Astrophysics

We investigate the stellar populations for a sample of 161 massive, mainly quiescent galaxies at ??z obs ??0.8 with deep Keck/DEIMOS rest-frame optical spectroscopy (HALO7D survey). With the fully Bayesian framework Prospector, we simultaneously fit the spectroscopic and photometric data with an advanced physical model (including non-parametric star-formation histories, emission lines, variable dust attenuation law, and dust and AGN emission) together with an uncertainty and outlier model. We show that both spectroscopy and photometry are needed to break the dust-age-metallicity degeneracy. We find a large diversity of star-formation histories: although the most massive ( M ??>2? 10 11 M ??) galaxies formed the earliest (formation redshift of z f ????0 with a short star-formation timescale of ? SF ?? Gyr ), lower-mass galaxies have a wide range of formation redshifts, leading to only a weak trend of z f with M ??. Interestingly, several low-mass galaxies with have formation redshifts of z f ???? . Star-forming galaxies evolve about the star-forming main sequence, crossing the ridgeline several times in their past. Quiescent galaxies show a wide range and continuous distribution of quenching timescales ( ? quench ???? Gyr ) with a median of ??? quench ?? 1.0 +0.8 ??.9 Gyr and of quenching epochs of z quench ??.8??.0 ( ??z quench ?? 1.3 +0.7 ??.4 ). This large diversity of quenching timescales and epochs points toward a combination of internal and external quenching mechanisms. In our sample, rejuvenation and "late bloomers" are uncommon. In summary, our analysis supports the "grow & quench" framework and is consistent with a wide and continuously-populated diversity of quenching timescales.

We derive the mass-radius relation and mass function of molecular clumps in the Large Magellanic Cloud (LMC) and interpret them in terms of the simple feedback model proposed by Fall, Krumholz, and Matzner (FKM). Our work utilizes the dendrogram-based catalog of clumps compiled by Wong et al. from 12 CO and 13 CO maps of six giant molecular clouds in the LMC observed with the Atacama Large Millimeter Array (ALMA). The Magellanic Clouds are the only external galaxies for which this type of analysis is possible at the necessary spatial resolution ( ?? pc). We find that the mass-radius relation and mass function of LMC clumps have power-law forms, R??M α and dN/dM??M β , with indices α=0.36±0.03 and β=??.8±0.1 over the mass ranges 10 2 M ???�M??10 5 M ??and 10 2 M ???�M??10 4 M ??, respectively. With these values of α and β for the clumps (i.e., protoclusters), the predicted index for the mass function of young LMC clusters from the FKM model is β??.7 , in good agreement with the observed index. The situation portrayed here for clumps and clusters in the LMC replicates that in the Milky Way.

We present an analysis of the R??.5 kpc core regions of seven simulated Milky Way mass galaxies, from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, for a finely sampled period ( ?t=2.2 Myr) of 22 Myr at z?? , and compare them with star formation rate (SFR) and gas surface density observations of the Milky Way's Central Molecular Zone (CMZ). Despite not being tuned to reproduce the detailed structure of the CMZ, we find that four of these galaxies are consistent with CMZ observations at some point during this 22 Myr period. The galaxies presented here are not homogeneous in their central structures, roughly dividing into two morphological classes; (a) several of the galaxies have very asymmetric gas and SFR distributions, with intense (compact) starbursts occurring over a period of roughly 10 Myr, and structures on highly eccentric orbits through the CMZ, whereas (b) others have smoother gas and SFR distributions, with only slowly varying SFRs over the period analyzed. In class (a) centers, the orbital motion of gas and star-forming complexes across small apertures ( R??50 pc, analogously |l|< 1 ??in the CMZ observations) contributes as much to tracers of star formation/dense gas appearing in those apertures, as the internal evolution of those structures does. These asymmetric/bursty galactic centers can simultaneously match CMZ gas and SFR observations, demonstrating that time-varying star formation can explain the CMZ's low star formation efficiency.

The classical definition of the virial temperature of a galaxy halo excludes a fundamental contribution to the energy partition of the halo: the kinetic energy of non-thermal gas motions. Using simulations from the FOGGIE project (Figuring Out Gas & Galaxies In Enzo) that are optimized to resolve low-density gas, we show that the kinetic energy of non-thermal motions is roughly equal to the energy of thermal motions. The simulated FOGGIE halos have ??? lower bulk temperatures than expected from a classical virial equilibrium, owing to significant non-thermal kinetic energy that is formally excluded from the definition of T vir . We derive a modified virial temperature explicitly including non-thermal gas motions that provides a more accurate description of gas temperatures for simulated halos in virial equilibrium. Strong bursts of stellar feedback drive the simulated FOGGIE halos out of virial equilibrium, but the halo gas cannot be accurately described by the standard virial temperature even when in virial equilibrium. Compared to the standard virial temperature, the cooler modified virial temperature implies other effects on halo gas: (i) the thermal gas pressure is lower, (ii) radiative cooling is more efficient, (iii) O VI absorbing gas that traces the virial temperature may be prevalent in halos of a higher mass than expected, (iv) gas mass estimates from X-ray surface brightness profiles may be incorrect, and (v) turbulent motions make an important contribution to the energy balance of a galaxy halo.

Quasars behind the Galactic plane (GPQs) are important astrometric references and useful probes of Milky Way gas. However, the search for GPQs is difficult due to large extinctions and high source densities in the Galactic plane. Existing selection methods for quasars developed using high Galactic latitude (high- b ) data cannot be applied to the Galactic plane directly because the photometric data obtained from high- b regions and the Galactic plane follow different probability distributions. To alleviate this dataset shift problem for quasar candidate selection, we adopt a Transfer Learning Framework at both data and algorithm levels. At the data level, to make a training set in which dataset shift is modeled, we synthesize quasars and galaxies behind the Galactic plane based on SDSS sources and Galactic dust map. At the algorithm level, to reduce the effect of class imbalance, we transform the three-class classification problem for stars, galaxies, and quasars to two binary classification tasks. We apply XGBoost algorithm on Pan-STARRS1 (PS1) and AllWISE photometry for classification, and additional cut on Gaia proper motion to remove stellar contaminants. We obtain a reliable GPQ candidate catalog with 160,946 sources located at |b|??20 ??in PS1-AllWISE footprint. Photometric redshifts of GPQ candidates achieved with XGBoost regression algorithm show that our selection method can identify quasars in a wide redshift range ( 0<z?? ). This study extends the systematic searches for quasars to the dense stellar fields and shows the feasibility of using astronomical knowledge to improve data mining under complex conditions in the Big Data era.

Rotating outflows from protostellar disks might trace extended magneto-hydrodynamic (MHD) disk winds (DWs), providing a solution to the angular momentum problem in disk accretion for star formation. In the jet system HH 212, a rotating outflow was detected in SO around an episodic jet detected in SiO. Here we spatially resolve this SO outflow into three components: a collimated jet aligned with the SiO jet, the wide-angle disk outflow, and an evacuated cavity in between created by a large jet-driven bowshock. Although it was theoretically predicted before, it is the first time that such a jet-DW interaction is directly observed and resolved, and it is crucial for the proper interpretation and modeling of non-resolved DW candidates. The resolved kinematics and brightness distribution both support the wide-angle outflow to be an extended MHD DW dominating the local angular momentum extraction out to 40 au, but with an inner launching radius truncated to ?? au. Inside 4 au, where the DW may not exist, the magneto-rotational instability (MRI) might be transporting angular momentum outwards. The jet-DW interaction in HH 212, potentially present in other similar systems, opens an entirely new avenue to probe the large-scale magnetic field in protostellar disks.

Several publications highlight the importance of the observations of bow shocks to learn more about the surrounding interstellar medium and radiation field. We revisit the most prominent dusty and gaseous bow shock source, X7, close to the supermassive black hole, Sgr~A*, using multiwavelength analysis. For the purpose of this study, we use SINFONI (H+K-band) and NACO ( L ??- and M ??-band) data-sets between 2002 and 2018 with additional COMIC/ADONIS+RASOIR ( L ??-band) data of 1999. By analyzing the line maps of SINFONI, we identify a velocity of ??00 km/s from the tip to the tail. Furthermore, a combination of the multiwavelength data of NACO and SINFONI in the H -, K -, L ??-, and M ??-band results in a two-component black-body fit that implies that X7 is a dust-enshrouded stellar object. The observed ongoing elongation and orientation of X7 in the Br γ line maps and the NACO L ??-band continuum indicate a wind arising at the position of Sgr~A* or at the IRS16 complex. Observations after 2010 show that the dust and the gas shell seems to be decoupled in projection from its stellar source S50. The data also implies that the tail of X7 gets thermally heated up due to the presence of S50. The gas emission at the tip is excited because of the related forward scattering (Mie-scattering), which will continue to influence the shape of X7 in the near future. In addition, we find excited [FeIII] lines, which underline together with the recently analyzed dusty sources and the Br γ -bar the uniqueness of this source.

Using a large sample of sub-L ??galaxies, with similar UV magnitudes, M UV ?��?19 at z?? , extracted from the FirstLight simulations, we show the diversity of galaxies at the end of the reionization epoch. We find a factor ??40 variation in the specific star-formation rate (sSFR). This drives a ??1 dex range in equivalent width of the [OIII] λ 5007 line. Variations in nebular metallicity and ionization parameter within HII regions lead to a scatter in the equivalent widths and [OIII]/H α line ratio at a fixed sSFR. [OIII]-bright emitters ([OIII]/H α >1) have higher ionization parameters and/or higher metallicities than H α -bright ([OIII]/H α <1) galaxies. According to the surface brightness maps in both [OIII] and H α , [OIII]-bright emitters are more compact than H α -bright galaxies. H α luminosity is higher than [OIII] if star formation is distributed over extended regions. OIII dominates if it is concentrated in compact clumps. In both cases, the H α -emitting gas is significantly more extended than [OIII].

The presence of massive black holes (BHs) with masses of order 10 9 M ??, powering bright quasars when the Universe was less than 1 Gyr old, poses strong constraints on their formation mechanism. Several scenarios have been proposed to date to explain massive BH formation, from the low-mass seed BH remnants of the first generation of stars to the massive seed BHs resulting from the rapid collapse of massive gas clouds. However, the plausibility of some of these scenarios to occur within the progenitors of high-z quasars has not yet been thoroughly explored. In this work, we investigate, by combining dark-matter only N-body simulations with a semi-analytic framework, whether the conditions for the formation of massive seed BHs from synchronised atomic-cooling halo pairs and/or dynamically-heated mini-haloes are fulfilled in the overdense regions where the progenitors of a typical high-redshift quasar host form and evolve. Our analysis shows that the peculiar conditions in such regions, i.e. strong halo clustering and high star formation rates, are crucial to produce a non-negligible number of massive seed BH host candidates: we find ??400 dynamically heated metal-free mini-haloes, including one of these which evolves to a synchronised pair and ends up in the massive quasar-host halo by z=6 . This demonstrates that the progenitors of high-redshift quasar host haloes can harbour early massive seed BHs. Our results further suggest that multiple massive seed BHs may form in or near the quasar host's progenitors, potentially merging at lower redshifts and yielding gravitational wave events.

Our Galaxy and the nearby Andromeda galaxy (M31) are the most massive members of the Local Group, and they seem to be a bound pair, despite the uncertainties on the relative motion of the two galaxies. A number of studies have shown that the two galaxies will likely undergo a close approach in the next 4 ??5 Gyr. We used direct N -body simulations to model this interaction to shed light on the future of the Milky Way - Andromeda system and for the first time explore the fate of the two supermassive black holes (SMBHs) that are located at their centers. We investigated how the uncertainties on the relative motion of the two galaxies, linked with the initial velocities and the density of the diffuse environment in which they move, affect the estimate of the time they need to merge and form ``Milkomeda''. After the galaxy merger, we follow the evolution of their two SMBHs up to their close pairing and fusion. Upon the fiducial set of parameters, we find that Milky Way and Andromeda will have their closest approach in the next 4.3 Gyr and merge over a span of 10 Gyr. Although the time of the first encounter is consistent with other predictions, we find that the merger occurs later than previously estimated. We also show that the two SMBHs will spiral in the inner region of Milkomeda and coalesce in less than 16.6 Myr after the merger of the two galaxies. Finally, we evaluate the gravitational-wave emission caused by the inspiral of the SMBHs, and we discuss the detectability of similar SMBH mergers in the nearby Universe ( z?? ) through next-generation gravitational-wave detectors.