Local Viscosity Control Printing for High-Throughput Additive Manufacturing of Polymers

Fused deposition modeling's (FDM) throughput is limited by process physics as well as practical considerations favoring single-head polymer extrusion. To expedite the thermoplastic additive manufacturing process, we propose a parallelized material deposition process called local viscosity control (LVC) additive manufacturing. LVC prints an entire layer in one step by selectively modulating the viscosity of polymer feedstock in contact with a heated wire mesh. Layers of molten polymer are contact printed, with the relative motion between the wire mechanism and a build plate allowing wetting and surface tension to transfer selectively heated, lower viscosity regions of polymer to a fixed substrate. Experiments demonstrate the viability of this process using a single cell depositing layered polycarbonate pixels. Theoretical analysis shows this process may offer similar capabilities in resolution to conventional FDM with a significantly higher production rate for commonly available input power

Discovery of A Very Bright, Strongly-Lensed z=2 Galaxy in the SDSS DR5

We report on the discovery of a very bright z = 2.00 star-forming galaxy that is strongly lensed by a foreground z=0.422 luminous red galaxy (LRG). This system was found in a systematic search for bright arcs lensed by LRGs and brightest cluster galaxies in the Sloan Digital Sky Survey Data Release 5 sample. Follow-up observations on the Subaru 8.2m telescope on Mauna Kea and the Astrophysical Research Consortium 3.5m telescope at Apache Point Observatory confirmed the lensing nature of this system. A simple lens model for the system, assuming a singular isothermal ellipsoid mass distribution, yields an Einstein radius of 3.82 +/- 0.03 arcsec or 14.8 +/- 0.1 kpc/h at the lens redshift. The total projected mass enclosed within the Einstein radius is 2.10 +/- 0.03 x 10^12 M_sun/h, and the magnification factor for the source galaxy is 27 +/- 1. Combining the lens model with our gVriz photometry, we find an (unlensed) star formation rate for the source galaxy of 32 M_sun/h / yr, adopting a fiducial constant star formation rate model with an age of 100 Myr and E(B-V) = 0.25. With an apparent magnitude of r = 19.9, this system is among the very brightest lensed z >= 2 galaxies, and provides an excellent opportunity to pursue detailed studies of the physical properties of an individual high-redshift star-forming galaxy.Comment: 31 pages, 12 figures, 4 tables, submitted to Ap

The HETDEX Pilot Survey III: The Low Metallicities of High-Redshift Lyman Alpha Galaxies

We present Keck/NIRSPEC spectroscopic observations of three Lyman alpha emitting galaxies (LAEs) at z ~ 2.3 discovered with the HETDEX pilot survey. We detect Halpha, [OIII], and Hbeta emission from two galaxies at z = 2.29 and 2.49, designated HPS194 and HPS256, respectively, representing the first detection of multiple rest-frame optical emission lines in galaxies at high-redshift selected on the basis of their Lyman alpha emission. The redshifts of the Lyman alpha emission from these galaxies are offset redward of the systemic redshifts by Delta_v = 162 +/- 37 (photometric) +/- 42 (systematic) km/s for HPS194, and Delta_v = 36 +/- 35 +/- 18 km/s for HPS256. An interpretation for HPS194 is that a large-scale outflow may be occurring in its interstellar medium. The emission line ratios imply that neither LAE hosts an active galactic nucleus. Using the upper limits on the [NII] emission we place meaningful constraints on the gas-phase metallicities in these two LAEs of Z < 0.17 and < 0.28 Zsol (1 sigma). Measuring the stellar masses of these objects via spectral energy distribution (SED) fitting (~ 10^10 and 6 x 10^8 Msol, respectively), we study the nature of LAEs in a mass-metallicity plane. At least one of these two LAEs appears to be more metal poor than continuum-selected star-forming galaxies at the same redshift and stellar mass, implying that objects exhibiting Lyman alpha emission may be systematically less chemically enriched than the general galaxy population. We use the SEDs of these two galaxies to show that neglecting the emission lines when fitting stellar population models to the observed photometry can result in overestimates of the population age by orders of magnitude, and the stellar mass by a factor of ~ 2. This effect is particularly important at z > 7, where similarly strong emission lines may masquerade in the photometry as a 4000 A break (abridged).Comment: Replaced with the accepted (to ApJ) versio

How Do Disks Survive Mergers?

We develop a physical model for how galactic disks survive and/or are destroyed in interactions. Based on dynamical arguments, we show gas primarily loses angular momentum to internal torques in a merger. Gas within some characteristic radius (a function of the orbital parameters, mass ratio, and gas fraction of the merging galaxies), will quickly lose angular momentum to the stars sharing the perturbed disk, fall to the center and be consumed in a starburst. A similar analysis predicts where violent relaxation of the stellar disks is efficient. Our model allows us to predict the stellar and gas content that will survive to re-form a disk in the remnant, versus being violently relaxed or contributing to a starburst. We test this in hydrodynamic simulations and find good agreement as a function of mass ratio, orbital parameters, and gas fraction, in simulations spanning a wide range in these properties and others, including different prescriptions for gas physics and feedback. In an immediate sense, the amount of disk that re-forms can be understood in terms of well-understood gravitational physics, independent of details of ISM gas physics or feedback. This allows us to explicitly quantify the requirements for such feedback to (indirectly) enable disk survival, by changing the pre-merger gas content and distribution. The efficiency of disk destruction is a strong function of gas content: we show how and why sufficiently gas-rich major mergers can, under general conditions, yield systems with small bulges (B/T<0.2). We provide prescriptions for inclusion of our results in semi-analytic models.Comment: 32 pages, 16 figures, accepted to ApJ (minor revisions to match accepted version

Large-scale collaboration reveals landscape-level effects of land-use on turtle demography

Freshwater turtles and tortoises are declining worldwide and currently represent one of the most imperiled major vertebrate groups. Identifying the conditions that promote long-term viable populations is a critical conservation need. However, for most species, there is relatively little or no empirical information about the factors influencing population demographics. Large-scale population monitoring efforts necessary to acquire such information remain rare due to the logistic challenges associated with low and variable detectability, which generally preclude large monitoring initiatives by any single entity. The development of collaborative population monitoring programs represents one potential strategy for overcoming these challenges. Our goal was to leverage partnerships to identify the potential factors and relevant scales affecting wood turtle (Glyptemys insculpta) population demographics. Through a large-scale collaborative multi-institutional monitoring effort, we conducted 983 spring stream surveys at 293 sites across the northeastern United States. Wood turtle abundance was negatively associated with agriculture (300 m and 5500 m) and road traffic (5500 m) and positively associated with mature forest (5500 m). Juvenile proportion displayed strong negative relationships with stream gradient and imperviousness (300 m). Sex ratios were more male-skewed with higher mature forest cover (90 m) and road density (5500 m) and less undeveloped land (300 m). These findings suggest that effective conservation of demographically robust turtle populations will require consideration of multiple spatial scales. Landscape-level conservation may be particularly important for ensuring long-term viable populations. This study highlights the valuable role that collaboration across institutions and jurisdictions can play in the conservation of cryptic taxa

High-Redshift Starbursting Dwarf Galaxies Revealed by GRB Afterglows

We present a study of 15 long-duration gamma-ray burst (GRB) host galaxies at z>2. The GRBs are selected with available early-time afterglow spectra in order to compare interstellar medium (ISM) absorption-line properties with stellar properties of the host galaxies. In addition to five previously studied hosts, we consider new detections for the host galaxies of GRB050820 and GRB060206 and place 2-sigma upper limits to the luminosities of the remaining unidentified hosts. We examine the nature of the host galaxy population and find that (1) the UV luminosity distribution of GRB host galaxies is consistent with expectations from a UV luminosity weighted random galaxy population with a median luminosity of =0.1 L*; (2) there exists a moderate correlation between UV luminosity and SiII 1526 absorption width, which together with the observed large line widths of W(1526)>1.5 Ang for a large fraction of the objects suggests a galactic outflow driven velocity field in the host galaxies; (3) there is tentative evidence for a trend of declining ISM metallicity with decreasing galaxy luminosity in the star-forming galaxy population at z=2-4; (4) the interstellar UV radiation field is found ~ 35-350 times higher in GRB hosts than the Galactic mean value; and (5) additional galaxies are found at < 2" from the GRB host in all fields with known presence of strong MgII absorbers, but no additional faint galaxies are found at < 2" in fields without strong MgII absorbers. Our study confirms that the GRB host galaxies (with known optical afterglows) are representative of unobscured star-forming galaxies at z>2, and demonstrates that high spatial resolution images are necessary for an accurate identification of GRB host galaxies in the presence of strong intervening absorbers.Comment: 24 emulateapj pages, 24 figures, ApJ in press; full-resolution version available at http://lambda.uchicago.edu/public/tmp/ghost.pd

Mergers, AGN, and 'Normal' Galaxies: Contributions to the Distribution of Star Formation Rates and Infrared Luminosity Functions

We use a novel method to predict the contribution of normal star-forming galaxies, merger-induced bursts, and obscured AGN, to IR luminosity functions (LFs) and global SFR densities. We use empirical halo occupation constraints to populate halos with galaxies and determine the distribution of normal and merging galaxies. Each system can then be associated with high-resolution hydrodynamic simulations. We predict the distribution of observed luminosities and SFRs, from different galaxy classes, as a function of redshift from z=0-6. We provide fitting functions for the predicted LFs, quantify the uncertainties, and compare with observations. At all redshifts, 'normal' galaxies dominate the LF at moderate luminosities ~L* (the 'knee'). Merger-induced bursts increasingly dominate at L>>L*; at the most extreme luminosities, AGN are important. However, all populations increase in luminosity at higher redshifts, owing to increasing gas fractions. Thus the 'transition' between normal and merger-dominated sources increases from the LIRG-ULIRG threshold at z~0 to bright Hyper-LIRG thresholds at z~2. The transition to dominance by obscured AGN evolves similarly, at factor of several higher L_IR. At all redshifts, non-merging systems dominate the total luminosity/SFR density, with merger-induced bursts constituting ~5-10% and AGN ~1-5%. Bursts contribute little to scatter in the SFR-stellar mass relation. In fact, many systems identified as 'ongoing' mergers will be forming stars in their 'normal' (non-burst) mode. Counting this as 'merger-induced' star formation leads to a stronger apparent redshift evolution in the contribution of mergers to the SFR density.Comment: 16 pages, 9 figures (+appendices), accepted to MNRAS. A routine to return the galaxy merger rates discussed here is available at http://www.cfa.harvard.edu/~phopkins/Site/mergercalc.htm

The physical scale of the far-infrared emission in the most luminous submillimetre galaxies II: evidence for merger-driven star formation

We present high-resolution 345 GHz interferometric observations of two extreme luminous (L_{IR}>10^{13} L_sun), submillimetre-selected galaxies (SMGs) in the COSMOS field with the Submillimeter Array (SMA). Both targets were previously detected as unresolved point-sources by the SMA in its compact configuration, also at 345 GHz. These new data, which provide a factor of ~3 improvement in resolution, allow us to measure the physical scale of the far-infrared in the submillimetre directly. The visibility functions of both targets show significant evidence for structure on 0.5-1 arcsec scales, which at z=1.5 translates into a physical scale of 5-8 kpc. Our results are consistent with the angular and physical scales of two comparably luminous objects with high-resolution SMA followup, as well as radio continuum and CO sizes. These relatively compact sizes (<5-10 kpc) argue strongly for merger-driven starbursts, rather than extended gas-rich disks, as the preferred channel for forming SMGs. For the most luminous objects, the derived sizes may also have important physical consequences; under a series of simplifying assumptions, we find that these two objects in particular are forming stars close to or at the Eddington limit for a starburst.Comment: 9 pages, 3 Figures, submitted to MNRA

The Astropy Problem

The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Despite this, the project has always been and remains to this day effectively unfunded. Further, contributors receive little or no formal recognition for creating and supporting what is now critical software. This paper explores the problem in detail, outlines possible solutions to correct this, and presents a few suggestions on how to address the sustainability of general purpose astronomical software

Mergers in Lambda-CDM: Uncertainties in Theoretical Predictions and Interpretations of the Merger Rate

Different methodologies lead to order-of-magnitude variations in predicted galaxy merger rates. We examine and quantify the dominant uncertainties. Different halo merger rates and subhalo 'destruction' rates agree to within a factor ~2 given proper care in definitions. If however (sub)halo masses are not appropriately defined or are under-resolved, the major merger rate can be dramatically suppressed. The dominant differences in galaxy merger rates owe to baryonic physics. Hydrodynamic simulations without feedback and older models that do not agree with the observed galaxy mass function propagate factor ~5 bias in the resulting merger rates. However, if the model matches the galaxy mass function, properties of central galaxies are sufficiently converged to give small differences in merger rates. But variations in baryonic physics of satellites also have dramatic effects. The known problem of satellite 'over-quenching' in most semi-analytic models (SAMs), whereby SAM satellites are too efficiently stripped of gas, could lead to order-of-magnitude under-estimates of merger rates for low-mass, gas-rich galaxies. Fixing the satellite properties to observations tends to predict higher merger rates, but with factor ~2 empirical uncertainties. Choice of mass ratio definition matters: at low masses, most true major mergers (in baryonic/dynamical galaxy mass) will appear to be minor mergers in their stellar or luminosity mass ratio. Observations and models using these criteria may underestimate major merger rates by factors ~5. Orbital parameters and gas fractions also introduce factor ~3 differences in amount of bulge formed by mergers, even for fixed mass ratio encounters.Comment: 32 Pages, 15 figures, accepted to ApJ (revised to match accepted version and correct Fig. 12