arXiv subjects

Alistair R. Walker

Publications and source records attributed to Alistair R. Walker.

At least 55 records · Page 3Linked to original sources

arXiv2012

The impact of camera optical alignments on weak lensing measures for the Dark Energy Survey

Telescope Point Spread Function (PSF) quality is critical for realising the potential of cosmic weak lensing observations to constrain dark energy and test General Relativity. In this paper we use quantitative weak gravitational lensing measures to inform the precision of lens optical alignment, with specific reference to the Dark Energy Survey (DES). We compute optics spot diagrams and calculate the shear and flexion of the PSF as a function of position on the focal plane. For perfect optical alignment we verify the high quality of the DES optical design, finding a maximum PSF contribution to the weak lensing shear of 0.04 near the edge of the focal plane. However this can be increased by a factor of approximately three if the lenses are only just aligned within their maximum specified tolerances. We calculate the E and B-mode shear and flexion variance as a function of de-centre or tilt of each lens in turn. We find tilt accuracy to be a few times more important than de-centre, depending on the lens considered. Finally we consider the compound effect of de-centre and tilt of multiple lenses simultaneously, by sampling from a plausible range of values of each parameter. We find that the compound effect can be around twice as detrimental as when considering any one lens alone. Furthermore, this combined effect changes the conclusions about which lens is most important to align accurately. For DES, the tilt of the first two lenses is the most important.

astro-ph.CO↗

arXiv2012

The Resolved Stellar Population in 50 Regions of M83 from HST/WFC3 Early Release Science Observations

We present a multi-wavelength photometric study of ~15,000 resolved stars in the nearby spiral galaxy M83 (NGC5236, D=4.61Mpc) based on Hubble Space Telescope Wide Field Camera 3 observations using four filters: F336W, F438W, F555W, and F814W. We select 50 regions (an average size of 260 pc by 280 pc) in the spiral arm and inter-arm areas of M83, and determine the age distribution of the luminous stellar populations in each region. This is accomplished by correcting for extinction towards each individual star by comparing its colors with predictions from stellar isochrones. We compare the resulting luminosity weighted mean ages of the luminous stars in the 50 regions with those determined from several independent methods, including the number ratio of red-to-blue supergiants, morphological appearance of the regions, surface brightness fluctuations, and the ages of clusters in the regions. We find reasonably good agreement between these methods. We also find that young stars are much more likely to be found in concentrated aggregates along spiral arms, while older stars are more dispersed. These results are consistent with the scenario that star formation is associated with the spiral arms, and stars form primarily in star clusters and then disperse on short timescales to form the field population. The locations of Wolf-Rayet stars are found to correlate with the positions of many of the youngest regions, providing additional support for our ability to accurately estimate ages. We address the effects of spatial resolution on the measured colors, magnitudes, and age estimates. While individual stars can occasionally show measurable differences in the colors and magnitudes, the age estimates for entire regions are only slightly affected.

astro-ph.GA↗

arXiv2011

Star formation in 30 Doradus

Using observations obtained with the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST), we have studied the properties of the stellar populations in the central regions of 30 Dor, in the Large Magellanic Cloud. The observations clearly reveal the presence of considerable differential extinction across the field. We characterise and quantify this effect using young massive main sequence stars to derive a statistical reddening correction for most objects in the field. We then search for pre-main sequence (PMS) stars by looking for objects with a strong (> 4 sigma) Halpha excess emission and find about 1150 of them over the entire field. Comparison of their location in the Hertzsprung-Russell diagram with theoretical PMS evolutionary tracks for the appropriate metallicity reveals that about one third of these objects are younger than ~4Myr, compatible with the age of the massive stars in the central ionising cluster R136, whereas the rest have ages up to ~30Myr, with a median age of ~12Myr. This indicates that star formation has proceeded over an extended period of time, although we cannot discriminate between an extended episode and a series of short and frequent bursts that are not resolved in time. While the younger PMS population preferentially occupies the central regions of the cluster, older PMS objects are more uniformly distributed across the field and are remarkably few at the very centre of the cluster. We attribute this latter effect to photoevaporation of the older circumstellar discs caused by the massive ionising members of R136.

astro-ph.SR↗

arXiv2011

Using H-alpha Morphology and Surface Brightness Fluctuations to Age-Date Star Clusters in M83

We use new WFC3 observations of the nearby grand design spiral galaxy M83 to develop two independent methods for estimating the ages of young star clusters. The first method uses the physical extent and morphology of Halpha emission to estimate the ages of clusters younger than tau ~10 Myr. It is based on the simple premise that the gas in very young (tau < few Myr) clusters is largely coincident with the cluster stars, is in a small, ring-like structure surrounding the stars in slightly older clusters (e.g., tau ~5 Myr), and is in a larger ring-like bubble for still older clusters (i.e., ~5-10 Myr). The second method is based on an observed relation between pixel-to-pixel flux variations within clusters and their ages. This method relies on the fact that the brightest individual stars in a cluster are most prominent at ages around 10 Myr, and fall below the detection limit (i.e., M_V < -3.5) for ages older than about 100 Myr. These two methods are the basis for a new morphological classification system which can be used to estimate the ages of star clusters based on their appearance. We compare previous age estimates of clusters in M83 determined from fitting UBVI Halpha measurements using predictions from stellar evolutionary models with our new morphological categories and find good agreement at the ~95% level. The scatter within categories is ~0.1 dex in log tau for young clusters (<10 Myr) and ~0.5 dex for older (>10 Myr) clusters. A by-product of this study is the identification of 22 "single-star" HII regions in M83, with central stars having ages ~4 Myr.

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arXiv2011

Large-scale shock-ionized and photo-ionized gas in M83: the impact of star formation

We investigate the ionization structure of the nebular gas in M83 using the line diagnostic diagram, [O III](5007 \degA)/Hβ vs. [S II](6716 °A+6731 °A)/Hα with the newly available narrowband images from the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope (HST). We produce the diagnostic diagram on a pixel-by-pixel (0.2" x 0.2") basis and compare it with several photo- and shock-ionization models. For the photo-ionized gas, we observe a gradual increase of the log([O III]/Hβ) ratios from the center to the spiral arm, consistent with the metallicity gradient, as the H II regions go from super solar abundance to roughly solar abundance from the center out. Using the diagnostic diagram, we separate the photo-ionized from the shock-ionized component of the gas. We find that the shock-ionized Hα emission ranges from ~2% to about 15-33% of the total, depending on the separation criteria used. An interesting feature in the diagnostic diagram is an horizontal distribution around log([O III]/Hβ) ~ 0. This feature is well fit by a shock-ionization model with 2.0 Z\odot metallicity and shock velocities in the range of 250 km/s to 350 km/s. A low velocity shock component, < 200 km/s, is also detected, and is spatially located at the boundary between the outer ring and the spiral arm. The low velocity shock component can be due to : 1) supernova remnants located nearby, 2) dynamical interaction between the outer ring and the spiral arm, 3) abnormal line ratios from extreme local dust extinction. The current data do not enable us to distinguish among those three possible interpretations. Our main conclusion is that, even at the HST resolution, the shocked gas represents a small fraction of the total ionized gas emission at less than 33% of the total. However, it accounts for virtually all of the mechanical energy produced by the central starburst in M83.

astro-ph.CO↗

arXiv2011

Detection of brown dwarf-like objects in the core of NGC3603

We use near-infrared data obtained with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope to identify objects having the colors of brown dwarfs (BDs) in the field of the massive galactic cluster NGC 3603. These are identified through use of a combination of narrow and medium band filters spanning the J and H bands, and which are particularly sensitive to the presence of the 1.3-1.5μm H2O molecular band - unique to BDs. We provide a calibration of the relationship between effective temperature and color for both field stars and for BDs. This photometric method provides effective temperatures for BDs to an accuracy of {\pm}350K relative to spectroscopic techniques. This accuracy is shown to be not significantly affected by either stellar surface gravity or uncertainties in the interstellar extinction. We identify nine objects having effective temperature between 1700 and 2200 K, typical of BDs, observed J-band magnitudes in the range 19.5-21.5, and that are strongly clustered towards the luminous core of NGC 3603. However, if these are located at the distance of the cluster, they are far too luminous to be normal BDs. We argue that it is unlikely that these objects are either artifacts of our dataset, normal field BDs/M-type giants or extra-galactic contaminants and, therefore, might represent a new class of stars having the effective temperatures of BDs but with luminosities of more massive stars. We explore the interesting scenario in which these objects would be normal stars that have recently tidally ingested a Hot Jupiter, the remnants of which are providing a short-lived extended photosphere to the central star. In this case, we would expect them to show the signature of fast rotation.

astro-ph.SR↗

arXiv2011

The Hubble Space Telescope Wide Field Camera 3 Early Release Science data: Panchromatic Faint Object Counts for 0.2-2 microns wavelength

We describe the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) Early Release Science (ERS) observations in the Great Observatories Origins Deep Survey (GOODS) South field. The new WFC3 ERS data provide calibrated, drizzled mosaics in the UV filters F225W, F275W, and F336W, as well as in the near-IR filters F098M (Ys), F125W (J), and F160W (H) with 1-2 HST orbits per filter. Together with the existing HST Advanced Camera for Surveys (ACS) GOODS-South mosaics in the BViz filters, these panchromatic 10-band ERS data cover 40-50 square arcmin at 0.2-1.7 μm in wavelength at 0.07-0.15" FWHM resolution and 0.090" Multidrizzled pixels to depths of AB\simeq 26.0-27.0 mag (5-σ) for point sources, and AB\simeq 25.5-26.5 mag for compact galaxies. In this paper, we describe: a) the scientific rationale, and the data taking plus reduction procedures of the panchromatic 10-band ERS mosaics; b) the procedure of generating object catalogs across the 10 different ERS filters, and the specific star-galaxy separation techniques used; and c) the reliability and completeness of the object catalogs from the WFC3 ERS mosaics. The excellent 0.07-0.15" FWHM resolution of HST/WFC3 and ACS makes star- galaxy separation straightforward over a factor of 10 in wavelength to AB\simeq 25-26 mag from the UV to the near-IR, respectively.

astro-ph.CO↗

arXiv2010

Anatomy of a post-starburst minor merger: a multi-wavelength WFC3 study of NGC 4150

(Abridged) We present a spatially-resolved near-UV/optical study of NGC 4150, using the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope. Previous studies of this early-type galaxy (ETG) indicate that it has a large reservoir of molecular gas, exhibits a kinematically decoupled core (likely indication of recent merging) and strong, central H_B absorption (indicative of young stars). The core of NGC 4150 shows ubiquitous near-UV emission and remarkable dusty substructure. Our analysis shows this galaxy to lie in the near-UV green valley, and its pixel-by-pixel photometry exhibits a narrow range of near-UV/optical colours that are similar to those of nearby E+A (post-starburst) galaxies. We parametrise the properties of the recent star formation (age, mass fraction, metallicity and internal dust content) in the NGC 4150 pixels by comparing the observed near-UV/optical photometry to stellar models. The typical age of the recent star formation (RSF) is around 0.9 Gyrs, consistent with the similarity of the near-UV colours to post-starburst systems, while the morphological structure of the young component supports the proposed merger scenario. The RSF metallicity, representative of the metallicity of the gas fuelling star formation, is around 0.3 - 0.5 Zsun. Assuming that this galaxy is a merger and that the gas is sourced mainly from the infalling companion, these metallicities plausibly indicate the gas-phase metallicity (GPM) of the accreted satellite. Comparison to the local mass-GPM relation suggests (crudely) that the mass of the accreted system is around 3x10^8 Msun, making NGC 4150 a 1:20 minor merger. A summation of the pixel RSF mass fractions indicates that the RSF contributes about 2-3 percent of the stellar mass. This work reaffirms our hypothesis that minor mergers play a significant role in the evolution of ETGs at late epochs.

astro-ph.CO↗

arXiv2010

Hubble Space Telescope Imaging of Lyman Alpha Emission at z=4.4

We present the highest redshift detections of resolved Lyman alpha emission, using Hubble Space Telescope/ACS F658N narrowband-imaging data taken in parallel with the Wide Field Camera 3 Early Release Science program in the GOODS CDF-S. We detect Lyman alpha emission from three spectroscopically confirmed z = 4.4 Lyman alpha emitting galaxies (LAEs), more than doubling the sample of LAEs with resolved Lyman alpha emission. Comparing the light distribution between the rest-frame ultraviolet continuum and narrowband images, we investigate the escape of Lyman alpha photons at high redshift. While our data do not support a positional offset between the Lyman alpha and rest-frame ultraviolet (UV) continuum emission, the half-light radii in two out of the three galaxies are significantly larger in Lyman alpha than in the rest-frame UV continuum. This result is confirmed when comparing object sizes in a stack of all objects in both bands. Additionally, the narrowband flux detected with HST is significantly less than observed in similar filters from the ground. These results together imply that the Lyman alpha emission is not strictly confined to its indigenous star-forming regions. Rather, the Lyman alpha emission is more extended, with the missing HST flux likely existing in a diffuse outer halo. This suggests that the radiative transfer of Lyman alpha photons in high-redshift LAEs is complicated, with the interstellar-medium geometry and/or outflows playing a significant role in galaxies at these redshifts.

astro-ph.CO↗

arXiv2010

The Luminosity, Mass, and Age Distributions of Compact Star Clusters in M83 Based on HST/WFC3 Observations

The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL \propto L^{alpha}, with alpha = -2.04 +/- 0.08, down to M_V ~ -5.5. We test the sensitivity of the luminosity function to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in alpha. We estimate ages and masses for the clusters by comparing their measured UBVI,Halpha colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power-law, dN/dt propto t^{gamma}, with gamma=-0.9 +/- 0.2, for M > few x 10^3 Msun and t < 4x10^8 yr. This indicates that clusters are disrupted quickly, with ~80-90% disrupted each decade in age over this time. The mass function of clusters over the same M-t range is a power law, dN/dM propto M^{beta}, with beta=-1.94 +/- 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, M_C, or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e. mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters.

astro-ph.GA↗

arXiv2010

Progressive star formation in the young galactic super star cluster NGC 3603

Early release science observations of the cluster NGC3603 with the WFC3 on the refurbished HST allow us to study its recent star formation history. Our analysis focuses on stars with Halpha excess emission, a robust indicator of their pre-main sequence (PMS) accreting status. The comparison with theoretical PMS isochrones shows that 2/3 of the objects with Halpha excess emission have ages from 1 to 10 Myr, with a median value of 3 Myr, while a surprising 1/3 of them are older than 10 Myr. The study of the spatial distribution of these PMS stars allows us to confirm their cluster membership and to statistically separate them from field stars. This result establishes unambiguously for the first time that star formation in and around the cluster has been ongoing for at least 10-20 Myr, at an apparently increasing rate.

astro-ph.SR↗

arXiv2010

Supernova Remnants and the Interstellar Medium of M83: Imaging & Photometry with WFC3 on HST

We present Wide Field Camera 3 images taken with the Hubble Space Telescope within a single field in the southern grand design star-forming galaxy M83. Based on their size, morphology and photometry in continuum-subtracted H$α$, [\SII], H$β$, [\OIII] and [\OII] filters, we have identified 60 supernova remnant candidates, as well as a handful of young ejecta-dominated candidates. A catalog of these remnants, their sizes and, where possible their H$α$ fluxes are given. Radiative ages and pre-shock densities are derived from those SNR which have good photometry. The ages lie in the range $2.62 < log(τ_{\rm rad}/{\rm yr}) < 5.0$, and the pre-shock densities at the blast wave range over $0.56 < n_0/{\rm cm^{-3}} < 1680$. Two populations of SNR have been discovered. These divide into a nuclear and spiral arm group and an inter-arm population. We infer an arm to inter-arm density contrast of 4. The surface flux in diffuse X-rays is correlated with the inferred pre-shock density, indicating that the warm interstellar medium is pressurised by the hot X-ray plasma. We also find that the interstellar medium in the nuclear region of M83 is characterized by a very high porosity and pressure and infer a SNR rate of one per 70-150 yr for the nuclear ($R<300 $pc) region. On the basis of the number of SNR detected and their radiative ages, we infer that the lower mass of Type II SNe in M83 is $M_{\rm min} = 16^{+7}_ {-5}$ M$_{\odot}$. Finally we give evidence for the likely detection of the remnant of the historical supernova, SN1968L.

astro-ph.CO↗

arXiv2003

A Survey for EHB Stars in the Galactic Bulge

We present a progress report on an extensive survey to find and characterize all types of blue horizontal-branch stars in the nuclear bulge of the Galaxy. We have obtained wide, shallow imaging in UBV of ~12 square degrees in the bulge, with follow-up spectroscopy for radial velocities and metal abundance determinations. We have discovered a number of metal-rich blue HB stars, whose presence in the bulge is expected by the interpretation of the extragalactic ultraviolet excess. Very deep images have been obtained in UBV and SDSS u along the bulge minor axis, which reveal a significant number of EHB candidates fainter than B = 19, i.e., with the same absolute magnitudes as EHB stars in several globular clusters.

astro-ph↗

arXiv2003

The Large Synoptic Survey Telescope (LSST) and its Impact on Variable Star Research

The Large Synoptic Survey Telescope (LSST) is conceived as an 8.4-m telescope with CCD or CMOS focal plane covering most of a field 0.6 m in diameter, the latter exceeding the size of the largest photographic plates ever used in astronomy. The telescope design is driven by the desire to cover much of the accessible sky to faint magnitudes on a timescale of $\sim$ one night, with major science drivers the search for near Earth-orbit asteroids down to a diameter of 300 m, discovering supernovae to z $\sim 2$, and mass tomography of the Universe via gravitational weak lensing. Here the suitability of this facility for more ``traditional'' variable star research is investigated.

astro-ph↗

arXiv2003

Distances to Local Group Galaxies

Distances to galaxies in the Local Group are reviewed. In particular, the distance to the Large Magellanic Cloud is found to be $(m-M)_0 = 18.52 \pm 0.10$, corresponding to $50,600 \pm 2,400$ pc. The importance of M31 as an analog of the galaxies observed at greater distances is stressed, while the variety of star formation and chemical enrichment histories displayed by Local Group galaxies allows critical evaluation of the calibrations of the various distance indicators in a variety of environments.

astro-ph↗

arXiv2000

Hot Horizontal Branch Stars in the Galactic Bulge. I

We present the first results of a survey of blue horizontal branch (BHB) stars in the Galactic bulge. 164 candidates with 15 < V < 17.5 in a field 7.5deg from the Galactic Center were observed in the blue at 2.4A FWHM resolution with the AAT 2dF spectrograph. Radial velocities were measured for all stars. For stars with strong Balmer lines, their profiles were matched to theoretical spectrum calculations to determine stellar temperature Teff and gravity log g; matches to metal lines yielded abundances. CTIO UBV photometry then gave the reddening and distance to each hot star. Reddening was found to be highly variable, with E(B-V) from 0.0 to 0.55 around a mean of 0.28. Forty-seven BHB candidates were identified with Teff >= 7250K, of which seven have the gravities of young stars, three are ambiguous, and 37 are HB stars. They span a wide metallicity range, from solar to 1/300 solar. The warmer BHB's are more metal-poor and loosely concentrated towards the Galactic center, while the cooler ones are of somewhat higher metallicity and closer to the center. Their red B-V colors overlap main-sequence stars, but the U-B vs. B-V diagram separates them until E(B-V) > 0.5. We detect two cool solar-metallicity HB stars in the bulge of our own Galaxy, the first such stars known. Still elusive are their hot counterparts, the metal-rich sdB/O stars causing excess UV light in metal-rich galaxies; they have V ~ 20.5 in the Bulge.

astro-ph↗

arXiv1999

CCD Photometry of Galactic Globular Clusters V. NGC 2808

We present a deep color magnitude diagram for the galactic globular cluster NGC 2808 (C0911-646), reaching 3 magnitudes below the main sequence turnoff. A set of local photometric standards are provided, and the mean cluster reddening and metallicity are constrained using several photometric techniques. The unusual horizontal branch morphology discovered by Sosin et al. (1997, ApJ, 480, L35) is confirmed. No significant radial population gradients are identified to within the limits of the data. We investigate the redwards color spread on the main sequemce found by Ferraro et al. (1997, A&A, 327, 598) in a field centered 6.3 arcmin from the cluster center, attributed to binaries. We conclude that the spread is in the most part due to a non-Gaussian distribution of photometric errors about the main sequence ridgeline.

astro-ph↗

arXiv1998

Properties of a Proper-Motion Selected Sample of Giants in the Small Magellanic Cloud near NGC 121

We present both low and high-dispersion abundance analyses of giants drawn from the zero proper motion sample near NGC 121. For the sample of 35 stars, the mean metallicity of the SMC field near NGC 121 is [Fe/H]=-1.3 with a real dispersion of 0.4 dex. The most metal-rich stars are at -0.5 dex and the most metal-poor stars at -2.1 dex. The velocity dispersion of the sample is 25 km/s. The dispersion of various kinematic samples in the SMC is independent of age. The RGB colors are consistent with Galactic globular clusters of similar metallicity. The field population, however, does not have an extended blue horizontal branch, and the main sequence turnoff extends 1 magnitude brighter than the turnoff in M5. Evidently active star formation in this region started roughly at the age of the Galactic globular cluster M5 and extended for about 6 Gyrs, and then stopped.

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