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Register a new userCHAOS V: Recombination Line Carbon Abundances in M101
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The CHAOS project is building a large database of LBT H II region spectra in nearby spiral galaxies to use direct abundances to better determine the dispersion in metallicity as a function of galactic radius. Here, we present CHAOS LBT observations of C II $lambda$4267 emission detected in 10 H II regions in M 101, and, using a new photoionization model based ionization correction factor, we convert these measurements into total carbon abundances. A comparison with M101 C II recombination line observations from the literature shows excellent agreement, and we measure a relatively steep gradient in log(C/H) of -0.37 +/- 0.06 dex/R_e. The C/N observations are consistent with a constant value of log(C/N) = 0.84 with a dispersion of only 0.09 dex, which, given the different nucleosynthetic sources of C and N, is challenging to understand. We also note that when plotting N/O versus O/H, all of the H II regions with detections of CII $lambda$4267 present N/O abundances at the minimum of the scatter in N/O at a given value of O/H. If the high surface brightness necessary for the detection of the faint recombination lines is interpreted as an indicator of H II region youth, then this may point to a lack of nitrogen pollution in the youngest H II regions. In the future, we anticipate that the CHAOS project will significantly increase the total number of C II $lambda$4267 measurements in extragalactic H II regions.
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Recombination lines (RLs) of C II, N II, and O II in planetary nebulae (PNs) have been found to give abundances that are much larger in some cases than abundances from collisionally-excited forbidden lines (CELs). The origins of this abundance discrepancy are highly debated. We present new spectroscopic observations of O II and C II recombination lines for six planetary nebulae. With these data we compare the abundances derived from the optical recombination lines with those determined from collisionally-excited lines. Combining our new data with published results on RLs in other PNs, we examine the discrepancy in abundances derived from RLs and CELs. We find that there is a wide range in the measured abundance discrepancy Delta(O+2) = log O+2(RL) - log O+2(CEL), ranging from approximately 0.1 dex up to 1.4 dex. Most RLs yield similar abundances, with the notable exception of O II multiplet V15, known to arise primarily from dielectronic recombination, which gives abundances averaging 0.6 dex higher than other O II RLs. We compare Delta(O+2) against a variety of physical properties of the PNs to look for clues as to the mechanism responsible for the abundance discrepancy. The strongest correlations are found with the nebula diameter and the Balmer surface brightness. An inverse correlation of Delta(O+2) with nebular density is also seen. Similar results are found for carbon in comparing C II RL abundances with ultraviolet measurements of C III].
The CHemical Abundances of Spirals (CHAOS) project leverages the combined power of the Large Binocular Telescope with the broad spectral range and sensitivity of the Multi Object Double Spectrograph (MODS) to measure direct abundances in large samples of HII regions in spiral galaxies. We present LBT MODS observations of 109 Hii regions in NGC5457, of which 74 have robust measurements of key auroral lines, a factor of 3 larger than all previous published detections of auroral lines in the HII regions of NGC5457. Comparing the temperatures derived from the different ionic species we find: (1) strong correlations of T[NII] with T[SIII] and T[OIII], consistent with little or no intrinsic scatter; (2) a correlation of T[SIII] with T[OIII], but with significant intrinsic dispersion; (3) overall agreement between T[NII], T[SII], and T[OII], as expected, but with significant outliers; (4) the correlations of T[NII] with T[SIII] and T[OIII] match the predictions of photoionization modeling while the correlation of T[SIII] with T[OIII] is offset from the prediction of photoionization modeling. Based on these observations, which include significantly more observations of lower excitation HII regions, missing in many analyses, we inspect the commonly used ionization correction factors (ICFs) for unobserved ionic species and propose new empirical ICFs for S and Ar. We have discovered an unexpected population of HII regions with a significant offset to low values in Ne/O, which defies explanation. We derive radial gradients in O/H and N/O which agree with previous studies. Our large observational database allows us to examine the dispersion in abundances, and we find intrinsic dispersions of 0.074 in O/H and 0.095 in N/O (at a given radius). We stress that this measurement of the intrinsic dispersion comes exclusively from direct measurements of HII regions in NGC5457.
We report the direct abundances for the galaxy NGC 2403 as observed by the CHemical Abundances Of Spirals (CHAOS) project. Using the Multi-Object Double Spectrograph on the Large Binocular Telescope, we observe two fields with H II regions that cover an R$_g$/R$_e$ range of 0.18 to 2.31. 32 H II regions contain at least one auroral line detection, and we detect a total of 122 temperature-sensitive auroral lines. Here, for the first time, we use the intrinsic scatter in the T$_e$-T$_e$ diagrams, added in quadrature to the uncertainty on the measured temperature, to determine the uncertainty on an electron temperature inferred for one ionization zone from a measurement in a different ionization zone. We then use all available temperature data within an H II region to obtain a weighted average temperature within each ionization zone. We re-derive the oxygen abundances of all CHAOS galaxies using this new temperature prioritization method, and we find that the gradients are consistent with the results of Berg et al. (2020). For NGC 2403, we measure a direct oxygen abundance gradient of -0.09($pm$0.03) dex/Re, with an intrinsic dispersion of 0.037($pm$0.017) dex, and an N/O abundance gradient of -0.17($pm$0.03) dex/Re with an intrinsic dispersion of 0.060($pm$0.018) dex. For direct comparison, we use the line intensities from the study of NGC 2403 by Berg et al. (2013), and find their recomputed values for the O/H and N/O gradients are consistent with ours.
Carbon radio recombination lines (RRLs) at low frequencies (<=500 MHz) trace the cold, diffuse phase of the interstellar medium, which is otherwise difficult to observe. We present the detection of carbon RRLs in absorption in M82 with LOFAR in the frequency range of 48-64 MHz. This is the first extragalactic detection of RRLs from a species other than hydrogen, and below 1 GHz. Since the carbon RRLs are not detected individually, we cross-correlated the observed spectrum with a template spectrum of carbon RRLs to determine a radial velocity of 219 +- 9 km/s . Using this radial velocity, we stack 22 carbon-{alpha} transitions from quantum levels n = 468-508 to achieve an 8.5 sigma detection. The absorption line profile exhibits a narrow feature with peak optical depth of 0.003 and FWHM of 31 km/s. Closer inspection suggests that the narrow feature is superimposed on a broad, shallow component. The total line profile appears to be correlated with the 21 cm H I line profile reconstructed from H I absorption in the direction of supernova remnants in the nucleus. The narrow width and centroid velocity of the feature suggests that it is associated with the nuclear starburst region. It is therefore likely that the carbon RRLs are associated with cold atomic gas in the direction of the nucleus of M82.
We present new Jansky Very Large Array (VLA) images of the central region of the W49A star-forming region at 3.6~cm and at 7~mm at resolutions of 0farcs15 (1650 au) and 0farcs04 (440 au), respectively. The 3.6~cm data reveal new morphological detail in the ultracompact ion{H}{2} region population, as well as several previously unknown and unresolved sources. In particular, source A shows elongated, edge-brightened, bipolar lobes, indicative of a collimated outflow, and source E is resolved into three spherical components. We also present VLA observations of radio recombination lines at 3.6~cm and 7~mm, and IRAM Northern Extended Millimeter Array (NOEMA) observations at 1.2~mm. Three of the smallest ultracompact ion{H}{2} regions (sources A, B2 and G2) all show broad kinematic linewidths, with $Delta$V$_{FWHM}gtrsim$40~km~s$^{-1}$. A multi-line analysis indicates that broad linewidths remain after correcting for pressure broadening effects, suggesting the presence of supersonic flows. Substantial changes in linewidth over the 21 year time baseline at both 3.6 cm and 7 mm are found for source G2. At 3.6 cm, the linewidth of G2 changed from 31.7$pm$1.8 km s$^{-1}$ to 55.6$pm$2.7 km s$^{-1}$, an increase of $+$23.9$pm$3.4 km s$^{-1}$. The G2 source was previously reported to have shown a 3.6~cm continuum flux density decrease of 40% between 1994 and 2015. This source sits near the center of a very young bipolar outflow whose variability may have produced these changes.
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