The formation, evolution and death of massive stars release large quantities of energy and momentum into the gas surrounding the sites of star formation. This process, generically termed 'feedback', ...inhibits further star formation either by removing gas from the galaxy, or by heating it to temperatures that are too high to form new stars. Observations reveal feedback in the form of galactic-scale outflows of gas in galaxies with high rates of star formation, especially in the early Universe. Feedback in faint, low-mass galaxies probably facilitated the escape of ionizing radiation from galaxies when the Universe was about 500 million years old, so that the hydrogen between galaxies changed from neutral to ionized-the last major phase transition in the Universe.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
We report new observations of SL2S J021737-051329, a lens system consisting of a bright arc at z = 1.84435, magnified ∼17× by a massive galaxy at z = 0.65. SL2S0217 is a low-mass (M < 109 M ), ...low-metallicity (Z ∼ 1/20 Z ) galaxy, with extreme star-forming conditions that produce strong nebular UV emission lines in the absence of any apparent outflows. Here we present several notable features from rest-frame UV Keck/LRIS spectroscopy: (1) Very strong narrow emission lines are measured for C iv λλ1548, 1550, He ii λ1640, O iii λλ1661, 1666, Si iii λλ1883, 1892, and C iii λλ1907, 1909. (2) Double-peaked Ly emission is observed with a dominant blue peak and centered near the systemic velocity. (3) The low- and high-ionization absorption features indicate very little or no outflowing gas along the sight line to the lensed galaxy. The relative emission-line strengths can be reproduced with a very high ionization, low-metallicity starburst with binaries, with the exception of He ii, which indicates that an additional ionization source is needed. We rule out large contributions from active galactic nuclei and shocks to the photoionization budget, suggesting that the emission features requiring the hardest radiation field likely result from extreme stellar populations that are beyond the capabilities of current models. Therefore, SL2S0217 serves as a template for the extreme conditions that are important for reionization and thought to be more common in the early universe.
Abstract
Ultraviolet nebular emission lines are important for understanding the time evolution and nucleosynthetic origins of their associated elements, but the underlying trends of their relative ...abundances are unclear. We present UV spectroscopy of 20 nearby low-metallicity, high-ionization dwarf galaxies obtained using the
Hubble Space Telescope
. Building upon previous studies, we analyze the C/O relationship for a combined sample of 40 galaxies with significant detections of the UV O
+2
/C
+2
collisionally excited lines and direct-method oxygen abundance measurements. Using new analytic carbon ionization correction factor relationships, we confirm the flat trend in C/O versus O/H observed for local metal-poor galaxies. We find an average log(C/O) = −0.71 with an intrinsic dispersion of
σ
= 0.17 dex. The C/N ratio also appears to be constant at log(C/N) = 0.75, plus significant scatter (
σ
= 0.20 dex), with the result that carbon and nitrogen show similar evolutionary trends. This large and real scatter in C/O over a large range in O/H implies that measuring the UV C and O emission lines alone does not provide a reliable indicator of the O/H abundance. By modeling the chemical evolution of C, N, and O of individual targets, we find that the C/O ratio is very sensitive to both the detailed star formation history and to supernova feedback. Longer burst durations and lower star formation efficiencies correspond to low C/O ratios, while the escape of oxygen atoms in supernovae winds produces decreased effective oxygen yields and larger C/O ratios. Further, a declining C/O relationship is seen with increasing baryonic mass due to increasing effective oxygen yields.
Abstract
Stellar population models produce radiation fields that ionize oxygen up to O
+2
, defining the limit of standard H
ii
region models (<54.9 eV). Yet, some extreme emission-line galaxies, or ...EELGs, have surprisingly strong emission originating from much higher ionization potentials. We present UV HST/COS and optical LBT/MODS spectra of two nearby EELGs that have very high-ionization emission lines (e.g., He
ii
λλ
1640,4686 C
iv
λλ
1548,1550, Fe
v
λ
4227, Ar
iv
λλ
4711,4740). We define a four-zone ionization model that is augmented by a very high-ionization zone, as characterized by He
+2
(>54.4 eV). The four-zone model has little to no effect on the measured total nebular abundances, but does change the interpretation of other EELG properties: we measure steeper central ionization gradients; higher volume-averaged ionization parameters; and higher central
T
e
,
n
e
, and log
U
values. Traditional three-zone estimates of the ionization parameter can underestimate the average log
U
by up to 0.5 dex. Additionally, we find a model-independent dichotomy in the abundance patterns, where the
α
/H abundances are consistent but N/H, C/H, and Fe/H are relatively deficient, suggesting these EELGs are
α
/Fe-enriched by more than three times. However, there still is a high-energy ionizing photon production problem (HEIP
3
). Even for such
α
/Fe enrichment and very high log
U
s, photoionization models cannot reproduce the very high-ionization emission lines observed in EELGs.
Abstract
In the last few years, prominent high-ionization nebular emission lines (i.e., O
iii
, C
iii
, C
iv
, and He
ii
) have been observed in the deep UV spectra of
z
∼ 5–7 galaxies, indicating ...that extreme radiation fields characterize reionization-era systems. These lines have been linked to the leakage of Lyman continuum photons (necessary for reionization) both theoretically and observationally. Consequently, high-ionization UV emission lines present our best probe to detect and characterize the most distant galaxies that we will observe in the coming years, and are key to understanding the sources of reionization, yet the physics governing their production is poorly understood. Here we present recent high-resolution
Hubble Space Telescope
spectra of two nearby extreme UV emission-line galaxies, J104457 and J141851. We report the first observations of intense nebular He
ii
and double-peaked, resonantly scattered C
iv
emission, a combination that suggests these galaxies both produce and transmit a significant number of very high-energy ionizing photons (
E
> 47.89 eV) through relatively low column densities of high-ionization gas. This suggests that, in addition to photons at the H-ionizing edge, the very hard ionizing photons that escape from these galaxies may provide a secondary source of ionization that is currently unconstrained observationally. Simultaneous radiative transfer models of Ly
α
and C
iv
are needed to understand how ionizing radiation is transmitted through both low- and high-ionization gas. Future rest-frame far-UV observations of galaxies within the epoch of reionization using the
James Webb Space Telescope
or extremely large telescopes (ELTs) will allow us to constrain the escape of helium-ionizing photons and provide an estimate for their contribution to the reionization budget.
Motivated by the Increasing use of the Kennicutt-Schmidt (K-S) star formation law to interpret observations of high-redshift galaxies, the importance of gas accretion to galaxy formation, and the ...recent observations of chemical abundances in galaxies at z similar to 2 -3, I use simple analytical models to assess the consistency of these processes of galaxy evolution with observations and with each other. I derive the time dependence of star formation Implied by the K-S law and show that the sustained high star formation rates observed in galaxies at z similar to 2 -3 require the accretion of additional gas. A model in which the gas accretion rate is approximately equal to the combined star formation and outflow rates broadly reproduces the observed trends of star formation rate with galaxy age. Using an analytical description of chemical evolution, I also show that this model, further constrained to have an outflow rate roughly equal to the star formation rate, reproduces the observed mass-metallicity relation at z similar to 2.
Abstract
The resonantly scattered Ly
α
line illuminates the extended halos of neutral hydrogen in the circumgalactic medium of galaxies. We present integral field Keck Cosmic Web Imager observations ...of double-peaked, spatially extended Ly
α
emission in 12 relatively low-mass (
M
⋆
∼ 10
9
M
⊙
)
z
∼ 2 galaxies characterized by extreme nebular emission lines. Using individual spaxels and small bins as well as radially binned profiles of larger regions, we find that for most objects in the sample the Ly
α
blue-to-red peak ratio increases, the peak separation decreases, and the fraction of flux emerging at line center increases with radius. We use new radiative transfer simulations to model each galaxy with a clumpy, multiphase outflow with radially varying outflow velocity, and self-consistently apply the same velocity model to the low-ionization interstellar absorption lines. These models reproduce the trends of peak ratio, peak separation, and trough depth with radius, and broadly reconcile outflow velocities inferred from Ly
α
and absorption lines. The galaxies in our sample are well-described by a model in which neutral, outflowing clumps are embedded in a hotter, more highly ionized inter-clump medium (ICM), whose residual neutral content produces absorption at the systemic redshift. The peak ratio, peak separation, and trough flux fraction are primarily governed by the line-of-sight component of the outflow velocity, the H
i
column density, and the residual neutral density in the ICM respectively. The azimuthal asymmetries in the line profile further suggest nonradial gas motions at large radii and variations in the H
i
column density in the outer halos.
The study of carbon and oxygen abundances yields information on the time evolution and nucleosynthetic origins of these elements, yet they remain relatively unexplored. At low metallicities, ...(12+log(O/H) < 8.0), nebular carbon measurements are limited to rest-frame UV collisionally excited emission lines. Therefore, we present the UV spectrophotometry of 12 nearby low-metallicity high-ionization H II regions in dwarf galaxies obtained using the Cosmic Origins Spectrograph on the Hubble Space Telescope. We present the first analysis of the C/O ratio in local galaxies based solely on simultaneous significant detections of the UV O super(+2) and C super(+2) collisionally excited lines in seven of our targets and five objects from the literature to create a final sample of 12 significant detections. Our sample is complemented by optical SDSS spectra, from which we measured the nebular physical conditions and oxygen abundances using the direct method. At low metallicity, (12+log(O/H) < 8.0), no clear trend is evident in C/O versus O/H for the present sample given the large dispersion observed. When combined with recombination line observations at higher values of O/H, a general trend of increasing C/O with increasing O/H is also viable but with some significant outliers. Additionally, we find the C/N ratio appears to be constant (but with significant scatter) over a large range in oxygen abundance, indicating that carbon is predominantly produced by similar nucleosynthetic mechanisms as nitrogen. If true, and our current understanding of nitrogen production is correct, this would indicate that primary production of carbon (a flat trend) dominates at low metallicity, but quasi-secondary production (an increasing trend) becomes prominent at higher metallicities. A larger sample will be needed to determine the true nature and dispersion of the relation.
Abstract
Ly
α
photons are resonantly scattered by neutral hydrogen, and may therefore trace both the spatial extent and the kinematics of the gas surrounding galaxies. We present new observations of ...the extended
Ly
α
halo of Q2343-BX418, a low-mass (
M
⋆
=
5
×
10
8
M
⊙
), low-metallicity (
Z
≈ 0.25
Z
⊙
) star-forming galaxy at
z
= 2.3. Using the Keck Cosmic Web Imager (KCWI), the blue-sensitive optical integral field spectrograph recently installed on the Keck II telescope, we detect
Ly
α
in emission to a radius of 23 kpc, and measure an exponential scale length of 6 kpc in the outer region of the extended halo. We study the double-peaked spectroscopic
Ly
α
profile in individual spectral pixels (“spaxels”) over a ∼25 × 30 kpc region, finding significant variations in the peak ratio and peak separation. The profile is dominated by the red peak in the central regions, while in the outskirts of the extended halo the red and blue peak strengths are roughly equal; these observations are consistent with a model in which the peak ratio is largely determined by the radial component of the outflow velocity. We find a gradient of 300
km
s
−
1
in the
Ly
α
peak separation across the extended halo, indicating variations in the column density, covering fraction, or velocity range of the gas. These new observations emphasize the need for realistic, spatially resolved models of
Ly
α
radiative transfer in the halos of galaxies.
We present the first comprehensive evolutionary analysis of the rest-frame UV spectroscopic properties of star-forming galaxies at z ∼ 2-4. We match samples at different redshifts in UV luminosity ...and stellar mass, and perform systematic measurements of spectral features and stellar population modeling. By creating composite spectra grouped according to Ly equivalent width (EW) and various galaxy properties, we study the evolutionary trends among Ly , low- and high-ionization interstellar (LIS and HIS) absorption features, and integrated galaxy properties. We also examine the redshift evolution of Ly and LIS absorption kinematics, and fine-structure emission EWs. The connections among the strengths of Ly , LIS lines, and dust extinction are redshift independent, as is the decoupling of the Ly and HIS line strengths, and the bulk outflow kinematics as traced by the LIS lines. Stronger Ly emission is observed at higher redshift at fixed UV luminosity, stellar mass, SFR, and age. Much of this variation in the average Ly strength with redshift, and the variation in Ly strength at fixed redshift, can be explained in terms of variations in the neutral gas covering fraction and/or dust content in the ISM and CGM. However, based on the connection between Ly and C iii emission strengths, we additionally find evidence for variations in the intrinsic production rate of Ly photons at the highest Ly EWs. The challenge now is to understand the observed evolution of the neutral gas covering fraction and dust extinction within a coherent model for galaxy formation, and make robust predictions for the escape of ionizing radiation at z > 6.
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