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Title: Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina

Abstract

Here, we present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the Carina II (Car II) and Carina III (Car III) dwarf galaxy candidates, recently discovered in the Magellanic Satellites Survey (MagLiteS). We identify 18 member stars in Car II, including two binaries with variable radial velocities and two RR Lyrae stars. The other 14 members have a mean heliocentric velocity $${v}_{\mathrm{hel}}=477.2\pm 1.2$$ $$\mathrm{km}\,{{\rm{s}}}^{-1}$$ and a velocity dispersion of $${\sigma }_{v}={3.4}_{-0.8}^{+1.2}$$ $$\mathrm{km}\,{{\rm{s}}}^{-1}$$. Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of $${1.0}_{-0.4}^{+0.8}\times {10}^{6}$$ $${M}_{\odot }$$, indicating a mass-to-light ratio of $${369}_{-161}^{+309}$$ $${M}_{\odot }$$/$${L}_{\odot }$$. From equivalent width measurements of the calcium triplet lines of nine red giant branch (RGB) stars, we derive a mean metallicity of $${\rm{[Fe/H]}}=-2.44\pm 0.09$$ with dispersion $${\sigma }_{{\rm{[Fe/H]}}}={0.22}_{-0.07}^{+0.10}$$. Considering both the kinematic and chemical properties, we conclude that Car II is a dark-matter-dominated dwarf galaxy. For Car III, we identify four member stars, from which we calculate a systemic velocity of $${v}_{\mathrm{hel}}={284.6}_{-3.1}^{+3.4}$$ $$\mathrm{km}\,{{\rm{s}}}^{-1}$$. The brightest RGB member of Car III has a metallicity of $${\rm{[Fe/H]}}\,=-1.97\pm 0.12$$. Due to the small size of the Car III spectroscopic sample, we cannot conclusively determine its nature. Although these two systems have the smallest known physical separation ($${\rm{\Delta }}d\sim 10\,\mathrm{kpc}$$) among Local Group satellites, the large difference in their systemic velocities, $$\sim 200\,\mathrm{km}\,{{\rm{s}}}^{-1}$$, indicates that they are unlikely to be a bound pair. One or both systems are likely associated with the Large Magellanic Cloud (LMC), and may remain LMC satellites today. No statistically significant excess of γ-ray emission is found at the locations of Car II and Car III in eight years of Fermi-LAT data.

Authors:
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Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States); Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE; National Aeronautic and Space Administration (NASA)
Contributing Org.:
MagLiteS Collaboration
OSTI Identifier:
1438564
Alternate Identifier(s):
OSTI ID: 1439046
Report Number(s):
arXiv:1802.06810; FERMILAB-PUB-18-110-AE-PPD
Journal ID: ISSN 1538-4357
Grant/Contract Number:
AC02-07CH11359; AC02-76SF00515
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 857; Journal Issue: 2; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; 47 OTHER INSTRUMENTATION; dark matter; galaxies: dwarf; galaxies: individual (Carina II, Carina III); galaxies: stellar content; Local Group; stars: abundances

Citation Formats

Li, T. S., Simon, J. D., Pace, A. B., Torrealba, G., Kuehn, K., Drlica-Wagner, A., Bechtol, K., Vivas, A. K., Marel, R. P. van der, Wood, M., Yanny, B., Belokurov, V., Jethwa, P., Zucker, D. B., Lewis, G., Kron, R., Nidever, D. L., Sánchez-Conde, M. A., Ji, A. P., Conn, B. C., James, D. J., Martin, N. F., Martinez-Delgado, D., and Noël, N. E. D.. Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina. United States: N. p., 2018. Web. doi:10.3847/1538-4357/aab666.
Li, T. S., Simon, J. D., Pace, A. B., Torrealba, G., Kuehn, K., Drlica-Wagner, A., Bechtol, K., Vivas, A. K., Marel, R. P. van der, Wood, M., Yanny, B., Belokurov, V., Jethwa, P., Zucker, D. B., Lewis, G., Kron, R., Nidever, D. L., Sánchez-Conde, M. A., Ji, A. P., Conn, B. C., James, D. J., Martin, N. F., Martinez-Delgado, D., & Noël, N. E. D.. Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina. United States. doi:10.3847/1538-4357/aab666.
Li, T. S., Simon, J. D., Pace, A. B., Torrealba, G., Kuehn, K., Drlica-Wagner, A., Bechtol, K., Vivas, A. K., Marel, R. P. van der, Wood, M., Yanny, B., Belokurov, V., Jethwa, P., Zucker, D. B., Lewis, G., Kron, R., Nidever, D. L., Sánchez-Conde, M. A., Ji, A. P., Conn, B. C., James, D. J., Martin, N. F., Martinez-Delgado, D., and Noël, N. E. D.. Wed . "Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina". United States. doi:10.3847/1538-4357/aab666.
@article{osti_1438564,
title = {Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina},
author = {Li, T. S. and Simon, J. D. and Pace, A. B. and Torrealba, G. and Kuehn, K. and Drlica-Wagner, A. and Bechtol, K. and Vivas, A. K. and Marel, R. P. van der and Wood, M. and Yanny, B. and Belokurov, V. and Jethwa, P. and Zucker, D. B. and Lewis, G. and Kron, R. and Nidever, D. L. and Sánchez-Conde, M. A. and Ji, A. P. and Conn, B. C. and James, D. J. and Martin, N. F. and Martinez-Delgado, D. and Noël, N. E. D.},
abstractNote = {Here, we present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the Carina II (Car II) and Carina III (Car III) dwarf galaxy candidates, recently discovered in the Magellanic Satellites Survey (MagLiteS). We identify 18 member stars in Car II, including two binaries with variable radial velocities and two RR Lyrae stars. The other 14 members have a mean heliocentric velocity ${v}_{\mathrm{hel}}=477.2\pm 1.2$ $\mathrm{km}\,{{\rm{s}}}^{-1}$ and a velocity dispersion of ${\sigma }_{v}={3.4}_{-0.8}^{+1.2}$ $\mathrm{km}\,{{\rm{s}}}^{-1}$. Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of ${1.0}_{-0.4}^{+0.8}\times {10}^{6}$ ${M}_{\odot }$, indicating a mass-to-light ratio of ${369}_{-161}^{+309}$ ${M}_{\odot }$/${L}_{\odot }$. From equivalent width measurements of the calcium triplet lines of nine red giant branch (RGB) stars, we derive a mean metallicity of ${\rm{[Fe/H]}}=-2.44\pm 0.09$ with dispersion ${\sigma }_{{\rm{[Fe/H]}}}={0.22}_{-0.07}^{+0.10}$. Considering both the kinematic and chemical properties, we conclude that Car II is a dark-matter-dominated dwarf galaxy. For Car III, we identify four member stars, from which we calculate a systemic velocity of ${v}_{\mathrm{hel}}={284.6}_{-3.1}^{+3.4}$ $\mathrm{km}\,{{\rm{s}}}^{-1}$. The brightest RGB member of Car III has a metallicity of ${\rm{[Fe/H]}}\,=-1.97\pm 0.12$. Due to the small size of the Car III spectroscopic sample, we cannot conclusively determine its nature. Although these two systems have the smallest known physical separation (${\rm{\Delta }}d\sim 10\,\mathrm{kpc}$) among Local Group satellites, the large difference in their systemic velocities, $\sim 200\,\mathrm{km}\,{{\rm{s}}}^{-1}$, indicates that they are unlikely to be a bound pair. One or both systems are likely associated with the Large Magellanic Cloud (LMC), and may remain LMC satellites today. No statistically significant excess of γ-ray emission is found at the locations of Car II and Car III in eight years of Fermi-LAT data.},
doi = {10.3847/1538-4357/aab666},
journal = {The Astrophysical Journal (Online)},
number = 2,
volume = 857,
place = {United States},
year = {Wed Apr 25 00:00:00 EDT 2018},
month = {Wed Apr 25 00:00:00 EDT 2018}
}

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