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Scott Chapman

Publications and source records attributed to Scott Chapman.

At least 145 records · Page 8Linked to original sources
arXiv2006

A near zero velocity dispersion stellar component in the Canes Venatici dwarf spheroidal galaxy

We present a spectroscopic survey of the newly-discovered Canes Venatici dwarf galaxy using the Keck/DEIMOS spectrograph. Two stellar populations of distinct kinematics are found to be present in this galaxy: an extended, metal-poor component, of half-light radius 7'.8(+2.4/-2.1), which has a velocity dispersion of 13.9(+3.2/-2.5) km/s, and a more concentrated (half-light radius 3'.6(+1.1/-0.8) metal-rich component of extremely low velocity dispersion. At 99% confidence, the upper limit to the central velocity dispersion of the metal-rich population is 1.9 km/s. This is the lowest velocity dispersion ever measured in a galaxy. We perform a Jeans analysis on the two components, and find that the dynamics of the structures can only be consistent if we adopt extreme (and unlikely) values for the scale length and velocity dispersion of the metal-poor population. With a larger radial velocity sample and improved measurements of the density profile of the two populations, we anticipate that it will be possible to place strong constraints on the central distribution of the dark matter in this galaxy.

astro-ph↗
arXiv2006

Resolving the Stellar Outskirts of M31 and M33

Many clues about the galaxy assembly process lurk in the faint outer regions of galaxies. The low surface brightnesses of these parts pose a significant challenge for studies of diffuse light, and few robust constraints on galaxy formation models have been derived to date from this technique. Our group has pioneered the use of extremely wide-area star counts to quantitatively address the large-scale structure and stellar content of external galaxies at very faint light levels. We highlight here some results from our imaging and spectroscopic surveys of M31 and M33.

astro-ph↗
arXiv2005

Rest-frame optical and far-infrared observations of extremely bright Lyman-break galaxy candidates at z ~ 2.5

We have investigated the rest-frame optical and far-infrared properties of a sample of extremely bright candidate Lyman-break galaxies (LBG) identified in the Sloan Digital Sky Survey. Their high ultraviolet luminosities and lack of strong ultraviolet emission lines are suggestive of massive starbursts, although it is possible that they are more typical luminosity LBGs which have been highly magnified by strong gravitational lensing. Alternatively, they may be an unusual class of weak-lined quasars. If the ultraviolet and submillimetre properties of these objects mirror those of less luminous, starburst LBGs, then they should have detectable rest-frame far-infrared emission. However, our submm photometry fails to detect such emission, indicating that these systems are not merely scaled-up (either intrinsically or as a result of lensing) examples of typical LBGs. In addition we have searched for the morphological signatures of strong lensing, using high-resolution, near-infrared imaging, but we find none. Instead, near-infrared spectroscopy reveals that these systems are, in fact, a rare class of broad absorption-line (BAL) quasars.

astro-ph↗
arXiv2005

Optical and Near-Infrared Integral Field Spectroscopy of the SCUBA Galaxy N2-850.4

We present optical and near-infrared integral field spectroscopy of the SCUBA galaxy SMM J163650.43+405734.5 (ELAIS N2 850.4) at z=2.385. We combine Ly-alpha and H-alpha emission line maps and velocity structure with high resolution HST ACS and NICMOS imaging to probe the complex dynamics of this vigorous star-burst galaxy. The imaging data shows a complex morphology, consisting of at least three components separated by ~1'' (8kpc) in projection. When combined with the H-alpha velocity field from UKIRT UIST IFU observations we identify two components whose redshifts are coincident with the systemic redshift, measured from previous CO observations, one of which shows signs of AGN activity. A third component is offset by 220+/-50km/s from the systemic velocity. The total star formation rate of the whole system (estimated from the narrow-line H-alpha and uncorrected for reddening) is 340+/-50Mo/yr. The Ly-alpha emission mapped by the GMOS IFU covers the complete galaxy and is offset by +270+/-40km/s from the systemic velocity. This velocity offset is comparable to that seen in rest-frame UV-selected galaxies at similar redshifts and usually interpreted as a star-burst driven wind. The extended structure of the Ly-alpha emission suggests that this wind is not a nuclear phenomenon, but is instead a galactic scale outflow. Our observations suggest that the vigorous activity in N2 850.4 is arising as a result of an interaction between at least two dynamically-distinct components, resulting in a strong starburst, a starburst-driven wind and actively-fuelled AGN activity. [abridged]

astro-ph↗
arXiv2004

An 850-micron SCUBA map of the Groth Strip and reliable source extraction

We present an 850-micron map and list of candidate sources in a 70 arcmin^2 sub-area of the Groth Strip observed using SCUBA. We initially detect 7 candidate sources with signal-to-noise ratios (SNRs) between 3.0 and 3.5 and 4 candidate sources with SNR > 3.5. Simulations suggest that on average in a map this size one expects 1.6 false positive sources for SNR > 3.5 and 4.5 for 3 < SNR < 3.5. Flux boosting in maps is a well known effect and we have developed a simple Bayesian prescription for estimating the unboosted flux distribution and used this method to determine the best flux estimates of our sources. This method is easily adapted for any other modest signal-to-noise survey in which there is prior knowledge of the source counts. We performed follow-up photometry in an attempt to confirm or reject 5 of our source candidates. We confirm the reality of 2 of the SCUBA sources, although at lower levels than suggested in the map and we find that the photometry results are consistent with and confirm the de-boosted map fluxes. Our final candidate source list contains 3 sources, including the 2 confirmed detections and 1 further candidate source with SNR > 3.5 which has a reasonable chance of being real.

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arXiv2004

The Rest-frame Optical Spectra of SCUBA Galaxies

We present near-infrared spectroscopy and narrow-band imaging at the wavelength of redshifted H-alpha for a sample of 30 high-redshift, far-infrared luminous galaxies. This sample is selected from surveys in the sub-millimeter, millimeter and radio wavebands and has complete redshift coverage with a median redshift of z~2.4. Removing obvious AGN, we find that the predicted H-alpha star formation rates in this diverse population are suppressed (by ~10x) compared to those derived from their far-infrared luminosities. We estimate that AGN are present in at least 40% of the galaxies in our sample. To further investigate this, we construct a composite rest-frame spectrum for both the entire sample and for those galaxies which individually show no signs of nuclear activity. We find [NII]/H-alpha ratios for both composite spectra which suggest that the energy output of the galaxies is star-formation- rather than AGN-dominated. The median H-alpha line width for our sample (removing obvious AGN) is 400+/-70km/s (FWHM), and the typical spatial extent of the H-alpha emission in our narrow-band observations is < 4-8kpc, which indicates a dynamical mass of 1-2 x 10^11 Mo. This estimate is larger than recent estimates of the dynamical masses of UV-selected galaxies at similar redshifts derived in an identical manner. We also compared our H-alpha and far-infrared luminosities with deep Chandra observations of a subset of our survey fields and use these data to further assess their AGN content. We conclude that these high-redshift, far-infrared luminous galaxies represent a population of massive, metal-rich, merging systems with high instantaneous star formation rates, strong dust obscuration and actively-fueled AGN which are likely to be the progenitors of massive local elliptical galaxies. (abridged)

astro-ph↗
arXiv2004

A Keck/Deimos Survey of Red Giant Branch Stars in the Outskirts of M31

We are using the DEIMOS multi-object spectrograph on the Keck II 10m telescope to conduct a spectroscopic survey of red giant branch stars in the outskirts of M31. To date, velocities have been obtained for most of the major substructures in the halo as well as at several positions in the far outer disk and inner halo. First results concerning the giant stellar stream and major axis substructures are presented here.

astro-ph↗
arXiv2004

The First Measurements of Galaxy Clustering from IRAC Data of the Spitzer First Look Survey

We present the first results of the angular auto-correlation function of the galaxies detected by the Infrared Array Camera (IRAC) instrument in the First Look Survey (FLS) of the Spitzer Space Telescope. We detect significant signals of galaxy clustering within the survey area. The angular auto-correlation function of the galaxies detected in each of the four IRAC instrument channels is consistent with a power-law form $w(θ)=Aθ^{1-γ}$ out to $θ= 0.2\arcdeg$, with the slope ranging from $γ= 1.5$ to 1.8. We estimate the correlation amplitudes $A$ to be $2.95 \times 10^{-3}$, $2.03 \times 10^{-3}$, $4.53 \times 10^{-3}$, and $2.34 \times 10^{-3}$ at $θ=1\arcdeg$ for galaxies detected in the IRAC 3.6$μ$m, 4.5$μ$m, 5.8$μ$m, and 8.0$μ$m instrument channels, respectively. We compare our measurements at 3.6$μ$m with the previous K-band measurements, and discuss the implications of these results.

astro-ph↗
arXiv2003

Measurement of the Sunyaev-Zel'dovich Increment in Massive Galaxy Clusters

We have detected the Sunyaev-Zel'dovich (SZ) increment at 850 microns in two galaxy clusters (Cl 0016+16 and MS 1054.4-0321) using SCUBA (Sub-millimetre Common User Bolometer Array) on the James Clerk Maxwell Telescope. Fits to the isothermal βmodel yield a central Compton y parameter of (2.2 \pm 0.7) x 10^{-4} and a central 850 micron flux of ΔI_{0} = 2.2 \pm 0.7 mJy/beam in Cl 0016. This can be combined with decrement measurements to infer y = (2.38 \pm_{0.34}^{0.36}) x 10^{-4} and v_{pec} = 400 \pm_{1400}^{1900} km/s. In MS 1054 we find a peak 850 micron flux of ΔI_{0} = 2.0 \pm 1.0 mJy/beam and y = (2.0 \pm 1.0) x 10^{-4}. To be successful such measurements require large chop throws and non-standard data analysis techniques. In particular, the 450 micron data are used to remove atmospheric variations in the 850 micron data. An explicit annular model is fit to the SCUBA difference data in order to extract the radial profile, and separately fit to the model differences to minimize the effect of correlations induced by our scanning strategy. We have demonstrated that with sufficient care, SCUBA can be used to measure the SZ increment in massive, compact galaxy clusters.

astro-ph↗
arXiv2002

The origin of [Fe II] emission in NGC 4151

The centre of NGC 4151 has been observed in the J-band with the SMIRFS integral field unit (IFU) on the UK Infrared Telescope. A map of [Fe II] emission is derived, and compared with the distributions of the optical narrow line region and radio jet. We conclude that, because the [Fe II] emission is associated more closely with the visible narrow-line region than with the radio jet, it arises mainly through photoionization of gas by collimated X-rays from the Seyfert nucleus. The velocity field and strength with respect to [Pa B] are found to be consistent with this argument. The performance of the IFU is considered briefly, and techniques for observation and data analysis are discussed.

astro-ph↗
arXiv2002

A SCUBA Scanmap of the HDF: Measuring the bright end of the sub-mm source counts

Using the 850 micron SCUBA camera on the JCMT and a scanning technique different from other sub-mm surveys, we have obtained a 125 square arcminute map centered on the Hubble Deep Field. The one-sigma sensitivity to point sources is roughly 3 mJy and thus our map probes the brighter end of the sub-mm source counts. We find 6 sources with a flux greater than about 12 mJy (>4 sigma) and, after a careful accounting of incompleteness and flux bias, estimate the integrated density of bright sources N(>12 mJy)= 164 (+77/-58) per square degree (68 per cent confidence bounds).

astro-ph↗
arXiv2000

Known and unknown SCUBA sources

Summary and discussion of some projects to use SCUBA to target sources selected at other wavebands, as well as to find new sub-mm galaxies in `blank fields': FIRBACK galaxies; Lyman break galaxies and `the Blob'; HDF flanking fields and the Groth Strip; survey of lensing cluster fields.

astro-ph↗
arXiv2000

The brighter side of sub-mm source counts: a SCUBA scan-map of the Hubble Deep Field

We present an 11 X 11 arcminute map centred on the Hubble Deep Field taken at 850 microns with the SCUBA camera on the JCMT. The map has an average one-sigma sensitivity to point sources of about 2.3 mJy and thus probes the brighter end of the sub-mm source counts. We find 7 sources with a flux greater than 9 mJy (roughly 4 sigma), and therefore estimate N(>9 mJy)= 208 (+90/-72) per degree. This result is consistent with work from other groups, but improves the statistics at the bright end, and is suggestive of a steepening of the counts.

astro-ph↗
arXiv1999

Multiple object and integral field near-infrared spectroscopy using fibers

We describe a new system for multiple object spectroscopy and integral field spectroscopy at near-infrared wavelengths using optical fibers. Both modes of the SMIRFS instrument have been tested at the UK Infrared Telescope with the CGS4 infrared spectrograph. The modular system includes a common optical system to image the fiber slit onto the cold slit inside the CGS4 cryostat. The multiobject mode consists of 14 interchangeable fused silica or zirconium fluoride fibers each with a field of 4 arcsec. The integral field mode consists of 72 fused silica fibers coupled with a lenslet array to give a contiguous field of 6x4 arcsec with 0.6 arcsec sampling. We describe the performance of both modes. For the multiobject mode, the feasibility and desirability of using fluoride fibers to extend the wavelength range into the K-band is discussed. For the integral field mode, the performance is compared with theoretical expectation with particular attention to the effect of Focal Ratio Degradation in the fibers. These results demonstrate the feasibility of multiobject and integral field spectroscopy in the near-infrared using lenslet-coupled fiber systems. Although SMIRFS in an experimental system working with a spectrograph not designed for this purpose, the throughput and uniformity of response are good. SMIRFS points the way forward to systems with much larger numbers of elements.

astro-ph↗
arXiv1998

Shades of Black: Searching for Brown Dwarfs & Giant Planets

We have searched, so far in vain, for brown dwarfs or giant planets in the vicinity of several nearby stars using cameras built at Universite de Montreal attached to the CFHT Adaptive Optics system. Here, we show how pairs of images taken simultaneously through separate filters on, and off, the 1.6 micron methane absorption band, can be used to completely defeat speckles, the dominant noise source, while revealing cool brown dwarfs and giant planets in high contrast. We achieve the photon shot-noise limit for data taken without a focal plane mask but, so far, have had less success with data where the image is occulted by a mask.

astro-ph↗
arXiv1996

Realistic Expanding Source Model for Invariant One-Particle Multiplicity Distributions and Two-Particle Correlations in Relativistic Heavy-Ion Collisions

We present a realistic expanding source model with nine parameters that are necessary and sufficient to describe the main physics occuring during hydrodynamical freezeout of the excited hadronic matter produced in relativistic heavy-ion collisions. As a first test of the model, we compare it to data from central Si + Au collisions at p_lab/A = 14.6 GeV/c measured in experiment E-802 at the AGS. An overall chi-square per degree of freedom of 1.055 is achieved for a fit to 1416 data points involving invariant pi^+, pi^-, K^+, and K^- one-particle multiplicity distributions and pi^+ and K^+ two-particle correlations. The 99-percent-confidence region of parameter space is identified, leading to one-dimensional error estimates on the nine fitted parameters and other calculated physical quantities. Three of the most important results are the freezeout temperature, longitudinal proper time, and baryon density along the symmetry axis. For these we find values of 92.9 +/- 4.4 MeV, 8.2 +/- 2.2 fm/c, and 0.0222 + 0.0096 / - 0.0069 fm^-3, respectively.

nucl-th↗
arXiv1996

Realistic Expanding Source Model for Relativistic Heavy-Ion Collisions

We introduce a new realistic expanding source model for invariant one-particle multiplicity distributions and two-particle correlations in nearly central relativistic heavy-ion collisions that contains nine adjustable parameters, which are necessary and sufficient to properly characterize the gross properties of the source during its freezeout from a hydrodynamical fluid into a collection of noninteracting, free-streaming hadrons. These nine physically relevant parameters fall into three categories of three parameters each, with the first category corresponding to the source's longitudinal motion, the second category corresponding to its transverse motion, and the third category corresponding to its intrinsic properties. As an initial application, we apply our model to the analysis of invariant pi^+, pi^-, K^+, and K^- one-particle multiplicity distributions and pi^+ and K^+ two-particle correlations for nearly central Si + Au collisions at p_lab/A = 14.6 GeV/c. In a minimization with 1416 data points, the resulting chi-square is 1484.6, which corresponds to an acceptable chi-square per degree of freedom of 1.055. We list the values of the nine parameters determined this way plus several additional calculated freezeout quantities of physical interest, along with their uncertainties at 99% confidence limits.

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