分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Dwarf galaxies located in extremely under-dense cosmic voids are excellent test-beds for disentangling the effects of large-scale environment on galaxy formation and evolution. We present integral field spectroscopy for low-mass galaxies ($M_{\star}=10^{7}-10^{9}~M_{\odot}$) located inside (N=21) and outside (N=9) cosmic voids using the Keck Cosmic Web Imager (KCWI). Using measurements of stellar line-of-sight rotational velocity $v_{\mathrm{rot}}$ and velocity dispersion $\sigma_{\star}$, we test the tidal stirring hypothesis, which posits that dwarf spheroidal galaxies are formed through tidal interactions with more massive host galaxies. We measure low values of $v_{\mathrm{rot}}/\sigma_{\star}\lesssim2$ for our sample of isolated dwarf galaxies, and we find no trend between $v_{\mathrm{rot}}/\sigma_{\star}$ and distance from a massive galaxy $d_{L^{\star}}$ out to $d_{L^{\star}}\sim10$ Mpc. These suggest that dwarf galaxies can become dispersion-supported "puffy" systems even in the absence of environmental effects like tidal interactions. We also find indications of an upward trend between $v_{\mathrm{rot}}/\sigma_{\star}$ and galaxy stellar mass, perhaps implying that stellar disk formation depends on mass rather than environment. Although some of our conclusions may be slightly modified by systematic effects, our main result still holds: that isolated low-mass galaxies may form and remain as puffy systems rather than the dynamically cold disks predicted by classical galaxy formation theory.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Anthony J. Pahl
Alice Shapley
Charles C. Steidel
Naveen A. Reddy
Yuguang Chen
Gwen C. Rudie
Allison L. Strom
摘要： The connection between the escape fraction of ionizing radiation ($f_{esc}$) and the properties of galaxies, such as stellar mass (M*), age, star-formation rate (SFR), and dust content, are key inputs for reionization models, but many of these relationships remain untested at high redshift. We present an analysis of a sample of 96 z~3 galaxies from the Keck Lyman Continuum Spectroscopic Survey (KLCS). These galaxies have both sensitive Keck/LRIS spectroscopic measurements of the Lyman continuum (LyC) region, and multi-band photometry that places constraints on stellar population parameters. We construct composite spectra from subsamples binned as a function of galaxy property and quantify the ionizing-photon escape for each composite. We find a significant anti-correlation between $f_{esc}$ and M*, consistent with predictions from cosmological zoom-in simulations. We also find significant anti-correlation between $f_{esc}$ and E(B-V), encoding the underlying physics of LyC escape in our sample. We also find no significant correlation between $f_{esc}$ and either stellar age or specific SFR (=SFR/M*), challenging interpretations that synchronize recent star formation and favorable conditions for ionizing escape. The galaxy properties now shown to correlate with $f_{esc}$ in the KLCS are Ly$\alpha$ equivalent width, UV Luminosity, M*, SFR, and E(B-V), but not age or sSFR. To date, this is the most comprehensive analysis of galaxy properties and LyC escape at high redshift, and will be used to guide future models and observations of the reionization epoch.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present new spectroscopic observations of Lyman-$\alpha$ (Ly$\alpha$) Blob 2 ($z \sim$ 3.1). We observed extended Ly$\alpha$ emission in three distinct regions, where the highest Ly$\alpha$ surface brightness (SB) center is far away from the known continuum sources. We searched through the MOSFIRE slits that cover the high Ly$\alpha$ SB regions, but were unable to detect any significant nebular emission near the highest SB center. We further mapped the flux ratio of the blue peak to the red peak and found it is anti-correlated with Ly$\alpha$ SB with a power-law index of $\sim$ -0.4. We used radiative transfer models with both multiphase, clumpy and shell geometries and successfully reproduced the diverse Ly$\alpha$ morphologies. We found that most spectra suggest outflow-dominated kinematics, while 4/15 spectra imply inflows. A significant correlation exists between parameter pairs, and the multiphase, clumpy model may alleviate previously reported discrepancies. We also modeled Ly$\alpha$ spectra at different positions simultaneously and found that the variation of the inferred clump outflow velocities can be approximately explained by line-of-sight projection effects. Our results support the `central powering + scattering' scenario, i.e. the Ly$\alpha$ photons are generated by a central powering source and then scatter with outflowing, multiphase HI gas while propagating outwards. The infalling of cool gas near the blob outskirts shapes the observed blue-dominated Ly$\alpha$ profiles, but its energy contribution to the total Ly$\alpha$ luminosity is less than 10%, i.e. minor compared to the photo-ionization by star-forming galaxies and/or AGNs.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yuguang Chen
Charles C. Steidel
Dawn K. Erb
David R. Law
Ryan F. Trainor
Naveen A. Reddy
Alice E. Shapley
Anthony J. Pahl
Allison L. Strom
Noah R. Lamb
Zhihui Li
Gwen C. Rudie
摘要： We present the first statistical analysis of kinematically-resolved, spatially-extended Ly$\alpha$ emission around $z = 2-3$ galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies ($z_\mathrm{med} = 2.29$) comprises the subset with typical KCWI integration times of ~5 hours and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high resolution images were used to evaluate the azimuthal dependence of the diffuse Ly$\alpha$ emission with respect to the stellar continuum within projected galactocentric distances of $\lesssim 30$ proper kpc. We introduce cylindrically-projected 2D spectra (CP2D) that map the averaged Ly$\alpha$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of Ly$\alpha$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended Ly$\alpha$ emission along the galaxy principal axes are statistically indistinguishable, with residual asymmetry of $\le$ 2% ($\sim 2 \sigma$) of the integrated Ly$\alpha$ emission. The symmetry implies that the Ly$\alpha$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of Ly$\alpha$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak Ly$\alpha$ emission. We speculate that this weak directional dependence of Ly$\alpha$ emission becomes discernible only when the Ly$\alpha$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with Ly$\alpha$ radiative transfer modeling.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Tucker Jones
Ryan Sanders
Yuguang Chen
Xin Wang
Takahiro Morishita
Guido Roberts-Borsani
Tommaso Treu
Alan Dressler
Emiliano Merlin
Diego Paris
Paola Santini
Pietro Bergamini
Erin Huntzinger
Themiya Nanayakkara
Kristan Boyett
Marusa Bradac
Gabriel Brammer
Antonello Calabro
Karl Glazebrook
Kathryn Grasha
摘要： The abundance of carbon relative to oxygen (C/O) is a promising probe of star formation history in the early universe, as these elements are produced on different timescales. We present a measurement of $\log{\mathrm{(C/O)}} = -1.01\pm0.12$ (stat) $\pm0.10$ (sys) in a $z=6.23$ galaxy observed as part of the GLASS-JWST Early Release Science Program. Notably, we achieve good precision thanks to the detection of the rest-frame ultraviolet O III], C III], and C IV emission lines delivered by JWST/NIRSpec. The C/O abundance is $\sim$0.8 dex lower than the solar value and is consistent with the expected yield from core-collapse supernovae, indicating negligible carbon enrichment from intermediate-mass stars. This in turn implies rapid buildup of a young stellar population with age $\lesssim$100 Myr in a galaxy seen $\sim$900 million years after the Big Bang. Our chemical abundance analysis is consistent with spectral energy distribution modeling of JWST/NIRCam photometric data, which indicates a current stellar mass $\log\,\mathrm{M}_* / \mathrm{Msun} = 8.4^{+0.4}_{-0.2}$ and specific star formation rate sSFR $\simeq 20$ Gyr$^{-1}$. These results showcase the value of chemical abundances and C/O in particular to study the earliest stages of galaxy assembly.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Vivienne Langen
Sebastiano Cantalupo
Charles C. Steidel
Yuguang Chen
Gabriele Pezzulli
Sofia G. Gallego
摘要： The discovery of giant quasar Ly$\alpha$ nebulae at $z>2$ has opened up the possibility to directly study in emission the Circumgalactic and Intergalactic Medium (CGM/IGM). However, the resonant nature of the Ly$\alpha$ line and its different emission mechanisms hamper the ability to constrain both the kinematics and physical properties of the CGM/IGM. Here, we present results of a pilot project aiming at the detection of CGM H$\alpha$ emission, a line which does not suffer from these limitations. To this end, we first used KCWI to detect Ly$\alpha$ emission around three bright quasars with $2.2510^{12}$M$_{\odot}$).
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Dawn K. Erb
Zhihui Li
Charles C. Steidel
Yuguang Chen
Max Gronke
Allison L. Strom
Ryan F. Trainor
Gwen C. Rudie
摘要： The resonantly scattered Lyman-$\alpha$ 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$\alpha$ emission in 12 relatively low-mass ($M_{\star} \sim10^9 \, M_{\odot}$) $z\sim2$ 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$\alpha$ 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$\alpha$ 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 HI column density, and the residual neutral density in the ICM respectively. Azimuthal asymmetries in the line profile further suggest non-radial gas motions at large radii and variations in the HI column density in the outer halos.
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