Abstract
We present a new and simple method to measure the instantaneous mass and radial growth rates of the stellar discs of spiral galaxies, based on their star formation rate surface density (SFRD) profiles. Under the hypothesis that discs are exponential with time-varying scalelengths, we derive a universal theoretical profile for the SFRD, with a linear dependence on two parameters: the specific mass growth rate |$\nu _ {\rm M} \equiv \dot{M}_\star /M_\star$| and the specific radial growth rate |$\nu _ {\rm R} \equiv \dot{R}_\star /R_\star$| of the disc. We test our theory on a sample of 35 nearby spiral galaxies, for which we derive a measurement of νM and νR. 32/35 galaxies show the signature of ongoing inside-out growth (νR > 0). The typical derived e-folding time-scales for mass and radial growth in our sample are ∼10 and ∼30 Gyr, respectively, with some systematic uncertainties. More massive discs have a larger scatter in νM and νR, biased towards a slower growth, both in mass and size. We find a linear relation between the two growth rates, indicating that our galaxy discs grow in size at ∼0.35 times the rate at which they grow in mass; this ratio is largely unaffected by systematics. Our results are in very good agreement with theoretical expectations if known scaling relations of disc galaxies are not evolving with time.
Highlights
The theory of cosmological tidal torques (Peebles 1969) predicts the mean specific angular momentum of galaxies to be an increasing function of time
If applied to spiral galaxies, in which stars are mostly distributed on a rotating, centrifugally supported, disc, the theory suggests that the outer parts, with higher specific angular momenta, should form later than the inner ones (Larson 1976, the so-called inside-out formation scenario), implying that spirals should grow in size while they grow in mass
We have developed, from very simple assumptions, a model that predicts a universal shape for the radial profile of the SFRD of spiral galaxies
Summary
The theory of cosmological tidal torques (Peebles 1969) predicts the mean specific angular momentum of galaxies to be an increasing function of time. If applied to spiral galaxies, in which stars are mostly distributed on a rotating, centrifugally supported, disc, the theory suggests that the outer parts, with higher specific angular momenta, should form later than the inner ones (Larson 1976, the so-called inside-out formation scenario), implying that spirals should grow in size while they grow in mass Apart from this quite general prediction provided by cosmology, the details about how stellar discs form and grow in mass and size are not known from first principles and significant observational effort is still required to shed light on the missing links from structure formation to galaxy formation. The interpretation and comparison of these pioneering studies is made non-trivial by inhomogeneities among observations at different redshifts, as well as differences in sample definitions and analysis techniques (see e.g. Lange et al 2015); several subtle issues have been shown to significantly bias the results, most notably the selection effect due to cosmological
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