Title: [X/Fe] Marks the Spot: Mapping Chemical Azimuthal Variations in the Galactic Disk with APOGEE
Authors: Zoe Hackshaw, Keith Hawkins, Carrie Filion, Danny Horta, Chervin F. P. Laporte, Chris Carr, Adrian M. Price-Whelan
First Author’s Institution: Department of Astronomy, The University of Texas at Austin, 2515 Speedway Boulevard Austin, TX 78712, USA
Status: Submitted to AAS Journals [open access]
When you look up at the sky, for the most part, the stars all look similar – just tiny, twinkling dots. But, imagine you could see their chemistry. You’d see that some stars are dominated by iron, while others are mainly made of magnesium. Looking at the Milky Way as a whole, you’d see a broader pattern emerge: stars closer to the Galactic center tend to have higher metallicity than the ones further out in the disk. This so-called metallicity gradient has become a well-documented phenomenon in galactic archaeology.
However, due to the recent advent of large all-sky spectroscopic surveys like APOGEE, astronomers have noticed another interesting phenomenon among stars in the Milky Way: azimuthal chemical variations. These are deviations of the predicted metallicity gradients from the observed ones that manifest as a striped pattern of chemical oscillations. Astronomers have proposed multiple explanations for the existence of these chemical variations, like radial migration and interactions between stars and the Milky Way’s spiral arms. However none of these explanations have been conclusively verified! With that in mind, a team of astronomers from the University of Texas at Austin set out to measure these azimuthal chemical variations in the Milky Way in their own effort to investigate where they came from.
Step 1: Measuring the Metallicity Gradient
In order to study azimuthal variations, the team first needed to quantify the metallicity gradient. They used a sample of stars from SDSS-VI APOGEE between 3.5 and 15.5 kiloparsecs from the galactic center, which are plotted according to their position and [Fe/H] in the left panel of Figure 1. Through fitting, the relationship between these stars’ galactocentric radius and [Fe/H] was found to be linear with a gradient of ∆[Fe/H]/∆R = −0.066 ± 0.0004 dex/kpc, which is in good agreement with previous literature. Satisfied with this result, the authors moved onto quantifying the azimuthal variations.
Figure 1: On the far left, the cartesian galactic X and Y distribution of the paper’s sample is shown, colored by metallicity. In the middle, the same sample is shown, except it’s colored by the predicted metallicity, which is derived from the linear relationship that the authors measured. Finally on the right, this figure shows the residual, or result after subtracting the data and model. In this figure, it’s clear that there are striations where the model over and under predicts the observed [Fe/H]. These striations are the azimuthal variations. Figure 5 in the paper.
Step 2: Measuring the Azimuthal Variations
Using the linear relationship between galactocentric radius and [Fe/H] derived in the previous step, the authors predicted every star’s [Fe/H] using just their galactocentric radius, which is plotted in the center panel of Figure 1. They then subtracted the predicted [Fe/H] from the original observed [Fe/H], revealing the azimuthal variations in the Milky Way! This is shown in the right panel of Figure of 1.
Figure 2: These two figures show the comparison between the observed azimuthal variations, plotted in blue and red, and two models of the Milky Way’s spiral arms, plotted as black lines or contours. From visual inspection, it’s clear that neither spiral arm model lines up particularly well with the observed azimuthal variations. Figure 6 in the paper.
But why do these variations exist? Some works have suggested that these variations could be caused by the Milky Way’s spiral arms churning stars around like a galaxy-sized blender. To test this, the authors compared multiple models of spiral arms to the azimuthal variations they measured.. Unfortunately, none of the spiral arm models clearly lined up with the observed azimuthal oscillations. However, they did find that the strength of the variations appear to be correlated with stellar age!
Figure 3: These figures show the comparison between the observed azimuthal variations in the three age bins: 0-2 Gyr (left), 2-6 Gyr (center), and 6+ Gyr (right). The color indicates the strength of the variation. From this, the authors determined that the strength of the azimuthal variations seems to be correlated with stellar age. Figure 8 in the paper.
Separating their sample into three different age groups, shown in Figure 3 , the authors found that the oldest stars had the strongest azimuthal variations, which implies that their cause could be due to dynamical events during a star’s life as opposed to some intrinsic chemical factor in a star’s birth.
Today’s paper took a detailed dive into azimuthal chemical variations of stars in the Milky Way. Aptly titled ‘X marks the spot”, it seems like this paper’s “map” has led astronomers to not just treasure, but potentially a whole new exciting hunt.
Astrobite edited by: Cesiley King
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