109 research outputs found
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) 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 relation. At , we find that the scatter decreases
slightly with stellar mass from 0.35 dex at M
M to 0.30 dex at M M. The
scatter decreases from to by 0.05 dex at M M and by 0.15 dex for lower masses. We show that the scatter
at 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
( 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
tend to have been above/below the main sequence for 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 -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
-enhancement among star-forming galaxies at . The definition of
-enhancement in a composite stellar population is ambiguous. We
elucidate two definitions - termed 'mean' and 'galactic' -enhancement -
in more detail. While a star-forming galaxy has a high 'mean'
-enhancement when its stars formed rapidly, a galaxy with a large
'galactic' -enhancement generally had a delayed star formation history.
We find that absorption-line strengths of Mg and Fe correlate with variations
in -enhancement. These correlations are strongest for the 'galactic'
-enhancement. However, we show that these are mostly caused by other
effects which are cross-correlated with -enhancement, such as
variations in the light-weighted age. This severely complicates the retrieval
of -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 -enhancements. We suggest that
this more complex coupling between -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
Lyman- 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
deg. 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 LF between and ,
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 Mpc. Combined with our
wide-field SA22 measurements, our results indicate a non-Schechter-like bright
end of the LF at 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
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 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. 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 with H-alpha (Ha). We derive a simple empirical relation that robustly retrieves f as a function of Lya rest-frame EW (EW): f= 0.0048 EW 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 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) calibration to several sources at z>2.6 to find that star-forming LAEs have SFRs typically ranging from 0.1 to 20 M/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 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 Mpc. Our Lya emitters span a wide range in Lya luminosities, from faint to bright (L erg s) and rest-frame equivalent widths (EW~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 . 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 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 , respectively, uniformly selected down to a flux limit of erg s cm through a narrow band survey covering an area of deg 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 CII] emitters are consistent with a mixture of blue (UV slope ) 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 yr. CIII] emitters at trace a general population of SF galaxies, with , a variety of optical morphologies, including isolated and interacting galaxies and low BHAR (
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