Changing-look Active Galactic Nuclei (CL AGN) can be generally confirmed by the emergence (turn-on) or disappearance (turn-off) of broad emission lines, associated with a transient timescale (about ...\(100\sim5000\) days) that is much shorter than predicted by traditional accretion disk models. We carry out a systematic CL AGN search by cross-matching the spectra coming from the Dark Energy Spectroscopic Instrument and the Sloan Digital Sky Survey. Following previous studies, we identify CL AGN based on \(\rm{H}\alpha \), \(\rm{H}\beta\), and Mg\,{\sc ii} at \(z\leq0.75\) and Mg\,{\sc ii}, C\,{\sc iii}, and C\,{\sc iv} at \(z>0.75\). We present 130 CL AGN based on visual inspection and three selection criteria, including 2 \(\rm{H}\alpha\), 45 \(\rm{H}\beta\), 38 Mg\,{\sc ii}, 61 C\,{\sc iii}, and 10 C\,{\sc iv} CL AGN. Twenty cases show simultaneous appearances/disappearances of two broad emission lines while three AGN exhibit the concurrent appearance of three broad emission lines. We also present 91 CL AGN candidates with significant flux variation of broad emission lines but remaining strong broad components. In the confirmed CL AGN, 42 cases show additional CL candidate features for different lines. In this paper, we find 1) a 95:35 ratio of a turn-on to turn-off CL AGN; 2) the highest redshift CL AGN (\(z=3.56\)) ever discovered; 3) an upper limit transition timescale ranging from 244 to 5762 days in the rest-frame; 4) the majority of CL AGN follow the bluer-when-brighter trend. Our results greatly increase the current CL census (\(30\sim50\%\)) and would be conducive to explore the underlying physical mechanism.
MNRAS 512(3), 4352, 2022 The protocluster SPT2349$-$56 at $z\,{=}\,4.3$ contains one of the most
actively star-forming cores known, yet constraints on the total stellar mass of
this system are highly ...uncertain. We have therefore carried out deep optical
and infrared observations of this system, probing rest-frame ultraviolet to
infrared wavelengths. Using the positions of the spectroscopically-confirmed
protocluster members, we identify counterparts and perform detailed source
deblending, allowing us to fit spectral energy distributions in order to
estimate stellar masses. We show that the galaxies in SPT2349$-$56 have stellar
masses proportional to their high star-formation rates, consistent with other
protocluster galaxies and field submillimetre galaxies (SMGs) around redshift
4. The galaxies in SPT2349$-$56 have on average lower molecular gas-to-stellar
mass fractions and depletion timescales than field SMGs, although with
considerable scatter. We construct the stellar-mass function for SPT2349$-$56
and compare it to the stellar-mass function of $z\,{=}\,1$ galaxy clusters,
finding consistent shapes between the two. We measure rest-frame galaxy
ultraviolet half-light radii from our HST-F160W imaging, finding that on
average the galaxies in our sample are similar in size to typical star-forming
galaxies at these redshifts. However, the brightest HST-detected galaxy in our
sample, found near the luminosity-weighted centre of the protocluster core,
remains unresolved at this wavelength. Hydrodynamical simulations predict that
the core galaxies will quickly merge into a brightest cluster galaxy, thus our
observations provide a direct view of the early formation mechanisms of this
class of object.
Modern galaxy cluster science is a multi-wavelength endeavor with
cornerstones provided by X-ray, optical/IR, mm, and radio measurements. In
combination, these observations enable the construction of ...large, clean,
complete cluster catalogs, and provide precise redshifts and robust mass
calibration. The complementary nature of these multi-wavelength data
dramatically reduces the impact of systematic effects that limit the utility of
measurements made in any single waveband. The future of multi-wavelength
cluster science is compelling, with cluster catalogs set to expand by orders of
magnitude in size, and extend, for the first time, into the high-redshift
regime where massive, virialized structures first formed. Unlocking
astrophysical and cosmological insight from the coming catalogs will require
new observing facilities that combine high spatial and spectral resolution with
large collecting areas, as well as concurrent advances in simulation modeling
campaigns. Together, future multi-wavelength observations will resolve the
thermodynamic structure in and around the first groups and clusters,
distinguishing the signals from active and star-forming galaxies, and unveiling
the interrelated stories of galaxy evolution and structure formation during the
epoch of peak cosmic activity.
MNRAS, 495, 3124 (2020) We present an extensive ALMA spectroscopic follow-up programme of the
$z\,{=}\,4.3$ structure SPT2349$-$56, one of the most actively star-forming
proto-cluster cores known, to ...identify additional members using their C{\sc
ii} 158\,$\mu$m and \mbox{CO(4--3)} lines. In addition to robustly detecting
the 14 previously published galaxies in this structure, we identify a further
15 associated galaxies at $z\,{=}\,4.3$, resolving 55$\,{\pm}\,$5\,per cent of
the 870-$\mu$m flux density at 0.5\,arcsec resolution compared to 21\,arcsec
single-dish data. These galaxies are distributed into a central core containing
23 galaxies extending out to 300\,kpc in diameter, and a northern extension,
offset from the core by 400\,kpc, containing three galaxies. We discovered
three additional galaxies in a red {\it Herschel\/}-SPIRE source 1.5\,Mpc from
the main structure, suggesting the existence of many other sources at the same
redshift as SPT2349$-$56 that are not yet detected in the limited coverage of
our data. An analysis of the velocity distribution of the central galaxies
indicates that this region may be virialized with a mass of
(9$\pm$5)$\,{\times}\,$10$^{12}$\,M$_{\odot}$, while the two offset galaxy
groups are about 30 and 60\,per cent less massive and show significant velocity
offsets from the central group. We calculate the C{\sc ii} and far-infrared
number counts, and find evidence for a break in the C{\sc ii} luminosity
function. We estimate the average SFR density within the region of SPT2349$-$56
containing single-dish emission (a proper diametre of 720\,kpc), assuming
spherical symmetry, to be roughly
4$\,{\times}\,10^4$\,M$_{\odot}$\,yr$^{-1}$\,Mpc$^{-3}$; this may be an order
of magnitude greater than the most extreme examples seen in simulations.
Extremely metal-poor galaxies (XMPGs) at relatively low redshift are excellent laboratories for studying galaxy formation and evolution in the early universe. Much effort has been spent on ...identifying them from large-scale spectroscopic surveys or spectroscopic follow-up observations. Previous work has identified a few hundred XMPGs. In this work, we obtain a large sample of 223 XMPGs at \(z<1\) from the early data of the Dark Energy Spectroscopic Instrument (DESI). The oxygen abundance is determined using the direct \(T_{\rm e}\) method based on the detection of the O III\(\lambda\)4363 line. The sample includes 95 confirmed XMPGs based on the oxygen abundance uncertainty; remaining 128 galaxies are regarded as XMPG candidates. These XMPGs are only 0.01% of the total DESI observed galaxies. Their coordinates and other proprieties are provided in the paper. The most XMPG has an oxygen abundance of \(\sim 1/34 Z_{\odot}\), stellar mass of about \(1.5\times10^7 M_{\odot}\) and star formation rate of 0.22 \(M_{\odot}\) yr\(^{-1}\). The two most XMPGs present distinct morphologies suggesting different formation mechanisms. The local environmental investigation shows that XMPGs preferentially reside in relatively low-density regions. Many of them fall below the stellar mass-metallicity relations (MZRs) of normal star-forming galaxies. From a comparison of the MZR with theoretical simulations, it appears that XMPGs are good analogs to high-redshift star-forming galaxies. The nature of these XMPG populations will be further investigated in detail with larger and more complete samples from the on-going DESI survey.
The discoveries made over the past 20 years by Chandra and XMM-Newton surveys
in conjunction with multiwavelength imaging and spectroscopic data available in
the same fields have significantly ...changed the view of the supermassive black
hole (SMBH) and galaxy connection. These discoveries have opened up several
exciting questions that are beyond the capabilities of current X-ray telescopes
and will need to be addressed by observatories in the next two decades. As new
observatories peer into the early Universe, we will begin to understand the
physics and demographics of SMBH infancy (at $z>6$) and investigate the
influence of their accretion on the formation of the first galaxies ($\S$ 2.1).
We will also be able to understand the accretion and evolution over the cosmic
history (at $z\sim$1-6) of the full population of black holes in galaxies,
including low accretion rate, heavily obscured AGNs at luminosities beyond the
reach of current X-ray surveys ($\S$2.2 and $\S$2.3), enabling us to resolve
the connection between SMBH growth and their environment.
The protocluster SPT2349\(-\)56 at \(z\,{=}\,4.3\) contains one of the most actively star-forming cores known, yet constraints on the total stellar mass of this system are highly uncertain. We have ...therefore carried out deep optical and infrared observations of this system, probing rest-frame ultraviolet to infrared wavelengths. Using the positions of the spectroscopically-confirmed protocluster members, we identify counterparts and perform detailed source deblending, allowing us to fit spectral energy distributions in order to estimate stellar masses. We show that the galaxies in SPT2349\(-\)56 have stellar masses proportional to their high star-formation rates, consistent with other protocluster galaxies and field submillimetre galaxies (SMGs) around redshift 4. The galaxies in SPT2349\(-\)56 have on average lower molecular gas-to-stellar mass fractions and depletion timescales than field SMGs, although with considerable scatter. We construct the stellar-mass function for SPT2349\(-\)56 and compare it to the stellar-mass function of \(z\,{=}\,1\) galaxy clusters, finding consistent shapes between the two. We measure rest-frame galaxy ultraviolet half-light radii from our HST-F160W imaging, finding that on average the galaxies in our sample are similar in size to typical star-forming galaxies at these redshifts. However, the brightest HST-detected galaxy in our sample, found near the luminosity-weighted centre of the protocluster core, remains unresolved at this wavelength. Hydrodynamical simulations predict that the core galaxies will quickly merge into a brightest cluster galaxy, thus our observations provide a direct view of the early formation mechanisms of this class of object.
The past decade has seen an explosion of discoveries and new insights into the diffuse gas within galaxies, galaxy clusters, and the filaments composing the Cosmic Web. A new decade will bring fresh ...opportunities to further this progress towards developing a comprehensive view of the composition, thermal state, and physical processes of diffuse gas in the Universe. Ultraviolet (UV) spectroscopy, probing diffuse 10^4-10^6 K gas at high spectral resolution, is uniquely poised to (1) witness environmental galaxy quenching processes in action, such as strangulation and tidal- and ram-pressure stripping, (2) directly account for the baryon content of galaxy clusters in the cold-warm (T<10^6 K) gas, (3) determine the phase structure and kinematics of gas participating in the equilibrium-regulating exchange of energy at the cores of galaxy clusters, and (4) map cold streams and filaments of the Cosmic Web that feed galaxies and clusters. With a substantial UV undertaking beyond the Hubble Space Telescope, all of the above would be achievable over the entire epoch of galaxy cluster formation. Such capabilities, coupled with already-planned advancements at other wavelengths, will transform extragalactic astronomy by revealing the dominant formation and growth mechanisms of gaseous halos over the mass spectrum, settling the debate between early- and late-time metal enrichment scenarios, and revealing how the ecosystems in which galaxies reside ultimately facilitate their demise.
We present an extensive ALMA spectroscopic follow-up programme of the \(z\,{=}\,4.3\) structure SPT2349\(-\)56, one of the most actively star-forming proto-cluster cores known, to identify additional ...members using their C{\sc ii} 158\,\(\mu\)m and \mbox{CO(4--3)} lines. In addition to robustly detecting the 14 previously published galaxies in this structure, we identify a further 15 associated galaxies at \(z\,{=}\,4.3\), resolving 55\(\,{\pm}\,\)5\,per cent of the 870-\(\mu\)m flux density at 0.5\,arcsec resolution compared to 21\,arcsec single-dish data. These galaxies are distributed into a central core containing 23 galaxies extending out to 300\,kpc in diameter, and a northern extension, offset from the core by 400\,kpc, containing three galaxies. We discovered three additional galaxies in a red {\it Herschel\/}-SPIRE source 1.5\,Mpc from the main structure, suggesting the existence of many other sources at the same redshift as SPT2349\(-\)56 that are not yet detected in the limited coverage of our data. An analysis of the velocity distribution of the central galaxies indicates that this region may be virialized with a mass of (9\(\pm\)5)\(\,{\times}\,\)10\(^{12}\)\,M\(_{\odot}\), while the two offset galaxy groups are about 30 and 60\,per cent less massive and show significant velocity offsets from the central group. We calculate the C{\sc ii} and far-infrared number counts, and find evidence for a break in the C{\sc ii} luminosity function. We estimate the average SFR density within the region of SPT2349\(-\)56 containing single-dish emission (a proper diametre of 720\,kpc), assuming spherical symmetry, to be roughly 4\(\,{\times}\,10^4\)\,M\(_{\odot}\)\,yr\(^{-1}\)\,Mpc\(^{-3}\); this may be an order of magnitude greater than the most extreme examples seen in simulations.
Extending the earlier measurements reported in Hitomi collaboration (2016, Nature, 535, 117), we examine the atmospheric gas motions within the central 100~kpc of the Perseus cluster using ...observations obtained with the Hitomi satellite. After correcting for the point spread function of the telescope and using optically thin emission lines, we find that the line-of-sight velocity dispersion of the hot gas is remarkably low and mostly uniform. The velocity dispersion reaches maxima of approximately 200~km~s\(^{-1}\) toward the central active galactic nucleus (AGN) and toward the AGN inflated north-western `ghost' bubble. Elsewhere within the observed region, the velocity dispersion appears constant around 100~km~s\(^{-1}\). We also detect a velocity gradient with a 100~km~s\(^{-1}\) amplitude across the cluster core, consistent with large-scale sloshing of the core gas. If the observed gas motions are isotropic, the kinetic pressure support is less than 10\% of the thermal pressure support in the cluster core. The well-resolved optically thin emission lines have Gaussian shapes, indicating that the turbulent driving scale is likely below 100~kpc, which is consistent with the size of the AGN jet inflated bubbles. We also report the first measurement of the ion temperature in the intracluster medium, which we find to be consistent with the electron temperature. In addition, we present a new measurement of the redshift to the brightest cluster galaxy NGC~1275.
