arXiv subjects

G. F. Lewis

Publications and source records attributed to G. F. Lewis.

At least 145 records · Page 8Linked to original sources

arXiv2005

A radial velocity survey of low Galactic latitude structures: I. Kinematics of the Canis Major dwarf galaxy

As part of a radial velocity survey of low Galactic latitude structures that we undertook with the 2dF spectrograph on the AAT, we present the radial velocities of more than 1500 Red Giant Branch and Red Clump stars towards the centre of the Canis Major dwarf galaxy. With a mean velocity of 72\pm7 km/s at a Heliocentric distance of 5.5 kpc and 114\pm2 km/s at 8.5 kpc, these stars present a peculiar distance -- radial velocity relation that is unlike that expected from thin or thick disc stars. Moreover, they belong to a kinematically cold population with an intrinsic dispersion that may be as low as 11_{-1}^{+3} km/s. The radial velocity distribution is used to select Canis Major stars in the UCAC2.0 proper motion catalogue and derive proper motions in Galactic coordinates of (μ_l,μ_b)= (-3.6\pm0.8 mas/yr, 1.5\pm0.4 mas/yr) for the dwarf galaxy, which after correcting for the reflex solar motion along this line-of-sight gives (μ_l',μ_b')= (-6.8\pm0.8 mas/yr, 0.8\pm0.4 mas/yr)$, corresponding to a prograde orbit with a tangential velocity of \sim235 km/s at the average distance of \sim7.2 kpc. All these kinematic constraints can be reproduced in simulations of the accretion of a dwarf onto the Galactic disc. Such a process could also be responsible for the Monoceros Ring that has recently been shown to encompass the Galactic disc. However, without constraints on the kinematics of the tidal arms emerging from the Canis Major dwarf, it is not yet possible to definitively prove a link between the two structures.

astro-ph↗

arXiv2005

Correcting the influence of an asymmetric line spread function in 2-degree Field spectrograph data

We investigate the role of asymmetries in the line spread function of the 2-degree field spectrograph and the variations in these asymmetries with the CCD, the plate, the time of observation and the fibre. A data-reduction pipeline is developed that takes these deformations into account for the calibration and cross-correlation of the spectra. We show that, using the emission lines of calibration lamp observations, we can fit the line spread function with the sum of two Gaussian functions representing the theoretical signal and a perturbation of the system. This model is then used to calibrate the spectra and generate templates by downgrading high resolution spectra. Thus, we can cross-correlate the observed spectra with templates degraded in the same way. Our reduction pipeline is tested on real observations and provides a significant improvement in the accuracy of the radial velocities obtained. In particular, the systematic errors that were as high as ~20 km/s when applying the AAO reduction package 2dfDR are now reduced to ~5 km/s. Even though the 2-degree Field spectrograph is to be decommissioned at the end of 2005, the analysis of archival data and previous studies could be improved by the reduction procedure we propose here.

astro-ph↗

arXiv2005

A new population of extended, luminous star clusters in the halo of M31

We present three new clusters discovered in the halo of M31 which, although having globular-like colours and luminosities,have an unusually large half-light radii, ~30 pc. They lie at projected galactocentric distances of approx. 15 to 35 kpc. These objects begin to fill the gap in parameter space between globular clusters and dwarf spheroidals, and are unlike any clusters found in the Milky Way, or elsewhere to date. Basic photometric and derived King profile fit parameters are given, and we discuss possible origins and their relationships to other populations.

astro-ph↗

arXiv2004

Distances and Metallicities for 17 Local Group Galaxies

We have obtained Johnson V and Gunn i photometry for a large number of Local Group galaxies using the Isaac Newton Telescope Wide Field Camera (INT WFC). The majority of these galaxies are members of the M31 subgroup and the observations are deep enough to study the top few magnitudes of the red giant branch in each system. We previously measured the location of the tip of the red giant branch (TRGB) for Andromeda I, Andromeda II and M33 to within systematic uncertainties of typically < 0.05 mags (McConnachie et al. 2004). As the TRGB acts as a standard candle in old, metal poor stellar populations, we were able to derive distances to each of these galaxies. Here we derive TRGB distances to the giant spiral galaxy M31 and 13 additional dwarf galaxies - NGC205, NGC185, NGC147, Pegasus, WLM, LGS3, Cetus, Aquarius, AndIII, AndV, AndVI, AndVII and the newly discovered dwarf spheroidal AndIX. The observations for each of the dwarf galaxies were intentionally taken in photometric conditions. In addition to the distances, we also self-consistently derive the median metallicity of each system from the colour of their red giant branches. This allows us to take into account the small metallicity variation of the absolute I magnitude of the TRGB. The homogeneous nature of our data and the identical analysis applied to each of the 17 Local Group galaxies ensures that these estimates form a reliable set of distance and metallicity determinations that are ideal for comparative studies of Local Group galaxy properties.

astro-ph↗

arXiv2004

Why the Canis Major overdensity is not due to the Warp: analysis of its radial profile and velocities

In response to criticism by Momany et al. (2004), that the recently-identified Canis Major (CMa) overdensity could be simply explained by the Galactic warp, we present proof of the existence of a stellar population in the direction of CMa that cannot be explained by known Galactic components. By analyzing the radial distribution of counts of M-giant stars in this direction, we show that the Momany et al. (2004) warp model overestimates the number of stars in the Northern hemisphere, hence hiding the CMa feature in the South. The use of a better model of the warp has little influence on the morphology of the overdensity and clearly displays an excess of stars grouped at a distance of D=7.2\pm 0.3 kpc. To lend further support to the existence of a population that does not belong to the Galactic disc, we present radial velocities of M-giant stars in the centre of the CMa structure that were obtained with the 2dF spectrograph at the AAT. The extra population shows a radial velocity of vr=109\pm4 km/s, which is significantly higher than the typical velocity of the disc at the distance of CMa. This population also has a low dispersion (13\pm4 km/s). The Canis Major overdensity is therefore highly unlikely to be due to the Galactic warp, adding weight to the hypothesis that we are observing a disrupting dwarf galaxy or its remnants. This leads to questions on what part of CMa was previously identified as the Warp and how to possibly disentangle the two structures.

astro-ph↗

arXiv2004

Detection of the Canis Major galaxy at (l;b)=(244;-8) and in the background of Galactic open clusters

We report on the detection of Main Sequence stars belonging to the recently identified Canis Major galaxy in a field located at ~ 4.2 deg from the center of the stellar system. With Main Sequence fitting we obtain a distance modulus (m-M)_0=14.5 +/- 0.3 to the dwarf, corresponding to a distance of D_{sun} = 8.0 +/- 1.2 kpc, in full agreement with previous estimates based on the photometric parallax of M-giants. From the comparison with theoretical isochrones we constrain the age of the main population of the Canis Major system in the range \~ 4-10 Gyr. A blue plume of likely younger stars (age < 1-2 Gyr) is also identified. The available Colour Magnitude Diagrams of open clusters that may be projected onto the main body of Canis Major are also briefly analyzed. The position, distance and stellar population of the old open clusters AM-2 and Tombaugh 2 strongly suggest that they are physically associated with the Canis Major galaxy. Using our own photometry and data from 2MASS and the GSC2.2 surveys we demonstrate that the claim by Momany et al. that the CMa overdensity is entirely due to the Galactic warp is not supported by the existing observations, once all the available pieces of information are taken into account. It is shown that the CMa overdensity clearly emerges at a heliocentric distance of ~ 8 kpc above any overdensity possibly produced by the Galactic warp.

astro-ph↗

arXiv2004

Radio continuum imaging of FIR luminous QSOs at z>6

We present sensitive imaging at 1.4 GHz of the two highest redshift far-infrared (FIR) luminous QSOs SDSS J114816.65+525150.2 (z=6.42) and SDSS J104845.05+463718.3 (z=6.2). Radio continuum emission is detected from J1148+5251 with S_{1.4} = 55 \pm 12 uJy, while J1048+4637 is marginally detected with S_{1.4} = 26 \pm 12 uJy. Comparison of the radio and FIR luminosities shows that both sources follow the radio-FIR correlation for star forming galaxies, with implied (massive) star formation rates \sim 10^3 M_sun year^{-1}, although we cannot rule-out as much as 50% of the FIR luminosity being powered by the AGN. Five bright (> 22 mJy) radio sources are detected within 8' of J1148+5251. This is a factor 30 more than expected for a random field. Two sources have SDSS redshifts, including a z = 1.633 radio loud quasar and a z = 0.05 radio galaxy. However, we do not find evidence for a galaxy cluster in the SDSS data, at least out to z = 0.2. Considering the faint SDSS magnitudes of the remaining radio sources, we conclude that the over-density of radio sources could either be a statistical fluke, or a very large scale structure (> 8 Mpc comoving) at z > 1. We also consider the possibility of gravitational lensing by the closest (in angle) bright galaxy in the SDSS data at z = 0.05, and conclude that the galaxy provides negligible magnification.

astro-ph↗

arXiv2004

Determining the Location of the Tip of the Red Giant Branch in Old Stellar Populations: M33, Andromeda I & II

The absolute bolometric luminosity of the point of core Helium ignition in old, metal poor, red-giant stars is of roughly constant magnitude, varying only very slightly with mass or metallicity. It can thus be used as a standard candle. Here, we review the main difficulties in measuring this location in any real dataset and develop an empirical approach to optimise it for tip of the red giant branch (TRGB) analysis. We go on to present a new algorithm for the identification of the TRGB in nearby metal poor stellar systems. Our method uses a least-squares fit of a data-adaptive slope to the luminosity function in 1 magnitude windows. This finds the region of the luminosity function that shows the most significant decline in star counts as we go to brighter magnitudes; the base of this decline is attributed as the location of the tip. This technique then allows for the determination of realistic uncertainties which reflect the quality of the luminosity function used, but which are typically ~0.02 mags rms + ~ 0.03 mags systematic, a significant improvement upon previous methods that have used the tip as a standard candle. Finally, we apply our technique to the Local Group spiral galaxy M33 and the dwarf galaxies And I & II, and derive distance modulii of 24.50 +/- 0.06 (794 +/- 23 kpc), 24.33 +/- 0.07 (735 +/- 23 kpc) and 24.05 +/- 0.06 (645 +/- 19 kpc) respectively. The result for M33 is in excellent agreement with the Cepheid distances to this galaxy, and makes the possibility of a significant amount of reddening in this object unlikely.

astro-ph↗

arXiv2004

The Andromeda Stream

The existence of a stream of tidally stripped stars from the Sagittarius Dwarf galaxy demonstrates that the Milky Way is still in the process of accreting mass. More recently, an extensive stream of stars has been uncovered in the halo of the Andromeda galaxy (M31), revealing that it too is cannibalizing a small companion. This paper reports the recent observations of this stream, determining it spatial and kinematic properties, and tracing its three-dimensional structure, as well as describing future observations and what we may learn about the Andromeda galaxy from this giant tidal stream.

astro-ph↗

arXiv2003

A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion onto the Milky Way

We present an analysis of the asymmetries in the population of Galactic M-giant stars present in the 2MASS All Sky catalogue. Several large-scale asymmetries are detected, the most significant of which is a strong elliptical-shaped stellar over-density, close to the Galactic plane at (l=240, b=-8), in the constellation of Canis Major. A small grouping of globular clusters (NGC 1851, NGC 1904, NGC 2298, and NGC 2808), coincident in position and radial velocity, surround this structure, as do a number of open clusters. The population of M-giant stars in this over-density is similar in number to that in the core of the Sagittarius dwarf galaxy. We argue that this object is the likely dwarf galaxy progenitor of the ring-like structure that has recently been found at the edge of the Galactic disk. A numerical study of the tidal disruption of an accreted dwarf galaxy is presented. The simulated debris fits well the extant position, distance and velocity information on the ``Galactic Ring'', as well as that of the M-giant over-densities, suggesting that all these structures are the consequence of a single accretion event. The disrupted dwarf galaxy stream orbits close to the Galactic Plane, with a pericentre at approximately the Solar circle, an orbital eccentricity similar to that of stars in the Galactic thick disk, as well as a vertical scale height similar to that of the thick disk. This finding strongly suggests that the Canis Major dwarf galaxy is a building block of the Galactic thick disk, that the thick disk is continually growing, even up to the present time, and that thick disk globular clusters were accreted onto the Milky Way from dwarf galaxies in co-planar orbits.

astro-ph↗

arXiv2003

Further multiwavelength observations of the SSA22 Ly_alpha emitting `blob'

We present new follow-up observations of the sub-mm luminous Ly_alpha-emitting object in the SSA22 z=3.09 galaxy overdensity, referred to as `Blob 1' by Steidel et al.(2000). In particular we discuss high resolution Hubble Space Telescope optical imaging, Owens Valley Radio Observatory spectral imaging, Keck spectroscopy, VLA 20cm radio continuum imaging, and Chandra X-ray observations. We also present a more complete analysis of the existing James Clerk Maxwell Telescope sub-mm data. We detect several optical continuum components which may be associated with the core of the submillimeter emitting region. A radio source at the position of one of the HST components (22:17:25.94, +00:12:38.9) identifies it as the likely counterpart to the submillimeter source. We also tentatively detect the CO(4-3) molecular line, centered on the radio position. We use the CO(4-3) intensity to estimate a limit on the gas mass for the system. The optical morphology of sources within the Ly_alpha cloud appears to be filamentary, while the optical source identified with the radio source has a dense knot which may be an AGN or compact starburst. We obtain a Keck-LRIS spectrum of this object, despite its faintness (R=26.8). The spectrum reveals weak Ly_alpha emission, but no other obvious features, suggesting that the source is not an energetic AGN (or that it is extremely obscured). We use non-detections in deep Chandra X-ray images to constrain the nature of the `Blob'. Although conclusive evidence regarding the nature of the object remains hard to obtain at this redshift, the evidence presented here is at least consistent with a dust-obscured AGN surrounded by a starburst situated at the heart of this giant Ly_alpha cloud.

astro-ph↗

arXiv2003

The three dimensional structure of the giant stellar stream in Andromeda

The wide-field CCD camera at the CFH telescope was used to survey the giant stellar stream in the Andromeda galaxy, resolving stars down the red giant branch in M31 to I ~ 25, a magnitude deeper than our previous INT survey of this galaxy and extending 1 degree further out. The stream is seen to extend out to the south-east of M31 as far as we have surveyed (some 4.5 degrees, corresponding to a projected distance ~60 kpc). It is a linear structure in projection, and the eastern edge of the stream presents a sharp boundary in star counts suggesting that it remains a coherent structure. By analysing the luminosity function of the metal rich component of the stream we find that, at the furthest extent of our survey, the stream is 100 kpc further away along the line of sight than M31. It can then be traced to a point on the north-western side of the galaxy where it is some 30 kpc in front of M31, at which point the stream turns away from our survey area.

astro-ph↗

arXiv2003

A Molecular Einstein Ring: Imaging a Starburst Disk Surrounding a Quasi-Stellar Object

Images of the CO 2-1 line emission, and the radio continuum emission, from the redshift 4.12 gravitationally lensed quasi-stellar object (QSO) PSS J2322+1944 reveal an Einstein ring with a diameter of 1.5". These observations are modeled as a star forming disk surrounding the QSO nucleus with a radius of 2 kpc. The implied massive star formation rate is 900 M_sun/year. At this rate a substantial fraction of the stars in a large elliptical galaxy could form on a dynamical time scale of 10^8 years. The observation of active star formation in the host galaxy of a high-redshift QSO supports the hypothesis of coeval formation of supermassive black holes and stars in spheroidal galaxies.

astro-ph↗

arXiv2003

The bi-variate luminosity-color distribution of IRAS galaxies, and implications for the high redshift Universe

We present a characterization of the local luminosity-color, bi-variate distribution of IRAS galaxies from the 1.2Jy sample, selected at 60 micron. The S60/S100 infrared color is used as the best single parameter description of the IR spectral energy distribution of galaxies. We derive an analytical form of the distribution and use it to constrain the effect of the IR color distribution on evolution models for high redshift, farIR luminous galaxies. Our adopted evolution retains the locally observed correlation between luminosity and color, such that the larger characteristic luminosities at higher redshift have a warmer characteristic color. The width of the color distribution at a given luminosity remains constant for all redshifts. We demonstrate that there is the potential for both hotter and colder sources to be missed in cosmological surveys. An evolving bi-variate luminosity function coupled with the cold source bias of sub-mm selected surveys suggests the existence of a large population of cold sources appearing in such surveys. Likewise, a hot source bias for most SIRTF wavebands together with a bi-variate model suggests an excess of hot sources being selected. We test the evolutionary form against available data for higher redshift, farIR galaxies. The data do not reveal evidence for any strong evolution in the characteristic luminosity-color distribution as a function of redshift over 0<z<1. However, there is marginal evidence for a broadening of the color distribution at higher redshifts, consistent with our locally characterized trend of a broadening in the IR color distribution at the highest luminosities.

astro-ph↗

arXiv2003

One Ring to Encompass them All: A giant stellar structure that surrounds the Galaxy

We present evidence that the curious stellar population found by the Sloan Digital Sky Survey in the Galactic anticentre direction extends to other distant fields that skirt the plane of the Milky Way. New data, taken with the INT Wide Field Camera show a similar population, narrowly aligned along the line of sight, but with a Galactocentric distance that changes from ~15 kpc to \~20 kpc (over ~100 degrees on the sky). Despite being narrowly concentrated along the line of sight, the structure is fairly extended vertically out of the plane of the Disk, with a vertical scale height of 0.75+/-0.04 kpc. This finding suggests that the outer rim of the Galaxy ends in a low-surface brightness stellar ring. Presently available data do not allow us to ascertain the origin of the structure. One possibility is that it is the wraith of a satellite galaxy devoured long-ago by the Milky Way, though our favoured interpretation is that it is a perturbation of the disk, possibly the result of ancient warps. Assuming that the Ring is smooth and axisymmetric, the total stellar mass in the structure may amount to 2x10^8 up to 10^9 Solar masses.

astro-ph↗

arXiv2002

The Properties of Microjansky Radio Sources in the HDF-N, SSA13, and SSA22 Fields

(abridged) We present multiwavelength observations for a large sample of microjansky radio sources detected in ultradeep 1.4GHz maps centered on the Hubble Deep Field-North (HDF-N) and the Hawaii Survey Fields SSA13 and SSA22. Our spectroscopic redshifts for 169 radio sources reveal a flat median redshift distribution, and these sources are hosted by similarly luminous optical L* galaxies, regardless of redshift. This is a serious concern for radio estimates of the local star formation rate density, as a substantial fraction of the ultraviolet luminosity density is generated by sub-L* galaxies at low redshifts. From our submillimeter measurements for 278 radio sources, we find error-weighted mean 850micron fluxes of 1.72$\pm$0.09 mJy for the total sample, 2.37$\pm$0.13 mJy for the optically-faint (I>23.5) subsample, and 1.04$\pm$0.13 mJy for the optically-bright (I<23.5) subsample. We significantly (>3σ) detect in the submillimeter 50 of the radio sources, 38 with I>23.5. Spectroscopic redshifts for three of the I<23.5 submillimeter-detected radio sources are in the range z=1.0-3.4, and all show AGN signatures. Using only the submillimeter mapped regions we find that 69\pm9% of the submillimeter-detected radio population are at I>23.5. We also find that 66\pm7% of the S850>5 mJy (>4σ) sources are radio-identified. We find that millimetric redshift estimates at low redshifts are best made with a FIR template intermediate between a Milky Way type galaxy and a starburst galaxy, and at high redshifts with an Arp220 template.

astro-ph↗

arXiv2002

Luminous early-type field galaxies at z ~ 0.4 - II. Star-formation history and space density

We present a combined photometric and spectroscopic analysis of the star formation history and space density of a sample of 485 luminous, M_V - 5 log h < -20.5, field early-type galaxies at redshifts 0.3 < z < 0.6. The observed b_Jori colours as a function of redshift, mean absorption line strengths and [OII] 3727 emission properties are used to constrain the star formation history of the galaxies. The mean star formation history of the early-type galaxy sample is consistent with an old (z_f>1), passively evolving luminosity-weighted stellar population. Twenty-one percent of the sample possess detectable [OII] 3727 emission consistent with a low level (<1 M_solar/yr) of on-going star formation. Parametric and non-parametric estimates of the space density of the sample are derived. The integrated luminosity density at z ~ 0.4, allowing only for passive luminosity evolution, is in excellent agreement with the local, ( = 0.1), luminosity density of early-type galaxies. Overall, the sample properties are consistent with a galaxy formation scenario in which the majority of luminous field early-type galaxies formed at redshifts z>1 and have largely evolved passively since the formation epoch.

astro-ph↗

arXiv2002

A best fit evolution scenario for the SCUBA galaxies

We explore the evolution of the SCUBA sub-mm galaxy population using Monte Carlo simulations to generate synthetic sub-mm/radio color magnitude diagrams. To represent the local distribution of observed dust properties, we use a local far-infrared luminosity function derived from the IRAS 1.2 Jy catalog, bivariate in FIR luminosity and 60micron/100micron color. We assume a peak luminosity evolution scenario and a fixed cosmology, to fit a single parameter model to the existing sub-mm/radio data. Our best fitting model has a peak in the luminosity evolution at z=2.6.

astro-ph↗