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

On the variation in stellar alpha-enhancements of star-forming galaxies in the EAGLE simulation

The ratio of $\alpha$-elements to iron in galaxies holds valuable information about the star-formation history since their enrichment occurs on different timescales. The fossil record of stars in galaxies has mostly been excavated for passive galaxies, since the light of star-forming galaxies is dominated by young stars which have much weaker atmospheric absorption features. Here we use the cosmological EAGLE simulation to investigate the origin of variations in $\alpha$-enhancement among star-forming galaxies at $z=0$. The definition of $\alpha$-enhancement in a composite stellar population is ambiguous. We elucidate two definitions - termed 'mean' and 'galactic' $\alpha$-enhancement - in more detail. While a star-forming galaxy has a high 'mean' $\alpha$-enhancement when its stars formed rapidly, a galaxy with a large 'galactic' $\alpha$-enhancement generally had a delayed star formation history. We find that absorption-line strengths of Mg and Fe correlate with variations in $\alpha$-enhancement. These correlations are strongest for the 'galactic' $\alpha$-enhancement. However, we show that these are mostly caused by other effects which are cross-correlated with $\alpha$-enhancement, such as variations in the light-weighted age. This severely complicates the retrieval of $\alpha$-enhancements in star-forming galaxies. The ambiguity is not severe for passive galaxies and we confirm that spectral variations in these galaxies are caused by measurable variations in $\alpha$-enhancements. We suggest that this more complex coupling between $\alpha$-enhancement and star formation histories can guide the interpretation of new observations of star-forming galaxies.Comment: 16 pages, 12 figures, 4 tables. Submitted to MNRAS. Comments/hints for missing citations are very welcome

Identification of the brightest Ly\alpha\ emitters at z=6.6: implications for the evolution of the luminosity function in the re-ionisation era

Using wide field narrow-band surveys, we provide a new measurement of the $z=6.6$ Lyman-$\alpha$ Emitter (LAE) luminosity function (LF), which constraints the bright end for the first time. We use a combination of archival narrow-band NB921 data in UDS and new NB921 measurements in SA22 and COSMOS/UltraVISTA, all observed with the Subaru telescope, with a total area of $\sim 5$ deg$^2$. We exclude lower redshift interlopers by using broad-band optical and near-infrared photometry and also exclude three supernovae with data split over multiple epochs. Combining the UDS and COSMOS samples we find no evolution of the bright end of the Ly$\alpha$ LF between $z=5.7$ and $6.6$, which is supported by spectroscopic follow-up, and conclude that sources with \emph{Himiko}-like luminosity are not as rare as previously thought, with number densities of $\sim 1.5\times10^{-5}$ Mpc$^{-3}$. Combined with our wide-field SA22 measurements, our results indicate a non-Schechter-like bright end of the LF at $z=6.6$ and a different evolution of \emph{observed} faint and bright LAEs. This differential evolution is also seen in the spectroscopic follow-up of UV selected galaxies and is now also confirmed for Ly$\alpha$ emitters, and we argue that it may be an effect of re-ionisation. Using a toy-model, we show that such differential evolution of the LF is expected, since brighter sources are able to ionise their surroundings earlier, such that Ly$\alpha$ photons are able to escape. Our targets are excellent candidates for detailed follow-up studies and provide the possibility to give a unique view on the earliest stages in the formation of galaxies and re-ionisation process.Comment: 20 pages, main results shown in Fig. 6 and Fig. 7, accepted by MNRA

Predicting Lyman-alpha escape fractions with a simple observable:Lyman-alpha in emission as an empirically calibrated star formation rate indicator

Lyman-alpha (Lya) is intrinsically the brightest line emitted from active galaxies. While it originates from many physical processes, for star-forming galaxies the intrinsic Lya luminosity is a direct tracer of the Lyman-continuum (LyC) radiation produced by the most massive O- and early-type B-stars with lifetimes of a few Myrs. As such, Lya luminosity should be an excellent instantaneous star formation rate (SFR) indicator. However, its resonant nature and susceptibility to dust as a rest-frame UV photon makes Lya very hard to interpret due to the uncertain Lya escape fraction, f$_{esc,Ly\alpha}$. Here we explore results from the CALYMHA survey at z=2.2, follow-up of Lya emitters (LAEs) at z=2.2-2.6 and a z~0-0.3 compilation of LAEs to directly measure f$_{esc,Ly\alpha}$ with H-alpha (Ha). We derive a simple empirical relation that robustly retrieves f$_{esc,Ly\alpha}$ as a function of Lya rest-frame EW (EW$_0$): f$_{esc,Ly\alpha}$= 0.0048 EW$_0[{\AA}]\pm0.05$ and we show that the relation is driven by a tight sequence between high ionisation efficiencies and low dust extinction in LAEs. Observed Lya luminosities and EW$_0$ are easy measurable quantities at high redshift, thus making our relation a practical tool to estimate intrinsic Lya and LyC luminosities under well controlled and simple assumptions. Our results allow observed Lya luminosities to be used to compute SFRs for LAEs at z~0-2.6 within 0.2 dex of the Ha dust corrected SFRs. We apply our empirical SFR(Lya,EW$_0$) calibration to several sources at z>2.6 to find that star-forming LAEs have SFRs typically ranging from 0.1 to 20 M$_{\odot}$/yr and that our calibration might be even applicable for luminous LAEs within the epoch of re-ionisation. Our results imply higher than canonical ionisation efficiencies and low dust content in LAEs across cosmic time, and will be easily tested with future observations with JWST

The Lyman-alpha luminosity function at z=5.7-6.6 and the steep drop of the faint end:implications for reionization

We present new results from the widest narrow band survey search for Lyman-alpha (Lya) emitters at z=5.7, just after reionization. We survey a total of 7 deg$^2$ spread over the COSMOS, UDS and SA22 fields. We find over 11,000 line emitters, out of which 514 are robust Lya candidates at z=5.7 within a volume of 6.3x10$^6$ Mpc$^3$. Our Lya emitters span a wide range in Lya luminosities, from faint to bright (L$_{\rm Ly\alpha}\sim10^{42.5-44}$ erg s$^{-1}$) and rest-frame equivalent widths (EW$_0$~25-1000 \AA) in a single, homogeneous data-set. By combining all our fields we find that the faint end slope of the z=5.7 Lya luminosity function is very steep, with $\alpha=-2.3^{+0.4}_{-0.3}$. We also present an updated z=6.6 Lya luminosity function, based on comparable volumes and obtained with the same methods, which we directly compare with that at z=5.7. We find a significant decline of the number density of faint Lya emitters from z=5.7 to z=6.6 (by $0.5\pm0.1$ dex), but no evolution at the bright end/no evolution in L*. Faint Lya emitters at z=6.6 show much more extended haloes than those at z=5.7, suggesting that neutral Hydrogen plays an important role, increasing the scattering and leading to observations missing faint Lya emission within the epoch of reionization. All together, our results suggest that we are observing patchy reionization which happens first around the brightest Lya emitters, allowing the number densities of those sources to remain unaffected by the increase of neutral Hydrogen fraction from z~5 to z~7

A 1.4 deg^2 blind survey for CII], CIII] and CIV at z~0.7-1.5. I:Nature, morphologies and equivalent widths

While traditionally associated with active galactic nuclei (AGN), the properties of the CII], CIII] and CIV emission lines are still uncertain as large, unbiased samples of sources are scarce. We present the first blind, statistical study of CII], CIII] and CIV emitters at $z\sim0.68,1.05,1.53$, respectively, uniformly selected down to a flux limit of $\sim4\times10^{-17}$ erg s$^{-1}$ cm$^{-1}$ through a narrow band survey covering an area of $\sim1.4$ deg$^2$ over COSMOS and UDS. We detect 16 CII], 35 CIII] and 17 CIV emitters, whose nature we investigate using optical colours as well as HST, X-ray, radio and far infra-red data. We find that $z\sim0.7$ CII] emitters are consistent with a mixture of blue (UV slope $\beta=-2.0\pm0.4$) star forming galaxies with disky HST structure and AGN with Seyfert-like morphologies. Bright CII] emitters have individual X-ray detections as well as high average black hole accretion rates (BHAR) of $\sim0.1$ $M_{\odot}$ yr$^{-1}$. CIII] emitters at $z\sim1.05$ trace a general population of SF galaxies, with $\beta=-0.8\pm1.1$, a variety of optical morphologies, including isolated and interacting galaxies and low BHAR (