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Investigating inner Galactic Bulge stars through the near infrared

Su, Hung Shuo LU (2017) In Lund Observatory Examensarbeten ASTM31 20171
Lund Observatory
Department of Astronomy and Theoretical Physics
Abstract
Context: The formation of the Galactic Bulge is a topic of active research. There are many scenarios based on observations and Galactic evolution models. The key properties which need to be well constrained observationally are the metallicity distributions of stars and the spatial metallicity gradients. The metallicity distribution of stars in the inner Galactic Bulge (|b|<4o) is a subject of ongoing debate, with only a few spectroscopic studies based on small samples of stars. Many studies in the literature reported a narrow metallicity distribution, with one study which concluded a lack of a vertical metallicity gradient in the innermost regions (Rich et al. 2012). However, new studies from large scale surveys (Schultheis et al. 2015)... (More)
Context: The formation of the Galactic Bulge is a topic of active research. There are many scenarios based on observations and Galactic evolution models. The key properties which need to be well constrained observationally are the metallicity distributions of stars and the spatial metallicity gradients. The metallicity distribution of stars in the inner Galactic Bulge (|b|<4o) is a subject of ongoing debate, with only a few spectroscopic studies based on small samples of stars. Many studies in the literature reported a narrow metallicity distribution, with one study which concluded a lack of a vertical metallicity gradient in the innermost regions (Rich et al. 2012). However, new studies from large scale surveys (Schultheis et al. 2015) are beginning to challenge this with larger dispersions in the metallicity distributions reported. These large scale surveys utilise automated pipelines to analyse spectra, which are known to have issues. As such, one would like to investigate and validate the reported results.

Aims: Due to the large discrepancy in reported metallicity distribution in the inner Bulge and the challenging nature of spectral analysis (due to high extinction and cool effective temperatures), the stellar samples from two opposing studies (Rich et al. 2012; Schultheis et al. 2015) will be re-analysed to present a homogeneous dataset of stars in the inner Galactic Bulge. A benchmarked line by line analysis method would be used to determine the metallicities of the stars. One would also like to compare the results to those reported in an automated pipeline method, given there are known calibration issues with metallicities.

Method: A set of iron lines were benchmarked against the Sun, Arcturus, and a sample of 96 high SNR stars using spectra in the near infrared (H-band). The iron lines which were found to produce accurate abundances were utilised to determine the metallicity of the inner Galactic Bulge stars. This was conducted by the fitting of synthetic spectra to the observed spectra within the program Spectroscopy Made Easy (SME), which employs a chi-squared minimisation algorithm to determine parameters.

Results: The determined metallicities for the Schultheis et al. (2015) sample were within ~ 0.2 dex of the reported values. Both of the metallicity distribution were dominated by a peak at ~ -0.2 dex. The reported APOGEE values also show a bump in the distribution at ~ -0.8 dex, but is absent in the distribution based on the metallicities determined in this study. Preliminary tests on the Rich et al. (2012) sample gave large discrepancies between the synthetic and observed spectra. Investigation on the cause of this is still ongoing and hence no meaningful metallicities could be determined.

Conclusions: Overall, we corroborate with APOGEE on the result of a large spread in the metallicity distribution. However, some deviations are found at the metal poor ([Fe/H] < -1.0 dex) and metal rich ([Fe/H] > 0.1 dex) end of the distribution, depending on the methodology. Considering the low sample size and uncertainties, the presence of a metal poor population ([Fe/H] ~ -1.0dex), as claimed in Schultheis et al. (2015), is weakened based on the results of this study. (Less)
Popular Abstract (Swedish)
Sedan mänsklighetens början har det alltid funnits de som dragits till natthimlen och förundrats över vad som kan finnas bortom det mörkaste av mörker. Funderingarna om att vi människor endast är en del av något mycket större började gro ordentligt i samband med teleskopet som verktyg, och när personer som Galileo Galileis och William Herschels började observera himlen med nya tekniker. Det visade sig till slut att de oräkneliga antal stjärnor som de observerade utgjorde ett diskliknande system, det vi idag kallar Vintergatan, vår galax.

En av de mest fängslande frågorna som uppstått efter denna upptäckt är ”Hur blev Vintergatan till?”. Denna fråga har fascinerat astronomer sedan långt tillbaka i historien. Trots flera århundrande av... (More)
Sedan mänsklighetens början har det alltid funnits de som dragits till natthimlen och förundrats över vad som kan finnas bortom det mörkaste av mörker. Funderingarna om att vi människor endast är en del av något mycket större började gro ordentligt i samband med teleskopet som verktyg, och när personer som Galileo Galileis och William Herschels började observera himlen med nya tekniker. Det visade sig till slut att de oräkneliga antal stjärnor som de observerade utgjorde ett diskliknande system, det vi idag kallar Vintergatan, vår galax.

En av de mest fängslande frågorna som uppstått efter denna upptäckt är ”Hur blev Vintergatan till?”. Denna fråga har fascinerat astronomer sedan långt tillbaka i historien. Trots flera århundrande av forskning och försök att besvara frågan är det fortfarande ett väl studerat ämne. Det finns många teorier om möjliga mekanismer som ligger bakom Vintergatans tillblivelse men trots detta har vi ännu inte en fullständig bild. Det här projektet har som mål att kunna bidra till att besvara frågan, detta genom att använda stjärnor i den inre ”utbuktnigen”, eller som det heter på engelska ”bulge”, för att finna begränsningar i de teoretiska modellerna om hur Vintergatan formades.

Utifrån observationer kan man dela in Vintergatan i komponenter, så som ”the disk”, ”the halo” och ”the bulge”. The bulge är särskilt intressant eftersom extragalactic studier visar att inte alla galaxer har liknande strukturer och därför kan sakna en ”bulge”. Hur det kommer sig att Vintergatan har en ”bulge” är fortfarande inte utrett, för att besvara det behövs fler observationer av de inre regionerna av galaxens centrum.


Dock är observationer med hjälp av optiskt ljus svåra att genomföra på grund av den stora mängden gas och damm som finns i the bulge som absorberar ljuset. Inte förrän nyligen har nya tekniker gjort det möjlig att göra observationer med infrarött ljus, som enkelt kan färdas genom gas och damm så att spektroskopi kan göras.

Spektroskopi är en metod där man avgör ett material eller provs kemisk sammansättning genom att använda ljus. På liknande sätt som ett prisma sprider ljus i olika färger, fungerar spektroskopi genom att man splittrar ljuset genom ett prov och observerar de olika våglängderna som ljuset har.

När fotoner skapas i en stjärna genom nukleär bränning måste de fara genom stjärnans alla lager innan de kan observeras. Under tiden kan fotonerna bli absorberade av ämnen som skapas i en nukleär fusion, men bara vid särskilda våglängder. Dessa absorptioner lämnar distinkta mönster i observationerna som kan liknas vid fingeravtryck och på så sätt kan man avgöra strukturers kemiska sammansättning utifrån vilka våglängder som kan observeras.
Genom att experimentera med ett flertal ämnen i ett laboratorium och hitta våglängdsfingeravtryck och sedan jämföra med observationer kan man avgöra vilka ämnen som bygger upp en stjärna. Genom att ta reda på vilka kemiska sammansättningar som bygger upp stjärnor inom en region kan vi börja förstå hur dess stjärnor en gång blev till. Detta är ett avgörande steg för att förstå hur the bulge, och senare Vintergatan, skapades. (Less)
Please use this url to cite or link to this publication:
author
Su, Hung Shuo LU
supervisor
organization
course
ASTM31 20171
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Bulge, Milky Way, Galactic Centre, infrared, spectroscopy
publication/series
Lund Observatory Examensarbeten
report number
2017-EXA115
language
English
id
8912515
date added to LUP
2017-06-09 17:26:14
date last changed
2017-06-09 17:26:14
@misc{8912515,
  abstract     = {Context: The formation of the Galactic Bulge is a topic of active research. There are many scenarios based on observations and Galactic evolution models. The key properties which need to be well constrained observationally are the metallicity distributions of stars and the spatial metallicity gradients. The metallicity distribution of stars in the inner Galactic Bulge (|b|<4o) is a subject of ongoing debate, with only a few spectroscopic studies based on small samples of stars. Many studies in the literature reported a narrow metallicity distribution, with one study which concluded a lack of a vertical metallicity gradient in the innermost regions (Rich et al. 2012). However, new studies from large scale surveys (Schultheis et al. 2015) are beginning to challenge this with larger dispersions in the metallicity distributions reported. These large scale surveys utilise automated pipelines to analyse spectra, which are known to have issues. As such, one would like to investigate and validate the reported results.
 
Aims: Due to the large discrepancy in reported metallicity distribution in the inner Bulge and the challenging nature of spectral analysis (due to high extinction and cool effective temperatures), the stellar samples from two opposing studies (Rich et al. 2012; Schultheis et al. 2015) will be re-analysed to present a homogeneous dataset of stars in the inner Galactic Bulge. A benchmarked line by line analysis method would be used to determine the metallicities of the stars. One would also like to compare the results to those reported in an automated pipeline method, given there are known calibration issues with metallicities.
 
Method: A set of iron lines were benchmarked against the Sun, Arcturus, and a sample of 96 high SNR stars using spectra in the near infrared (H-band). The iron lines which were found to produce accurate abundances were utilised to determine the metallicity of the inner Galactic Bulge stars. This was conducted by the fitting of synthetic spectra to the observed spectra within the program Spectroscopy Made Easy (SME), which employs a chi-squared minimisation algorithm to determine parameters.
 
Results: The determined metallicities for the Schultheis et al. (2015) sample were within ~ 0.2 dex of the reported values. Both of the metallicity distribution were dominated by a peak at ~ -0.2 dex. The reported APOGEE values also show a bump in the distribution at ~ -0.8 dex, but is absent in the distribution based on the metallicities determined in this study. Preliminary tests on the Rich et al. (2012) sample gave large discrepancies between the synthetic and observed spectra. Investigation on the cause of this is still ongoing and hence no meaningful metallicities could be determined.
 
Conclusions: Overall, we corroborate with APOGEE on the result of a large spread in the metallicity distribution. However, some deviations are found at the metal poor ([Fe/H] < -1.0 dex) and metal rich ([Fe/H] > 0.1 dex) end of the distribution, depending on the methodology. Considering the low sample size and uncertainties, the presence of a metal poor population ([Fe/H] ~ -1.0dex), as claimed in Schultheis et al. (2015), is weakened based on the results of this study.},
  author       = {Su, Hung Shuo},
  keyword      = {Bulge,Milky Way,Galactic Centre,infrared,spectroscopy},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lund Observatory Examensarbeten},
  title        = {Investigating inner Galactic Bulge stars through the near infrared},
  year         = {2017},
}