分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Hiddo Algera
Hanae Inami
Pascal Oesch
Laura Sommovigo
Rychard Bouwens
Michael Topping
Sander Schouws
Mauro Stefanon
Daniel Stark
Manuel Aravena
Laia Barrufet
Elisabete da Cunha
Pratika Dayal
Ryan Endsley
Andrea Ferrara
Yoshinobu Fudamoto
Valentino Gonzalez
Luca Graziani
Jacqueline Hodge
Alexander Hygate
摘要： Cosmic dust is an essential component shaping both the evolution of galaxies and their observational signatures. How quickly dust builds up in the early Universe remains an open question that requires deep observations at (sub-)millimeter wavelengths to resolve. Here we use Atacama Large Millimeter Array observations of 45 galaxies from the Reionization Era Bright Emission Line Survey (REBELS) and its pilot programs, designed to target [CII] and dust emission in UV-selected galaxies at $z\sim7$, to investigate the dust content of high-redshift galaxies through a stacking analysis. We find that the typical fraction of obscured star formation $f_\mathrm{obs} = \mathrm{SFR}_\mathrm{IR} / \mathrm{SFR}_\mathrm{UV + IR}$ depends on stellar mass, similar to what is observed at lower redshift, and ranges from $f_\mathrm{obs} \approx 0.3 - 0.6$ for galaxies with $\log_{10}\left(M_\star / M_\odot\right) = 9.4 - 10.4$. We further adopt the $z\sim7$ stellar mass function from the literature to extract the obscured cosmic star formation rate density (SFRD) from the REBELS survey. Our results suggest only a modest decrease in the SFRD between $3\lesssim z \lesssim 7$, with dust-obscured star formation still contributing $\sim30\%$ at $z\sim7$. While we extensively discuss potential caveats, our analysis highlights the continued importance of dust-obscured star formation even well into the epoch of reionization.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Mengtao Tang
Daniel P. Stark
Zuyi Chen
Charlotte Mason
Michael Topping
Ryan Endsley
Peter Senchyna
Adèle Plat
Ting-Yi Lu
Lily Whitler
Brant Robertson
Stéphane Charlot
摘要： 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Hiddo Algera
Hanae Inami
Laura Sommovigo
Yoshinobu Fudamoto
Raffaella Schneider
Luca Graziani
Pratika Dayal
Rychard Bouwens
Manuel Aravena
Elisabete da Cunha
Andrea Ferrara
Alexander Hygate
Ivana van Leeuwen
Ilse De Looze
Marco Palla
Andrea Pallottini
Renske Smit
Mauro Stefanon
Michael Topping
Paul van der Werf
摘要： We present new ALMA Band 8 (rest-frame $90\,\mu$m) observations of three massive ($M_\star \approx 10^{10}\,M_\odot$) galaxies at $z\approx7$ previously detected in [CII]$158\,\mu$m and underlying dust continuum emission in the Reionization Era Bright Emission Line Survey (REBELS). We detect the dust continuum emission of two of our targets in Band 8 (REBELS-25 and REBELS-38), while REBELS-12 remains undetected. Through modified blackbody fitting we determine cold dust temperatures ($T_\mathrm{dust} \approx 30 - 35\,$K) in both of the dual-band detected targets, given a fiducial model of optically thin emission with $\beta = 2.0$. Their dust temperatures are lower than most $z\sim7$ galaxies in the literature, and consequently their dust masses are higher ($M_\mathrm{dust} \approx 10^{8}\,M_\odot$). Nevertheless, these large dust masses are still consistent with predictions from models of dust production in the early Universe. In addition, we target and detect [OIII]$88\,\mu$m emission in both REBELS-12 and REBELS-25, and find $L_\mathrm{[OIII]} / L_\mathrm{[CII]}$ ratios of approximately unity, low compared to the $L_\mathrm{[OIII]} / L_\mathrm{[CII]} \gtrsim 2 - 10$ observed in the known $z\gtrsim6$ population thus far. We argue the lower line ratios are due to a comparatively weaker ionizing radiation field resulting from the less starbursty nature of our targets. This low burstiness supports the cold dust temperatures and below average $\mathrm{[OIII]}\lambda\lambda4959,5007 + \mathrm{H}\beta$ equivalent widths of REBELS-25 and REBELS-38, compared to the known high-redshift population. Overall, this provides evidence for the existence of a massive, dust-rich galaxy population at $z\approx7$ which has previously experienced vigorous star formation, but is currently forming stars in a steady, as opposed to bursty, manner.
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