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The Elusive Active Nucleus of NGC 4945
Astrophysics · Submitted 11 Feb 2000
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim An important conclusion is that powerful AGNs can be hidden even at mid-infrared wavelengths and, therefore, the nature of luminous dusty galaxies cannot be always characterized by long-wavelength data alone but must be complemented with sensitive hard X-ray observations.
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The abstract, sentence by sentence
We present new HST NICMOS observations of NGC 4945, a starburst galaxy hosting a highly obscured active nucleus that is one of the brightest extragalactic sources at 100 keV. The HST data are complemented with ground based [FeII] line and mid--IR observations. A 100pc-scale starburst ring is detected in Pa alpha, while H_2 traces the walls of a super bubble opened by supernova-driven winds. The conically shaped cavity is particularly prominent in Pa alpha equivalent width and in the Pa alpha/H_2 ratio. Continuum images are heavily affected by dust extinction and the nucleus of the galaxy is located in a highly reddened region with an elongated, disk-like morphology. No manifestation of the active nucleus is found, neither a strong point source nor dilution in CO stellar features, which are expected tracers of AGN activity. Even if no AGN traces are detected in the near-IR, with the currently available data it is still not possible to establish whether the bolometric luminosity of the object is powered by the AGN or by the starburst: we demonstrate that the two scenarios constitute equally viable alternatives. However, the absence of any signature other than in the hard X-rays implies that, in both scenarios, the AGN is non-standard: if it dominates, it must be obscured in all directions, conversely, if the starburst dominates, the AGN must lack UV photons with respect to X-rays. An important conclusion is that powerful AGNs can be hidden even at mid-infrared wavelengths and, therefore, the nature of luminous dusty galaxies cannot be always characterized by long-wavelength data alone but must be complemented with sensitive hard X-ray observations.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We present new HST NICMOS observations of NGC 4945, a starburst galaxy hosting a highly obscured active nucleus that is one of the brightest extragalactic sources at 100 keV. | contribution 0.95 | 0.91 | 0.04 | asserted 1.00 |
| s1 | The HST data are complemented with ground based [FeII] line and mid--IR observations. | method 0.99 | 0.62 | 0.01 | asserted 0.98 |
| s2 | A 100pc-scale starburst ring is detected in Pa alpha, while H_2 traces the walls of a super bubble opened by supernova-driven winds. | result 1.00 | 0.90 | 0.03 | asserted 1.00 |
| s3 | The conically shaped cavity is particularly prominent in Pa alpha equivalent width and in the Pa alpha/H_2 ratio. | result 1.00 | 0.89 | 0.03 | asserted 1.00 |
| s4 | Continuum images are heavily affected by dust extinction and the nucleus of the galaxy is located in a highly reddened region with an elongated, disk-like morphology. | result 0.99 | 0.85 | 0.03 | asserted 1.00 |
| s5 | No manifestation of the active nucleus is found, neither a strong point source nor dilution in CO stellar features, which are expected tracers of AGN activity. | result 1.00 | 0.94 | 0.12 | asserted 1.00 |
| s6 | Even if no AGN traces are detected in the near-IR, with the currently available data it is still not possible to establish whether the bolometric luminosity of the object is powered by the AGN or by the starburst: we demonstrate that the two scenarios constitute equally viable alternatives. | interpretation 0.40 | 0.86 | 0.23 | asserted 0.81 |
| s7 | However, the absence of any signature other than in the hard X-rays implies that, in both scenarios, the AGN is non-standard: if it dominates, it must be obscured in all directions, conversely, if the starburst dominates, the AGN must lack UV photons with respect to X-rays. | interpretation 1.00 | 0.78 | 0.19 | asserted 0.78 |
| s8 | An important conclusion is that powerful AGNs can be hidden even at mid-infrared wavelengths and, therefore, the nature of luminous dusty galaxies cannot be always characterized by long-wavelength data alone but must be complemented with sensitive hard X-ray observations. | interpretation 0.98 | 0.60 | 0.80 | asserted 0.95 |
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Original abstract
We present new HST NICMOS observations of NGC 4945, a starburst galaxy hosting a highly obscured active nucleus that is one of the brightest extragalactic sources at 100 keV. The HST data are complemented with ground based [FeII] line and mid--IR observations. A 100pc-scale starburst ring is detected in Pa alpha, while H_2 traces the walls of a super bubble opened by supernova-driven winds. The conically shaped cavity is particularly prominent in Pa alpha equivalent width and in the Pa alpha/H_2 ratio. Continuum images are heavily affected by dust extinction and the nucleus of the galaxy is located in a highly reddened region with an elongated, disk-like morphology. No manifestation of the active nucleus is found, neither a strong point source nor dilution in CO stellar features, which are expected tracers of AGN activity. Even if no AGN traces are detected in the near-IR, with the currently available data it is still not possible to establish whether the bolometric luminosity of the object is powered by the AGN or by the starburst: we demonstrate that the two scenarios constitute equally viable alternatives. However, the absence of any signature other than in the hard X-rays implies that, in both scenarios, the AGN is non-standard: if it dominates, it must be obscured in all directions, conversely, if the starburst dominates, the AGN must lack UV photons with respect to X-rays. An important conclusion is that powerful AGNs can be hidden even at mid-infrared wavelengths and, therefore, the nature of luminous dusty galaxies cannot be always characterized by long-wavelength data alone but must be complemented with sensitive hard X-ray observations.
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It is also one of the nearest galaxies known to host both an AGN and active starburst regions in coexistence [ref] .
When the Shadow Meets Its Measure: Assessing the Feasibility of Submillimeter Black Hole Shadow Imaging in Megamaser Disk AGN · NGC 4945 · 0.85
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