A Long Time Ago in a Dusty Galaxy Far Away

Dust is a very important element present everywhere in the Universe. In particular, dust attenuation absorbs, scatters and re-emits light.  Nevertheless, not much is known on how dust affects light from galaxies at high redshift, since it is difficult to observe them. Mónica Tress, a final-year-PhD explains more about how dust attenuation effects are studied in galaxies far away.

Dust is an important element in the universe. It is sometimes seen as a little bit of an inconvenience, since it affects the light we receive from stars and galaxies. For this reason, astrophysicists have to account for the effect of dust by correcting their observations when deriving relevant parameters like stellar masses, ages and star formation rates. Dust is formed by particles ranging from 0.001 to 1 μm and it is present everywhere in the universe, particularly in the star-forming regions. Dust is an essential component in the formation and evolution of stars and planetary systems since stars are born within clouds of dust and gas.

The way dust affects light depends on the wavelength of the incoming photons. In the optical and ultraviolet spectral regions, photons can be absorbed and scattered, with those with shorter wavelengths being more prone to interactions with dust particles. This means dust makes light appear redder by absorbing more blue light. At longer wavelengths these processes are less likely, but dust particles, heated by the absorbed photons in the optical and UV range, can reemit the absorbed energy, so that at infrared wavelengths we see the contribution from dust in emission.

 

How to become a star
The Barnard 68 dark nebula. Credit: ESO

In the image above, we can see the effects of dust absorption on a dark nebula. This is Barnard 68 located towards the Ophiuchus constellation, where we can observe optical light being absorbed by dust. You can even glimpse the strong reddening if you look at stars along the fuzzy edges of the nebula!

Pleiades_large
Pleiades Cluster. Credit: NASA/ESA/AURA/Caltech

Scattering of light preferentially affects blue light, so we have images like the Pleiades (above). This cluster is an example where dust scatters the starlight and makes it blue. This process is similar to Rayleigh scattering, which is the same case we observe in the sky: light from the Sun gets scattered in the atmosphere but the blue photons are more affected by this process, giving the sky its blue colour. Moreover, as the Sun sets, its light has to traverse a longer distance through the atmosphere, producing a more intense scattering of (blue) photons away from the line of sight to the Sun, making it appear quite red.

In order to account for these effects, astrophysicists use a wavelength dependent function to describe them and to be able to apply dust corrections. Previously, this has been a fixed generic function called the dust attenuation law. In the UV and optical region, this law is mainly a smooth curve with a prominent absorption feature at 2,175 Å. This generic universal function is then applied to correct for dust in many different environments. Nevertheless, this idea on a fixed dust curve has been changing. The work I have been doing for my PhD research project at MSSL explores this view of the non-universality of the dust attenuation law.

I have been working with galaxy photometry from the Survey of High-z Absorption Red and Dead Sources (SHARDS). This survey took very deep imaging of a small region of the sky with a large telescope (the 10.4m GTC in La Palma, Canary islands). From this set, we selected a set of star-forming galaxies at redshift from 1.5 to 3.0 (very distant sources). These observations were complemented with additional imaging, mainly from the Hubble Space Telescope to measure the dust properties of more than 1,700 galaxies. The dust properties we explored are the colour excess E(B-V), the strength of the 2,175 Å absorption bump (B) and the total-to-selective ratio (Rv), that controls whether the dust law has a very strong dependence of wavelength (small Rv) or a weak one (Rv). E(B-V) accounts for the amount of dust present. B depends on the presence of benzene-type dust particles, such as Polycyclic Aromatic Hydrocarbons (PAH). Rv can be linked with the size of the dust grains or with the distribution of the dust in the galaxy. This last association can tell us about the “geometry” or clumpiness of dust.

Our main results are shown in the plot below, where we show the correlations found between these three dust parameters E(B-V) , Rv and B. We observe a mild relation with the amount of dust and the strength of the NUV bump, with less amount of dust suggesting a stronger NUV bump.

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Correlations between dust parameters. Credit: https://doi.org/10.1093/mnras/stx3334

The main correlation we observe is with Rv and B. Specifically, we see that weaker (stronger) NUV bump is linked with a smaller (larger) Rv. Since Rv, total-to-selective ratio, can be associated to either the dust size grains or to the distribution of dust, the explanation for this trend can be twofold. First, it could be that small dust particles are related to a stronger NUV bump. This possibility is backed by studies that associates the possible NUV bump carrier, PAHs, to small particles. On the other hand, Rv could be connected to the distribution of dust, so the observed correlations would be due to different arrangements of dust and stars within galaxies.

In order to disentangle this degeneracy, we make use of a phenomenological model. We employ synthetic population models and obtain their dust properties as before, but now we assume a fixed dust composition and a time-dependent amount of dust (E(B-V)). In this manner, we wanted to explore if it was possible to obtain correlations similar to the ones mentioned before, even if we assume the same type of dust in all galaxies. The E(B-V) time dependency was expressed with two different functions: a linear relation given by some slope μ and an exponentially decaying dust, expressed by a timescale τ. In both cases, the parameter gives us an idea of the pace of dust destruction in the model. What we found is shown in the following plots, where we compare the Rv and B dust parameters obtained. We observe a similar correlation as with the real data, with a negative trend between Rv and B. Since we assumed a fixed dust composition, we can infer that the distribution of dust within galaxies in a time-dependent way is a driver of these types of trends. In conclusion, not only the dust composition but also its distribution inside galaxies influences how dust affects light.

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dust5
Credit: https://doi.org/10.1093/mnras/stx3334

References:

Tress, M., Mármol-Queraltó, E., Ferreras, I., et al., 2018, “SHARDS: constraints on the dust attenuation law of star-forming galaxies at z~2 “, MNRAS, 475, 2363

Tress M., Ferreras I., Pérez-González P. G., Bressan A., Barro G., Domínguez- Sánchez H., and Eliche-Moral C. (2019), “A deeper look at the dust attenuation law of star-forming galaxies at high redshift .” Submitted to MNRAS, arXiv:1904.10025

Featured Image Credit: NASA/ESA/AURA/Caltech

 

 

 

 

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