Ca ii triplet spectroscopy of RGB stars in NGC 6822: kinematics and metallicities

Swan, Jesse; Cole, A. A.; Tolstoy, Eline; Irwin, M. J.

doi:10.1093/mnras/stv2774

Cited by 15 publications

(18 citation statements)

References 68 publications

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“…That is, metal-rich populations are more centrally concentrated. Such a relation was predicted by Swan et al (2016) and can be seen in the young versus old stellar population distributions of de Blok & Walter (2006).…”

Section: Gm R Rsupporting

confidence: 52%

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NGC 6822 as a Probe of Dwarf Galactic Evolution*

Belland

Kirby

Boylan-Kolchin

et al. 2020

ApJ

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NGC 6822 is the closest isolated dwarf irregular galaxy to the Milky Way. Its proximity and stellar mass (M 10 8  , large for a dwarf galaxy) allow for a detailed study of its kinematic properties. The red giant branch (RGB) stars at the galaxy's center are particularly interesting because they are aligned on an axis perpendicular to the galaxy's more extended H I disk. We detected a velocity gradient among the RGB population using spectra from Keck/ Deep Imaging Multi-Object Spectrograph (DEIMOS). This rotation is aligned with the H I disk, but the sense of rotation is about the major axis of the central RGB population. We measured the rotation velocity (v) and velocity dispersion (σ) of the RGB population in five metallicity bins. We found an increase of rotation support (v/σ) with increasing metallicity, driven primarily by decreasing dispersion. We also deduced an increasing radial distance for lower metallicity stars at −0.5kpc dex −1 by relating the observed stellar kinematics to position via NGC 6822ʼs H I velocity curve. While the inverted metallicity gradient-like distribution could be interpreted as evidence for an outside-in formation scenario, it may instead indicate that stellar feedback disturbed a centrally star-forming galaxy over time.

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Section: Gm R Rsupporting

confidence: 52%

“…To understand Figure 3 and the kinematics of NGC 6822 in more detail, we split the rotation support into contributions from rotation and from dispersion in Figure 4. Dispersion notably decreases with increasing metallicity, as noted by Swan et al (2016). That study separated the galaxy into two populations: metal-rich and metal-poor.…”

Section: Discussionmentioning

confidence: 52%

NGC 6822 as a Probe of Dwarf Galactic Evolution*

Belland

Kirby

Boylan-Kolchin

et al. 2020

ApJ

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“…Two dIrrs with detailed studies are WLM (Leaman et al 2012(Leaman et al , 2013 and NGC6822 (Swan et al 2016), which have stellar masses of 4.5×10 7 M ☉ and 1.7×10 8 M ☉ (Woo et al 2008), both considerably more massive than the dIrrs studied in our work. Leaman et al (2012) presented a rich dynamical data set for WLM.…”

Section: Summary and Discussionmentioning

confidence: 63%

Chemistry and Kinematics of the Late-Forming Dwarf Irregular Galaxies Leo A, Aquarius, and Sagittarius Dig*

Kirby

Rizzi

Held

et al. 2016

ApJ

100

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We present Keck/DEIMOS spectroscopy of individual stars in the relatively isolated Local Group dwarf galaxies LeoA, Aquarius, and the Sagittarius dwarf irregular galaxy. The three galaxies-but especially LeoA and Aquarius-share in common delayed star formation histories (SFHs) relative to many other isolated dwarf galaxies. The stars in all three galaxies are supported by dispersion. We found no evidence of stellar velocity structure, even for Aquarius, which has rotating H I gas. The velocity dispersions indicate that all three galaxies are dark-matter-dominated, with dark-to-baryonic mass ratios ranging from -+ 4.4 0.8 1.0 (SagDIG) to -+ 9.6 1.8 2.5 (Aquarius). LeoA and SagDIG have lower stellar metallicities than Aquarius, and they also have higher gas fractions, both of which would be expected if Aquarius were further along in its chemical evolution. The metallicity distribution of LeoA is inconsistent with a closed or leaky box model of chemical evolution, suggesting that the galaxy was preenriched or acquired external gas during star formation. The metallicities of stars increased steadily for all three galaxies, but possibly at different rates. The [α/Fe] ratios at a given [Fe/H] are lower than that of the Sculptor dwarf spheroidal galaxy, which indicates more extended SFHs than Sculptor, consistent with photometrically derived SFHs. Overall, the bulk kinematic and chemical properties for the late-forming dwarf galaxies do not diverge significantly from those of less delayed dwarf galaxies, including dwarf spheroidal galaxies.

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“…In Fornax, four out of five GCs fall in this range, and in NGC 147 this is true for four of the five bright GCs for which we have spectroscopic metallicities. In NGC 6822 we only have data for two GCs, but again both of these fall below the peak of the field star metallicity distribution at [Fe/H] = −0.84 (Swan et al 2016). Nevertheless, it may be notable that none of the GCs for which we have obtained accurate metallicity determinations until now have metallicities below [Fe/H] ≈ −2.5.…”

Section: Metallicitiesmentioning

confidence: 99%

Detailed abundance analysis of globular clusters in the Local Group

Larsen

Brodie

Wasserman

et al. 2018

A&A

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Context. Globular clusters (GCs) are emerging as powerful tracers of the chemical composition of extragalactic stellar populations. Aims. We present new abundance measurements for eleven GCs in the Local Group galaxies NGC 147, NGC 6822, and Messier 33. These are combined with previously published observations of four GCs in the Fornax and WLM galaxies. Methods. The abundances were determined from analysis of integrated-light spectra, obtained with the HIRES spectrograph on the Keck I telescope and with UVES on the VLT. We used our analysis technique that has been developed for this purpose and tested on Milky Way GCs. Results. We find that the clusters with [Fe/H] < −1.5 are all α-enhanced at about the same level as Milky Way GCs. Their Na abundances are also generally enhanced relative to Milky Way halo stars, suggesting that these extragalactic GCs resemble their Milky Way counterparts in containing significant fractions of Na-rich stars. For [Fe/H] > −1.5, the GCs in M33 are also α-enhanced, while the GCs that belong to dwarfs (NGC 6822 SC7 and Fornax 4) have closer to Solar-scaled α-element abundances. The abundance patterns in SC7 are remarkably similar to those in the Galactic GC Ruprecht 106, including significantly sub-solar [Na/Fe] and [Ni/Fe] ratios. In NGC 147, the GCs with [Fe/H] < −2.0 account for about 6% of the total luminosity of stars in the same metallicity range, a lower fraction than those previously found in the Fornax and WLM galaxies, but substantially higher than in the Milky Way halo. Conclusions. At low metallicities, the abundance patterns suggest that GCs in the Milky Way, dwarf galaxies, and M33 experienced similar enrichment histories and/or processes. At higher metallicities, the lower levels of α-enhancement in the GCs found in dwarf galaxies resemble the abundance patterns observed in field stars in nearby dwarfs. Constraining the presence of multiple populations in the GCs is complicated by the lack of information about detailed abundances in field stars of the corresponding metallicities. We suggest that correlations such as [Na/Fe] vs. [Ni/Fe] may prove useful for this purpose if an accuracy of ∼ 0.1 dex or better can be reached for the integrated-light measurements.

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Ca ii triplet spectroscopy of RGB stars in NGC 6822: kinematics and metallicities

Cited by 15 publications

References 68 publications

NGC 6822 as a Probe of Dwarf Galactic Evolution*

NGC 6822 as a Probe of Dwarf Galactic Evolution*

Chemistry and Kinematics of the Late-Forming Dwarf Irregular Galaxies Leo A, Aquarius, and Sagittarius Dig*

Detailed abundance analysis of globular clusters in the Local Group

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