110 research outputs found
Modelling and interpreting spectral energy distributions of galaxies with BEAGLE
We present a new-generation tool to model and interpret spectral energy
distributions (SEDs) of galaxies, which incorporates in a consistent way the
production of radiation and its transfer through the interstellar and
intergalactic media. This flexible tool, named BEAGLE (for BayEsian Analysis of
GaLaxy sEds), allows one to build mock galaxy catalogues as well as to
interpret any combination of photometric and spectroscopic galaxy observations
in terms of physical parameters. The current version of the tool includes
versatile modeling of the emission from stars and photoionized gas, attenuation
by dust and accounting for different instrumental effects, such as
spectroscopic flux calibration and line spread function. We show a first
application of the BEAGLE tool to the interpretation of broadband SEDs of a
published sample of galaxies at redshifts . We find that the constraints derived on photometric redshifts
using this multi-purpose tool are comparable to those obtained using public,
dedicated photometric-redshift codes and quantify this result in a rigorous
statistical way. We also show how the post-processing of BEAGLE output data
with the Python extension PYP-BEAGLE allows the characterization of systematic
deviations between models and observations, in particular through posterior
predictive checks. The modular design of the BEAGLE tool allows easy extensions
to incorporate, for example, the absorption by neutral galactic and
circumgalactic gas, and the emission from an active galactic nucleus, dust and
shock-ionized gas. Information about public releases of the BEAGLE tool will be
maintained on http://www.jacopochevallard.org/beagle.Comment: added missing term in equation 4.1 (Erratum submitted to MNRAS
The [OIII]H Equivalent Width Distribution at z7: Implications for the Contribution of Galaxies to Reionization
We quantify the distribution of [OIII]+H line strengths at z7
using a sample of 20 bright (M 21) galaxies. We
select these systems over wide-area fields (2.3 deg total) using a new
colour-selection which precisely selects galaxies at z6.636.83, a
redshift range where blue Spitzer/IRAC [3.6][4.5] colours unambiguously
indicate strong [OIII]H emission. These 20 galaxies suggest a
log-normal [OIII]H EW distribution with median EW =
759 and standard deviation =
0.26 dex. We find no evidence for strong variation in this EW
distribution with UV luminosity. The typical [OIII]+H EW at z7
implied by our sample is considerably larger than that in massive star forming
galaxies at z2, consistent with a shift toward larger average sSFR (4.4
Gyr) and lower metallicities (0.16 Z). We also find evidence for
the emergence of a population with yet more extreme nebular emission
([OIII]+H EW1200 ) that is rarely seen at
lower redshifts. These objects have extremely large sSFR (30 Gyr), as
would be expected for systems undergoing a burst or upturn in star formation.
While this may be a short-lived phase, our results suggest that 20% of the
z7 population has such extreme nebular emission, implying that galaxies
likely undergo intense star formation episodes regularly at z6. We argue
that this population may be among the most effective ionizing agents in the
reionization era, both in terms of photon production efficiency and escape
fraction. We furthermore suggest that galaxies passing through this large sSFR
phase are likely to be very efficient in forming bound star clusters.Comment: 20 pages, 11 figures. Accepted in MNRAS with minor revision
Effect of primordial non-Gaussianities on the far-UV luminosity function of high-redshift galaxies: implications for cosmic reionization
[Abridged] Understanding how the intergalactic medium (IGM) was reionized at
z > 6 is one of the big challenges of current high redshift astronomy. It
requires modelling the collapse of the first astrophysical objects (Pop III
stars, first galaxies) and their interaction with the IGM, while at the same
time pushing current observational facilities to their limits. The
observational and theoretical progress of the last few years have led to the
emergence of a coherent picture in which the budget of hydrogen-ionizing
photons is dominated by low-mass star-forming galaxies, with little
contribution from Pop III stars and quasars. The reionization history of the
Universe therefore critically depends on the number density of low-mass
galaxies at high redshift. In this work, we explore how changes in the
statistical properties of initial density fluctuations affect the formation of
early galaxies. Following Habouzit et al. (2014), we run 5 N-body simulations
with Gaussian and (scale-dependent) non-Gaussian initial conditions, all
consistent with Planck constraints. By appealing to a galaxy formation model
and to a population synthesis code, we compute the far-UV galaxy luminosity
function down to M_UV = -14 at redshift 7 < z < 15. We find that models with
strong primordial non-Gaussianities on < Mpc scales show a far-UV luminosity
function significantly enhanced in low-mass galaxies. We adopt a reionization
model calibrated from state-of-the-art hydrodynamical simulations and show that
such non-Gaussianities leave a clear imprint on the Universe reionization
history and electron Thomson scattering optical depth tau_E. Although current
uncertainties in the physics of reionization and on the determination of tau_E
still dominate the signatures of non-Gaussianities, our results suggest that
tau_E could ultimately be used to constrain the statistical properties of
initial density fluctuations.Comment: 18 pages, 12 figures, accepted for publication in MNRA
Testing primordial non-Gaussianities on galactic scales at high redshift
Primordial non-Gaussianities provide an important test of inflationary
models. Although the Planck CMB experiment has produced strong limits on
non-Gaussianity on scales of clusters, there is still room for considerable
non-Gaussianity on galactic scales. We have tested the effect of local
non-Gaussianity on the high redshift galaxy population by running five
cosmological N-body simulations down to z=6.5. For these simulations, we adopt
the same initial phases, and either Gaussian or scale-dependent non-Gaussian
primordial fluctuations, all consistent with the constraints set by Planck on
clusters scales. We then assign stellar masses to each halo using the halo -
stellar mass empirical relation of Behroozi et al. (2013). Our simulations with
non-Gaussian initial conditions produce halo mass functions that show clear
departures from those obtained from the analogous simulations with Gaussian
initial conditions at z>~10. We observe a >0.3 dex enhancement of the low-end
of the halo mass function, which leads to a similar effect on the galaxy
stellar mass function, which should be testable with future galaxy surveys at
z>10. As cosmic reionization is thought to be driven by dwarf galaxies at high
redshift, our findings may have implications for the reionization history of
the Universe.Comment: 6 pages, 3 figures, 1 table, MNRAS (Letters) in pres
Ultraviolet spectra of extreme nearby star-forming regions --- approaching a local reference sample for JWST
Nearby dwarf galaxies provide a unique laboratory in which to test stellar
population models below . Such tests are particularly important for
interpreting the surprising high-ionization UV line emission detected at
in recent years. We present HST/COS ultraviolet spectra of ten nearby
metal-poor star-forming galaxies selected to show He II emission in SDSS
optical spectra. The targets span nearly a dex in gas-phase oxygen abundance
() and present uniformly large specific star
formation rates (sSFR ). The UV spectra confirm
that metal-poor stellar populations can power extreme nebular emission in
high-ionization UV lines, reaching C III] equivalent widths comparable to those
seen in systems at . Our data reveal a marked transition in UV
spectral properties with decreasing metallicity, with systems below
() presenting minimal
stellar wind features and prominent nebular emission in He II and C IV. This is
consistent with nearly an order of magnitude increase in ionizing photon
production beyond the -ionizing edge relative to H-ionizing flux
as metallicity decreases below a fifth solar, well in excess of standard
stellar population synthesis predictions. Our results suggest that often
neglected sources of energetic radiation such as stripped binary products and
very massive O-stars produce a sharper change in the ionizing spectrum with
decreasing metallicity than expected. Consequently, nebular emission in C IV
and He II powered by these stars may provide useful metallicity constraints in
the reionization era.Comment: 27 pages, 13 figures, 11 tables, accepted for publication in MNRA
Lyman-alpha and CIII] Emission in z=7-9 Galaxies: Accelerated Reionization Around Luminous Star Forming Systems?
We discuss new Keck/MOSFIRE spectroscopic observations of four luminous
galaxies at z~7-9 selected to have intense optical line emission by
Roberts-Borsani et al. (2016). Previous follow-up has revealed Lyman-alpha in
two of the four galaxies. Our new MOSFIRE observations confirm that Lyman-alpha
is present in the entire sample. We detect Lyman-alpha emission in COS-zs7-1,
confirming its redshift as z=7.154, and we detect Lyman-alpha in EGS-zs8-2 at
z=7.477, verifying a tentative detection presented in an earlier study. The
ubiquity of Lyman-alpha in this sample is puzzling given that the IGM is likely
significantly neutral over 7<z<9. To investigate this result in more detail, we
have initiated a campaign to target UV metal emission in the four Lyman-alpha
emitters as a probe of both the radiation field and the velocity offset of
Lyman-alpha. Here we present the detection of intense CIII] emission in
EGS-zs8-1, a galaxy from this sample previously shown to have Lyman-alpha at
z=7.73. Photoionization models indicate that an intense radiation field and low
metallicity are required to reproduce the intense CIII] and optical line
emission. We argue that this extreme radiation field is likely to affect the
local environment, increasing the transmission of Lyman-alpha through the
galaxy. Moreover, the centroid of CIII] indicates that Lyman-alpha is
redshifted from the systemic value by 340 km/s. This velocity offset is larger
than that seen in less luminous systems, providing an additional explanation
for the transmission of Lyman-alpha emission through the IGM. Since the
transmission is further enhanced by the likelihood that such systems are also
situated in the densest regions with the largest ionized bubbles, the
visibility of Lyman-alpha at z>7 is expected to be strongly
luminosity-dependent, with the most effective transmission occurring in systems
with intense star formation.Comment: Submitted to MNRAS, 13 pages, 8 figure
Encoding the infrared excess (IRX) in the NUVrK color diagram for star-forming galaxies
We present an empirical method of assessing the star formation rate (SFR) of
star-forming galaxies based on their locations in the rest-frame color-color
diagram (NUV-r) vs (r-K). By using the Spitzer 24 micron sample in the COSMOS
field (~16400 galaxies with 0.2 < z < 1.3) and a local GALEX-SDSS-SWIRE sample
(~700 galaxies with z = <
L_IR / L_UV > can be described by a single vector, NRK, that combines the two
colors. The calibration between and NRK allows us to recover the IR
luminosity, L_IR, with an accuracy of ~0.21 dex for the COSMOS sample and ~0.27
dex for the local one. The SFRs derived with this method agree with the ones
based on the observed (UV+IR) luminosities and on the spectral energy
distribution fitting for the vast majority (~85 %) of the star-forming
population. Thanks to a library of model galaxy SEDs with realistic
prescriptions for the star formation history, we show that we need to include a
two-component dust model (i.e., birth clouds and diffuse ISM) and a full
distribution of galaxy inclinations in order to reproduce the behavior of the
stripes in the NUVrK diagram. In conclusion, the NRK method, based only
on rest-frame UV and optical colors available in most of the extragalactic
fields, offers a simple alternative of assessing the SFR of star-forming
galaxies in the absence of far-IR or spectral diagnostic observations.Comment: 21 pages, 22 figures, in publication in Astronomy & Astrophysic
Black hole formation and growth with non-Gaussian primordial density perturbations
Quasars powered by massive black holes (BHs) with mass estimates above a
billion solar masses have been identified at redshift 6 and beyond. The
existence of such BHs requires almost continuous growth at the Eddington limit
for their whole lifetime, of order of one billion years. In this paper, we
explore the possibility that positively skewed scale-dependent non-Gaussian
primordial fluctuations may ease the assembly of massive BHs. In particular,
they produce more low-mass halos at high redshift, thus altering the production
of metals and ultra-violet flux, believed to be important factors in BH
formation. Additionally, a higher number of progenitors and of nearly
equal-mass halo mergers would boost the mass increase provided by BH-BH mergers
and merger-driven accretion. We use a set of two cosmological simulations, with
either Gaussian or scale-dependent non-Gaussian primordial fluctuations to
perform a proof-of-concept experiment to estimate how BH formation and growth
are altered. We estimate the BH number density and the fraction of halos where
BHs form, for both simulations and for two popular scenarios of BH formation
(remnants of the first generation of stars and direct collapse in the absence
of metals and molecular hydrogen). We find that the fractions of halos where
BHs form are almost identical, but that non-Gaussian primordial perturbations
increase the total number density of BHs for the both BH formation scenarios.
We also evolve BHs using merger trees extracted from the simulations and find
that non-Gaussianities increase both the BH masses and the number of the most
massive BHs.Comment: 11 pages, 10 figures, MNRAS accepte
Searching for z > 6.5 Analogs Near the Peak of Cosmic Star Formation
Strong [OIII]4959,5007+H emission appears to be
typical in star-forming galaxies at z > 6.5. As likely contributors to cosmic
reionization, these galaxies and the physical conditions within them are of
great interest. At z > 6.5, where Ly is greatly attenuated by the
intergalactic medium, rest-UV metal emission lines provide an alternative
measure of redshift and also constraints on the physical properties of
star-forming regions and massive stars. We present the first statistical sample
of rest-UV line measurements in z 2 galaxies selected as analogs of
those in the reionization era based on [OIII]4959,5007 EW or
rest-frame U-B color. Our sample is drawn from the 3D-HST Survey and spans the
redshift range 1.36 z 2.49. We find that the median
Ly and CIII]1907,1909 EWs of our sample are
significantly greater than those of z 2 UV-continuum-selected
star-forming galaxies. Measurements from both individual and composite spectra
indicate a monotonic, positive correlation between CIII] and [OIII], while a
lack of trend is observed between Ly and [OIII] at [OIII] EW <
1000\unicode{x212B}. At higher [OIII] EW, extreme Ly emission starts
to emerge. Using stacked spectra, we find that Ly and CIII] are
significantly enhanced in galaxies with lower metallicity. Two objects in our
sample appear comparable to z > 6.5 galaxies with exceptionally strong rest-UV
metal line emission. These objects have significant
CIV1548,1550, HeII1640, and
OIII]1661,1665 emission in addition to intense Ly or
CIII]. Detailed characterization of these lower-redshift analogs provides
unique insights into the physical conditions in z > 6.5 star-forming regions,
motivating future observations of reionization-era analogs at lower redshifts.Comment: 22 pages, 9 figures, accepted for publication in Ap
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