分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Multi-wavelength images from the Hubble Space Telescope covering the wavelength range 0.27-1.6 $\mu$m show that the central area of the nearby dwarf galaxy NGC4449 contains several tens of compact sources that are emitting in the hydrogen recombination line Pa$\beta$ (1.2818 $\mu$m) but are only marginally detected in H$\alpha$ (0.6563 $\mu$m) and undetected at wavelengths $\lambda\le$0.55 $\mu$m. An analysis of the spectral energy distributions (SEDs) of these sources indicates that they are likely relatively young star clusters heavily attenuated by dust. The selection function used to identify the sources prevents meaningful statistical analyses of their age, mass, and dust extinction distributions. However, these cluster candidates have ages $\sim$5-6 Myr and A$_V>$6 mag, according to their SED fits, and are extremely compact, with typical deconvolved radii of 1 pc. The dusty clusters are located at the periphery of dark clouds within the galaxy and appear to be partially embedded. Density and pressure considerations indicate that the HII regions surrounding these clusters may be stalled, and that pre-supernova feedback has not been able to clear the clusters of their natal cocoons. These findings are in potential tension with existing models that regulate star formation with pre-supernova feedback, since pre-supernova feedback acts on short timescales, $\lesssim$4 Myr, for a standard Stellar Initial Mass function. The existence of a population of dusty star clusters with ages $>$4 Myr, if confirmed by future observations, paints a more complex picture for the role of stellar feedback in controlling star formation.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The Mg II 2796, 2803 doublet has been suggested to be a useful indirect indicator for the escape of Ly-alpha and Lyman continuum (LyC) photons in local star-forming galaxies. However, studies to date have focused on small samples of galaxies with strong Mg II or strong LyC emission. Here we present the first study of Mg II probing a large dynamic range of galaxy properties, using newly obtained high signal-to-noise, moderate-resolution spectra of Mg II for a sample of 34 galaxies selected from the Low-redshift Lyman Continuum Survey. We show that the galaxies in our sample have Mg II profiles ranging from strong emission to P-Cygni profiles, and to pure absorption. We find there is a significant trend (with a possibility of spurious correlations of ~ 2%) that galaxies detected as strong LyC Emitters (LCEs) also show larger equivalent widths of Mg II emission, and non-LCEs tend to show evidence of more scattering and absorption features in Mg II We then find Mg II strongly correlates with Ly-alpha in both equivalent width and escape fraction, regardless of whether the emission or absorption dominates the Mg II profiles. Furthermore, we present that, for galaxies categorized as Mg II emitters (MgE), one can adopt the information of Mg II, metallicity, and dust to estimate the escape fraction of LyC within a factor of 3. These findings confirm that Mg II lines can be used as a tool to select galaxies as LCEs and to serve as an indirect indicator for the escape of Ly-alpha and LyC.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Interstellar dust captures a significant fraction of elements heavier than helium in the solid state and is an indispensable component both in theory and observations of galaxy evolution. Dust emission is generally the primary coolant of the interstellar medium (ISM) and facilitates the gravitational collapse and fragmentation of gas clouds from which stars form, while altering the emission spectrum of galaxies from ultraviolet (UV) to far-infrared wavelengths through the reprocessing of starlight. However, the astrophysical origin of various types of dust grains remains an open question, especially in the early Universe. Here we report direct evidence for the presence of carbonaceous grains from the detection of the broad UV absorption feature around $2175 \, \mathring{\rm A}$ in deep near-infrared spectra of galaxies up to the first billion years of cosmic time, at a redshift ($z$) of $\sim 7$. This dust attenuation feature has previously only been observed spectroscopically in older, more evolved galaxies at redshifts of $z < 3$. The carbonaceous grains giving rise to this feature are often thought to be produced on timescales of hundreds of millions of years by asymptotic giant branch (AGB) stars. Our results suggest a more rapid production scenario, likely in supernova (SN) ejecta.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present emission line ratios from a sample of 26 Lyman break galaxies from $z\sim5.5-9.5$ with $-17.05.5$ spectra. We find that the emission line ratios exhibited by these $z\sim5.5-9.5$ galaxies occupy clearly distinct regions of line-ratio space compared to typical z~0-3 galaxies, instead being more consistent with extreme populations of lower-redshift galaxies. This is best illustrated by the [OIII]/[OII] ratio, tracing interstellar medium (ISM) ionisation, in which we observe more than half of our sample to have [OIII]/[OII]>10. Our high signal-to-noise spectra reveal more than an order of magnitude of scatter in line ratios such as [OII]/H$\beta$ and [OIII]/[OII], indicating significant diversity in the ISM conditions within the sample. We find no convincing detections of [NII] in our sample, either in individual galaxies, or a stack of all G395M/F290LP spectra. The emission line ratios observed in our sample are generally consistent with galaxies with extremely high ionisation parameters (log $U\sim-1.5$), and a range of metallicities spanning from $\sim0.1\times Z_\odot$ to higher than $\sim0.3\times Z_\odot$, suggesting we are probing low-metallicity systems undergoing periods of rapid star-formation, driving strong radiation fields. These results highlight the value of deep observations in constraining the properties of individual galaxies, and hence probing diversity within galaxy population.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Local galaxies are known to broadly follow a bimodal distribution: actively star forming and quiescent systems (i.e. galaxies with no or negligible star formation activity at the epoch of observation). Why, when and how such bimodality was established, and whether it has been associated with different processes at different cosmic epochs, is still a key open question in extragalactic astrophysics. Directly observing early quiescent galaxies in the primordial Universe is therefore of utmost importance to constraining models of galaxy formation and transformation. Early quiescent galaxies have been identified out to redshift $z 10^{10}~M_{\odot}$). Here we report the discovery of a quiescent galaxy at z$=$7.3, when the Universe was only 700 Myr old - about 5% of its current age. The JWST/NIRSpec spectrum of this galaxy from our JADES programme exhibits a complete absence of nebular emission lines, while the Balmer break and Ly$\alpha$ drop are unambiguously detected. We infer that this galaxy experienced a short and intense burst of star formation followed by rapid quenching, about 10-20 Myr before the epoch of observation. Particularly interesting is that the mass of this quiescent galaxy is only $\sim$4-6$\times 10^8~M_{\odot}$. This mass range is sensitive to various feedback mechanisms that can result in temporary or permanent quiescence. Therefore this galaxy represents a unique opportunity to learn more about galaxy formation and transformation in the early Universe.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discovery of a remarkable Ly$\alpha$ emitting galaxy at $z=7.278$, JADES-GS+53.16746-27.7720 (shortened to JADES-GS-z7-LA), with EW$_0$(Ly$\alpha$) $\approx400 \pm 90$A and UV magnitude $-16.7$. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], H$\beta$ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Ly$\alpha$ line is detected in both lower resolution ($R\sim100$) PRISM as well as medium resolution ($R\sim1000$) G140M grating spectra. The Ly$\alpha$ FWHM in the grating is $\approx360$ km s$^{-1}$ and the line peaks within $120$ km s$^{-1}$ of the systemic redshift, indicative of very little neutral gas or dust within the galaxy. We estimate the Ly$\alpha$ escape fraction to be $\sim100\%$. JADES-GS-z7-LA has a [O III]/[O II] ratio (O32) of $8.8 \pm 1.1$ and ([O III]+[O II])/H$\beta$ ratio (R23) of $9.6\pm2.2$, consistent with low metallicity and high ionization parameters. Deep NIRCam imaging also revealed a close companion source (separated by $0.23''$), which exhibits similar photometry to that of JADES-GS-z7-LA, with a photometric excess in the F410M NIRCam image consistent with [O III]+H$\beta$ emission at the same redshift. The spectral energy distribution of JADES-GS-z7-LA indicates a `bursty' star-formation history, with a low stellar mass of $10^{7.15}$ M$_\odot$. The only explanation of the high EW Ly$\alpha$ emission seen in JADES-GS-z7-LA is if it resides in an ionized bubble with radius $\gtrsim 3$ pMpc. Owing to the faint nature of JADES-GS-z7-LA, we show that it is incapable of single-handedly ionizing a bubble large enough. Therefore, we suggest that JADES-GS-z7-LA (and possibly the companion source) may be a part of a larger overdensity, presenting direct evidence of overlapping ionized bubbles at $z>7$, allowing us to study the process of reionization across both small and large scales.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present a comparative study of X-ray and IR AGNs at $z\approx2$ to highlight the important AGN selection effects on the distributions of host galaxy properties. Compared with non-AGN star-forming galaxies (SFGs) on the main sequence, X-ray AGNs have similar median star formation (SF) properties, but their incidence (q$_{\rm{AGN}}$) is higher among galaxies with either enhanced or suppressed SF, and among galaxies with larger stellar mass surface density, regardless if it is measured within half-light radius ($\Sigma_e$) or central 1kpc ($\Sigma_{\rm{1kpc}}$). Unlike X-ray AGNs, IR AGNs are less massive, and have enhanced SF and similar distributions of colors, $\Sigma_e$ and $\Sigma_{\rm{1kpc}}$ relative to non-AGN SFGs. Given that $\Sigma_e$ and $\Sigma_{\rm{1kpc}}$ strongly correlate with M$_*$, we introduce the fractional mass within central 1kpc ($\rm{\frac{M_{1kpc}}{M_*}}$), which only weakly depends on M$_*$, to quantify galaxy compactness. Both AGN populations have similar $\rm{\frac{M_{1kpc}}{M_*}}$ distributions compared to non-AGN SFGs'. While q$_{\rm{AGN}}$ increases with $\Sigma_e$ and $\Sigma_{\rm{1kpc}}$, it remains constant with $\rm{\frac{M_{1kpc}}{M_*}}$, indicating that the trend of increasing q$_{\rm{AGN}}$ with $\rm{\Sigma}$ is driven by M$_*$ more than morphology. While our findings are not in conflict with the scenario of AGN quenching, they do not imply it either, because the incidence of AGNs hosted in transitional galaxies depends crucially on AGN selections. Additionally, despite the relatively large uncertainty of AGN bolometric luminosities, their very weak correlation, if any, with SF activities, regardless of AGN selections, also argues against a direct causal link between the presences of AGNs and the quenching of massive galaxies at $z\sim2$.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We study the relationship between the morphology and star formation history (SFH) of 361 quiescent galaxies (QGs) at redshift $\langle z_{obs}\rangle\approx 2$, with stellar mass $\log M_*\ge10.3$, selected with the UVJ technique. Taking advantage of panchromatic photometry covering the rest-frame UV-to-NIR spectral range ($\approx40$ bands), we reconstruct the non-parametric SFH of the galaxies with the fully Bayesian SED fitting code Prospector. We find that the half-light radius $R_e$, observed at $z_{obs}$, depends on the formation redshift of the galaxies, $z_{form}$, and that this relationship depends on stellar mass. At $\log M_*11$, the relationship between $R_e$ and $z_{form}$ flattens, suggesting that mergers become increasingly important for the size growth of more massive galaxies after they quenched. We also find that the relationship between $z_{form}$ and galaxy compactness similarly depends on stellar mass. While no clear trend is observed for QGs with $\log M_*>11$, lower-mass QGs that formed earlier, i.e. with larger $z_{form}$, have larger central stellar mass surface densities, both within the $R_e$ ($\Sigma_e$) and central 1 kpc ($\Sigma_{1kpc}$), and also larger $M_{1kpc}/M_*$, the fractional mass within the central 1 kpc. These trends between $z_{form}$ and compactness, however, essentially disappear, if the progenitor effect is removed by normalizing the stellar density with the cosmic density at $z_{form}$. Our findings highlight the importance of reconstructing the SFH of galaxies before attempting to infer their intrinsic structural evolution.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We use the SED-fitting code Prospector to reconstruct the nonparametric star formation history (SFH) of massive ($\log M_*>10.3$) star-forming galaxies (SFGs) and quiescent galaxies (QGs) at redshift $z_{\rm{obs}}\sim2$ to investigate the joint evolution of star-formation activity and structural properties. We find significant correlations between the SFH of the galaxies and their morphology. Compared to extended SFGs, compact SFGs are more likely to have experienced multiple star-formation episodes, with the fractional mass formed during the older ($\ge1$ Gyr) episode being larger, suggesting that high-redshift SFGs assembled their central regions earlier and then kept growing in central mass as they become more compact. The SFH of compact QGs does not significantly differ from the average for this category, and shows an early burst followed by a gradual decline of the star formation rate. The SFH of extended QGs, however, is similar to that of post-starburst galaxies and their morphology is also frequently disturbed. Knowledge of the SFH also enables us to empirically reconstruct the structural evolution of individual galaxies. While the progenitor effect is clearly observed and accounted for in our analysis, it alone is insufficient to explain the observed structural evolution. We show that, as they evolve from star-forming phase to quiescence, galaxies grow massive dense stellar cores. Quenching begins at the center and then propagates outward to the rest of the structure. We discuss possible physical scenarios for the observed evolution and find that our empirical constraints are in good quantitative agreement with the model predictions from dissipative accretion of gas to the center followed by massive starbursts before final quiescence (wet compaction).
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present a study of spatially-resolved star formation histories (SFHs) for 60 $z\sim2.3$ main-sequence, star-forming galaxies selected from the MOSDEF spectroscopic survey in the GOODS-N field, with median stellar mass $\log(M_\star/M_\odot)=9.75$ and spanning the range $8.6<\log(M_\star/M_\odot)<11.5$. Photometry is decomposed into a central and outer spatial component using observed $z_\mathrm{F850LP}-H_\mathrm{F160W}$ colors. The Prospector code is used to model spectral energy distributions for the center, outskirt, and integrated galaxy using HST/ACS and WFC3, Spitzer/IRAC, and ground-based photometry, with additional constraints on gas-phase metallicity and spectroscopic redshift from MOSDEF spectroscopy. For the low-resolution bands, spatially-resolved photometry is determined with an iterative approach. The reconstructed SFHs indicate that the majority of galaxies with $\log(M_\star/M_\odot)<10.5$ are observed while their central regions undergo relatively recent ($<100$ Myr) bursts of star formation, while the outskirts have a smooth, quasi-steady SFH that gently increases towards the redshift of observation. The enhanced star formation activity of the central parts is broadly consistent with the idea that it is produced by highly dissipative gas compaction and accretion. The wide range of central densities and sizes observed in the sample suggests that for the selected galaxies such a process has started but is still far from being completed. The implication would be that selecting star-forming galaxies at cosmic noon frequently includes systems in an "evolved" evolutionary phase where the centers have recently started a burst of star formation activity that will likely initiate inside-out quenching in the next several hundred million years.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The UltraViolet imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey Fields (UVCANDELS) program provides HST/UVIS F275W imaging for four CANDELS fields. We combine this UV imaging with existing HST/near-IR grism spectroscopy from 3D-HST+AGHAST to directly compare the resolved rest-frame UV and H$\alpha$ emission for a sample of 979 galaxies at $0.71.5$), suggesting that bursty star-formation is likely prevalent in the outskirts of even the most massive galaxies but is likely over-shadowed by their brighter cores. Furthermore, we present the UV-to-H$\alpha$ ratio as a function of galaxy surface brightness, a proxy for stellar mass surface density, and find that regions below $\sim10^8~M_\odot~kpc^{-2}$ are consistent with bursty star-formation, regardless of their galaxy stellar mass, potentially suggesting that local star-formation is independent of global galaxy properties at the smallest scales.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present JEMS (JWST Extragalactic Medium-band Survey), the first public
medium-band imaging survey carried out using JWST/NIRCam and NIRISS. These
observations use $\sim2\mu$m and $\sim4\mu$m medium-band filters (NIRCam F182M,
F210M, F430M, F460M, F480M; and NIRISS F430M & F480M in parallel) over 15.6
square arcminutes in the Hubble Ultra Deep Field (UDF), thereby building on the
deepest multi-wavelength public datasets available anywhere on the sky. We
describe our science goals, survey design, NIRCam and NIRISS image reduction
methods, and describe our first data release of the science-ready mosaics. Our
chosen filters create a JWST imaging survey in the UDF that enables novel
analysis of a range of spectral features potentially across the redshift range
of $0.31$ magnitude)
across redshifts $1.5
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discovery of an extreme galaxy overdensity at $z = 5.4$ in the GOODS-S field using JWST/NIRCam imaging from JADES and JEMS alongside JWST/NIRCam wide field slitless spectroscopy from FRESCO. We identified potential members of the overdensity using HST+JWST photometry spanning $\lambda = 0.4-5.0\ \mu\mathrm{m}$. These data provide accurate and well-constrained photometric redshifts down to $m \approx 29-30\,\mathrm{mag}$. We subsequently confirmed $N = 96$ potential members of the overdensity using JWST slitless spectroscopy over $\lambda = 3.9-5.0\ \mu\mathrm{m}$ through a targeted line search for $\mathrm{H} \alpha$ around the best-fit photometric redshift. We verified that $N = 53$ galaxies reside in the field at $z = 5.2-5.5$ while $N = 43$ galaxies reside in an overdensity at $z = 5.4$ around $\sim 10-12$ times that of a random volume. Stellar populations for these galaxies were inferred from the photometry and used to construct the star-forming main sequence, where protocluster members appeared more massive and exhibited earlier star formation (and thus older stellar populations) when compared to their field galaxy counterparts. We estimate the total halo mass of this large-scale structure to be $13.0 \lesssim \mathrm{log}_{10} \left( M_{\mathrm{halo}}/M_{\odot} \right) \lesssim 13.5$ using an empirical stellar mass to halo mass relation, although this is likely an underestimate as a result of incompleteness. Our discovery demonstrates the power of JWST at constraining dark matter halo assembly and galaxy formation at very early cosmic times.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The relationship between galaxy characteristics and the reionization of the universe remains elusive, mainly due to the observational difficulty in accessing the Lyman continuum (LyC) at these redshifts. It is thus important to identify low-redshift LyC-leaking galaxies that can be used as laboratories to investigate the physical processes that allow LyC photons to escape. The weakness of the [S II] nebular emission lines relative to typical star-forming galaxies has been proposed as a LyC predictor. In this paper, we show that the [S II]-deficiency is an effective method to select LyC-leaking candidates using data from the Low-redshift LyC Survey, which has detected flux below the Lyman edge in 35 out of 66 star-forming galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. We show that LyC leakers tend to be more [S II]-deficient and that the fraction of their detections increases as [S II]-deficiency becomes more prominent. Correlational studies suggest that [S II]-deficiency complements other LyC diagnostics (such as strong Lyman-$\alpha$ emission and high [O III]/[O II]). Our results verify an additional technique by which reionization-era galaxies could be studied.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Star-forming galaxies are considered the likeliest source of the H I ionizing Lyman Continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above z >~ 6, the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of indirect indicators for LyC that can be calibrated at lower redshifts and applied in the Epoch of Reionization. Emission from Mg II \ly\ly 2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for 8 LyC emitter candidates, selected to have strong Mg II emission lines. We securely detect LyC emission in 50% (4/8) galaxies with 2$\sigma$ significance. This high detection rate suggests that strong Mg II emitters might be more likely to leak LyC than similar galaxies without strong Mg II. Using photoionization models, we constrain the escape fraction of Mg II as ~ 15 -- 60%. We confirm that the escape fraction of Mg II correlates tightly with that of Lyman-alpha (LyA), which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg II emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg II emission can be adopted to estimate the escape fraction of LyA and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the Epoch of Reionization.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present JWST NIRCam 9-band near-infrared imaging of the luminous $z=10.6$ galaxy GN-z11 from the JWST Advanced Deep Extragalactic Survey (JADES) of the GOODS-N field. We find a spectral energy distribution (SED) entirely consistent with the expected form of a high-redshift galaxy: a clear blue continuum from 1.5 to 4 microns with a complete dropout in F115W. The core of GN-z11 is extremely compact in JWST imaging. We analyze the image with a two-component model, using a point source and a S\'{e}rsic profile that fits to a half-light radius of 200 pc and an index $n=0.9$. We find a low-surface brightness haze about $0.4''$ to the northeast of the galaxy, which is most likely a foreground object but might be a more extended component of GN-z11. At a spectroscopic redshift of 10.60 (Bunker et al. 2023), the comparison of the NIRCam F410M and F444W images spans the Balmer jump. From population synthesis modeling, here assuming no light from an active galactic nucleus, we reproduce the SED of GN-z11, finding a stellar mass of $\sim$$10^{9}~M_{\odot}$, a star-formation rate of $\sim$$20~M_{\odot}~\mathrm{yr}^{-1}$ and a young stellar age of $\sim$$20~\mathrm{Myr}$. As massive galaxies at high redshift are likely to be highly clustered, we search for faint neighbors of GN-z11, finding 9 galaxies out to $\sim$5 comoving Mpc transverse with photometric redshifts consistent with $z=10.6$, and a 10$^{\rm th}$ more tentative dropout only $3''$ away.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present JADES JWST/NIRSpec spectroscopy of GN-z11, the most luminous candidate $z>10$ Lyman break galaxy in the GOODS-North field with $M_{UV}=-21.5$. We derive a redshift of $z=10.603$ (lower than previous determinations) based on multiple emission lines in our low and medium resolution spectra over $0.8-5.3\,\mu$m. We significantly detect the continuum and measure a blue rest-UV spectral slope of $\beta=-2.4$. Remarkably, we see spatially-extended Lyman-$\alpha$ in emission (despite the highly-neutral IGM expected at this early epoch), offset 555 km/s redward of the systemic redshift. From our measurements of collisionally-excited lines of both low- and high-ionization (including [O II] $\lambda3727$, [Ne III] $\lambda 3869$ and C III] $\lambda1909$) we infer a high ionization parameter ($\log U\sim -2$). We detect the rarely-seen N IV] $\lambda1486$ and N III]$\lambda1748$ lines in both our low and medium resolution spectra, with other high ionization lines seen in low resolution spectrum such as He II (blended with O III]) and C IV (with a possible P-Cygni profile). Based on the observed rest-UV line ratios, we cannot conclusively rule out photoionization from AGN. The high C III]/He II ratios, however, suggest a likely star-formation explanation. If the observed emission lines are powered by star formation, then the strong N III] $\lambda1748$ observed may imply an unusually high $N/O$ abundance. Balmer emission lines (H$\gamma$, H$\delta$) are also detected, and if powered by star formation rather than an AGN we infer a star formation rate of $\sim 20-30 M_{\odot}\,\rm yr^{-1}$ (depending on the IMF) and low dust attenuation. Our NIRSpec spectroscopy confirms that GN-z11 is a remarkable galaxy with extreme properties seen 430 Myr after the Big Bang.