Segue 2: The Least Massive Galaxy

Segue 2 velocity distribution Segue 2 is a tiny galaxy that orbits the Milky Way.  It has only about 2000 stars.  For comparison, the Milky Way has about 200 billion stars.  There are other galaxies with even fewer stars than Segue 2, but what makes Segue 2 unique is its total mass.  The mass of galaxies comes not just from stars but also dark matter.  It turns out that Segue 2 has the least amount of dark matter of any known galaxy.

The amount of dark matter in a galaxy is measured by the motions of stars in the galaxy.  The more mass the galaxy has, the faster the stars move.  The unshaded histogram on the left shows the velocities of stars in Segue 2, measured with the Keck 2 telescope.  There is some uncertainty in the measurements, so the differences in velocities of the stars in the unshaded histogram don't necessarily indicate that the stars have different velocities.  In fact, the measurements are consistent with a single velocity for all of the stars.  That is, the measurements are consistent with the absence of dark matter.

In detail, my collaborators and I placed an upper limit of 2.2 km/s on the velocity dispersion of stars in Segue 2.  The inferred mass is less than 150,000 times the mass of the sun.  That may sound like a lot of mass, but it's actually 10 million times less massive than the Milky Way.

This discovery has gotten a lot of attention in the press, including from the LA Times, NY Times, Time, New Scientist, and the OC Register.

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Metals Lost from Dwarf Galaxies

Metals lost from dwarf galaxies Galaxies have one job to do: Turn gas into stars.  Dwarf galaxies are really bad at their jobs.  Take Ursa Minor, for example.  It contains about half a million stars.  However, in the past it probably contained much more mass in gas than its present mass in stars.  That's because it lost a great deal of gas over its lifetime.

Galaxies lose gas in a variety of ways.  The most common ways for dwarf galaxies to lose gas is "stellar feedback" and "tidal stripping."  Stellar feedback is the loss of gas from the galaxy due to energy input from the stars.  For example, when some stars end their lives, they blow up as supernovae.  One supernova puts out as much energy in one second as the the Sun does in several billion years!  That much energy can expel a lot of gas from the galaxy.

Tidal stripping is a mechanism of gas loss unique to dwarf galaxies, which are satellites of larger galaxies.  When the dwarf galaxy gets close enough to its large host, the much stronger gravity of the larger galaxy can rip gas out of the smaller galaxy.  In the case of Ursa Minor and the other galaxies represented in the plot to the left, that large host galaxy is our Galaxy, the Milky Way.

Massive stars are nuclear reactors, fusing hydrogen and helium into heavier elements like iron.  To illustrate how much material is lost from galaxies, let's again consider Ursa Minor.  Over its lifetime, Ursa Minor synthesized a couple thousand times the Sun's mass in iron.  That number is represent by the hollow circled labeled "UMi."  My measurements show that Ursa Minor presently contains less than a thousand solar masses of iron.  That's the filled black circle.  That means the puny galaxy lost 99.8% of the iron that its stars produced.  That's like getting a 0.2% on your performance review.

More massive galaxies lost a slightly smaller percentage of their iron.  Fornax, which is about 35 times the mass of Ursa Minor, lost 96% of the iron that its stars produced.  And it's not just iron.  The metals lost from the galaxy also include magnesium, silicon, and calcium.  Even a galaxy like NGC 1569, which is even more massive than Fornax, lost nearly all of the metals that its stars produced.  That means that only the largest galaxies in the Universe, like our Milky Way, do a good job of turning their gas into stars.

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The Chemistry and Dynamics of VV124

The metallicity and velocity distributions of VV124 Say hello to the newest member of the Local Group of galaxies, VV124.  The Local Group is a collection of mostly dwarf galaxies, but it also includes big ones like the Milky Way and Andromeda.  Until a few years ago, a big catalog mistakenly listed VV124 as being about 10 times more distant than it actually is.  When the mistake was noticed, the true distance confirmed VV124 as a member of the Local Group.

My faculty contact at Caltech, Judy Cohen, and the Italian astronomer Michele Bellazzini, had the idea to look at VV124 with the Keck II telescope.  Judy and I observed this galaxy in January 2011 with the amazing spectrograph called DEIMOS.  We spent an entire night staring at this galaxy, and our reward was beautiful stellar spectra.  These spectra contain information about the individual stars that make up VV124.  The information we're interested in includes the velocities of the stars and their compositions.

The top panel of the figure to the left shows the distribution of iron among the stars of VV124.  A pretty basic model of galactic chemical evolution describes this distribution pretty well.  The blue line (overlapping exactly with the red line) is that simple model.  Adding complexity to the model gives a slightly better description of the iron distribution.  The red and green lines represent more complex models, and the green line fits the data a little better than the blue line.  Therefore, VV124 is a simple galaxy that started with pure hydrogen gas, and it turned some of that gas into stars.  It may have acquired a small amount of gas while it was forming stars.

The velocity distribution of the stars presents additional evidence that VV124 is a simple galaxy.  The velocities are distributed according to a "Gaussian," a bell curve.  What's more is that the stars on one side of the galaxy don't show any different average velocity than the stars on the other side of the galaxy.  That means that the galaxy does not rotate.  The stars move around randomly without any ordered motion.  That's typical of dwarf galaxies.

The strange thing is that the gas in the galaxy does have velocity structure.  Why the gas is ordered but the stars are not remains a mystery.

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Lithium-Rich Red Giants

Lithium evolution in red giants Lithium is a fairly simple element.  It has just three protons, one more than helium and two more than hydrogen.  The Big Bang manufactured nearly all of the lithium in the Universe.  From there, the lithium content of the Universe has decreased because stars destroy lithium in the course of normal setllar evolution.  In the plot to the left, the gray points show the amount of lithium in stars at different points in their lives.  Stellar evolution moves stars to the right on this diagram.  At a certain point, when the star transitions from being a dwarf star to a giant star, over 90% of the lithium is converted into helium.  Another episode of lithium destruction takes care of the rest of the lithium.

Almost all stars obey this simple picture.  However, a few giant stars are renegades.  A very small number of red giant stars--about 1%--have a lot lithium.  Some of them have even more lithium than they started with.  That means that these stars didn't just save their lithium from destruction.  They created it.

A master's student (Xiaoting Fu) and I found 14 red giants in dwarf galaxies that belong to this special class of giants.  I don't know how these stars get all that lithium, but I'm not alone.  Some smart theorists have some good ideas, but none of them is completely consistent with the data.  Regardless, this new sample of 14 metal-poor, lithium-rich giants is larger than all of the previous samples combined, and it will help those theorists figure out what exactly is making all that lithium.

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Dwarf Galaxy Chemical Evolution Modeling

Sculptor's star formation and gas flow history The measurement of alpha element abundances can reveal the history of star formation in a galaxy.  I previously measured the alpha element distribution in the dwarf galaxy Sculptor.  Now I have devised a very basic model of chemical evolution to describe those measurements.  The model is very simple compared to some great models in the literature.  However, I kept it simple intentionally so that it wouldn't take very long to run on my computer.  That way, I could quickly see the results of changing the input parameters.  That was important because I wanted to make a model that described my alpha element measurements as best I could, so I had to try a lot of different input parameters.

The figure to the left illustrates the history of star formation and gas flow in Sculptor.  This figure is one way to interpret the elemental distributions that I measured.

The top panel shows how Sculptor accumulated gas over the one billion years that it formed stars.  This information comes mostly from the number of stars at different metallicites.  That is, the infall rate depends mostly on the [Fe/H] distribution, not [α/Fe].

The second panel shows the mass of Sculptor, separated into gas (blue) and stars (red).  The peak gas mass is about ten times the final stellar mass.  That means that Sculptor required a great deal of gas to catalyze the star formation of a relatively small mass of stars.  The post above talks about galaxies having one job to do: Turn gas into stars.  Sculptor was so bad at it that it had to go through tons and tons of gas to make just a few stars.

The third panel shows how fast Sculptor turned gas into stars.  This is another way to see that Sculptor was terrible at its job.  At its peak, its star formation rate was 0.026 solar masses per year.  For comparison, the star formation rate of the Milky Way is about one solar mass per year.

The bottom two panels show the composition of the gas in Sculptor as a function of time.  The steady decline of [Mg/Fe] with time indicates that Type Ia supernovae dominated the chemical evolution of Sculptor.  Those supernovae are big iron bombs that add a bunch of iron (Fe) but no magnesium (Mg).  As a result,those supernovae rapidly decrease [Mg/Fe].

This model isn't the only way to interpret the measured chemical abundances of Sculptor, but this model does roughly describe the observations.  In total, Sculptor formed stars for just one billion years.  That actually isn't that much.  Consider that the Earth has been around for over four billion years.  The history of star formation in Sculptor is much shorter than Earth's geological history.

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Alpha Element Distribution as a Function of Dwarf Satellite Galaxy Luminosity

The alpha and iron abundance distributions in several Milky Way dwarf satellite galaxies of varying luminosityThe distribution of different elements across the stars in a galaxy reveals the history of star formation in the galaxy.  The next section explains the basic idea as it applies to one specific dwarf galaxy.  It is also enlightening to compare different dwarf galaxies to each other.  In particular, the trends in the element abundance distributions may change with dwarf galaxy size.  In this case, "size" actually refers to the number of stars in the galaxy, which can equivalently be thought of as luminosity, or how brightly the galaxy shines as a whole.

The legend in the plot on the left lists eight dwarf galaxies, satellites of the Milky Way, in order from brightest (most stars) to faintest (fewest stars).  As a point of reference, it also shows the trend in our own Milky Way galaxy, which is many times larger than the eight galaxies I have included in my own study.  It is clear that the [α/Fe] ratio drops more steeply with increasing [Fe/H] in the dwarf galaxies than in the Milky Way.  However, at low metallicity ([Fe/H] < -1.5) [α/Fe] drops at about the same rate in the dwarf galaxies, regardless of their final sizes.

One interpretation of the similarity in [α/Fe] at low [Fe/H] is that the very first episodes of star formation in dwarf galaxies are inefficient.  The galaxies' initially meager supplies of interstellar gas, from which stars are formed, keep star formation at a low level.  As a result, the long-lived Type Ia supernovae have enough time to explode and depress the [α/Fe] ratio as the galaxies evolve and become enriched in iron (moving right along the x-axis).

That all of these galaxies show similar [α/Fe] at a given [Fe/H] is unexpected.  Larger dwarf galaxies, like Fornax (dark red), are expected to have more intense star formation than smaller dwarf galaxies, like Ursa Minor (dark blue).  As a result, the larger galaxies should have been able to keep their [α/Fe] high for a broader range of [Fe/H].  In fact, the average star formation efficiency over the entire galaxy lifetimes is certainly larger for the larger galaxies.  However, the decline in [α/Fe] is manifest only for [Fe/H] < -1.5, corresponding to the earliest epochs.  Therefore, this plot shows that the early star formation histories of all eight galaxies was similar.  At later times, corresponding to [Fe/H] > -1.5, other methods of determining star formation histories show that the evolutionary paths of the galaxies diverge.

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Alpha Element Distribution in Sculptor

The distribution of alpha elements relative to iron in SculptorIn the process of star formation, some stars explode as supernovae, releasing newly created elements into the interstellar gas.  New stars form from this gas, and the composition of those new stars incorporates the newly created elements.  Some of the supernovae explode very quickly after star formation begins.  These are called Type II supernove, and they show a predictable pattern of elements.  In particular, the ratio of light "alpha" elements, like magnesium (Mg) and silicon (Si), to iron (Fe) in the ejecta is high.  Other supernovae explode only after a long time, perhaps as long as several billion years.  These are Type Ia supernovae, and the alpha-to-iron ratio in their ejecta is low.  The difference in time to explosion means that the timescale over which stars formed in the gaalxy is imprinted in the [α/Fe] ratio of the surviving stars in these galaxies.

This plot shows the [α/Fe] ratio for several hundred stars in the Sculptor dwarf galaxy, a satellite of the Milky Way.  The x-axis is the "metallicity" of the star.  You can think of metallicity as a proxy for the birth order of the star.  Older stars have lower metallicity, or [Fe/H], whereas younger stars have higher [Fe/H].  The y-axis, [α/Fe], is also a time indicator, but it is not strictly age.  Rather, it indicates the frequency of star formation.  Galaxies that form stars quickly ramp up [Fe/H] fast enough that Type Ia supernovae do not have time to explode.  As a result, [α/Fe] remains high over the full range of [Fe/H].  Galaxies that form stars at a leisurely pace over billions of years have time for Type Ia supernovae to dilute the [α/Fe] ratio.

In the case of Sculptor, a steady decline in [α/Fe] as a function of [Fe/H] means that the galaxy took its time in forming stars, at least a billion years.  The star formation age of Sculptor is important because dwarf galaxies like Sculptor are thought form part of our own Milky Way galaxy.  In theory, dwarf galaxies are disrupted by the Milky Way and form a diffuse halo of stars around it.  However, the [α/Fe] patterns of Milky Way halo stars indicates that they formed very quickly.  Therefore, Sculptor is not representative of the galaxies that may have built up the majority of the Milky Way halo.  However, galaxies like Sculptor may have a significant, ongoing role in the continuing creation of the outer parts of the Milky Way halo.

This plot has been made possible by the techniques of my Ph.D. dissertation.  Specifically, I measured [Fe/H] and [α/Fe] from medium-resolution spectra obtained with the DEIMOS instrument on the Keck II telescope.  This plot has helped to determine a more precise history of star formation in Sculptor in a paper that I recently submitted to the Astrophysical Journal.  The distribution of element abundances has helped us learn how many stars formed in Sculptor at different historical epochs.

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Discovery of Extremely Metal-Poor Stars in Milky Way dSphs

comparison of medium resolution and high resolution [Fe/H]

A diffuse halo of stars and dark matter surrounds the Milky Way.  Most astronomers believe that some of the oldest stars in the Universe—nearly as old as the Universe itself—inhabit the stellar halo.  Part of the reason to expect these stars are so old is that they contain very little metals.  ("Metals" means every element except hydrogen and helium.)  Metals exist in the Universe because nuclear burning in stars processed hydrogen and helium into heavier elements.  That means that the first generations of stars had no metals, or at least very small amounts of metals.

One widely accepted theory of the formation of the Milky Way's stellar halo posits that small "building blocks" called dwarf spheroidal galaxies (dSphs) fell into each other by their mutual gravitational attraction.  Those building blocks are no longer distinct entities but instead a single enormous diffuse cloud of stars that surrounds the Milky Way.  However, some dSphs still exist today.  In the hierarchical assembly model, these dSphs would be the rare handful of latecomers to the Milky Way system which have not yet suffered the gravitational interactions that pull the small galaxies apart and melds them into the stellar halo.

Recently, some astronomers claimed that the stars in the halo are different enough from the stars in the surviving dSphs to cast doubt on the hierarchical assembly model.  Specifically, a small fraction of stars in the halo are extremely metal-poor.  Some have one ten thousandth the amount of metals that the Sun has.  Some have even less.  If dSphs built the stellar halo, then even the surviving dSphs should also have some extremely metal-poor stars.  The claim is that, on the other hand, some surviving dSphs—specifically the large, luminous ones—do not contain stars that are quite so metal-poor.

By measuring the metal content of very tiny, very faint surviving dSphs with a new technique, my group has discovered that certain dSphs do in fact contain stars nearly as metal-poor as those in the stellar halo.  The cumulative distribution of metallicity ([Fe/H]) shown above demonstrates that the extremely metal-poor tail of the faintest surviving dSphs has a shape similar to that of the Milky Way stellar halo after all.  The discrepancy at [Fe/H] > -2.3 is a result of excluding the more luminous surviving dSphs, whose stars are on average more metal-rich than the very faint dSphs represented by the black line.

While the discovery of extremely metal-poor stars in some surviving dSphs bolsters the hierarchical assembly model, it does not prove that all dSphs contain extremely metal-poor stars.  Therefore, the claim that the luminous dSphs are devoid of very metal-poor stars may be accurate.  The discovery of very metal-poor stars in the faint dSphs warrants a new look at the luminous dSphs with the same technique.  Presently, it is unclear whether the hierarchical assembly model depends on the existence of extremely metal-poor stars in luminous dSphs.  It is possible that the extremely metal-poor tail of the halo metallicity distribution formed from tiny dSphs alone.  Future work will apply the same technique used to discover the extremely metal-poor stars in small, faint dSphs to the stars in large, luminous dSphs.

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Metallicities and Alpha Enhancements of Red Giants with Medium Resolution Spectra

Comparison of DEIMOS [Fe/H] to high-resolution [Fe/H] for globular cluster stars

Detailed tests of theories of hierarchical structure formation require chemical abundances of many stars at large distances. In additional to overall metallicity, models predict the alpha enhancement of different dynamical components of the Milky Way and M31.  High dispersion spectroscopy can probe very few stars in the Milky Way satellite galaxies and no stars in M31.

My dissertation employs a technique using medium resolution spectroscopy and spectral synthesis to extract metallicity and [α/Fe] for large samples of individual stars at large distances.  I have found that R ~ 6000 spectra reproduce high resolution [Fe/H] and [α/Fe] measurements of Galactic globular cluster stars. At signal-to-noise ratios for which Milky Way satellites are easily accessible, the typical error on metallicity is 0.15 dex, and the typical error on [α/Fe] is 0.2 dex. Unlike empirical metallicity estimators, such as the equivalent width of the Ca II triplet, this synthetic method does not depend on the metallicity range or alpha enhancement of calibrators.

This plot shows the mean metllaicities of nine Milky Way globular clusters.  The horizontal axis is the result from high dispersion spectroscopy, and the vertical axis is my result from DEIMOS spectra.  Three clusters (NGC 2419, M79, and NGC 7492) have fewer than five stars observed in high resolution spectroscopy.  The measurements with DEIMOS are based on 32, 16, and 21 stars, respectively.  Despite the lower spectral resolution, the DEIMOS results may be more precise.

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