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\textbf{Thesis Progress Report}
\textbf{Introduction} \\
The goal of this thesis is to compare the stellar mass growth rate vs
current mass
at various redshift of local group dwarf galaxies with distant galaxies. This work was started in the fall, but was registered with the astronomy department. As this is the first term registered in the Physics department, this progress report will cover work done since the fall.
\textbf{Data} \\
The early part of the project was
dominated by trying to understand spent familiarizing myself with the data that would be
used. used, and the techniques used to gather that data.
The data used for distant galaxies was presented in \cite{Tomczak_2014}. This paper contains a table of mass vs number density (number of galaxies per unit volume) for $0.2 < z < 3$ which we use to construct the stellar mass function
(SMF). (SMF) which shows the frequency of galaxies at different masses and redshifts. However, while we plotted this data and various subsets of it (only star forming or quiescent galaxies) we do not use the raw data. Instead, we use a
fitting function - a paramaterised double Schechter function
- from \cite{Leja_2015} which smooths the data and ensures the number density at each mass is monotonically increasing as $z \rightarrow 0$. With this done, graphs showing the mass over $0.2 < z < 3$ for various start masses were constructed.
The local group dwarf galaxy data was taken from \cite{Weisz_2014}. This paper determines the mass of a subset of the known dwarf galaxies between $0 < z <
2.6$ 2.6$. It does this by
calculating determining the star formation history
(sfh). (SFH) by constructing a color magnitude diagram and determining using know properties of stars when star formation occurred. As with the distant galaxies, graphs showing the percentage of mass over time for various groups of the galaxies (grouped by galaxy shape or location) were plotted to better understand the data.
A second distant galaxy data set was introduced later in the project. This data, taken from \cite{Whitaker_2014} was used to confirm that the comparison between the two main data sets were reasonable and conformed to other data. Again, a
paramaterisation parameterization rather than raw data was used.
\textbf{Analysis} \\
A number of corrections must be applied to these data sets before they can be compared.
The first correction is for mergers. The local group data is based on galaxies that have not undergone
mergers \textbf{is this true?}, mergers, while some of the distant galaxies will have.
This has the effect of reducing the total number of galaxies in the sample over time, reducing the overall number density. We make the merger correction to the SMF using the method shown in \cite{Gomez_2015}. Supporting material such as plots of expected merger rates at various mass ratios and redshifts were also constructed to ensure that we were applying this correction correctly.
We also apply a correction for mass loss to both the local group and \cite{Whitaker_2014} data. As these both determine mass by integrating the star formation rate over time, the data shows the total stellar mass formed by a certain time, rather than the total stellar mass present at that time. Much of this mass loss is caused by the death of high mass, short lifespan ($ < 100Myr$) stars and so can be approximated as instantaneous using a multiplicative factor. The \cite{Tomczak_2014}
determines mass data is from
observed luminosity instantaneous mass and
is not
star formation rates, calculated from the SFH and so this
correction is not
applied there. applied.
Finally, a environmental correction was applied to the local group data.
If we split the known local group We expect that galaxies
into groups based on their location - satellites in different places in the Local Group (satellites of the Milky Way, satellites of M31 (Andromeda) and
those galaxies attached to neither of
these two - we find that the main galaxies) would have different growth rates. However, the \cite{Weisz_2014}
data, which data contains only a subset of approximately half of all
galaxies, known local group galaxies and does not sample evenly from these three
groups. environments (for observational reasons). To correct for this, we weight galaxies to ensure that at each mass and redshift the different environments are correctly weighted.
A significant analysis was also performed on the errors on the local group data reported by \cite{Weisz_2014}. These errors were calculated using methods defined in \cite{Dolphin_2012} (systematic) and \cite{Dolphin_2013} (random) but are considered extremely conservative. A method to determine a more reasonable set of uncertainties was not found and so we adopt the
literature convention
used in a later paper established by Weisz \cite{Weisz__2014} and simply apply a 50\% fractional uncertainty to all masses. This method is also considered conservative but significantly improves on the original.
\textbf{Comparison} \\
Having made the corrections discussed above, we compare the data sets. We find that the \cite{Tomczak_2014} data
tends to be within broadly agrees with the
(large) error bars of \cite{Whitaker_2014} data, but that both show higher growth rates than the local group data.
Compared to If this
data set, the local group data underestimates continues to hold as the
rate of growth at all masses final corrections are applied and
reshifts by ~0.3 dex. However, we find that the
\cite{Whitaker_2014} data work is
significatly different to both other data sets. checked, this is an interesting result as it would should galaxy strangulation; a process by which stellar mass growth rates are slowed in the presence of large gravitational fields.
\textbf{Conclusion} \\
While we have
preliminarily done most of the corrections needed to compare these data sets, there is still work to do. In particular,
work will the merger correction is not perfectly understood and may be
needed to explain the source of some of the discrepancy between the
\cite{Whitaker_2014} data and the other two
distant galaxy data sets.
A significant amount of work will Tests to show that the code used for analysis is performing correctly also
need to be
needed written. Finally, this work also needs to
write be written up and
present this work. presented.