen in the Vela C Centre-Ridge sub-region (Hill et al. 2011), and consistent with regulation by self-gravity (Ballesteros-Paredes et al. 2011).
The Pillars of Creation sub-region has a column density PDF that resembles a log-normal shape which has been seen in clouds which are initially quiescent and where turbulence is likely the main shaping mechanism (Kainulainen et al. 2009). Note, however that feedback effects, such as those from a nearby OB cluster, have yet to be factored into turbulence simulations from which model PDFs are drawn. The suggestion of turbulence here is consistent with the observations of Pound (1998, 12CO, 13CO and C18 who found that the Pillars of Creation are not gravitationally bound and are being ripped apart.
The sub-region hosting the Pillars of Creation (Fig. B.1, blue) attains higher dust temperatures than the other sub-regions, including that of the Spitzer identified MYSO. This difference shows that the NGC 6611 cluster has had more of an impact on the cloud temperature than the current star formation process. The M 16-N and Spire sub-regions have similar dust temperature PDF slopes (>21 K), which likely reflects their similar distance from the NGC 6611 cluster, though the M 16-N sub-region contains colder gas in the northern part of the region, including the ridge itself. The M 16-NW and M 16-E sub-regions span only small ranges in temperature. The median column density and dust temperature for each sub-region, determined from the PDFs, are given in Table 1.
It is interesting to note that while the M 16-E and Spire sub-regions have similar column density PDF profiles, they display quite different dust temperature PDF profiles. This suggests that they are both a part of the same cloud structure, but that the Spire is closer to and thus more readily externally heated by the NGC 6611 cluster than M 16-E. The three sub-regions closest to the OB cluster (Pillars of Creation, Spire, MYSO) have similar dust temperature profiles, but quite different column densities.
There is no clear trend in the column density PDFs to suggest or represent an effect of the winds or radiation from the NGC 6611 cluster on the sub-regions. The median temperature of each region correlates closely with the projected distance from the cluster, suggesting no strongly unbalanced projection effects for the cloud surrounding the NGC 6611 cluster.
The probability distribution functions (Fig. B.1) confirm that the column density in each sub-region in M 16 is indeed different, which influences the extent of the heating effect of the NGC 6611 cluster on each sub-region, as seen in the dust temperature PDFs. At zero-order and above AV > 15 mag there is no difference seen in the structure of the cloud with distance from the NGC 6611 cluster. That is, the column density PDFs of sub-regions near to the cluster, and thus heavily impacted, and those in more remote portions of the map, display similar column density PDF slopes.
It is also worth noting, that though normalised, the Pillars of Creation, Spire and MYSO sub-regions are comprised of a small number of pixels compared with the other sub-regions in the complex, which could thus affect their interpretation.
Upper left: column density map of NGC 6611 with radial segments as indicated. Upper-middle to lower-right: dust temperature (black) and column density (red) PA-averaged radial profiles in each segment. Segments are arranged clockwise starting with the Pillars of Creation (upper-middle) to the cavity region (lower-right) as identified by numbers. The profiles are the median values with projected distance in each segment to avoid biases by the presence of protostars. The radial lengths of each segment were defined by a drop in column density (as seen in the upper-left panel).
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© ESO, 2012
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