The origin of scatter in the star formation rate - stellar mass relation

Observations have revealed that the star formation rate (SFR) and stellar mass (M$_{\rm star}$) of star-forming galaxies follow a tight relation known as the galaxy main sequence. However, what physical information is encoded in this relation is under debate. Here, we use the EAGLE cosmological hydrodynamical simulation to study the mass dependence, evolution and origin of scatter in the SFR-M$_{\rm star}$ relation. At $z=0$, we find that the scatter decreases slightly with stellar mass from 0.35 dex at M$_{\rm star} \approx 10^9$ M$_{\odot}$ to 0.30 dex at M$_{\rm star} \gtrsim 10^{10.5}$ M$_{\odot}$. The scatter decreases from $z=0$ to $z=5$ by 0.05 dex at M$_{\rm star} \gtrsim 10^{10}$ M$_{\odot}$ and by 0.15 dex for lower masses. We show that the scatter at $z=0.1$ originates from a combination of fluctuations on short time-scales (ranging from 0.2-2 Gyr) that are presumably associated with self-regulation from cooling, star formation and outflows, but is dominated by long time-scale ($\sim 10$ Gyr) variations related to differences in halo formation times. Shorter time-scale fluctuations are relatively more important for lower-mass galaxies. At high masses, differences in black hole formation efficiency cause additional scatter, but also diminish the scatter caused by different halo formation times. While individual galaxies cross the main sequence multiple times during their evolution, they fluctuate around tracks associated with their halo properties, i.e. galaxies above/below the main sequence at $z = 0.1$ tend to have been above/below the main sequence for $\gg1$ Gyr.Comment: Accepted for publication in MNRAS. Updated comparison to observations. More detailed investigation of the relative importance of SFH-fluctuation time-scales on the SFR(Mstar) scatter (S 4.3, Figs. 6 & 7

Star-forming galaxies are predicted to lie on a fundamental plane of mass, star formation rate and \alpha-enhancement

Observations show that star-forming galaxies reside on a tight three-dimensional plane between mass, gas-phase metallicity and star formation rate (SFR), which can be explained by the interplay between metal-poor gas inflows, SFR and outflows. However, different metals are released on different time-scales, which may affect the slope of this relation. Here, we use central, star-forming galaxies with M$_{\rm star}=10^{9.0-10.5}$ M$_{\odot}$ from the EAGLE hydrodynamical simulation to examine three-dimensional relations between mass, SFR and chemical enrichment using absolute and relative C, N, O and Fe abundances. We show that the scatter is smaller when gas-phase $\alpha$-enhancement is used rather than metallicity. A similar plane also exists for stellar $\alpha$-enhancement, implying that present-day specific SFRs are correlated with long time-scale star formation histories. Between $z=0$ and 1, the $\alpha$-enhancement plane is even more insensitive to redshift than the plane using metallicity. However, it evolves at $z>1$ due to lagging iron yields. At fixed mass, galaxies with higher SFRs have star formation histories shifted toward late times, are more $\alpha$-enhanced and this $\alpha$-enhancement increases with redshift as observed. These findings suggest that relations between physical properties inferred from observations may be affected by systematic variations in $\alpha$-enhancements.Comment: 6 pages, 4 figures; Accepted for publication in MNRAS Letter

The Significance of Transport Costs in Africa

The success of Africa's exports, as well as its spatial development, depends on lowering transport costs. In this Policy Brief, we address a number of pertinent questions on transport costs in Africa, such as 'what are transport costs?', 'do transport costs matter for trade?', 'how important are transport costs in practice?', and 'why are Africa's transport costs so high?' We present a case study of the firm location decisions of exporters in South Africa to illustrate the significance in particular of domestic transport costs for manufactured exports. The message from this Policy Brief is that Africa's international transport costs are significantly higher than that of other regions, and its domestic transport costs could be just as significant. Moreover we show how domestic transport costs influence the location, the quantity, and the diversity of manufactured exports. Various policy options to reduce transport costs in Africa are discussed.transport, costs, trade, Africa, infrastructure, roads, rail, ports, harbour, South Africa, goods, location, freight,

Export Diversity and Regional Growth: Empirical Evidence from South Africa

This paper provides empirical evidence on the relationship between exports, and in particular export diversity, and regional growth in a developing country context. Using export data for 19 sectors from 354 subnational (magisterial) districts of South Africa, we construct various measures of subnational export diversity. We find that it is not only how much that is exported, but also important is what is exported. Regions with less specialization and more diversified exports generally experienced higher economic growth rates and contributed much more to overall exports from South Africa. We also find that distance (and thus transport costs) may matter for export diversity. Estimating a cubic-spline density function for the various measures of export diversity we find that export diversity declines as the distance from a port (export hub) increases. Most magisterial districts with high export diversity values are located within 100 km of the nearest port. Furthermore, comparing the cubic-spline density functions for 2004 with that of 1996 shows that distance (transport costs) have become more important (under greater openness), with fewer diverse magisterial districts located further away from ports in 2004 than in 1996. One possible explanation for this changing pattern of export diversity may be due to the impact of greater foreign direct investment in South Africa since 1996.exports, export diversification, export variety, regional growth, new economic geography

New MBA

Once again, the USB’s MBA has been audited and approved by two international accreditation agencies. But nothing remains static. AMANDA MATTHEE finds out how the USB has updated its MBA for 2009

The Geographical Location of Manufacturing Exporters in South Africa

This paper provides empirical evidence on the location of export-oriented manufacturing firms in Africa (South Africa), and on how the patterns of location has changed over the past decade after the country embarked on trade liberalization. It is found that (a) the proximity to a port is an important consideration in most export-oriented manufacturing firms' location, with more than 70 per cent of manufacturing exports in South Africa originating from a band of 100 km from a port; and (b) there is a second band of location of these firms at a distance of between 200 and 400 km from the port. Between 1996 and 2004, manufactured exports in the band between 200 and 400 km from the nearest port increased. Various possible explanations for this dispersion of export location are discussed. These include (a) changes in international and domestic transport costs; (b) an increase in manufactured exports that depend on natural resources due to demand factors; and/or (c) inflation in land-rents or wage rates in the vicinity of hubs; and/or (d) the increasing productivity of export plants due to scale effects.geographical economics, manufacturing exports, domestic transport costs, South Africa

Princípio de precaução e preocupação ética: um casamento racional?

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The Optimal Distance to Port for Exporting Firms

Success in international trade depends, amongst other things, on distance from markets. Most new economic geography models focus on the distance between countries. In contrast much less theorizing and empirical analysis have focused on how distances within a country?for instance due to the location behaviour of exporting firms?matter to international trade. In this paper we contribute to the literature on the latter by offering a theoretical model to explain the optimal distance that an export-oriented firm would locate from a port. We present empirical evidence from South Africa in support of the model.distance, transport costs, manufactured exports

Labour demand and the distribution of wages in South African manufacturing exporters

This paper contributes to the understanding of the linkages between exporting, labour demand, and wages in South Africa. We disentangle labour market differences between exporters and non-exporters and find that exporters employ more people and pay higher wages. Given these higher wages we investigate how this wage premium is distributed within the exporting firm. There appears to be a wide dispersion of wages within exporters (particularly international/non-African exporters). However, almost all of that dispersion (particularly amongst continuing exporters) is explained by the labour productivity and size of these firms. This suggests that there is thus a large degree of dispersion for these variables for these firm groups (relative to non-exporters). Wage inequality within exporters is not driven by exporting but rather by characteristics associated with the types of firms which participate in the export market

Export diversity and regional growth: Empirical evidence from South Africa

This paper provides empirical evidence on the relationship between exports, and in particular export diversity, and regional growth in a developing country context. Using export data for 19 sectors from 354 subnational (magisterial) districts of South Africa, we construct various measures of subnational export diversity. We find that it is not only how much that is exported, but also important is what is exported. Regions with less specialization and more diversified exports generally experienced higher economic growth rates and contributed much more to overall exports from South Africa. We also find that distance (and thus transport costs) may matter for export diversity. Estimating a cubic-spline density function for the various measures of export diversity we find that export diversity declines as the distance from a port (export hub) increases. Most magisterial districts with high export diversity values are located within 100 km of the nearest port. Furthermore, comparing the cubic-spline density functions for 2004 with that of 1996 shows that distance (transport costs) have become more important (under greater openness), with fewer diverse magisterial districts located further away from ports in 2004 than in 1996. One possible explanation for this changing pattern of export diversity may be due to the impact of greater foreign direct investment in South Africa since 1996