Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: With JWST, new opportunities to study the evolution of galaxies in the early Universe are emerging. Spitzer constraints on rest-optical properties of $z\gtrsim7$ galaxies demonstrated the power of using galaxy stellar masses and star formation histories (SFHs) to indirectly infer the cosmic star formation history. However, only the brightest individual $z\gtrsim8$ objects could be detected with Spitzer, making it difficult to robustly constrain activity at $z\gtrsim10$. Here, we leverage the greatly improved rest-optical sensitivity of JWST at $z\gtrsim8$ to constrain the ages of seven UV-bright ($M_{UV}\lesssim-19.5$) galaxies selected to lie at $z\sim8.5-11$, then investigate implications for $z\gtrsim15$ star formation. We infer the properties of individual objects with two spectral energy distribution modelling codes, then infer a distribution of ages for bright $z\sim8.5-11$ galaxies. We find a median age of $\sim20$ Myr, younger than that inferred at $z\sim7$ with a similar analysis, consistent with an evolution towards larger specific star formation rates at early times. The age distribution suggests that only $\sim3$ percent of bright $z\sim8.5-11$ galaxies would be similarly luminous at $z\gtrsim15$, implying that the number density of bright galaxies declines by at least an order of magnitude between $z\sim8.5-11$ and $z\sim15$. This evolution is challenging to reconcile with some early JWST results suggesting the abundance of bright galaxies does not significantly decrease towards very early times, but we suggest this tension may be eased if young stellar populations form on top of older stellar components, or if bright $z\sim15$ galaxies are observed during a burst of star formation.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We report the results of analyses of galactic outflows in a sample of 45 low-redshift starburst galaxies in the COS Legacy Archive Spectroscopic SurveY (CLASSY), augmented by five additional similar starbursts with COS data. The outflows are traced by blueshifted absorption-lines of metals spanning a wide range of ionization potential. The high quality and broad spectral coverage of CLASSY data enable us to disentangle the absorption due to the static ISM from that due to outflows. We further use different line multiplets and doublets to determine the covering fraction, column density, and ionization state as a function of velocity for each outflow. We measure the outflow's mean velocity and velocity width, and find that both correlate in a highly significant way with the star-formation rate, galaxy mass, and circular velocity over ranges of four orders-of-magnitude for the first two properties. We also estimate outflow rates of metals, mass, momentum, and kinetic energy. We find that, at most, only about 20% of silicon created and ejected by supernovae in the starburst is carried in the warm phase we observe. The outflows' mass-loading factor increases steeply and inversely with both circular and outflow velocity (log-log slope $\sim$ -1.6), and reaches $\sim 10$ for dwarf galaxies. We find that the outflows typically carry about 10 to 100% of the momentum injected by massive stars and about 1 to 20% of the kinetic energy. We show that these results place interesting constraints on, and new insights into, models and simulations of galactic winds.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: Reionisation-era galaxies often display intense nebular emission lines, both in rest-frame optical ([OIII]+H$\beta$) and ultraviolet (UV; CIII], CIV). How such strong nebular emission is powered remains unclear, with both active galactic nuclei (AGN) and hot stars considered equally viable. The UV continuum slopes of these early systems tend to be very blue ($\beta<-2$), reflecting minimal dust obscuration, young ages, and low metallicities. This contrasts with narrow-lined AGN at $z\sim2-3$, whose UV slopes are significantly redder ($\beta>-1$) than typical star-forming systems in the reionisation era. To investigate the properties of AGN in the reionisation era, we have conducted a search for potential examples of rare analogues with blue continua at intermediate redshift ($z\sim2-3$). Our goals are to determine whether AGN with intense line emission and blue continua exist and thereby to establish the range of rest-frame UV and optical line ratios in this population. In this paper we report the detection of a X-ray luminous AGN at $z=3.21$ (UDS-24561) with extreme [OIII]+H$\beta$ line emission (EW $=1300$ \r{A}) and a blue UV continuum slope ($\beta=-2.34$). MMT/Binospec and Keck/MOSFIRE spectra indicate rest-frame UV line ratios consistent with AGN photoionisation models and rest-frame optical lines with both a narrow component (FWHM $=154$ km$/$s) and extended broad wings (FWHM $=977$ km$/$s), consistent with outflowing gas. We describe how such objects can be identified in future JWST emission line surveys in the reionisation era, thereby providing a valuable census of AGN activity at $z>6$ and understanding their contribution to cosmic reionisation.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We report the discovery of a triply-imaged active galactic nucleus (AGN), lensed by the galaxy cluster MACS J0035.4-2015 ($z_{\mathrm{d}}=0.352$). The object is detected in Hubble Space Telescope (HST) imaging taken for the RELICS program. It appears to have a quasi-stellar nucleus consistent with a point-source, with a de-magnified radius of $r_e\lesssim100$ pc. The object is spectroscopically confirmed to be an AGN at $z_{\mathrm{spec}}=2.063\pm0.005$ showing broad rest-frame UV emission lines, and is detected in both X-ray observations with \textit{Chandra} and in ALCS ALMA band 6 (1.2 mm) imaging. It has a relatively faint rest-frame UV luminosity for a quasar-like object, $M_{\mathrm{UV},1450}=-19.7\pm0.2$. The object adds to just a few quasars or other X-ray sources known to be multiply lensed by a galaxy cluster. Some faint, diffuse emission from the host galaxy is also seen around the nucleus, and nearby there is another fainter object sharing the same multiple-imaging symmetry and geometric redshift, which may be an interacting galaxy or a star-forming knot in the host. We present an accompanying lens model, calculate the magnifications and time delays, and infer physical properties for the source. We find the rest-frame UV continuum and emission lines to be dominated by the AGN, and the optical emission to be dominated by the relatively young ($\sim100$ Myr) host galaxy of modest stellar mass $M_{\star}\simeq10^{9.2} \mathrm{M}_{\odot}$. We also observe variations in the AGN's emission, which may suggest that the AGN used to be more active. This object adds a low-redshift counterpart to several relatively faint AGN recently uncovered at high redshifts with HST and JWST.
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: The ultraviolet (UV) continuum slope
($\beta~$where$~f_\lambda\propto\lambda^\beta$) of galaxies is sensitive to a
variety of properties, from the metallicity and age of the stellar population
to dust attenuation throughout the galaxy. Considerable attention has focused
on identifying reionization-era galaxies with very blue UV slopes ($\beta<-3$).
Not only do such systems provide a signpost of low-metallicity stars, but they
also identify galaxies likely to leak ionizing photons from their HII regions
as such blue UV slopes require the reddening effect of nebular continuum to be
diminished. In this paper we present a search for reionization-era galaxies
with very blue UV colors in recent JWST/NIRCam imaging of the EGS field. We
characterize UV slopes for a large sample of$~z\simeq7-11~$galaxies, finding a
median of$~\beta=-2.0$. Two lower luminosity (M$_{\rm{UV}}\simeq-19.5$) and
lower stellar mass (6-10$\times10^7$M$_\odot$) systems exhibit extremely blue
UV slopes ($\beta=-2.9~$to$~-3.1$) and rest-optical photometry indicating weak
nebular line emission. Each system is very compact (r$_e\lesssim$260pc) with
very high star-formation-rate surface densities. We model the SEDs with a suite
of BEAGLE models with varying levels of ionizing photon escape. The SEDs cannot
be reproduced with our fiducial (f$_{\rm{esc,HII}}$=0) or alpha-enhanced
(Z$_\star
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: The onset of the {\it JWST}-era provides a much-improved opportunity to
characterize the resolved structure of early star forming systems. Previous
{\it Spitzer} observations of $z\gtrsim 6$ galaxies revealed the presence of
old stars and luminous HII regions (via [OIII]+H$\beta$ emission), but the poor
resolution stunted our ability to map their locations with respect to the star
forming regions identified in the rest-UV. In this paper, we investigate the
internal structure of 12 of the most luminous $z\simeq 6-8$ galaxies in the EGS
field observed with recent {\it JWST}/NIRCam imaging. The systems appear clumpy
in the rest-UV, with more than half of the light coming from $\simeq 10^7$ to
10$^{9}$ M$_\odot$ star forming complexes that are $\simeq 150$ - 480 pc in
size. The clumps tend to be dominated by young stars (median = 36 Myr), but we
also find large variations in clump ages within individual galaxies. The
[OIII]+H$\beta$ EW varies significantly across individual galaxies (reflecting
differences in stellar and gas properties), but the HII regions largely track
the UV-bright complexes. Perhaps surprisingly, the rest-optical continuum is
just as clumpy as the UV, and we do not find older (and redder) nuclear stellar
components that were previously undetected or faint in the UV. The majority of
the stellar mass in bright $6
Peer Review Status:Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Abstract: We describe new JWST/NIRSpec observations of galaxies at $z\gtrsim7$ taken as part of the CEERS survey of the EGS field. Previous observations of this area have revealed associations of Ly$\alpha$ emitters at redshifts ($z=7.5$, $7.7$, $8.7$) where the IGM is thought mostly neutral, leading to suggestions that these systems are situated in large ionized bubbles. We identify 21 $z\gtrsim7$ galaxies with robust redshifts in the CEERS dataset, including 10 in the Ly$\alpha$ associations. Their spectra are indicative of very highly ionized and metal poor gas, with line ratios (O32 $=17.84$ and Ne3O2 $=0.89$) and metallicity ($12+\log{[\rm{O/H}]}=7.84$) that are rarely seen at lower redshifts. We find that the most extreme spectral properties are found in the six $z\gtrsim7$ Ly$\alpha$ emitting galaxies in the sample. Each have hard ionizing spectra indicating that their visibility is likely enhanced by efficient ionizing photon production. Ly$\alpha$ velocity offsets are found to be very large ($\gtrsim300$ km s$^{-1}$), likely also contributing to their detectability. We find that Ly$\alpha$ in $z\gtrsim7$ galaxies is $6-12\times$ weaker than in lower redshift samples with matched rest-frame optical spectral properties. If the bubbles around the Ly$\alpha$ emitters are relatively small ($\lesssim0.5-1$ pMpc), we may expect such significant attenuation of Ly$\alpha$ in these ionized regions. We discuss several other effects that may contribute to weaker Ly$\alpha$ emission at $z\gtrsim7$. Deep spectroscopy of fainter galaxies in the vicinity of the Ly$\alpha$ emitters will better characterize the physical scale of the ionized bubbles in this field.
Peer Review Status:Awaiting Review