Sukanya Chakrabarti

The Sagittarius impact as an architect of spirality and outer rings in the Milky Way

Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand our Galaxy’s morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Centre in our model that are reminiscent of the low-latitude arcs observed in the same area of the Milky Way. Previous models have focused on Sgr itself to reproduce the dwarf’s orbital history and place associated constraints on the shape of the Milky Way gravitational potential, treating the Sgr impact event as a trivial influence on the Galactic disk. Our results show that the Milky Way’s morphology is not purely secular in origin and that low-mass minor mergers predicted to be common throughout the Universe probably have a similarly important role in shaping galactic structure.

Feedback-Driven Evolution of the Far-Infrared Spectral Energy Distributions of Luminous and Ultraluminous Infrared Galaxies

We calculate infrared spectral energy distributions (SEDs) from simulations of major galaxy mergers, and study the effect of active galactic nucleus (AGN) and starburst-driven feedback on the evolution of the SED as a function of time. We use a self-consistent three-dimensional radiative equilibrium code to calculate the emergent SEDs and to make images. To facilitate a simple description of our findings, we describe our results in reference to an approximate analytic solution for the far-IR SED. We focus mainly on the luminous infrared galaxy (LIRG) and ultraluminous infrared galaxy (ULIRG) phases of evolution. We contrast the SEDs of simulations performed with AGN feedback to simulations performed with starburst-driven wind feedback. We find that the feedback processes critically determine the evolution of the SED. Changing the source of illumination (whether stellar or AGN) has virtually no impact on the reprocessed far-infrared SED. We find that AGN feedback is particularly effective at dispersing gas and rapidly injecting energy into the ISM. The observational signature of such powerful feedback is a warm SED. In general, simulations performed with starburst-driven winds have colder spectra and reprocess more of their emission into the infrared, resulting in higher infrared to bolometric luminosities compared to (otherwise equivalent) simulations performed with AGN feedback. We depict our results in IRAS, as well as in Spitzers MIPS bands, and in Herschels PACS bands.

Tidal imprints of a dark subhalo on the outskirts of the Milky Way

We study the dynamical evolution of spiral structure in the s tellar disks of isolated galaxies using high resolution Smoothed Particle Hydrodynamics (SP H) simulations that treat the evolution of gas, stars, and dark matter self-consistently. We focus this study on the question of self-excited spiral structure in the stellar disk and inves tigate the dynamical coupling between the cold, dissipative gaseous component and the stellar com ponent. We find that angular momentum transport from the gas to the stars inside of corotati on leads to a roughly time-steady spiral structure in the stellar disk. To make this point clea r, we contrast these results with otherwise identical simulations that do not include a cold gase ou component that is able to cool radiatively and dissipate energy, and find that spiral struc ture, when it is incipient, dies out more rapidly in simulations that do not include gas. We also e mploy a standard star formation prescription to convert gas into stars and find that our resul ts hold for typical gas consumption time scales that are in accord with the Kennicutt-Schmidt re lation. We therefore attribute the long-lived roughly time steady spiral structure in the stel lar disk to the dynamical coupling between the gas and the stars and the resultant torques that t he self-gravitating gaseous disk is able to exert on the stars due to an azimuthal phase shift be tween the collisionless and dissipative components.

An Evolutionary Model for Submillimeter Galaxies

We calculate multi-wavelength spectral energy distributions (SEDs) (spanning optical to millimeter wavelengths) from simulations of major galaxy mergers with black hole feedback which produce submillimeter bright galaxies (SMGs), using a self-consistent three-dimensional radiative transfer code. These calculations allow us to predict multiwavelength correlations for this important class of galaxies. We review star formation rates, the time evolution of the 850 � m fluxes, along with the time evolution of the MBH Mstar relation of the SMGs formed in the mergers. We reproduce correlations for local AGN observed in Spitzer Space Telescope’s IRAC bands, and make definitive predictions for infrared X-ray correlations that should be testable by combining observations by Spitzer and the upcoming Herschel mission with X-ray surveys. To aid observational studies, we quantify the far-infrared X-ray correlations. Our dynamical approach allows us to directly correlate observed clustering in the data as seen in IRAC color-color plots with the relative amount of time the system spends in a region of color-color space. We also find that this clustering is positively correlated with the stars dominating in their contribution to the total bolometric luminosity. The merger simulations also allow us to directly correlate the 850 � m flux with the ratio of the black hole luminosity to the total luminosity, which is an inherent and testable feature of our model. We present photo albums spanning the lifetime of SMGs, from their infancy in the pre-merger phase to the final stage as an elliptical galaxy, as seen in the observed 3.6 � m and 450 � m band to visually illustrate some of the morphological differences between mergers of differing orbital inclination and progenitor redshift. We compare our SEDs from the simulations to observations of SMGs and find good agreement. We find that SMGs are a broader class of systems than starbursts or quasars. We introduce a simple, heuristic classification scheme on the basis of the LIR/Lx ratios of these galaxies, which may be interpreted qualitatively as an evolutionary scheme, as these galaxies evolve in LIR/Lx while transiting from the pre-merger stage, through the quasar phase, to a merger remnant. Subject headings: galaxies: formation—galaxies: AGN—infrared: galaxies—radiative transfer—stars: formation

PANCHROMATIC SPECTRAL ENERGY DISTRIBUTIONS OF DUSTY GALAXIES WITH RADISHE. I. PREDICTIONS FOR HERSCHEL: CORRELATING COLORS WITH GALACTIC ENERGY SOURCES

We present three-dimensional, self-consistent radiative transfer solutions with a new Monte Carlo radiative equilibrium code. The code RADISHE can be applied to calculate the emergent spectral energy distributions (SEDs) and broadband images from optical to millimeter wavelengths of arbitrary density geometries with distributed sources of radiation. One of the primary uses of this code has been to interface with hydrodynamical codes to calculate emergent SEDs along a simulation time sequence. We focus the applications of this code in this paper on infrared bright dusty galaxies, but RADISHE is also ideal for calculating emission from star clusters or protostellar environments. The primary methodological focus of this paper is on the radiative equilibrium temperature calculation. We find that an iterative calculation of the temperature, which takes the sum of photon flight paths as the Monte Carlo estimator for the mean free intensity, is significantly faster than relaxation temperature calculation methods, particularly when large numbers of grid cells are required (i.e., in modeling three-dimensional geometries such as the dust envelopes of turbulent massive protostellar cores or infrared-bright galaxies). An accurate long-wavelength SED and corresponding temperature calculation will be essential for analyzing upcoming Herschel Space Observatory observations. We present simulated color-color plots for infrared-bright galaxies at a range of redshifts, and unfold these plots as color versus the fractional active galactic nucleus (AGN) luminosity, to demonstrate that Herschel will be able to effectively discriminate between submillimeter galaxies, where the energy source is dominated by AGN, and those where star formation dominates. We demarcate in particular the Class II or energetically active AGN evolutionary phase in Herschel color-color plots.

The Nature of CO Emission from z~6 Quasars

We investigate the nature of molecular gas emission from z ~ 6 quasars via the commonly observed tracer of H2, carbon monoxide (CO). We achieve this by combining non-LTE radiative transfer calculations with merger-driven models of z ~ 6 quasar formation that arise naturally in Λ cold dark matter structure formation simulations. Motivated by observational constraints, we consider four representative z ~ 6 quasars formed in the halo mass range ~1012-1013 M☉ from different merging histories. Our main results are as follows. We find that, owing to massive starbursts and funneling of dense gas into the nuclear regions of merging galaxies, the CO is highly excited during both the hierarchical buildup of the host galaxy and the quasar phase, and the CO flux density peaks between J = 5 and 8. The CO morphology of z ~ 6 quasars often exhibits multiple CO emission peaks which arise from molecular gas concentrations which have not yet fully coalesced. Both of these results are found to be consistent with the sole CO detection at z ~ 6, in quasar J1148+5251. Quasars which form at z ~ 6 display a large range of sight line-dependent line widths. The sight line dependencies are such that the narrowest line widths are when the rotating molecular gas associated with the quasar is viewed face-on (when the LB is largest) and broadest when the quasar is seen edge-on (and the LB is lowest). Thus, we find that for all models selection effects exist such that quasars selected for optical luminosity are preferentially seen to be face-on which may result in CO detections of optically luminous quasars at z ~ 6 having line widths narrower than the median. The mean sight line-averaged line width is found to be reflective of the circular velocity of the host halo and thus scales with halo mass. For example, the mean line width for the ~1012 M☉ halo is σ ~ 300 km s−1, while the median for the ~1013 M☉ quasar host is σ ~ 650 km s−1. Depending on the host halo mass, approximately 2%-10% of sight lines in our modeled quasars are found to have narrow line widths compatible with observations of J1148+5251. When considering the aforementioned selection effects, these percentages increase to 10%-25% for quasars selected for optical luminosity. When accounting for both temporal evolution of CO line widths in galaxies, as well as the redshift evolution of halo circular velocities, these models can self-consistently account for the observed line widths of both submillimeter galaxies and quasars at z ~ 2. Finally, we find that the dynamical mass derived from the mean sight line-averaged line widths provide a good estimate of the total mass and allow for a massive molecular reservoir, supermassive black hole, and stellar bulge, consistent with the local MBH-Mbul relation.

FINDING DWARF GALAXIES FROM THEIR TIDAL IMPRINTS

We describe ongoing work on a new method that allows one to approximately determine the mass and relative position (in galactocentric radius and azimuth) of galactic companions purely from analysis of observed disturbances in gas disks. We demonstrate the validity of this method, which we call Tidal Analysis, by applying it to local spirals with known optical companions, namely M51 and NGC 1512. These galaxies span the range from having a very low mass companion (~one-hundredth the mass of the primary galaxy) to a fairly massive companion (~one-third the mass of the primary galaxy). This approach has broad implications for many areas of astrophysics—for the indirect detection of dark matter (or dark-matter-dominated dwarf galaxies), and for galaxy evolution in its use to decipher the dynamical impact of satellites on galactic disks. Here, we provide a proof of principle of the method by applying it to infer and quantitatively characterize optically visible galactic companions of local spirals, from the analysis of observed disturbances in outer gas disks.

The AGORA High-resolution Galaxy Simulations Comparison Project II: Isolated disk test

Using an isolated Milky Way-mass galaxy simulation, we compare results from nine state-of-the-art gravito-hydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e.g., radiative cooling and UV background by the standardized package Grackle) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt–Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly formed stellar clump mass functions show more significant variation (difference by up to a factor of ~3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low-density region, and between more diffusive and less diffusive schemes in the high-density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes.

Far-Infrared Spectral Energy Distributions and Photometric Redshifts of Dusty Galaxies

We infer the large-scale source parameters of dusty galaxies from their observed spectral energy distributions (SEDs) using the analytic radiative transfer methodology presented by Chakrabarti and McKee. For local ultraluminous infrared galaxies (ULIRGs), we show that the millimeter to far-infrared (FIR) SEDs can be well fit using the standard dust opacity index of 2 when self-consistent radiative transfer solutions are employed, indicating that the cold dust in local ULIRGs can be described by a single-grain model. We develop a method for determining photometric redshifts of ULIRGs and submillimeter galaxies from the millimeter-FIR SED; the resulting value of 1 + z is typically accurate to about 10% . As such, it is comparable to the accuracy of near-IR photometric redshifts and provides a complementary means of deriving redshifts from far-IR data, such as those from the upcoming Herschel Space Observatory. Since our analytic radiative transfer solution is developed for homogeneous, spherically symmetric, centrally heated, dusty sources, it is relevant for infrared bright galaxies that are primarily powered by compact sources of luminosity that are embedded in a dusty envelope. We discuss how deviations from spherical symmetry may affect the applicability of our solution, and we contrast our self-consistent analytic solution with standard approximations to demonstrate the main differences.

Phase wrapping of epicyclic perturbations in the Wobbly Galaxy

We use test-particle integrations to show that epicyclic motions excited by a pericentre passage of a dwarf galaxy could account for bulk vertical velocity streaming motions recently observed in the Galactic stellar disc near the Sun. We use fixed potential test-particle integrations to isolate the role of phase wrapping of epicyclic perturbations from bending and breathing waves or modes, which require self-gravity to oscillate. Perturbations from a fairly massive Sagittarius dwarf galaxy,