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

The mean free path of hydrogen ionizing photons during the epoch of reionization

We use the Aurora radiation-hydrodynamical simulations to study the mean free path (MFP) for hydrogen ionizing photons during the epoch of reionization. We directly measure the MFP by averaging the distance 1 Ry photons travel before reaching an optical depth of unity along random lines-of-sight. During reionization the free paths tend to end in neutral gas with densities near the cosmic mean, while after reionizaton the end points tend to be overdense but highly ionized. Despite the increasing importance of discrete, over-dense systems, the cumulative contribution of systems with $N_{\rm{HI}} \lesssim 10^{16.5}~{\rm cm^{-2}}$ suffices to drive the MFP at $z \approx 6$, while at earlier times higher column densities are more important. After reionization the typical size of HI systems is close to the local Jeans length, but during reionization it is much larger. The mean free path for photons originating close to galaxies, $\rm{MFP_{gal}}$, is much smaller than the cosmic MFP. After reionization this enhancement can remain significant up to starting distances of $\sim 1$ comoving Mpc. During reionization, however, $\rm{MFP_{gal}}$ for distances $\sim 10^2 - 10^3$ comoving kpc typically exceeds the cosmic MFP. These findings have important consequences for models that interpret the intergalactic MFP as the distance escaped ionizing photons can travel from galaxies before being absorbed and may cause them to under-estimate the required escape fraction from galaxies, and/or the required emissivity of ionizing photons after reionization.Comment: 13 pages, 9 figures, 1 table; submitted to MNRA

Predictions for the relation between strong HI absorbers and galaxies at redshift 3

We combine cosmological, hydrodynamical simulations with accurate radiative transfer corrections to investigate the relation between strong HI absorbers (N_HI >~ 10^17 /cm^2) and galaxies at redshift z = 3. We find a strong anti-correlation between the column density and the impact parameter that connects the absorber to the nearest galaxy. The median impact parameters for Lyman Limit (LL) and Damped Lyman-{\alpha} (DLA) systems are ~10 and ~1 proper kpc, respectively. If normalized to the size of the halo of the nearest central galaxy, the median impact parameters for LL and DLA systems become ~1 and ~10^-1 virial radii, respectively. At a given HI column density, the impact parameter increases with the mass of the closest galaxy, in agreement with observations. We predict most strong HI absorbers to be most closely associated with extremely low-mass galaxies, M_star < 10^8 M_sun and star formation rate <10^-1 M_sun/yr. We also find a correlation between the column density of absorbers and the mass of the nearest galaxy. This correlation is most pronounced for DLAs with N_HI > 10^21 /cm^2 which are typically close to galaxies with M_star >~ 10^9 M_sun. Similar correlations exist between column density and other properties of the associated galaxies such as their star formation rates, halo masses and HI content. The galaxies nearest to HI absorbers are typically far too faint to be detectable with current instrumentation, which is consistent with the high rate of (often unpublished) non-detections in observational searches for the galaxy counterparts of strong HI absorbers. Moreover, we predict that the detected nearby galaxies are typically not the galaxies that are most closely associated with the absorbers, thus causing the impact parameters, star formation rates and stellar masses of the observed counterparts to be biased high.Comment: 21 pages, 14 figures; Accepted for publication in MNRA

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

An evaluation of performance information procurement system (PIPS)

Since procurement is seen as crucial for project success, many\ud methods have been developed and papers written about this issue. A remarkable\ud contribution in this field comes from Dean Kashiwagi (Arizona State University,\ud USA) who underpins his support for the Performance Information Procurement\ud System (PiPS) with claims of high project performance and client satisfaction.\ud Kashiwagi’s explanation for PiPS’s effects is based upon a theoretical\ud framework that relates to staff members’ ability to deal appropriately with\ud information by making sound decisions based not just on implicit expectation\ud and tacit experience. This is not, however, a satisfactory explanation. This paper\ud provides an overview of New Institutional Economics perspectives which are\ud better able to explain the effects of PiPS. The linking of these theories to\ud innovative PiPS elements clears the path to effectively select and apply PiPS\ud elements within suitable projects in the Dutch construction industry. This will\ud enhance industry performance and is of interest to all stakeholders