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New Spitzer Results for Neon and Sulphur in
NGC 6822
Reggie Dufour
AU 10/07/2009
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Neon and Sulphur Emission Lines in H II Regions
Optical: [Ne III] 3869, 3969 A
[S II] 6717, 6730 A
[S III] 6311, 9069, 9531 A
(compare against H, H, etc.)
InfraRed: [Ne II] 12.81 m
[Ne III] 15.56 m
[S III] 18.71 m
[S IV] 10.51 m
(compare against H(7-6) 12.37 m)
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MNRAS, 377, 1407 (2007)
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Lebouteiller et al. 2008
Comparison Spitzer IR Observations
Wu et al. (2008)
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M83 HII Regions
M33 HII Regions
Blue Compact Galaxies
NGC 3603 HII Regions
LMC 30 Doradus
SMC N66
M33 fit
Orion
BCG fit
SMC-LMC fit
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M83 & M33 papers Summary:
Ne & S in Extragalactic HII Regions
1. The radial abundance gradients in Ne & S in M33 and M83
are similar to those of O => Ne/S, Ne/O, S/O are constant
across the disks, as expected from massive star nucleosynthesis models.
2. The average Ne/S ratio in the M33 H II regions is 16.9, which
is similar to that in the Orion Nebula (14.3), NGC 3603 (14.6),
30 Dor (11.4), SMC N66 (10.1), and BCDs (14.0).
3. The nebular Ne/S ratios found for H II regions (10-17) is
significantly higher than recent determinations of the solar
value (~5) and somewhat higher than recent nucleosynthesis
and GCE models (~9).
4. Our ionic abundance ratios of Ne+, Ne++, S++, & S+3 were
used to test the SEDs of various stellar atmospheric models
and we found that the supergiant atmospheric models of
Pauldrach et al. (2001) gave the best fits to the observations.
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*NEW* Spitzer IRS Observations of NGC 6822 (Cycle 4 program)
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Why NGC 6822?
Metal-Poor Im irregular galaxy -- Z ≈ 0.25Zsun
Close -- distance = 490 ± 40 pc (Mateo 1998)
Local Group Member (between SMC & LMC in properties)
Many Recent Studies: (11+ since 1999)
-- star formation processes & history
-- kinematics & dark matter distribution
-- ISM - HI & HII
-- Abundances - A stars, HII regions, planetary nebulae
-- a Reggie “pet galaxy”
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Wyder (2003)
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Cannon et al. (2006)
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De Blok & Walter
(2006)
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Lee, Skillman & Venn (2006)
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KD 5e
KD 28e
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NGC 6822 HII Regions & Photoionization SEDs
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Preliminary NGC 6822 Results
Fig. 1ab: Plot of Ne/S vs. Ne++/Ne+ [left panel] and S3+/S++ [right panel]. Our prior M33 results are shown as black stars for the 22
sources where we detected all four lines. The solid line is the linear least-squares fit to the stars with a gradient that is not statistically
significant. The results from our prior M83 study are shown as circles. No line fit is done. These data demonstrate a huge variation in
the inferred Ne/S ratio at low ionization. The orange squares show the Wu et al. (2008) data for blue compact dwarf galaxies, as
reanalyzed with our program. We show only 9 points, those objects where they actually detected all four lines: [S IV], [Ne II], [Ne III],
and [S III]. The dotted line is the linear least-squares fit to the squares with a statistically significant negative slope (3.8 σ). The median
Ne/S for the 9 galaxies is 14.0 very close to the Orion value of 14.3 (Simpson et al. 2004) shown as the dashed line. The Lebouteiller
et al. (2008) data were also reanalyzed and are presented as follows: NGC 3603 (red asterisks), 30 Dor (green triangles), and N 66
(blue diamonds). The median Ne/S ratios for each are 14.6, 11.4, and 10.1, respectively, possibly indicating a decreasing trend with
lower metallicity. The dash-dot line is a fit to the 23 points in 30 Dor and N 66 that show the highest ionization. The 4-sided violet
stars show our new Spitzer preliminary results for NGC 6822. Only five H II regions labeled in black with their KD number (Killen
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Dufour 1982), have all four lines measured. Our remeasure of the four SINGS giant H II regions are labeled in violet.
Summary: Spitzer Observations of Ne & S in H II Regions of
NGC 6822
1. The radial abundance gradients in Ne & S in M33 and M83
are similar to those of O => Ne/S, Ne/O, S/O are constant
across the disks, as expected from massive star nucleosynthesis models. Ditto for the HII Regions in NGC6822.
2. The average Ne/S ratio in the M33 H II regions is 16.9, which
is similar to that in the Orion Nebula (14.3), NGC 3603 (14.6),
30 Dor (11.4), SMC N66 (10.1), and BCDs (14.0).
For NGC 6822 we get Ne/S ~9.5.
3. The nebular Ne/S ratios found for most extragalactic HII regions
(10-17) is significantly higher than recent determinations of the
solar value (~5) and somewhat higher than recent
nucleosynthesis and GCE models (~9). Except for NGC 6822?
4. Our ionic abundance ratios of Ne+, Ne++, S++, & S+3 were
used to test the SEDs of various stellar atmospheric models
and we found that the supergiant atmospheric models of
Pauldrach et al. (2001) gave the best fits to the observations.
This is true for NGC 6822 HII regions as well.
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The future: SOFIA (2009?)
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