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Register a new userX-ray emission from the nuclear region of Arp 220
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We present an imaging and spectral analysis of the nuclear region of the ULIRG merger Arp 220, using deep textit{Chandra}-ACIS observations summing up to (sim 300mbox{ ks}). Narrow-band imaging with sub-pixel resolution of the innermost nuclear region reveals two distinct Fe-K emitting sources, coincident with the infrared and radio nuclear clusters. These sources are separated by 1 ((sim 380) pc). The X-ray emission is extended and elongated in the eastern nucleus, like the disk emission observed in millimeter radio images, suggesting starburst dominance in this region. We estimate Fe-K equivalent width (gtrsim 1) keV for both sources, and observed 2-10 keV luminosities (sim 2times{10}^{40}mbox{ erg}mbox{ s}^{-1}) (W) and (sim 3 times {10}^{40}mbox{ erg}mbox{ s}^{-1}) (E). In the 6-7 keV band the emission from these regions is dominated by the 6.7 keV Fe textsc{xxv} line, suggesting contribution from collisionally ionized gas. The thermal energy content of this gas is consistent with kinetic energy injection in the interstellar medium by Type II SNe. However, nuclear winds from hidden AGN ((varvsim 2000 mbox{ km}mbox{ s}^{-1})) cannot be excluded. The (3sigma) upper limits on the neutral Fe-K(alpha) flux of the nuclear regions correspond to intrinsic AGN 2-10 keV luminosities (< 1times {10}^{42}mbox{ erg}mbox{ s}^{-1}) (W) and (< 0.4times {10}^{42}mbox{ erg}mbox{ s}^{-1}) (E). For typical AGN SEDs the bolometric luminosities are (< 3times {10}^{43}mbox{ erg}mbox{ s}^{-1}) (W) and (< 8times {10}^{43}mbox{ erg}mbox{ s}^{-1}) (E), and black hole masses (<1times{10}^5 M_{astrosun}) (W) and (< 5times{10}^5 M_{astrosun}) (E) for Eddington limited AGNs with a standard 10% efficiency.
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Extragalactic cosmic ray populations are important diagnostic tools for tracking the distribution of energy in nuclei and for distinguishing between activity powered by star formation versus active galactic nuclei (AGNs). Here, we compare different diagnostics of the cosmic ray populations of the nuclei of Arp 220 based on radio synchrotron observations and the recent gamma-ray detection. We find the gamma-ray and radio emission to be incompatible; a joint solution requires at minimum a factor of 4 - 8 times more energy coming from supernovae and a factor of 40 - 70 more mass in molecular gas than is observed. We conclude that this excess of gamma-ray flux in comparison to all other diagnostics of star-forming activity indicates that there is an AGN present that is providing the extra cosmic rays, likely in the western nucleus.
We present a high spatial resolution optical and infrared study of the circumnuclear region in Arp 220, a late-stage galaxy merger. Narrowband imaging using HST/WFC3 has resolved the previously observed peak in H$alpha$+[NII] emission into a bubble-shaped feature. This feature measures 1.6 in diameter, or 600 pc, and is only 1 northwest of the western nucleus. The bubble is aligned with the western nucleus and the large-scale outflow axis seen in X-rays. We explore several possibilities for the bubble origin, including a jet or outflow from a hidden active galactic nucleus (AGN), outflows from high levels of star formation within the few hundred pc nuclear gas disk, or an ultraluminous X-ray source. An obscured AGN or high levels of star formation within the inner $sim$100 pc of the nuclei are favored based on the alignment of the bubble and energetics arguments.
We present an analysis of the diffuse soft X-ray emission from the nuclear region of M51 combining both XMM-Newton RGS and Chandra data. Most of the RGS spectrum of M51 can be fitted with a thermal model with a temperature of $sim0.5$ keV except for the OVII triplet, which is forbidden-line dominated. The Fe L-shell lines peak around the southern cloud, where the OVIII and NVII Lya lines also peak. In contrast, the peak of the OVII forbidden line is about 10$$ offset from that of the other lines, indicating that it is from a spatially distinct component. The spatial distribution of the OVII triplet mapped by the Chandra data shows that most of the OVII triplet flux is located at faint regions near edges, instead of the southern cloud where other lines peak. This distribution of the OVII triplet is inconsistent with the photoionization model. Other mechanisms that could produce the anomalous OVII triplet, including a recombining plasma and charge exchange X-ray emission, are discussed.
Recent analyses of the gamma-ray spectrum from the ultra-luminous infrared galaxy Arp 220 have revealed a discrepancy in the cosmic ray energy injection rates derived from the gamma-rays versus the radio emission. While the observed radio emission is consistent with the star formation rate inferred from infrared observations, a significantly higher cosmic ray population is necessary to accurately model the measured gamma-ray flux. To resolve this discrepancy between the radio and gamma-ray observations, we find that we must increase the cosmic ray energy injection rate and account for an infrared optical depth greater than unity. Raising the energy injection rate naturally raises the total gamma-ray flux but also raises the radio flux unless there is also an increase in the energy loss rate for cosmic ray leptons. A optically thick medium results in an increase in energy losses via inverse Compton for cosmic ray leptons and preserves agreement with submillimeter, millimeter, and infrared wavelength observations.
We present the first very-long-baseline interferometry (VLBI) detections of Zeeman splitting in another galaxy. We used Arecibo Observatory, the Green Bank Telescope, and the Very Long Baseline Array to perform dual-polarization observations of OH maser lines in the merging galaxy Arp 220. We measured magnetic fields of $sim$1-5 mG associated with three roughly parsec-sized clouds in the nuclear regions of Arp 220. Our measured magnetic fields have comparable strengths and the same direction as features at the same velocity identified in previous Zeeman observations with Arecibo alone. The agreement between single dish and VLBI results provides critical validation of previous Zeeman splitting observations of OH megamasers that used a single large dish. The measured magnetic field strengths indicate that magnetic energy densities are comparable to gravitational energy in OH maser clouds. We also compare our total intensity results to previously published VLBI observations of OH megamasers in Arp 220. We find evidence for changes in both structure and amplitude of the OH maser lines that are most easily explained by variability intrinsic to the masing region, rather than variability produced by interstellar scintillation. Our results demonstrate the potential for using high-sensitivity VLBI to study magnetic fields on small spatial scales in extragalactic systems.
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