Image of the Week

Mapping the interstellar medium using the Gaia RVS spectra

Figure 1. Upper left panel: the animation shows how the intensity, or strength, of the DIB at 862 nm gets stronger at increasing stellar distance along a given line of sight. The distance of each star is indicated in the plot, while the black line shows the best model description. Lower left panel: here we show a measure of the strength of the DIB, by measuring its equivalent width, or EW), at changing distance, and we can see a clear correlation between the strength and the distance from Earth. The uncertainty on the measured EW is shown as well: as the distance increases, so does the noise on the spectrum and the EWs become more difficult to measure precisely. Right panel: we move from a single line-of sight to all the possible line-of-sights explored by Gaia spectra. We show a face-on view of the Galaxy (sketch created by R. Benjamin, in Churchwell et al. 2009) superimposing the spatial distribution of the DIB EWs. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO, based on images published in Gaia Collaboration, Schultheis, et al. 2022.

The spectrum of a star is a gold mine of information, not only on the star itself but also on the dusty interstellar medium that is between us and the star. Both the star and the interstellar medium absorb the light that is emitted from the stellar surface, and the result is the presence of absorption features in the spectrum.

Having different origins, the 2 types of features (from star and for the medium) behave in a different way. The position of the stellar features changes according to the radial velocity of the star as seen from the observer (the well known Doppler effect) while the interstellar medium features are at a fixed position in the spectrum, i.e. their wavelength does not change. The features originating from stars have long been identified and are quite well understood, while there are still many things that remain unknown concerning the interstellar features.

The most famous interstellar feature is due to sodium (its atomic name is Na) but there are many, more subtle, features that have since been found. The interstellar medium is everywhere in space, and its properties, density and chemical composition can vary a lot in different regions of the Galaxy. But the farther we look, the stronger is its signature on the spectra. This is because the light of the star statistically crosses a larger amount of interstellar medium.

Diffuse interstellar bands (DIBs) are weak and have very broad absorption features, found at specific wavelengths, that originate from the interstellar medium. One example is shown in the left panel in the above figure, where the same feature with different intensities (or strengths) is shown as an animated image. They were first detected more than 100 years ago in 1919 by Mary L. Heger at Lick Observatory when she studied the interstellar sodium lines. She noticed that the absorption features remained stationary in the spectrum, while the lines originating from the star moved over time. These were later identified as Na lines.

So far, astronomers have found more than 600 DIBs but only one DIB has been successfully reproduced by laboratory measurements, tracing its origin to absorption by a complex molecule Buckminsterfullerene (C60+). However we still don’t know the origin of all the other DIBs, including the DIB observed at 862 nm in Gaia RVS spectra. This is what we call the longest mystery in stellar spectroscopy.

In the Gaia RVS wavelength range, DIBs are present and can be measured. Gaia Data Release 3 provides the first homogeneous all-sky survey of the DIB at 862 nm containing nearly half a million sources. This is so far the largest catalogue of DIBs.

The animation provided above shows in different ways how the strength of the DIB depends on the distance of the star. To build the plot on the left panels, we selected a specific line of sight, i.e. a direction in the sky, and extracted a sequence of Gaia RVS spectra from stars in this direction having measurable DIBs. Of course, Gaia also provides a solid determination of the distance.

On the upper left panel, we show an animated sequence of Gaia spectra zoomed in on the DIB region at 862 nm. We can clearly see how the DIB at 862 nm gets stronger at increasing stellar distance, which is also reported on the plot.

The lower left panel shows this correlation more clearly, by plotting the equivalent width (EW) of this feature, i.e. a measure of the strength of this feature, versus the distance of the star. The uncertainty of the equivalent width (EW) measurement is also shown in the plot: we receive less light from a more distant star, and therefore the noise on the Gaia spectra becomes stronger and the measure of the equivalent width gets more uncertain.

The animation in the right panel extends this concept to the entire Galactic plane visible by Gaia, using all the DIB EWs available in Gaia DR3, i.e. all the lines of sight. The animation represents (a projection of) the first 3-D map of DIB absorptions. Thanks to the homogeneity, high quality and uniformity of the Gaia DIB database, for the first time detailed spatial structures of the DIB distribution are revealed.

To build the animation shown above, we have selected only the high quality sample of DIBs (see additional information below) provided in the Gaia Data Release 3, as discussed and presented in Gaia Collaboration, Schultheis et al. (2022).

In Gaia Collaboration, Schultheis et al. (2022) we go further in the analysis. We have demonstrated a tight relation with the amount of interstellar dust, i.e. the interstellar reddening, showing the similarities between the DIB 862nm carrier, whose origin is still unknown, and the dust distribution in the Milky Way. This is possible due to the independent measures of interstellar reddening available for these stars, as measured from Gaia XP spectra and available in Gaia DR3. We also show that the picture is still quite complex, since there are important differences with the dust distribution, such as the presence of the DIBs in the so-called Local Bubble (a zone devoid of dust).

Due to the large number of high quality DIB measures, we could assess the rest-frame central wavelength of the DIB with an unprecedented precision. This is a fundamental step to try to reproduce these DIB features with detailed laboratory measurements to unveil the origin of the still unknown carrier of the DIB at 862 nm.

Additional information

• The high-quality sample consists of DIB measurements with a quality flag QFlag <= 2 and where the uncertainty of the DIB strength is better than 50%. In total this sample consists of 141 103 stars. The details on the flagging system are presented in Recio-Blanco et al. (2022).
• An introduction on the nature of the DIBs will be published in the Gaia Pills of Science
• A related article on the interstellar medium was published on the Gaia Data Release 3 publication day.

Credits: ESA/Gaia/DPAC, article written by Mathias Schultheis, Rosanna Sordo, and Orlagh Creevey

[Published: 06/10/2022] ​​​​​​