Galactic fly-bys: New source of lithium production

Abstract

Observations of low-metallicity halo stars have revealed a puzzling result: the abundance of 7 Li in these stars is at least three times lower than their predicted primordial abundance. It is unclear whether the cause of this disagreement is a lack of understanding of lithium destruction mechanisms in stars or the non-standard physics behind the Big Bang Nucleosynthesis (BBN). Uncertainties related to the destruction of lithium in stars can be circumvented if lithium abundance is measured in the "pristine" gas of the low metallicity systems. The first measurement in one such system, the Small Magellanic Cloud (SMC), was found to be at the level of the pure expected primordial value, but is on the other hand, just barely consistent with the expected galactic abundance for the system at the SMC metallicity, where important lithium quantity was also produced in interactions of galactic cosmic rays (GCRs) and presents an addition to the already present primordial abundance. Due to the importance of the SMC lithium measurement for the resolution of the lithium problem, we here draw attention to the possibility of another post-BBN production channel of lithium, which could present an important addition to the observed SMC lithium abundance. Besides standard galactic cosmic rays, additional post-BBN production of lithium might come from cosmic rays accelerated in galaxy-galaxy interactions. This might be important for a system such is the SMC, which has experienced galaxy harassment in its history. Within a simplified but illustrative framework we demonstrate that large-scale tidal shocks from a few galactic fly-bys can possibly produce lithium in amounts comparable to those expected from the interactions of galactic cosmic-rays produced in supernovae over the entire history of a system. In case of the SMC, we find that only two such fly-bys could possibly account for as much lithium as the standard, GCR production channel. However, adding any a new mechanism for post-BBN production of lithium, like the one proposed here, would contribute to the observed SMC lithium abundance, causing this measurement to be more in tension with the primordial abundance predicted by the standard BBN.

Cosmic Rays: A View Into Galactic Interactions and the New Physics

“…Prodanovic et al 2013 have pointed out that it would be sufficient for the entire gas of the SMC to be shocked only twice, to accelerate enough TCRs to produce as much lithium as galactic cosmic-rays (GCRs) have made throughout its history. This is consistent with the fact that SMC has already experienced at least two close galactic fly-bys and with the anomalously high observed abundance ratio between the two lithium isotopes [5]. Therefore, observations of lithium abundance in the SMC indicate that additional cosmic-ray population may have been produced at some epoch.…”

“…The presence of an new cosmic-ray population, in addition to already present galactic cosmic rays, can have important consequences for light element abundances [5] and the FIR-radio parameter measured in interacting, star-forming galaxies [7]. In terms of light elements, specifically lithium abundance measurements, observations in the Small Magellanic Cloud have found anomalous isotopic ratios of lithium, consistent with its ISM being exposed to additional comic-ray population, which in turn would indicate that the discrepancy between primordial lithium abundance predicted by the Big Bang Nucleosynthesis and that observed in low-metallicity halo stars, should be sought for in the form of the new physics [5]. Analysis of the FIR-radio correlation in interacting galaxies indicates that the FIR-radio parameter would vary across different interaction stages due to effects of tidal shock, causing a dispersion in the FIR-radio correlation of star-forming galaxies.…”

“…However, it was pointed out [5] that galactic interactions can also be a new source of cosmic rays accelerated [6] in large-scale tidal shocks âȂŞ tidal cosmic rays (TCRs). The possible effects of TCRs have been explored by looking at how they would impact light element abundances [5] and non-thermal radio emission [7] of interacting galaxy. Both studies have demonstrated that these effects can be significant.…”

Possible breaking of the FIR–radio correlation in tidally interacting galaxies

“…It was pointed out that tidal shocks that accompany these interactions can give rise to a population of tidal cosmic rays (TCRs). This can have a potentially significant impact on nucleosynthesis of light elements such as lithium (Prodanović et al 2013). Moreover, it is clear that the presence of such cosmic-ray population will also result in an enhanced radio emission (Prodanović et al 2013).…”

“…This can have a potentially significant impact on nucleosynthesis of light elements such as lithium (Prodanović et al 2013). Moreover, it is clear that the presence of such cosmic-ray population will also result in an enhanced radio emission (Prodanović et al 2013). As a result, the FIR-radio correlation will be impacted.…”