arXiv subjects

Timothy Miller

Publications and source records attributed to Timothy Miller.

28 records · Page 2Linked to original sources

arXiv2020

HST/COS Observations of Quasar Outflows in the 500 -- 1050 Å Rest Frame: II The Most Energetic Quasar Outflow Measured to Date

We present a study of the BAL outflows seen in quasar SDSS J1042+1646 (z = 0.978) in the rest-frame 500 -- 1050 $Å$ (EUV500) region. The results are based on the analysis of recent Hubble Space Telescope/Cosmic Origins Spectrograph observations. Five outflow systems are identified, where in total they include $\sim$70 outflow troughs from ionic transitions. These include the first non-solar detections from transitions of O V*, Ne V*, Ar VI, Ca VI, Ca VII, and Ca VIII. The appearance of very high-ionization species (e.g., Ne VIII, Na IX, and Mg X) in all outflows necessitates at least two-ionization phases for the observed outflows. We develop an interactive Synthetic Spectral Simulation method to fit the multitude of observed troughs. Detections of density sensitive troughs (e.g., S IV* $λ$ 657.32 $Å$ and the O V* multiplet) allow us to determine the distance of the outflows ($R$) as well as their energetics. Two of the outflows are at $R$ $\simeq$ 800 pc and one is at $R$ $\simeq$ 15 pc. One of the outflows has the highest kinetic luminosity on record ($\dot{E_{k}}$ $ = 5\times 10^{46}$ erg s$^{-1}$), which is 20% of its Eddington luminosity. Such a large ratio suggests that this outflow can provide the energy needed for active galactic nucleus feedback mechanisms.

astro-ph.GA↗

arXiv2020

HST/COS Observations of Quasar Outflows in the 500 -- 1050 Å Rest Frame: VI Wide, Energetic Outflows in SDSS J0755+2306

We present the analysis of two outflows (S1 at --5500 km s$^{-1}$ and S2 at --9700 km s$^{-1}$) seen in recent HST/COS observations of quasar SDSS J0755+2306 (z = 0.854). The outflows are detected as absorption troughs from both high-ionization species, including N III, O III, and S IV, and very high-ionization species, including Ar VIII, Ne VIII, and Na IX. The derived photoionization solutions show that each outflow requires a two ionization-phase solution. For S1, troughs from S IV* and S IV allow us to derive an electron number density, $n_{e}$ = 1.8$\times$10$^4$ cm$^{-3}$, and its distance from the central source of $R$ = 270 pc. For S2, troughs from O III* and O III yield $n_{e}$ = 1.2$\times$10$^3$ cm$^{-3}$ and $R$ = 1600 pc. The kinetic luminosity of S2 is $>$ 12% of the Eddington luminosity for the quasar and therefore can provide strong AGN feedback effects. Comparison of absorption troughs from O III and O VI in both outflow systems supports the idea that for a given element, higher ionization ions have larger covering fractions than lower ionization ones.

astro-ph.GA↗

arXiv2019

Evidence that Emission and Absorption Outflows in Quasars Are Related

We analyze VLT/X-shooter data for 7 quasars, where we study the relationships between their broad absorption line (BAL) and emission line outflows. We find: 1) the luminosity of the [OIII] $λ$5007 emission profile decreases with increasing electron number density (n$_e$) derived from the BAL outflow in the same quasar, 2) the measured velocity widths from the [OIII] emission features and CIV absorption troughs in the same object are similar, and 3) the mean radial velocity derived from the BAL outflow is moderately larger than the one from the [OIII] emission outflow. These findings can be explained by the physical interpretation that the [OIII] and BAL outflow are different manifestations of the same wind. When we have outflows with smaller distances to the central source, their n$_e$ is higher. Therefore, the [OIII] emission is collisionally de-excited and the [OIII] luminosity is then suppressed. Comparisons to previous studies show that the objects in our sample exhibit broad [OIII] emission features similar to the ones in extremely red quasars (ERQs). This might imply that BAL quasars and ERQs have the same geometry of outflows or are at a similar evolutionary stage. We found that the physical parameters derived from the BAL outflows can explain the amount of observed [OIII] luminosity, which strengthens our claim of both BAL and [OIII] outflows are from the same wind. These estimates can be tested with upcoming James Webb Space Telescope observations.

astro-ph.GA↗

arXiv2019

Two-stage Federated Phenotyping and Patient Representation Learning

A large percentage of medical information is in unstructured text format in electronic medical record systems. Manual extraction of information from clinical notes is extremely time consuming. Natural language processing has been widely used in recent years for automatic information extraction from medical texts. However, algorithms trained on data from a single healthcare provider are not generalizable and error-prone due to the heterogeneity and uniqueness of medical documents. We develop a two-stage federated natural language processing method that enables utilization of clinical notes from different hospitals or clinics without moving the data, and demonstrate its performance using obesity and comorbities phenotyping as medical task. This approach not only improves the quality of a specific clinical task but also facilitates knowledge progression in the whole healthcare system, which is an essential part of learning health system. To the best of our knowledge, this is the first application of federated machine learning in clinical NLP.

cs.IR↗

arXiv2018

FADL:Federated-Autonomous Deep Learning for Distributed Electronic Health Record

Electronic health record (EHR) data is collected by individual institutions and often stored across locations in silos. Getting access to these data is difficult and slow due to security, privacy, regulatory, and operational issues. We show, using ICU data from 58 different hospitals, that machine learning models to predict patient mortality can be trained efficiently without moving health data out of their silos using a distributed machine learning strategy. We propose a new method, called Federated-Autonomous Deep Learning (FADL) that trains part of the model using all data sources in a distributed manner and other parts using data from specific data sources. We observed that FADL outperforms traditional federated learning strategy and conclude that balance between global and local training is an important factor to consider when design distributed machine learning methods , especially in healthcare.

cs.CY↗

arXiv2018

Depth-bounding is effective: Improvements and evaluation of unsupervised PCFG induction

There have been several recent attempts to improve the accuracy of grammar induction systems by bounding the recursive complexity of the induction model (Ponvert et al., 2011; Noji and Johnson, 2016; Shain et al., 2016; Jin et al., 2018). Modern depth-bounded grammar inducers have been shown to be more accurate than early unbounded PCFG inducers, but this technique has never been compared against unbounded induction within the same system, in part because most previous depth-bounding models are built around sequence models, the complexity of which grows exponentially with the maximum allowed depth. The present work instead applies depth bounds within a chart-based Bayesian PCFG inducer (Johnson et al., 2007b), where bounding can be switched on and off, and then samples trees with and without bounding. Results show that depth-bounding is indeed significantly effective in limiting the search space of the inducer and thereby increasing the accuracy of the resulting parsing model. Moreover, parsing results on English, Chinese and German show that this bounded model with a new inference technique is able to produce parse trees more accurately than or competitively with state-of-the-art constituency-based grammar induction models.

cs.CL↗

arXiv2018

Learning Patient Representations from Text

Mining electronic health records for patients who satisfy a set of predefined criteria is known in medical informatics as phenotyping. Phenotyping has numerous applications such as outcome prediction, clinical trial recruitment, and retrospective studies. Supervised machine learning for phenotyping typically relies on sparse patient representations such as bag-of-words. We consider an alternative that involves learning patient representations. We develop a neural network model for learning patient representations and show that the learned representations are general enough to obtain state-of-the-art performance on a standard comorbidity detection task.

cs.CL↗

arXiv2018

A Mini-BAL Outflow at 900 pc from the Central Source: VLT/X-shooter Observations

We determine the physical conditions and location of the outflow material seen in the mini-BAL quasar SDSS J1111+1437 (z = 2.138). These results are based on the analysis of a high S/N, medium-resolution VLT/X- shooter spectrum. The main outflow component spans the velocity range -1500 to -3000 km s$^{-1}$ and has detected absorption troughs from both high-ionization species: C IV , N V , O VI , Si IV , P V , and S IV ; and low-ionization species: H I , C II , Mg II , Al II , Al III , Si II , and Si III . Measurements of these troughs allow us to derive an accurate photoionization solution for this absorption component: a hydrogen column density, log ( N H ) = 21.47 $^{+0.21}_{-0.27}$ cm$^{-2}$ and ionization parameter, log (U_H) = -1.23$^{+0.20}_{-0.25}$. Troughs produced from the ground and excited states of S IV combined with the derived U_H value allow us to determine an electron number density of log (n_e) = 3.62$^{+0.09}_{-0.11}$ cm$^{-3}$ and to obtain the distance of the ionized gas from the central source: R = 880$^{+220}_{-260}$ pc.

astro-ph.GA↗

arXiv2018

VLT/X-Shooter Survey of BAL Quasars: Large Distance Scale and AGN Feedback

We conducted a survey of quasar outflows using the VLT/X-shooter spectrograph. When choosing the 14 BAL and mini-BALs comprising this sample, the data did not cover the S IV and S IV* troughs, whose ratio can be used to determine the distance of the outflows from the central source (R). Therefore, this "Blind Survey" is unbiased towards a particular distance scale. Out of the eight outflows where R can be measured, six have R > 100 pc (spanning the range 100-4500 pc), one has R > 10 pc, and only one (at R < 60 pc) is compatible with a much smaller R scale. At least two of the outflows have a kinetic luminosity greater than 0.5% of their Eddington luminosity, implying that they are able to provide significant AGN feedback. The outflows span a range of 0 to -10000 km s$^{-1}$ in velocity; total column density between 10$^{20}$ - 10$^{22.5}$ cm$^{-2}$ ; ionization parameter ($U_H$ ) in the range 0.01 - 1; and electron number density between 10$^3$ - 10$^{5.5}$ cm$^{-3}$ , with one upper and one lower limit. The results of this survey can be extrapolated to the majority of BAL outflows, implying that most of these outflows are situated far away from the AGN accretion disk; and that a significant portion of them can contribute to AGN feedback processes.

astro-ph.GA↗

arXiv2018

Unsupervised Grammar Induction with Depth-bounded PCFG

There has been recent interest in applying cognitively or empirically motivated bounds on recursion depth to limit the search space of grammar induction models (Ponvert et al., 2011; Noji and Johnson, 2016; Shain et al., 2016). This work extends this depth-bounding approach to probabilistic context-free grammar induction (DB-PCFG), which has a smaller parameter space than hierarchical sequence models, and therefore more fully exploits the space reductions of depth-bounding. Results for this model on grammar acquisition from transcribed child-directed speech and newswire text exceed or are competitive with those of other models when evaluated on parse accuracy. Moreover, gram- mars acquired from this model demonstrate a consistent use of category labels, something which has not been demonstrated by other acquisition models.

cs.CL↗