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.
physics & astronomy
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.
Interstellar dust is still a dominant uncertainty in Astronomy, limiting precision in e.g., cosmological distance estimates and models of how light is re-processed within a galaxy. When a foreground galaxy serendipitously overlaps a more distant one, the latter backlights the dusty structures in the nearer foreground galaxy. Such an overlapping or occulting galaxy pair can be used to measure the distribution of dust in the closest galaxy with great accuracy. The STARSMOG program uses Hubble to map the distribution of dust in foreground galaxies in fine (<100 pc) detail. Integral Field Unit (IFU) observations will map the effective extinction curve, disentangling the role of fine-scale geometry and grain composition on the path of light through a galaxy. The overlapping galaxy technique promises to deliver a clear understanding of the dust in galaxies: geometry, a probability function of dimming as a function of galaxy mass and radius, and its dependence on wavelength.
The 7 Ms Chandra X-ray Observatory exposure on the Chandra
Deep Field-South (CDF-S) has provided the most sensitive
extragalactic X-ray survey by a wide margin. About 1050
X-ray sources have been detected, primarily distant active
galactic nuclei (AGNs) and starburst/normal galaxies. The
unmatched deep multiwavelength coverage for these sources
allows superb follow-up investigations, revealing the
details of supermassive black hole growth over most of
cosmic time. I will briefly describe the sources in the
7 Ms CDF-S and highlight some exciting science results.
The latter will include (1) evidence for black-hole vs.
bulge co-evolution in the distant universe; (2) constraints
on supermassive black hole growth in the first galaxies as
revealed by direct detection and stacking; and (3) the
discovery of representatives of a new population of faint,
fast X-ray transient sources. Finally, I will discuss some
future prospects for X-ray surveys of AGNs in the distant
universe, including the ongoing 5 Ms XMM-SERVS survey of
the LSST Deep Drilling Fields and new X-ray missions.