Galactic cartography with SkyMapper - I. Population substructure and the stellar number density of the inner halo

Zhen Wan, Prajwal R. Kafle, Geraint F. Lewis, Dougal Mackey, Sanjib Sharma, Rodrigo A. Ibata

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

The stars within our Galactic halo presents a snapshot of its ongoing growth and evolution, probing galaxy formation directly. Here, we present our first analysis of the stellar halo from detailed maps of Blue Horizontal Branch (BHB) stars drawn from the SkyMapper Southern Sky Survey. To isolate candidate BHBstars from the overall population, we develop a machinelearning approach through the application of an Artificial Neural Network (ANN), resulting in a relatively pure sample of target stars. From this, we derive the absolute u magnitude for the BHB sample to be ~2 mag, varying slightly with (v - g)0 and (u - v)0 colours. We examine the BHB number density distribution from 5272 candidate stars, deriving a double power law with a break radius of rs = 11.8 ± 0.3 kpc, and inner and outer slopes of αin = -2.5 ± 0.1 and αout = -4.5 ± 0.3, respectively. Through isochrone fitting of simulated BHB stars, we find a colour-age/metallicity correlation, with older/more metal-poor stars being bluer, and establish a parameter to indicate this age (or metallicity) variation. Using this, we construct the three-dimensional population distribution of BHB stars in the halo and identify significant substructure. Finally, in agreement with previous studies, we also identify a systemic age/metallicity shift spanning ~3 kpc to ~ 20 kpc in Galactocentric distance.

Original languageEnglish
Pages (from-to)1218-1228
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Volume480
Issue number1
DOIs
Publication statusPublished - 18 Jul 2018

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cartography
substructures
horizontal branch stars
blue stars
halos
metallicity
stars
population distribution
Southern sky
color
artificial neural network
galactic halos
power law
galactic evolution
density distribution
metal
slopes
radii
shift
metals

Cite this

Wan, Zhen ; Kafle, Prajwal R. ; Lewis, Geraint F. ; Mackey, Dougal ; Sharma, Sanjib ; Ibata, Rodrigo A. / Galactic cartography with SkyMapper - I. Population substructure and the stellar number density of the inner halo. In: Monthly Notices of the Royal Astronomical Society. 2018 ; Vol. 480, No. 1. pp. 1218-1228.
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Galactic cartography with SkyMapper - I. Population substructure and the stellar number density of the inner halo. / Wan, Zhen; Kafle, Prajwal R.; Lewis, Geraint F.; Mackey, Dougal; Sharma, Sanjib; Ibata, Rodrigo A.

In: Monthly Notices of the Royal Astronomical Society, Vol. 480, No. 1, 18.07.2018, p. 1218-1228.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Galactic cartography with SkyMapper - I. Population substructure and the stellar number density of the inner halo

AU - Wan, Zhen

AU - Kafle, Prajwal R.

AU - Lewis, Geraint F.

AU - Mackey, Dougal

AU - Sharma, Sanjib

AU - Ibata, Rodrigo A.

PY - 2018/7/18

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N2 - The stars within our Galactic halo presents a snapshot of its ongoing growth and evolution, probing galaxy formation directly. Here, we present our first analysis of the stellar halo from detailed maps of Blue Horizontal Branch (BHB) stars drawn from the SkyMapper Southern Sky Survey. To isolate candidate BHBstars from the overall population, we develop a machinelearning approach through the application of an Artificial Neural Network (ANN), resulting in a relatively pure sample of target stars. From this, we derive the absolute u magnitude for the BHB sample to be ~2 mag, varying slightly with (v - g)0 and (u - v)0 colours. We examine the BHB number density distribution from 5272 candidate stars, deriving a double power law with a break radius of rs = 11.8 ± 0.3 kpc, and inner and outer slopes of αin = -2.5 ± 0.1 and αout = -4.5 ± 0.3, respectively. Through isochrone fitting of simulated BHB stars, we find a colour-age/metallicity correlation, with older/more metal-poor stars being bluer, and establish a parameter to indicate this age (or metallicity) variation. Using this, we construct the three-dimensional population distribution of BHB stars in the halo and identify significant substructure. Finally, in agreement with previous studies, we also identify a systemic age/metallicity shift spanning ~3 kpc to ~ 20 kpc in Galactocentric distance.

AB - The stars within our Galactic halo presents a snapshot of its ongoing growth and evolution, probing galaxy formation directly. Here, we present our first analysis of the stellar halo from detailed maps of Blue Horizontal Branch (BHB) stars drawn from the SkyMapper Southern Sky Survey. To isolate candidate BHBstars from the overall population, we develop a machinelearning approach through the application of an Artificial Neural Network (ANN), resulting in a relatively pure sample of target stars. From this, we derive the absolute u magnitude for the BHB sample to be ~2 mag, varying slightly with (v - g)0 and (u - v)0 colours. We examine the BHB number density distribution from 5272 candidate stars, deriving a double power law with a break radius of rs = 11.8 ± 0.3 kpc, and inner and outer slopes of αin = -2.5 ± 0.1 and αout = -4.5 ± 0.3, respectively. Through isochrone fitting of simulated BHB stars, we find a colour-age/metallicity correlation, with older/more metal-poor stars being bluer, and establish a parameter to indicate this age (or metallicity) variation. Using this, we construct the three-dimensional population distribution of BHB stars in the halo and identify significant substructure. Finally, in agreement with previous studies, we also identify a systemic age/metallicity shift spanning ~3 kpc to ~ 20 kpc in Galactocentric distance.

KW - Colour-Magnitude Diagrams

KW - Galaxies: Halo

KW - Star: Horizontal Branch

KW - Survey: SkyMapper

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DO - 10.1093/MNRAS/STY1880

M3 - Article

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EP - 1228

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

SN - 0035-8711

IS - 1

ER -