122198042
ACIGA's high optical power test facility
Advanced laser interferometer detectors utilizing more than 100 W of laser power and with ~106 W circulating laser power present many technological problems. The Australian Consortium for Interferometric Gravitational Astronomy (ACIGA) is developing a high power research facility in Gingin, north of Perth, Western Australia, which will test techniques for the next generation interferometers. In particular it will test thermal lensing compensation and control strategies for optical cavities in which optical spring effects and parametric instabilities may present major difficulties.
144912
Multicolor cavity metrology.
Long-baseline laser interferometers used for gravitational-wave detection have proven to be very complicated to control. In order to have sufficient sensitivity to astrophysical gravitational waves, a set of multiple coupled optical cavities comprising the interferometer must be brought into resonance with the laser field. A set of multi-input, multi-output servos then lock these cavities into place via feedback control. This procedure, known as lock acquisition, has proven to be a vexing problem and has reduced greatly the reliability and duty factor of the past generation of laser interferometers. In this article, we describe a technique for bringing the interferometer from an uncontrolled state into resonance by using harmonically related external fields to provide a deterministic hierarchical control. This technique reduces the effect of the external seismic disturbances by 4 orders of magnitude and promises to greatly enhance the stability and reliability of the current generation of gravitational-wave detectors. The possibility for using multicolor techniques to overcome current quantum and thermal noise limits is also discussed.
14399224
The Formation of Polar Disk Galaxies
Polar ring galaxies, such as NGC 4650A, are a class of galaxies that have two kinematically distinct components that are inclined by almost 90° to each other. These striking galaxies challenge our understanding of how galaxies form; the origin of their distinct components has remained uncertain and is the subject of much debate. We use high-resolution cosmological simulations of galaxy formation to show that polar ring galaxies are simply an extreme example of the misalignment of angular momentum that occurs during the hierarchical structure formation characteristic of a cold dark matter cosmology. In our model, polar ring galaxies form through the continuous accretion of gas whose angular momentum is misaligned with the central galaxy.
119042953
Boron in the Small Magellanic Cloud: A Novel Test of Light-Element Production*
Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) observations of the B III resonance line at λ2066 have been obtained and analyzed for two Small Magellanic Cloud (SMC) B-type stars. While boron is not detected in either star, upper limits to the boron abundance are set, with 12 + log(B/H) ≤ 1.6 for both AV 304 and NGC 346-637. The upper limits are consistent with the relationship between boron and oxygen previously reported for Galactic disk stars. The SMC upper limits are discussed in light of that galaxy's star formation history, present oxygen abundance, and present cosmic-ray flux. The UV spectrum has also been used to determine the iron-group abundances in the SMC stars. For AV 304, [Fe/H] =-0.6 ± 0.2, from both an absolute and a differential analysis (with respect to the Galactic B-type star HD 36591). This is consistent with results from A-F supergiants in the SMC. A lower iron abundance is found for NGC 346-637, [Fe/H] =-1.0 ± 0.3, but this is in good agreement with the supergiant iron abundances in NGC 330, another young SMC cluster. We propose that NGC 346-637 might be an unrecognized binary, however, which complicates its spectral analysis.
51303810
The Comparative Chemical Evolution of an Isolated Dwarf Galaxy: A VLT and Keck Spectroscopic Survey of WLM
Building on our previous spectroscopic and photometric analysis of the isolated Local Group dwarf irregular (dIrr) galaxy WLM, we present a comparison of the metallicities of its red giant branch stars with respect to the well-studied Local Group dwarf spheroidal galaxies (dSphs) and Magellanic Clouds. We calculate a mean metallicity of [Fe/H] =–1.28 ± 0.02 and an intrinsic spread in metallicity of σ = 0.38 ± 0.04 dex, similar to the mean and spread observed in the massive dSph Fornax and the Small Magellanic Cloud. Thus, despite WLM's isolated environment, its global metallicity still follows expectations for mass and its global chemical evolution is similar to other nearby luminous dwarf galaxies (gas-rich or gas-poor). The data also show a radial gradient in [Fe/H] of d[Fe/H]/dr_c = –0.04 ± 0.04 dex r_c^(-1), which is flatter than that seen in the unbiased and spatially extended surveys of dSphs. Comparison of the spatial distribution of [Fe/H] in WLM, the Magellanic Clouds, and a sample of Local Group dSphs shows an apparent dichotomy in the sense that the dIrrs have statistically flatter radial [Fe/H] gradients than the low angular momentum dSphs. The correlation between angular momentum and radial metallicity gradient is further supported when considering the Local Group dEs. This chemodynamic relationship offers a new and useful constraint for environment-driven dwarf galaxy evolution models in the Local Group.
116908435
Cuspy No More: How Outflows Affect the Central Dark Matter and Baryon Distribution in Lambda CDM Galaxies
We examine the evolution of the inner dark matter (DM) and baryonic density profile of a new sample of simulated field galaxies using fully cosmological, Lambda CDM, high resolution SPH + N-Body simulations. These simulations include explicit H2 and metal cooling, star formation (SF) and supernovae (SNe) driven gas outflows. Starting at high redshift, rapid, repeated gas outflows following bursty SF transfer energy to the DM component and significantly flatten the originally `cuspy' central DM mass profile of galaxies with present day stellar masses in the 10^4.5 -- 10^9.8 Msolar range. At z=0, the central slope of the DM density profile of our galaxies (measured between 0.3 and 0.7 kpc from their centre) is well fitted by rhoDM propto r^alpha with alpha \simeq -0.5 + 0.35 log_10(Mstar/10^8Msolar) where Mstar is the stellar mass of the galaxy and 4 < log_10 Mstar < 9.4. These values imply DM profiles flatter than those obtained in DM--only simulations and in close agreement with those inferred in galaxies from the THINGS and LITTLE THINGS survey. Only in very small halos, where by z=0 star formation has converted less than ~ 0.03% of the original baryon abundance into stars, outflows do not flatten the original cuspy DM profile out to radii resolved by our simulations. The mass (DM and baryonic) measured within the inner 500 pc of each simulated galaxy remains nearly constant over four orders of magnitudes in stellar mass for Mstar 10^9 Msolar. This finding is consistent with estimates for faint Local Group dwarfs and field galaxies. These results address one of the outstanding problems faced by the CDM model, namely the strong discrepancy between the original predictions of cuspy DM profiles and the shallower central DM distribution observed in galaxies.
33569796
Analytic and numerical realisations of a disk galaxy
ABSTRACT Recentfocusontheimportanceofcold,unshockedgasaccretioningalaxyformation–notexplicitlyincludedinsemi-analyticstudies–motivatesthefollowingdetailedcom-parisonbetweentwoinherentlydiﬀerentmodellingtechniques:directhydrodynamicalsimulationandsemi-analyticmodelling.Byanalysingthephysicalassumptionsbuiltintothegasoline simulation,formulaefortheemergentbehaviourarederivedwhichallowimmediateandaccuratetranslationoftheseassumptionstothegalform semi-analyticmodel.Thesimulatedhalomergerhistoryisthenextractedandevolvedusingtheseequivalentequations,predictingastrikinglysimilargalacticsystem.Thisexer-cisedemonstratesthatitistheinitialconditionsandphysicalassumptionswhichareresponsibleforthepredictedevolution,notthechoiceofmodellingtechnique.Onthislevelplayingﬁeld,apreviously published galform modelisapplied(includingaddi-tionalphysicssuchaschemicalenrichmentandfeedbackfromactivegalacticnuclei)whichleadstostarklydiﬀerentpredictions. 1 INTRODUCTIONTwo diﬀerent approaches are widely used to test theoriesof galaxy formation. Both make use of developing compu-tational resources to integrate a set of coupled diﬀerentialequationsforwardintime,whereeachequationappliesphys-ical constraints, or empirical laws, to some of the system’sproperties.The semi-analytic method (White & Frenk 1991) canbecharacterisedbytheintenttoencapsulateasmuchofthephysicalbehaviouraspossibleintheequationsthemselves,chosentodescribesystematicproperties(virialisation,con-servationofangularmomentum,etc.)andthusleavingtheminimumtonumericalcalculation.Inherently diﬀerent to this is the approach taken bysimulations,wheretheethosistoapplyequationswhichareknown to govern the particular properties (e.g. the gravi-tational force between two particles) and demonstrate, byskilfully programmed numerical calculation, that these re-producemacroscopicproperties.Inpractice,adjustmentwillalwaysneedtobemadetoaccountforthefactthateachsimulatedparticlerepresentsmany,manyrealparticles.Intraditionalsimulationswhichconsideredonlygravity,thisadjustmentispurelymathemat-ical. When gravity is the dominant interaction, results aretherefore very robust and simulations are mostly acceptedasbeingafaithfulrealisationoftheUniverseatlargescales,wherethisisthecase.Anotablemeasureofthisacceptanceis that semi-analytic models have taken to using samplesof halos taken directly from simulations (Kauﬀmann et al.1999;Hellyetal.2003;Hattonetal.2003),ratherthansam-ples created statistically (Bower et al. 1993; Lacey & Cole1993) using methods based on the analytic arguments ofPress&Schechter(1974).Ingeneral,though,theremustcomeascalebelowwhichthegoverningequationsarenolongerdescribingparticlein-teractionsbutemergentphenomena.Atthesesmallerscales,the two largely contrasting approaches begin to look moresimilar. As an immediate example, simulations which en-compassanentiregalaxyhavenotattemptedtoresolvein-dividualstarsandmustthereforeapplyanequationwhichapproximatestheconversionrateoflargebodiesofgasintostars,basedonlocallyaverageddensity.Anequivalentandsometimesidenticalequationappearsinsemi-analyticmod-els(§3.7).Both approaches have had some success in accountingfortheobservedpropertiesofgalaxies.Predictivepower,ontheotherhand,reliesonsuretyintheunderlyingequations,oraclearunderstandingofhowdiﬀeringunderlyingassump-tionsmapontoﬁnalobservablequantities.To bolster this crucial understanding, it is extremelyhelpfultostudy,indetail,thepredictedevolutionofasingle,largegalaxyanditssatellites,asfollowedbythesetwodiﬀer-enttechniques.Iftheanalyticmodelbeginsfromtheprecisehalomergerhistoryfoundinthesimulation,andappliesthesame assumptions about the physics at smaller scales, willitpredictthesameobservablepropertiesand,ifnot,whereandwhydothetwocalculationsdiverge?1.1 Previous researchExistingquantitativecomparisonsbetweensimulationsandsemi-analytic models have focussed on samplesof galaxies.