For the past 150 years, the local interstellar medium's prevailing view has been based on a peculiarity known as the Gould Belt, an expanding ring of young stars, gas and dust, tilted about 20 degrees to the Galactic plane. However, the physical relationship between local gas clouds has remained unknown because the accuracy in distance measurements to such clouds is of the same order as, or larger than, their sizes. With the advent of large photometric surveys and the astrometric survey, in particular ESA Gaia, this situation has changed. In this talk, I will present the three-dimensional structure of all local cloud complexes. We find a narrow and coherent 2.7-kiloparsec arrangement of dense gas in the solar neighborhood that contains many of the clouds thought to be associated with the Gould Belt. This finding is inconsistent with the notion that these clouds are part of a ring, bringing the Gould Belt model into question. The structure comprises the majority of nearby star-forming regions, has an aspect ratio of about 1:20, and contains about three million solar masses of gas. Remarkably, this structure appears to be undulating, and its three-dimensional shape is well described by a damped sinusoidal wave on the plane of the Milky Way. I will also present ongoing work on the gas's space motion in the closest massive star factory, the Orion complex, and the dispersal of young stars into the Galactic field.
physics & astronomy
The cosmic downsizing of quasars is still a big puzzle in astronomy and it is commonly believed that the central active galactic nucleus (AGN) must have played a significant role in quenching itself, in a self-regulatory mechanism popularly termed “AGN feedback” . The AGN feedback also plays a crucial role in black hole and host galaxy co-evolution across cosmic time (the M-sigma relation). Here I will discuss the nature and impact of pc scale outflows from AGN, detected in X-rays. On the other hand, the feeding of the supermassive black hole (SMBH) at the center of AGNs is an equally interesting puzzle. We still do not know how matter from the host galaxy loses their angular momentum and falls into the accretion disk, finally feeding the SMBH. In an extensive X-ray spectral variability study of Compton-thin Type-II AGN, we found the presence of clumpy gas in the near vicinity (<pc) of the SMBH which are likely candidates of matter which fall into the black hole and feed them, creating the luminous AGN.
The Southern Stellar Stream Spectroscopic Survey (S5) is an ongoing spectroscopic program that maps the newly discovered stellar streams with the fiber-fed AAOmega spectrograph on the Anglo-Australian Telescope (AAT). S5 is the first systematic program pursuing a complete census of known streams in the Southern Hemisphere, providing a uniquely powerful sample for understanding the building blocks of the Milky Way's stellar halo, the progenitors and formation of stellar streams, the mass and shape of the Milky Way's halo, and ultimately the nature of dark matter. The survey started in Summer 2018 and has mapped ~20 streams with over 50 nights on AAT. In this talk, I will give a brief overview of the current status of the program, highlighting the latest science results from the survey, and end the talk with the public data release plan.
Low mass galaxies provide an essential testing ground for theoretical predictions of cosmology. Their number densities, structures, and internal dynamics can be extremely insightful for studying dark matter and galaxy formation on small scales. I will discuss recent results studying ultra-diffuse galaxies (UDGs). UDGs hold the promise of new constraints on low mass galaxies dynamics, as their spatial extent and often significant globular cluster populations provide probes on spatial scales where dark matter should dominate the kinematics. I will also discuss the dynamics of two UDGs that seem to lack most, if not all, of their dark matter and host an intriguing population of globular clusters. I will finish by presenting a new wide-field survey carried out with the 48-lens Dragonfly Telephoto Array. With an excellent photometric depth, the Dragonfly Wide Field Survey will provide an unprecedented view of the low surface brightness universe over a wide area of the sky (350 square degrees). The main goal of the survey is to provide information on the properties and statistics of the dwarf galaxy population beyond the Local Group but it will also provide a useful resource for other resolved, low surface brightness phenomena, such as stellar streams and tidal tails, stellar halos, intragroup light and the extent of massive galaxies.
The Cosmic Evolution of Galaxy Chemical Abundances and Baryon Cycling Over the Past 12 Billion Years
Understanding how galaxies assemble their baryonic content is one of the major open questions in galaxy formation and evolution. The key processes that govern the smooth secular growth of galaxies over time include gas accretion, star formation, energetic feedback, and outflows, collectively referred to as the cycle of baryons. The gas-phase metallicity of the interstellar medium in galaxies is sensitive to baryon cycling, and scaling relations between metallicity and global galaxy properties such as stellar mass and star-formation rate are valuable probes of gas flows and galaxy growth. I will present the latest observational constraints on the evolution of the mass-metallicity relation and the fundamental metallicity relation (mass-SFR-metallicity) from z=0 to z~3.3, spanning the past 12 Gyr of cosmic history. I will discuss the implications for outflow rates and mass loading factors of galactic winds. I will also examine future observational prospects to improve the connection between gas flows and the metal and gas content of galaxies, and to extend gas-phase abundance studies into the epoch of reionization with JWST.
Recent, high-resolution surveys of 21cm emission have revealed that neutral hydrogen (HI) in the local interstellar medium (ISM) contains a wonderful wealth of structures which reflect the complex interplay of Galactic dynamics and star formation feedback. Measuring the basic physical properties of these structures is crucial for understanding their origins, and also correcting observations of extragalactic light. However, constraining their temperature and density requires observations of 21cm absorption, which are severely limited. In this talk I will present our recent efforts to measure the temperature of HI across the sky using deep learning. We train a convolutional neural network using synthetic spectra from numerical simulations to predict quantities which formally require 21cm absorption — the true HI column density and the fraction of cold, optically thick HI along the line of sight — from 21cm emission alone. We validate the model using 21cm absorption observations from the literature, finding excellent accuracy. With this model, we construct the highest-resolution, highest-fidelity map of cold HI in the local ISM using 21cm emission data from the GALFA-HI and the HI4Pi surveys. This map characterizes the structure of neutral gas envelopes to molecular clouds with unprecedented resolution, and significantly improves dusty Galactic foreground estimation for extragalactic surveys. Via comparison with tracers of dust reddening (E(B-V)), we demonstrate that E(B-V)/N(HI) increases with increasing cold gas fraction, which will be leveraged to produce high-resolution, high-fidelity E(B-V) map at high latitudes.
The X-ray emission from active galactic nuclei (AGN) originates very close to the supermassive black hole at the centre of the host galaxy. The emission varies rapidly on timescales of hours and the spectrum reveals signatures of the extreme environment close to the black hole. Narrow-line Seyfert 1 galaxies (NLS1s) provide an enhanced view of the central region in AGN, revealed through reverberation lags, intense Fe La and Fe Ka relativistic emission, dynamic coronae, and ultrafast outflows. I will review recent work on NLS1s, highlighting their most interesting properties, and attempt to describe the NLS1 phenomenon in context of general AGN behavior.
The need for AGN feedback in the cores of galaxy clusters has been long established- without the energy injection by jetted AGN in the central galaxy, we believe that the intracluster medium (ICM) would undergo a cooling catastrophe, leading to prodigious star formation and galaxy building in contravention with observations. However, the actual physical mechanisms that govern the AGN feedback cycle remain elusive. In this talk, I will discuss the possible physical process by which the central AGN can heat the ICM. I will present a series of studies that, step-by-step, move us away from a simple hydrodynamic picture and force us to treat the ICM as a weakly collisional plasma with important properties governed by non-trivial kinetic physics.
The classical cooling-flow model of galaxy clusters fails in the absence of a non-gravitational heating mechanism needed to compensate for radiative cooling in the hot intra-cluster medium (ICM). Feedback from an active galactic nucleus (AGN) offset the cooling via the energy released from the bubbles inflated by radio jets launched from supermassive black holes (SMBH). However, it cannot completely offset the cooling as central cluster galaxies (BCGs) harbor a complex multiphase medium of extended warm and cold gas reservoirs, whose physical origin remains unknown. In the first part of this talk, I will present Atacama Large Millimeter Array (ALMA) and new Multi-Unit Spectroscopic Explorer (MUSE) observations of 15 central cluster galaxies to unveil the origin and life-cycle of these filamentary networks. In the second part of this talk, by extending the sample, including new MUSE observations of 15 central group galaxies (BGGs), I will explore the origin of the gas and the effect of AGN-feedback in the intermediate-mass range between individual galaxies and massive clusters.
Both stellar mass and supermassive black holes can vary in brightness extremely rapidly, changing by orders of magnitude within hours. This variability gives us a powerful tool to understand the accretion disks around black holes, and the relativistic winds that they can launch. Because the X-ray spectra are made up of multiple complex variable components, the observed variability can be strongly energy dependent. By calculating the variance of X-ray lightcurves as a function of energy, we can build a variance spectrum. These spectra have been used to qualitatively study black hole variability for many years, but are rarely used quantitatively. I will present recent results from an ongoing research program to model variance spectra of compact objects, including a new method for detecting ultra-fast outflows, implications for accretion disk physics and new constraints on AGN feedback.