197 research outputs found
Massive Star cluster formation under the microscope at z=6
We report on a superdense star-forming region with an effective radius (R_e)
smaller than 13 pc identified at z=6.143 and showing a star-formation rate
density \Sigma_SFR~1000 Msun/yr/kpc2 (or conservatively >300 Msun/yr/kpc2).
Such a dense region is detected with S/N>40 hosted by a dwarf extending over
440 pc, dubbed D1 (Vanzella et al. 2017b). D1 is magnified by a factor
17.4+/-5.0 behind the Hubble Frontier Field galaxy cluster MACS~J0416 and
elongated tangentially by a factor 13.2+/-4.0 (including the systematic
errors). The lens model accurately reproduces the positions of the confirmed
multiple images with a r.m.s. of 0.35", and the tangential stretch is well
depicted by a giant multiply-imaged Lya arc. D1 is part of an interacting
star-forming complex extending over 800 pc. The SED-fitting, the very blue
ultraviolet slope (\beta ~ -2.5, F(\lambda) ~ \lambda^\beta) and the prominent
Lya emission of the stellar complex imply that very young (< 10-100 Myr),
moderately dust-attenuated (E(B-V)<0.15) stellar populations are present and
organised in dense subcomponents. We argue that D1 (with a stellar mass of 2 x
10^7 Msun) might contain a young massive star cluster of M < 10^6 Msun and
Muv~-15.6 (or m_uv=31.1), confined within a region of 13 pc, and not dissimilar
from some local super star clusters (SSCs). The ultraviolet appearance of D1 is
also consistent with a simulated local dwarf hosting a SSC placed at z=6 and
lensed back to the observer. This compact system fits into some popular
globular cluster formation scenarios. We show that future high spatial
resolution imaging (e.g., E-ELT/MAORY-MICADO and VLT/MAVIS) will allow us to
spatially resolve light profiles of 2-8 pc.Comment: 21 pages, 14 figures, 1 table, MNRAS accepte
Ionising the Intergalactic Medium by Star Clusters: The first empirical evidence
We present a VLT/X-Shooter spectroscopy of the Lyman continuum (LyC) emitting
galaxy 'Ion2' at z=3.2121 and compare it to that of the recently discovered
strongly lensed LyC-emitter at z=2.37, known as the 'Sunburst' arc. Three main
results emerge from the X-Shooter spectrum: (a) the Lya has three distinct
peaks with the central one at the systemic redshift, indicating a ionised
tunnel through which both Lya and LyC radiation escape; (b) the large O32
oxygen index ([OIII]4959-5007 / [OII]3727-3729) of 9.18(-1.32/+1.82) is
compatible to those measured in local (z~0.4) LyC leakers; (c) there are narrow
nebular high-ionisation metal lines with \sigma_v < 20 km/s, which confirms the
presence of young hot, massive stars. The HeII1640 appears broad, consistent
with a young stellar component including Wolf-Rayet stars. Similarly, the
Sunburst LyC-emitter shows a triple-peaked Lya profile and from VLT/MUSE
spectroscopy the presence of spectral features arising from young hot and
massive stars. The strong lensing magnification, (\mu > 20), suggests that this
exceptional object is a gravitationally-bound star cluster observed at a
cosmological distance, with a stellar mass M <~ 10^7 Msun and an effective
radius smaller than 20 pc. Intriguingly, sources like Sunburst but without
lensing magnification might appear as Ion2-like galaxies, in which unresolved
massive star clusters dominate the ultraviolet emission. This work supports the
idea that dense young star clusters can contribute to the ionisation of the IGM
through holes created by stellar feedback.Comment: 13 pages, 9 figures and 1 table, MNRAS accepted. Some typos fixe
CLASH-VLT: Environment-driven evolution of galaxies in the z=0.209 cluster Abell 209
The analysis of galaxy properties and the relations among them and the
environment, can be used to investigate the physical processes driving galaxy
evolution. We study the cluster A209 by using the CLASH-VLT spectroscopic data
combined with Subaru photometry, yielding to 1916 cluster members down to a
stellar mass of 10^{8.6} Msun. We determine: i) the stellar mass function of
star-forming and passive galaxies; ii) the intra-cluster light and its
properties; iii) the orbits of low- and high-mass passive galaxies; and iv) the
mass-size relation of ETGs. The stellar mass function of the star-forming
galaxies does not depend on the environment, while the slope found for passive
galaxies becomes flatter in the densest region. The color distribution of the
intra-cluster light is consistent with the color of passive members. The
analysis of the dynamical orbits shows that low-mass passive galaxies have
tangential orbits, avoiding small pericenters around the BCG. The mass-size
relation of low-mass passive ETGs is flatter than that of high mass galaxies,
and its slope is consistent with that of field star-forming galaxies. Low-mass
galaxies are also more compact within the scale radius of 0.65 Mpc. The ratio
between stellar and number density profiles shows a mass segregation in the
center. The comparative analysis of the stellar and total density profiles
indicates that this effect is due to dynamical friction. Our results are
consistent with a scenario in which the "environmental quenching" of low-mass
galaxies is due to mechanisms such as harassment out to R200, starvation and
ram-pressure stripping at smaller radii, as supported by the analysis of the
mass function, of the dynamical orbits and of the mass-size relation of passive
early-types in different regions. Our analyses support the idea that the
intra-cluster light is formed through the tidal disruption of subgiant
galaxies.Comment: 17 pages, 20 figures, A&A in pres
Direct Lyman continuum and Lyman-alpha escape observed at redshift 4
We report on the serendipitous discovery of a z=4.0, M1500=-22.20
star-forming galaxy (Ion3) showing copious Lyman continuum (LyC) leakage (~60%
escaping), a remarkable multiple peaked Lya emission, and significant Lya
radiation directly emerging at the resonance frequency. This is the highest
redshift confirmed LyC emitter in which the ionising and Lya radiation possibly
share a common ionised cavity (with N_HI<10^17.2 cm^-2). Ion3 is spatially
resolved, it shows clear stellar winds signatures like the P-Cygni NV1240
profile, and has blue ultraviolet continuum (\beta = -2.5 +/- 0.25, F_\lambda~
\lambda^\beta) with weak low-ionisation interstellar metal lines. Deep
VLT/HAWKI Ks and Spitzer/IRAC 3.6um and 4.5um imaging show a clear photometric
signature of the Halpha line with equivalent width of 1000A rest-frame emerging
over a flat continuum (Ks-4.5um ~ 0). From the SED fitting we derive a stellar
mass of 1.5x10^9 Msun, SFR of 140 Msun/yr and age of ~10 Myr, with a low dust
extinction, E(B-V)< 0.1, placing the source in the starburst region of the
SFR-M^* plane. Ion3 shows similar properties of another LyC emitter previously
discovered (z=3.21, Ion2, Vanzella et al. 2016). Ion3 (and Ion2) represents
ideal high-redshift reference cases to guide the search for reionising sources
at z>6.5 with JWST.Comment: Accepted for publication in MNRAS. 5 pages, 4 figures, 1 tabl
The story of supernova 'Refsdal' told by MUSE
We present MUSE observations in the core of the HFF galaxy cluster MACS
J1149.5+2223, where the first magnified and spatially-resolved multiple images
of SN 'Refsdal' at redshift 1.489 were detected. Thanks to a DDT program with
the VLT and the extraordinary efficiency of MUSE, we measure 117 secure
redshifts with just 4.8 hours of total integration time on a single target
pointing. We spectroscopically confirm 68 galaxy cluster members, with redshift
values ranging from 0.5272 to 0.5660, and 18 multiple images belonging to 7
background, lensed sources distributed in redshifts between 1.240 and 3.703.
Starting from the combination of our catalog with those obtained from extensive
spectroscopic and photometric campaigns using the HST, we select a sample of
300 (164 spectroscopic and 136 photometric) cluster members, within
approximately 500 kpc from the BCG, and a set of 88 reliable multiple images
associated to 10 different background source galaxies and 18 distinct knots in
the spiral galaxy hosting SN 'Refsdal'. We exploit this valuable information to
build 6 detailed strong lensing models, the best of which reproduces the
observed positions of the multiple images with a rms offset of only 0.26". We
use these models to quantify the statistical and systematic errors on the
predicted values of magnification and time delay of the next emerging image of
SN 'Refsdal'. We find that its peak luminosity should should occur between
March and June 2016, and should be approximately 20% fainter than the dimmest
(S4) of the previously detected images but above the detection limit of the
planned HST/WFC3 follow-up. We present our two-dimensional reconstruction of
the cluster mass density distribution and of the SN 'Refsdal' host galaxy
surface brightness distribution. We outline the roadmap towards even better
strong lensing models with a synergetic MUSE and HST effort.Comment: 21 pages, 9 figures, 6 tables; accepted for publication in the
Astrophysical Journal - extra information on data analysis added, all model
predictions and results unchange
The 500 ks Chandra observation of the z = 6.31 QSO SDSS J1030+0524
We present the results from a ks Chandra observation of the
QSO SDSS J1030+0524. This is the deepest X-ray observation to date of
a QSO. The QSO is detected with a total of 125 net counts in the full
( keV) band and its spectrum can be modeled by a single power-law model
with photon index of and full band flux of
erg s cm. When compared with the data
obtained by XMM-Newton in 2003, our Chandra observation in 2017 shows a harder
() spectrum and a 2.5 times fainter flux. Such a
variation, in a timespan of yrs rest-frame, is unexpected for such a
luminous QSO powered by a black hole. The observed source
hardening and weakening could be related to an intrinsic variation in the
accretion rate. However, the limited photon statistics does not allow us to
discriminate between an intrinsic luminosity and spectral change, and an
absorption event produced by an intervening gas cloud along the line of sight.
We also report the discovery of diffuse X-ray emission that extends for 30"x20"
southward the QSO with a signal-to-noise ratio of 6, hardness ratio of
, and soft band flux of erg s cm, that is not
associated to a group or cluster of galaxies. We discuss two possible
explanations for the extended emission, which may be either associated with the
radio lobe of a nearby, foreground radio galaxy (at ), or
ascribed to the feedback from the QSO itself acting on its surrounding
environment, as proposed by simulations of early black hole formation.Comment: 13 pages, 9 figures, A&A accepte
Constraints on the [C II] luminosity of a proto-globular cluster at z ∼ 6 obtained with ALMA
We report on ALMA observations of D1, a system at z ∼ 6.15 with stellar mass M∼ 107M⊙ containing globular cluster (GC) precursors, stronglymagnified by the galaxy clusterMACS J0416.1-2403. Since the discovery of GC progenitors at high redshift, ours is the first attempt to probe directly the physical properties of their neutral gas through infrared observations. A careful analysis of our data set, performed with a suitable procedure designed to identify faint narrow lines and which can test various possible values for the unknown linewidth value, allowed us to identify a 4σ tentative detection of [CII] emission with intrinsic luminosity L[C II] = (2.9 ± 1.4) 106 L⊙, one of the lowest values ever detected at high redshift. This study offers a first insight on previously uncharted regions of the L[C II]-SFR relation. Despite large uncertainties affecting our measure of the star formation rate, if taken at face value our estimate lies more than ∼1 dex below the values observed in local and high redshift systems. Our weak detection indicates a deficiency of [C II] emission, possibly ascribed to various explanations, such as a low-density gas and/or a strong radiation field caused by intense stellar feedback, and a low metal content. From the non-detection in the continuum, we derive constraints on the dust mass, with 3σ upper limit values as low as ∼ a few 104 M⊙, consistent with the values measured in local metal-poor galaxies
High-resolution spectroscopy of a young, low-metallicity optically-thin L=0.02L* star-forming galaxy at z=3.12
We present VLT/X-Shooter and MUSE spectroscopy of an faint F814W=28.60+/-0.33
(Muv=-17.0), low mass (~<10^7 Msun) and compact (Reff=62pc) freshly
star-forming galaxy at z=3.1169 magnified (16x) by the Hubble Frontier Fields
galaxy cluster Abell S1063. Gravitational lensing allows for a significant jump
toward low-luminosity regimes, in moderately high resolution spectroscopy
(R=lambda/dlambda ~ 3000-7400). We measured CIV1548,1550, HeII1640,
OIII]1661,1666, CIII]1907,1909, Hbeta, [OIII]4959,5007, emission lines with
FWHM< 50 km/s and (de-lensed) fluxes spanning the interval 1.0x10^-19 -
2.0x10^-18 erg/s/cm2 at S/N=4-30. The double peaked Lya emission with
Delta_v(red-blue) = 280(+/-7)km/s and de-lensed fluxes
2.4_(blue)|8.5_(red)x10^-18 erg/s/cm2 (S/N=38_(blue)|110_(red)) indicate a low
column density of neutral hydrogen gas consistent with a highly ionized
interstellar medium as also inferred from the large [OIII]5007/[OII]3727>10
ratio. We detect CIV1548,1550 resonant doublet in emission, each component with
FWHM ~< 45 km/s, and redshifted by +51(+/-10)km/s relative to the systemic
redshift. We interpret this as nebular emission tracing an expanding
optically-thin interstellar medium. Both CIV1548,1550 and HeII1640 suggest the
presence of hot and massive stars (with a possible faint AGN). The ultraviolet
slope is remarkably blue, beta =-2.95 +/- 0.20 (F_lambda=lambda^beta),
consistent with a dust-free and young ~<20 Myr galaxy. Line ratios suggest an
oxygen abundance 12+log(O/H)<7.8. We are witnessing an early episode of
star-formation in which a relatively low NHI and negligible dust attenuation
might favor a leakage of ionizing radiation. This galaxy currently represents a
unique low-luminosity reference object for future studies of the reionization
epoch with JWST.Comment: 7 pages, 4 figures and 1 table; ApJL, accepted for publicatio
CLASH-VLT: Insights on the mass substructures in the Frontier Fields Cluster MACS J0416.1-2403 through accurate strong lens modeling
We present a detailed mass reconstruction and a novel study on the
substructure properties in the core of the CLASH and Frontier Fields galaxy
cluster MACS J0416.1-2403. We show and employ our extensive spectroscopic data
set taken with the VIMOS instrument as part of our CLASH-VLT program, to
confirm spectroscopically 10 strong lensing systems and to select a sample of
175 plausible cluster members to a limiting stellar mass of log(M_*/M_Sun) ~
8.6. We reproduce the measured positions of 30 multiple images with a
remarkable median offset of only 0.3" by means of a comprehensive strong
lensing model comprised of 2 cluster dark-matter halos, represented by cored
elliptical pseudo-isothermal mass distributions, and the cluster member
components. The latter have total mass-to-light ratios increasing with the
galaxy HST/WFC3 near-IR (F160W) luminosities. The measurement of the total
enclosed mass within the Einstein radius is accurate to ~5%, including
systematic uncertainties. We emphasize that the use of multiple-image systems
with spectroscopic redshifts and knowledge of cluster membership based on
extensive spectroscopic information is key to constructing robust
high-resolution mass maps. We also produce magnification maps over the central
area that is covered with HST observations. We investigate the galaxy
contribution, both in terms of total and stellar mass, to the total mass budget
of the cluster. When compared with the outcomes of cosmological -body
simulations, our results point to a lack of massive subhalos in the inner
regions of simulated clusters with total masses similar to that of MACS
J0416.1-2403. Our findings of the location and shape of the cluster dark-matter
halo density profiles and on the cluster substructures provide intriguing tests
of the assumed collisionless, cold nature of dark matter and of the role played
by baryons in the process of structure formation.Comment: 26 pages, 22 figures, 7 tables; accepted for publication in the
Astrophysical Journal. A high-resolution version is available at
https://sites.google.com/site/vltclashpublic/publications/Grillo_etal_2014.pd
CLASH-VLT: Substructure in the galaxy cluster MACS J1206.2-0847 from kinematics of galaxy populations
In the effort to understand the link between the structure of galaxy clusters
and their galaxy populations, we focus on MACSJ1206.2-0847 at z~0.44 and probe
its substructure in the projected phase space through the spectrophotometric
properties of a large number of galaxies from the CLASH-VLT survey. Our
analysis is mainly based on an extensive spectroscopic dataset of 445 member
galaxies, mostly acquired with VIMOS@VLT as part of our ESO Large Programme,
sampling the cluster out to a radius ~2R200 (4 Mpc). We classify 412 galaxies
as passive, with strong Hdelta absorption (red and blue galaxies, and with
emission lines from weak to very strong. A number of tests for substructure
detection are applied to analyze the galaxy distribution in the velocity space,
in 2D space, and in 3D projected phase-space. Studied in its entirety, the
cluster appears as a large-scale relaxed system with a few secondary, minor
overdensities in 2D distribution. We detect no velocity gradients or evidence
of deviations in local mean velocities. The main feature is the WNW-ESE
elongation. The analysis of galaxy populations per spectral class highlights a
more complex scenario. The passive galaxies and red strong Hdelta galaxies
trace the cluster center and the WNW-ESE elongated structure. The red strong
Hdelta galaxies also mark a secondary, dense peak ~2 Mpc at ESE. The emission
line galaxies cluster in several loose structures, mostly outside R200. The
observational scenario agrees with MACS J1206.2-0847 having WNW-ESE as the
direction of the main cluster accretion, traced by passive galaxies and red
strong Hdelta galaxies. The red strong Hdelta galaxies, interpreted as
poststarburst galaxies, date a likely important event 1-2 Gyr before the epoch
of observation. The emission line galaxies trace a secondary, ongoing infall
where groups are accreted along several directions.Comment: A&A accepted, 19 pages, 30 figures, minor language change
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