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ASTR 5610, Majewski [SPRING 2012]. Lecture Notes

ASTR 5610 (Majewski) Lecture Notes

CMDs, Population Synthesis and Dwarf Galaxies

Quantitative Analysis of the CMDs of Simple Composite Populations:

Star Formation Histories From Population Synthesis

Example: Study of the Leo I dwarf spheroidal by Gallart et al. (1999, AJ, 118, 2245)

Here is the CMD of the dSph Leo I.

CMD of the Leo I dSph from Gallart et al. (1999, AJ, 118, 2245).

Immediately you should be able to see the unusual nature of this CMD compared to the globular cluster CMDs we have looked at so far.

To understand this diagram in detail requires a more quantitative approach... Population Synthesis.

First, create library of SSPs of given age, [Fe/H], and IMF:

Models for a CMD computed for a population with constant SFR from 15 Gyr ago to the present (with Z=0.0004) and no binary stars. Note the sequence of ages in both the MS and the subgiant branch and, although less definite, also in the RC and HB. (Note: The models correspond to [Fe/H]=-1.7.) From Gallart et al. (1999, AJ, 118, 2245).

To mimic real data, one then subjects the SSP models to errors of the type one would expect.

Next, define CMD regions sensitive to age variations. For example:

Adapted from Gallart et al. (1999, AJ, 118, 2245).

The goal is to count and compare stars in the CMD regions between the real data and various combinations of superposed SSP models. For these combinations, the different SSPi are varied in their relative density normalization, a.

Then, the number of stars in given region j :

For each SFR(ti ) --> family of ai , compare to data N0,j :

As may be seen, this mechanism produces reasonable results!

Comparison of observed Leo I CMD to a model version created via population synthesis by Gallart et al. (1999). The inset in the right panel shows the derived SFR(t) = star formation history.


In reality, models used by Gallart et al. and other groups are more complex than described, and incorporate as variables:

Gallart et al. 50 combinations of Z, IMF, β; total of 6x107 models of ai <--> SFR

Other SFHs

Some examples of other various dwarf galaxy/composite stellar population systems with different SFHs including continuous and bursty versions.

Courtesy Carme Gallart.

As can be seen, many of the other Local Group galaxies also contain SFHs that are more complex:

(Left) Ground-based image of Sextans A by D. Hunter. (Right) HST WFPC2 image of Sextans A, courtesy Evan Skillman.

(Left) HST CMD of Sextans A by Skillman and collaborators. Boxes used to isolate specific regions for analysis are shown. (Right) Hodge Population Box derived from this CMD by Dolphin et al. (2005).

Another, more complicated, example from Siegel et al.'s (2007, ApJL, 667, L57) analysis of the Sagittarius dSph and its core (M54):

Here is the resulting "Hodge Population Box":

Dwarf Spheroidals, Dwarf Ellipticals and Dwarf Irregulars

(with thanks to Evan Skillman for helpful images and notes).

As established early in this course, there are a variety of dwarf galaxy types in the Local Group.

Comparison of properties:

Area-Resolved Population Synthesis and SFHs

A new level of CMD population synthesis, possible now with large area, deep surveys of nearby galaxies, is studying populations by region in a galaxy.

All material copyright © 2003,2006,2008,2010,2012 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 551 and Astronomy 5610 at the University of Virginia.
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