Calendar

Oct
28
Fri
Colloquium: Mark Wardle
Oct 28 @ 3:15 pm – 4:15 pm
Colloquium: Mark Wardle @ Biology Annex 102

Star formation in the vicinity of the supermassive black hole at the Galactic Centre

Dr. Mark Wardle, Macquarie University

The disruptive tidal field near supermassive black holes overcomes the self-gravity of objects that are less dense than the Roche density.  This was once expected to suppress star formation within several parsecs of  Sgr A*, the four million solar mass black hole at the centre of the Galaxy.   It has since become apparent that things are not this simple:  Sgr A* is surrounded by a sub-parsec-scale orbiting disk of massive stars, indicating a star formation event occurred a few million years ago.    And on parsec scales,  star formation seems to be happening now:  there are proplyd candidates and protostellar outflow candidates,  as well as methanol and water masers that in the galactic disk would be regarded as sure-fire signatures of star formation.  In this talk, I shall consider how star formation can occur so close to Sgr A*.

The stellar disk may be created through the partial capture of a molecular cloud as it swept through the inner few parsecs of the galaxy and temporarily engulfed Sgr A*.  This rather naturally creates a disk of gas with the steep surface density profile of the present stellar disk.  The inner 0.04 pc  is so optically thick that it cannot fragment, instead accreting onto Sgr A* in a few million years; meanwhile the outer disk fragments and creates the observed stellar disk.   The isolated young stellar objects found at larger distances, on the other hand,  can be explained by stabilisation of clouds or cloud cores by the high external pressure that permeates the inner Galaxy.   A virial analysis shows that clouds are indeed tidally disrupted within 0.5 pc of Sgr A*, but outside this the external pressure allows self-gravitating clouds to survive, providing the raw material for ongoing star formation.

 

May
5
Fri
Colloquium PhD Defense: Jacob Vander Vliet
May 5 @ 3:15 pm – 4:15 pm
Colloquium PhD Defense: Jacob Vander Vliet @ Domenici Hall 106

Observing the Baryon Cycle in Hydrodynamic Cosmological Simulations

Jacob Vander Vliet, NMSU

An understanding of galaxy evolution requires an understanding of the flow of baryons in and out of a galaxy. The accretion of baryons is required for galaxies to form stars, while stars eject baryons out of the galaxy through stellar feedback mechanisms such as supernovae, stellar winds, and radiation pressure. The interplay between outflowing and infalling material form the circumgalactic medium (CGM). Hydrodynamic simulations provide understanding in the connection between stellar feedback and the distribution and kinematics of baryons in the CGM. To compare simulations and observations properly the simulated CGM must be observed in the same manner as the real CGM. I have developed the Mockspec code to generate synthetic quasar absorption line observations of the CGM in cosmological hydrodynamic simulations. Mockspec generates synthetic spectra based on the phase, metallicity, and kinematics of CGM gas and mimics instrumental effects. Mockspec includes automatic analysis of the spectra and identifies the gas responsible for the absorption. Mockspec was applied to simulations of dwarf galaxies at low redshift to examine the observable effect different feedback models have on the CGM. While the different feedback models had strong effects on the galaxy, they all produced a similar CGM that failed match observations. Mockspec was applied to the VELA simulation suite of high redshift, high mass galaxies to examine the variance of the CGM across different galaxies in different environments. The observable CGM showed little variation between the different galaxies and almost no evolution from z=4 to z=1. The VELAs were not able to generate a CGM to match the observations. The properties of cells responsible for the absorption were compared to the derived properties from Voigt Profile decomposition. VP modeling was found to accurately describe the HI and MgII absorbing gas but failed for high ionization species such as CIV and OVI, which do not arise in the assumed coherent structures.  The technique of mock QAL is useful for testing the accuracy of the simulated CGM and for verifying observational techniques, but not for differentiating between feedback prescriptions in dwarf galaxies.

 

Jul
3
Mon
Colloquium PhD Defense: Nigel Mathes
Jul 3 @ 2:00 pm – 3:00 pm
Colloquium PhD Defense: Nigel Mathes

The Vulture Survey of MgII and CIV Absorbers: Feasting on the Bones of Spectra Left to Die

Nigel Mathes, NMSU

Abstract:

We present detailed measurements of the absorption properties and redshift evolution of MgII and CIV absorbers as measured in archival spectra from the UVES spectrograph at the Very Large Telescope (VLT/UVES) and the HIRES spectrograph at the Keck Telescope (Keck/HIRES) to equivalent width detection limits below 0.01 angstroms. This survey examines 860 high resolution spectra from various archival data sets representing 700 unique sightlines, allowing for detections of intervening MgII absorbers spanning redshifts 0.1 < z < 2.6 and intervening CIV absorbers spanning redshifts 1 < z < 5. We employ an accurate, automated approach to line detection which consistently detects redshifted absorption doublets. We observe three distinct epochs of evolution in the circumgalactic medium (CGM) as traced by MgII and CIV absorbers. At high redshifts, from 3 < z < 5, galaxies rapidly build up a metal enriched halo where, despite significant evolution in the ionizing background, the production of metals through star formation driven outflows dominates observed trends increasing the number of observed absorbers per redshift path length towards z = 3. At mid redshifts, from 2 < z < 3, a large cosmic increase in the global star formation rate drives large numbers of high column density outflows into the halos of galaxies. At this time, metal line absorption of all species is increased above all other epochs. At low redshifts, for z < 2, the universe becomes more quiescent in both star formation and ionizing background. Weak, low column density MgII absorbers proliferate, while strong MgII absorbers likely fragment or re-accrete onto their host galaxy. Strong CIV absorbers, at this time, still increase in number per absorption path, while their weaker counterparts begin to disappear. MgII and CIV absorbers appear to originate in star formation driven outflows, but their different evolutionary properties imply they represent two physically distinct phases of gas. These two phases comprise the CGM and contribute separately to the cycle of baryons into and out of galaxies.

Oct
23
Mon
Pizza Lunch: Kristian Finlator
Oct 23 @ 12:30 pm – 1:30 pm
Pizza Lunch: Kristian Finlator @ AY 119

Vastly Improved Simulations of the Hydrogen Reionization Epoch: Too Much for One Paper?

Nov
9
Thu
Special Pizza Lunch: Jane Rigby
Nov 9 @ 1:00 pm – 2:00 pm
Special Pizza Lunch: Jane Rigby @ AY 119

Galaxy Evolution in High Definition Via Gravitational Lensing

Dr. Jane Rigby

Deputy Project Scientist for JWST, NASA Goddard Space Flight Center

Abstract: In hundreds of known cases, “gravitational lenses” have deflected, distorted, and amplified images of galaxies or quasars behind them.  As such, gravitational lensing is a way to “cheat” at studying how galaxies evolve:  lensing can magnify galaxies by factors of 10–100 times, transforming them from objects we can barely detect to bright objects we can study in detail.   For such rare objects, we are studying how galaxies formed stars at redshifts of 1–4, the epoch when most of the Universe’s stars were formed. For lensed galaxies, we can obtained spectral diagnostics that are currently unavailable for the distant universe, but will become routine with next-generation telescopes.

In particular, I’ll discuss MEGaSaURA, The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas, which comprises high signal-to-noise, medium spectral resolution (R~3300) spectra of 15 extremely bright gravitationally lensed galaxies at redshifts of 1.6<z<3.6.   The sample, drawn from the SDSS Giant Arcs Survey, are many of the brightest lensed galaxies known.  The MEGaSaURA spectra reveal a wealth of spectral diagnostics: absorption from the outflowing wind; nebular emission lines that will be key diagnostics for JWST, GMT, and TMT; and photospheric absorption lines and P Cygni profiles from the massive stars that power the outflow.

Nov
13
Mon
Pizza Lunch: James Lewis
Nov 13 @ 12:30 pm – 1:30 pm
Pizza Lunch: James Lewis @ AY 119

Multivariate Analysis of the CGM

Mar
28
Wed
Colloquium PhD Thesis Defense: Ethan Dederick
Mar 28 @ 3:15 pm – 4:15 pm
Colloquium PhD Thesis Defense: Ethan Dederick @ Science Hall 109

Seismic Inferences of Gas Giant Planets: Excitation & Interiors

Ethan Dederick, NMSU

Seismology has been the premier tool of study for understanding the interior structure of the Earth, the Sun, and even other stars. In this thesis we develop the framework for the first ever seismic inversion of a rapidly rotating gas giant planet. We extensively test this framework to ensure that the inversions are robust and operate within a linear regime. This framework is then applied to Saturn to solve for its interior density and sound speed profiles to better constrain its interior structure. This is done by incorporating observations of its mode frequencies derived from Linblad and Vertical Resonances in Saturn’s C-ring. We find that although the accuracy of the inversions is mitigated by the limited number of observed modes, we find that Saturn’s core density must be at least 8.97 +/- 0.01 g cm^{-3} below r/R_S = 0.3352 and its sound speed must be greater than 54.09 +/- 0.01 km s^{-1} below r/R_S = 0.2237. These new constraints can aid the development of accurate equations of state and thus help determine the composition in Saturn’s core. In addition, we investigate mode excitation and whether the \kappa-Mechanism can excite modes on Jupiter. While we find that the \kappa-Mechanism does not play a role in Jovian mode excitation, we discover a different opacity driven mechanism, The Radiative Suppression Mechanism, that can excite modes in hot giant planets orbiting extremely close to their host stars if they receive a stellar flux greater than 10^9~erg cm^{-2} s^{-1}. Finally, we investigate whether moist convection is responsible for exciting Jovian modes. Mode driving can occur if, on average, one cloud column with a 1-km radius exists per 6423 km^2 or if ~43 storms with 200 columns, each with a radius of 25 km, erupt per day. While this seems unlikely given current observations, moist convection does have enough thermal energy to drive Jovian oscillations, should it be available to them.

Oct
19
Fri
Colloquium: Sanchayeeta Borthakur (Host: Kristian Finlator)
Oct 19 @ 3:15 pm – 4:15 pm
Colloquium:  Sanchayeeta Borthakur (Host: Kristian Finlator) @ BX102

Understanding How Galaxies Reionized the Universe

 Sanchayeeta Borthakur, Arizona State University

Identifying the population of galaxies that was responsible for the reionization of the universe is a long-standing quest in astronomy. While young stars can produce large amounts of ionizing photons, the mechanism behind the escape of Lyman continuum photons (wavelength < 912 A) from star-forming regions has eluded us. To identify such galaxies and to understand the process of the escape of Lyman continuum, we present an indirect technique known as the residual flux technique. Using this technique, we identified (and later confirmed) the first low-redshift galaxy that has an escape fraction of ionizing flux of 21%. This leaky galaxy provides us with valuable insights into the physics of starburst-driven feedback. In addition, since direct detection of ionizing flux is impossible at the epoch of reionization, the residual flux technique presents a highly valuable tool for future studies to be conducted with the upcoming large telescopes such as the JWST.

Mar
5
Tue
Public Talk: Janna Levin: Black Hole Blues
Mar 5 @ 7:30 pm – 8:45 pm
Dec
6
Fri
Colloquium: Elise Boera (Host: Kristian Finlator)
Dec 6 @ 3:15 pm – 4:15 pm
Colloquium: Elise Boera (Host: Kristian Finlator) @ BX102

Revealing reionization with the thermal history of the intergalactic medium

Elisa Boera, SISSA Trieste

During hydrogen reionization the UV radiation from the first luminous sources injected vast amount of energy into the intergalactic medium, photo-heating the gas to tens of thousands of degree Kelvin. This increase in temperature has left measurable `imprints’ in the thermal history of the cosmic gas: a peak in the temperature evolution at the mean density and a smoothing out of the gas in the physical space by the increased gas pressure following reionization (i.e. Jeans smoothing effect). The structures of the HI Lyman-alpha forest at high redshift are sensitive to both these effects and therefore represent a powerful tool to understand when and how reionization happened. I will present the most recent constraints on the thermal history of the intergalactic medium obtained using the Lyman-alpha forest flux power spectrum at z>5. I will show how these results can be used to obtain information on the timing and the sources of the reionization process and I will discuss their consistency with different possible reionization scenarios.