ABSTRACT The CHemical Abundances of Spirals (CHAOS) project leverages the combined power of the Large Binocular Telescope (LBT) with the broad spectral range and sensitivity of the Multi Object ...Double Spectrograph (MODS) to measure "direct" abundances (based on observations of the temperature-sensitive auroral lines) in large samples of H ii regions in spiral galaxies. We present LBT MODS observations of 62 H ii regions in the nearby spiral galaxy NGC 628, with an unprecedentedly large number of auroral lines measurements (18 O iii λ4363, 29 N ii λ5755, 40 S iiiλ6312, and 40 O ii λλ7320, 7330 detections) in 45 H ii regions. Comparing derived temperatures from multiple auroral line measurements, we find: (1) a strong correlation between temperatures based on S iii λ6312 and N ii λ5755; and (2) large discrepancies for some temperatures based on O ii λλ7320, 7330 and O iii λ4363. Both of these trends are consistent with other observations in the literature, yet, given the widespread use and acceptance of O iii λ4363 as a temperature determinant, the magnitude of the TO iii discrepancies still came as a surprise. Based on these results, we conduct a uniform abundance analysis prioritizing the temperatures derived from S iii λ6312 and N ii λ5755, and report the gas-phase abundance gradients for NGC 628. Relative abundances of S/O, Ne/O, and Ar/O are constant across the galaxy, consistent with no systematic change in the upper IMF over the sampled range in metallicity. These alpha-element ratios, along with N/O, all show small dispersions ( ∼ 0.1 dex) over 70% of the azimuthally averaged radius. We interpret these results as an indication that, at a given radius, the interstellar medium in NGC 628 is chemically well-mixed. Unlike the gradients in the nearly temperature-independent relative abundances, O/H abundances have a larger intrinsic dispersion of ∼0.165 dex. We posit that this dispersion represents an upper limit to the true dispersion in O/H at a given radius and that some of that dispersion is due to systematic uncertainties arising from temperature measurements.
CHAOS. VI. Direct Abundances in NGC 2403 Rogers, Noah S. J.; Skillman, Evan D.; Pogge, Richard W. ...
The Astrophysical journal,
07/2021, Letnik:
915, Številka:
1
Journal Article
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Abstract
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 0.18–2.31. Thirty-two 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 a H
ii
region to obtain a weighted-average temperature within each ionization zone. We rederive 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 a recent study of Berg et al. For NGC 2403, we measure a direct oxygen abundance gradient of −0.09(±0.03) dex/
R
e
, with an intrinsic dispersion of 0.037(±0.017) dex and a N/O abundance gradient of −0.17(±0.03) dex/
R
e
with an intrinsic dispersion of 0.060(±0.018) dex. For direct comparison, we use the line intensities from an earlier study of NGC 2403 by Berg et al. and find their recomputed values for the O/H and N/O gradients are consistent with ours.
Abstract
Currently, the primordial helium abundance is best estimated
through spectroscopic observations of H II regions in
metal-poor galaxies. However these determinations are limited by
several ...systematic uncertainties which ultimately limit our ability
to accurately ascertain the primordial abundance. In this study, we
improve the methodologies for solving for the reddening, the
emission contributions from collisional excitation of the H I
atoms, the effects underlying absorption in the H I and
He I emission lines, and the treatment of the blended H I and He I emission at λ3889 with the aim of
lowering the systematic uncertainties in helium abundance
determinations. To apply these methods, we have obtained
observations of the He I λ10830 emission line in the
brightest H II region in the extremely metal-poor (3%
Z
⊙
) galaxy Leo P with the LUCI1 instrument on the LBT. We
combine this measurement with previous MODS/LBT observations to
derive an improved helium abundance. In doing so, our present
analysis results in a decrease in the uncertainty in the helium
abundance of Leo P by approximately 70%. This result is combined
with data from other observations to estimate the primordial helium
mass fraction, Y
p
= 0.2453 ± 0.0034.
Abstract
Observations of high-redshift galaxies (
z
> 5) have shown that these galaxies have extreme emission lines with equivalent widths much larger than their local star-forming counterparts. ...Extreme emission line galaxies (EELGs) in the nearby universe are likely analogs to galaxies during the Epoch of Reionization and provide nearby laboratories to understand the physical processes important to the early universe. We use Hubble Space Telescope/Cosmic Origins Spectrograph and Large Binocular Telescope/Multi-Object Double Spectrographs spectra to study two nearby EELGs, J104457 and J141851. The far-UV spectra indicate that these two galaxies contain stellar populations with ages ≲10 Myr and metallicities ≤0.15
Z
⊙
. We use photoionization modeling to compare emission lines from models of single-age bursts of star formation to observed emission lines and find that the single-age bursts do not reproduce high-ionization lines including O
iii
or very-high-ionization lines like He
ii
or O
iv
. Photoionization modeling using the stellar populations fit from the UV continuum similarly is not capable of reproducing the very-high-energy emission lines. We add a blackbody to the stellar populations fit from the UV continuum to model the necessary high-energy photons to reproduce the very-high-ionization lines of He
ii
and O
iv
. We find that we need a blackbody of 80,000 K and ∼45%–55% of the luminosity from the blackbody and young stellar population to reproduce the very-high-ionization lines while simultaneously reproducing the low-, intermediate-, and high-ionization emission lines. Our self-consistent model of the ionizing spectra of two nearby EELGs indicates the presence of a previously unaccounted-for source of hard ionizing photons in reionization analogs.
We have identified a sample of 41 low-mass high-oxygen abundance outliers from the mass-metallicity relation of star-forming galaxies measured by Tremonti et al. (
We present the analysis of a binary microlensing event, KMT-2016-BLG-2052, for which the lensing-induced brightening of the source star lasted for two seasons. We determine the lens mass from the ...combined measurements of the microlens parallax, πE, and angular Einstein radius, θE. The measured mass indicates that the lens is a binary composed of M dwarfs with masses of M1 ∼ 0.34 M and M2 ∼ 0.17 M . The measured relative lens-source proper motion of ∼ 3.9 mas yr−1 is smaller than ∼5 mas yr−1 of typical Galactic lensing events, while the estimated angular Einstein radius of θE ∼ 1.2 mas is substantially greater than the typical value of ∼0.5 mas. Therefore, it turns out that the long timescale of the event is caused by the combination of the slow and large θE rather than the heavy mass of the lens. From the simulation of Galactic lensing events with very long timescales (tE 100 days), we find that the probabilities that long timescale events are produced by lenses with masses ≥1.0 M and ≥3.0 M are ∼19% and 2.6%, respectively, indicating that events produced by heavy lenses comprise a minor fraction of long timescale events. The results indicate that it is essential to determine lens masses by measuring both πE and θE in order to firmly identify heavy stellar remnants, such as neutron stars and black holes.
We present new observations leading to an improved black hole mass estimate for the Seyfert 1 galaxy NGC 4593 as part of a reverberation-mapping campaign conducted at the MDM Observatory. ...Cross-correlation analysis of the Hb emission-line light curve with the optical continuum light curve reveals an emission-line time delay of t sub(cent) = 3.73 c 0.75 days. By combining this time delay with the Hb line width, we derive a central black hole mass of M sub(BH) = (9.8 c 2.1) x 10 super(6) M sub( ), an improvement in precision of a factor of several over past results.
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