Galactic Astroarchaeology: Reconstructing the Bulge History by Means of the Newest Data
Authors: Gabriele Cescutti and Francesca Matteucci
Overview
This paper attempts to constrain star formation history by applying various initial mass functions (IMFs) to a chemical evolution model specific for the bulge of our galaxy. Results are compared to recent data of dwarf and giant starts from multiple observing groups.
Summary:
There are multiple scenarios for the formation of the bulge of our galaxy which include: 1. Accretion of extent stellar systems. 2. Accumulation of gas at the center of the galaxy, leading to star formation. 3. Accumulation of metal enriched gas from the halo or thick disk or thin disk. In order to constrain the conditions for bulge formation and identify the most likely formation scenario, the authors use multiple data sets of [?/Fe] from various observing groups.
Iron enrichment is due to Type Ia supernovae (binary systems) while ?-element enrichment is due Type II supernovae (core collapse). Due to the delay iron enrichment from Type 1 supernovae, different [?/Fe] vs. [Fe/H] relations are expected for different star formation rates. The bulge is assumed to have evolved quickly resulting in large [?/Fe] ratios over an extended value of [Fe/H]. There will be an abundance of ?-elements early on and then a large increase in iron once low mass stars have evolved to produce Type Ia supernovae changing the slope of [?/Fe] vs. [Fe/H].
The authors used chemical abundance data from Bensby et al. (2010), Alves-Brito et al (2010), and Ryde et al. (2009), which include observations from bulge dwarf and giant stars. They state that the chemical evolution model they used "closely follows that in Bellero et al. (2007)." They assume and instantaneous mixing approximation in the bulge, a high efficiency and short timescale for the star formation rate, consider several IMFs and various stellar lifetimes, primordial infalling gas, and detailed nucleosynthesis taken from Francois et al (2004), Maeder (1992), Maeder & Meynet (2002), and Cescuttie et al. (2006).
Employing IMFs from Bellero et al. (2007), Scalo (1986), and Salpeter (1995), they fit their model to the observations of giant and dwarf bulge stars. Results of modeled [?/Fe] vs. [Fe/H] using various IMFs are plotted against observed [?/Fe] vs. [Fe/H] are shown in Figure 1.
The shape of the function is due to the time delay in chemical enrichment from Type Ia as well as the star formation rate. A flatter function and greater than solar metallicity can be represents by a fast-forming bulge. The Scalo IMF does not fit well but the flatter functions of Salpeter and Ballero are better fits.
When looking at observed and modeled stellar metallicity distribution (Figure 2), Ballero is clearly the best fit. Scalo's IMF fits disk stars but not bulge stars. Further more, the fits to [O/Mg] vs [Mg/H] and [Fe/H] suggest mass loss is a factor in massive stars (figure 3). They conclude that this mass loss will play an important part for metallicities larger than solar.
The authors conclude that in order to accurately model the metallicity distribution function of the bulge, a flat IMF (flatter than Scalo) is needed and suggests that the bulge formed differently than the disk. The fits to abundance data suggests that the bulge formed very quickly, within the first 0.5 Gyrs.
Ties to Class Material
The paper discusses star formation rates which we have covered in class. This is the birthrate of stars which is the number of stars formed with mass m, per volume, V.
Question to Author
The blue giants in figure 4 do not fit models well while the dwarf stars do, what is the reason for this?
Cume Question
If this study were conducted using integrated light from distant galaxies, list a problem that would affect the observed metallicity distribution. Suggest one alternative that could be conducted to accure a metallicity distribution.
There is an age-metallicity degeneracy when observing the colors of stars. Both metal rich and older stars will be reddened. Rather than observing integrated color, one could observe spectral light.
Follow-up Research Project Idea
Given that the data for [O/Mg} vs [Mg/H] and [Fe/H] is poor for giants rather than dwarfs, more observations of dwarfs may help further constrain the chemical evolution models of the bulge stars.
