Abstract
AbstractThroughout the Hubble time, gas makes its way from the intergalactic medium into galaxies fuelling their star formation and promoting their growth. One of the key properties of the accreting gas is its angular momentum, which has profound implications for the evolution of, in particular, disc galaxies. Here, we discuss how to infer the angular momentum of the accreting gas using observations of present-day galaxy discs. We first summarize evidence forongoinginside-out growth of star forming discs. We then focus on the chemistry of the discs and show how the observed metallicity gradients can be explained if gas accretes onto a disc rotating with a velocity 20 – 30% lower than the local circular speed. We also show that these gradients are incompatible with accretion occurring at the edge of the discs and flowing radially inward. Finally, we investigate gas accretion from a hot corona with a cosmological angular momentum distribution and describe how simple models of rotating coronae guarantee the inside-out growth of disc galaxies.
Highlights
At variance with massive quiescent ellipticals, which assembled most of their mass a long time ago and experienced, at some points in the past, an abrupt decline of their star formation rate, the majority of presently star forming galaxies have been undergoing, for most of the cosmic time, a rather constant or gently declining star formation history (e.g. Pacifici et al 2016)
Throughout the Hubble time, gas makes its way from the intergalactic medium into galaxies fuelling their star formation and promoting their growth
We focus on the chemistry of the discs and show how the observed metallicity gradients can be explained if gas accretes onto a disc rotating with a velocity 20 − 30% lower than the local circular speed
Summary
At variance with massive quiescent ellipticals, which assembled most of their mass a long time ago and experienced, at some points in the past, an abrupt decline of their star formation rate, the majority of presently star forming galaxies have been undergoing, for most of the cosmic time, a rather constant or gently declining star formation history (e.g. Pacifici et al 2016). Pacifici et al 2016) This could in principle be explained either by a gradual consumption of a very large initial amount of cold gas, or by continuous accretion of new gas from the intergalactic medium. Both theory and observations strongly argue in favour of the second option, as i) gradual accretion is expected from the cosmological theory of structure formation (e.g. van den Bosch et al 2014); ii) star forming galaxies have relatively short depletions times (Saintonge et al 2011) and iii) preventing a huge initial reservoir of gas from very rapid exhaustion requires an implausibly low star formation efficiency, in stark contrast with observations (Kennicutt & Evans 2012; Fraternali & Tomassetti 2012).
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