Why Cosmic Dust Is Decreasing, In Spite Of The Increased Availability of Metals? (Cosmology / Astronomy)

Dust plays a crucial role in the thermal balance, dynamics and visibility of galaxies throughout cosmic times. Importantly, dust has a strong influence on the physical processes of the insterstellar medium (ISM) of galaxies in several ways.

Grain surfaces and the Polycyclic Aromatic Hydrocarbons (PAH) participate in a large number of chemical reaction networks in different phases of ISM, and act as catalyst for important chemical processes such as the formation of H2, which in turn drives molecular chemistry.

Dust governs the ISM thermal balance by providing photoelectric heating, and cooling which can alter the shape of the Initial Mass Function (IMF) by favouring cloud fragmentation, thus inhibiting the formation of massive stars and fostering the formation of low-mass stars.

Finally, grains absorb the stellar ultraviolet light and re-radiate it in the infrared, shielding the dense gas, and by these means triggering the formation of molecular clouds where new stars are born.

In spite of the almost 80-years history of dust studies, relatively little is known about the origin and build-up history of the solid component of the ISM. The naive expectation is that cosmic dust abundance should be tied to the metal abundance. However, recent data suggest that this is not the case. Indeed in the last 7−8 Gyr the dust abundance has significantly decreased despite of the increasing availability of heavy elements, the primary components of dust grains.

Several complementary observational evidences indicate that the cosmic dust mass density significantly drops from redshift z = 1 to z = 0. Clearly, and for the first time during cosmic evolution, dust must be destroyed more rapidly than it is formed.

Now, Ferrara and Peroux’s work is motivated by a single important question: why does the dust abundance – for the first time during cosmic evolution – decrease from z = 1 to z = 0 in spite of the increased availability of metals? They answered this question by combining new/recent observational data with simple but solid physical arguments. As a byproduct, they set novel constraints on dust destruction efficiency.

In simple terms, they have investigated the evolution of the cosmic dust density in the last ≈ 8 Gyr.

During this time stretch (corresponding to z = 1 → 0), observations showed that cosmic dust (Ωd) has decreased by about 37.5% in spite of the fact that the cosmic metal abundance has increased by about a factor 1.6. Thus, dust must have been efficiently destroyed during this period.

— wrote authors of the study.

So, by evaluating different dust destruction mechanisms, they concluded that : astration and supernova (SN) shocks in the ISM of galaxies are the dominant factors, with sputtering in hot gas playing a sub-dominant role in dust destruction. All these processes were obviously at work also at z > 1, but the decrease of “Ωd” at later times is driven by the declining cosmic star formation rate and associated metal production. Lets take a closer look:

Figure 1. Sketch of dust production and destruction mechanisms and corresponding yields. © Ferrara and Peroux

(a) Destruction by astration

Astration involves the incorporation of gas and dust into a stellar interior during star formation.

As stars forms in cold, neutral gas, they assume that the stellar build-up material has fd = fobs d = 0.31. Their data shows that there are increase of metals in stars is ∆Ω (Sys. obs, stat. Z) = 2.48 × 10¯5. This term contribute to the (negative) variation of dust density due to astration which is

This shows that the Astration contributes (11 − 49)% (for a high or low SN dust destruction efficiency, respectively) to the total amount of dust destruction; the rest is removed by the other two mechanisms.

(b) Destruction by hot gas

As already mentioned, they assumed that as dust gets embedded in the hot phase, it is – for their purposes – instantaneously and completely eroded by sputtering. This implies that dust associated with metals contained in the hot cosmic gas at z = 0 must be removed from the total budget. The metal content of hot gas has increased from z = 1 → 0 by ∆Ωobs (Sys. Z, stat. h) = 4.8 × 10¯6. This term contributes a negative variation equal to

corresponding to a mere (2 − 9)% of the total dust destruction budget.

(c) Destruction by Supernova (SN) shocks in galaxies

Finally, they considered dust destruction by SN shocks in the interstellar medium (ISM) of galaxies, which they identified here with the cold, neutral gas. Recent detailed numerical simulations of dust production and destruction in SN explosions find that, once the presence of a pre-supernova wind-driven cavity is properly included, dust destruction is strongly suppressed. The physical reason for this is that the dust is collected in a dense shell by the wind; the shell represents an almost insurmountable barrier that prevents the SN blast wave from processing the majority of the ambient dust protected by the shell. As a result they found, under typical ambient conditions (gas density n ≈ 1 cm¯3), the amount of dust destroyed per SN event is M_d,sn = 0.45M, i.e. 5−10 times less than usually assumed, with a hard upper limit of M_d,sn < 3.0M⊙ set by the available metal budget and maximal grain growth. This lower efficiency might be explained by effective sheilding of dust against shock processing in pre-supernova wind shells.


Reference: A. Ferrara, C. Peroux, “Late-time cosmic evolution of dust: solving the puzzle”, pp. 1-10, ArXiv, 2021. https://arxiv.org/abs/2103.06887


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