arXiv subjects

M. J. Irwin

Publications and source records attributed to M. J. Irwin.

At least 127 records · Page 7Linked to original sources

arXiv2006

The Origin of the Bifurcation in the Sagittarius Stream

The latest Sloan Digital Sky Survey data reveal a prominent bifurcation in the distribution of debris of the Sagittarius dwarf spheroidal (Sgr) beginning at a right ascension of roughly 190 degrees. Two branches of the stream (A and B) persist at roughly the same heliocentric distance over at least 50 degrees of arc. There is also evidence for a more distant structure (C) well behind the A branch. This paper provides the first explanation for the bifurcation. It is caused by the projection of the young leading (A) and old trailing (B) tidal arms of the Sgr, whilst the old leading arm (C) lies well behind A. This explanation is only possible if the halo is close to spherical, as the angular difference between the branches is a measure of the precession of the orbital plane.

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arXiv2006

The VLT-FLAMES Survey of Massive Stars: Stellar parameters and rotational velocities in NGC3293, NGC4755 and NGC6611

An analysis is presented of VLT-FLAMES spectroscopy for three Galactic clusters, NGC3293, NGC4755 and NGC6611. Non-LTE model atmosphere calculations have been used to estimate effective temperatures (from either the helium spectrum or the silicon ionization equilibrium) and gravities (from the hydrogen spectrum). Projected rotational velocities have been deduced from the helium spectrum (for fast and moderate rotators) or the metal line spectrum (for slow rotators). The observed Hertzsprung-Russell diagrams are compared with theoretical predictions and some discrepancies including differences in the main sequence luminosities are discussed. Cluster ages have been deduced and evidence for non-coeval star formation is found for all three of the clusters. Projected rotational velocities for targets in the older clusters, NGC3293 and NGC4755, have been found to be systematically larger than those for the field, confirming recent results in other similar age clusters. The distribution of projected rotational velocities are consistent with a Gaussian distribution of intrinsic rotational velocities. For the relatively unevolved targets in the older clusters, NGC3293 and NGC4755, the peak of the velocity distribution would be 250km/s with a full-width-half-maximum of approximately 180km/s. For NGC6611, the sample size is relatively small but implies a lower mean rotational velocity. This may be evidence for the spin-down effect due to angular momentum loss through stellar winds, although our results are consistent with those found for very young high mass stars. For all three clusters we deduce present day mass functions with Gamma-values in the range of -1.5 to -1.8, which are similar to other young stellar clusters in the Milky Way (Abridged).

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arXiv2006

The Field of Streams: Sagittarius and its Siblings

We use Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) u,g,r,i,z photometry to study Milky Way halo substructure in the area around the North Galactic Cap. A simple color cut (g-r < 0.4) reveals the tidal stream of the Sagittarius dwarf spheroidal, as well as a number of other stellar structures in the field. Two branches (A and B) of the Sagittarius stream are clearly visible in an RGB-composite image created from 3 magnitude slices, and there is also evidence for a still more distant wrap behind the A branch. A comparison of these data with numerical models suggests that the shape of the Galactic dark halo is close to spherical.

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arXiv2006

A New Milky Way Dwarf Satellite in Canes Venatici

In this Letter, we announce the discovery of a new dwarf satellite of the Milky Way, located in the constellation Canes Venatici. It was found as a stellar overdensity in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The satellite's color-magnitude diagram shows a well-defined red giant branch, as well as a horizontal branch. As judged from the tip of the red giant branch, it lies at a distance of ~220 kpc. Based on the SDSS data, we estimate an absolute magnitude of Mv ~ -7.9, a central surface brightness of mu_0,V ~ 28 mag arcsecond^-2, and a half-light radius of \~ 8.5' (~ 550 pc at the measured distance). The outer regions of Canes Venatici appear extended and distorted. The discovery of such a faint galaxy in proximity to the Milky Way strongly suggests that more such objects remain to be found.

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arXiv2006

Planetary Nebula Velocities in the Disk and Bulge of M31

We present radial velocities for a sample of 723 planetary nebulae (PNe) in the disk and bulge of M31, measured using the WYFFOS fibre spectrograph on the William Herschel telescope. Velocities are determined using the [OIII] 5007 Angstrom emission line. Rotation and velocity dispersion are measured to a radius of 50 arcminutes (11.5 kpc), the first stellar rotation curve and velocity dispersion profile for M31 to such a radius. Our kinematics are consistent with rotational support at radii well beyond the bulge effective radius of 1.4kpc, although our data beyond a radius of 5kpc are limited. We present tentative evidence for kinematic substructure in the bulge of M31 to be studied fully in a later work. This paper is part of an ongoing project to constrain the total mass, mass distribution and velocity anisotropy of the disk, bulge and halo of M31.

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arXiv2006

The internal kinematics of dwarf spheroidal galaxies

The status of kinematic observations in Local Group dwarf spheroidal galaxies (dSphs) is reviewed. Various approaches to the dynamical modelling of these data are discussed and some general features of dSph dark matter haloes based on simple mass models are presented.

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arXiv2006

A radial velocity survey of low Galactic latitude structures: III. The Monoceros Ring in front of the Carina and Andromeda galaxies

As part of our radial velocity survey of low Galactic latitude structures that surround the Galactic disc, we report the detection of the so called Monoceros Ring in the foreground of the Carina dwarf galaxy at Galactic coordinates (l,b)=(260,-22) based on VLT/FLAMES observations of the dwarf galaxy. At this location, 20 degrees in longitude greater than previous detections, the Ring has a mean radial velocity of 145+/-5 km/s and a velocity dispersion of only 17+/-5 km/s. Based on Keck/DEIMOS observations, we also determine that the Ring has a mean radial velocity of -75+/-4 km/s in the foreground of the Andromeda galaxy at (l,b)\sim(122,-22), along with a velocity dispersion of 26+/-3 km/s. These two kinematic detections are both highly compatible with known characteristics of the structure and, along with previous detections provide radial velocity values of the Ring over the 120<l<260 range. This should add strong constraints on numerical models of the accretion of the dwarf galaxy that is believed to be the progenitor of the Ring.

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arXiv2005

The core of the Canis Major galaxy as traced by Red Clump stars

The recently-discovered stellar system in Canis Major is analysed using He-burning Red Clump stars as tracers. Canis Major turns out to be the strongest and most spatially confined overdensity of the whole Galactic Disc, both in terms of number density and of statistical significance. When projected onto the Galactic Plane, it appears as an elongated and compact overdensity extending from l ~ 200 to l ~ 280 with a roundish core toward l ~ 240. We find that the main body of the system has an integrated absolute magnitude M_V=-14.4 +/- 0.8, a central surface brightness μ_{V,0} = 24.0 +/- 0.6 mag/arcsec^2 and a line-of-sight profile peaked at D_{sun}=7.2 +/- 1.0 kpc with Half Width at Half Maximum ~ 2.0 kpc, in excellent agreement with the results obtained with widely different tracers (M giants and Main Sequence stars) in previous analyses. The mean distance to the main body of Canis Major is observed to increase with increasing Galactic longitude, from D_{sun} ~ 6.3 kpc at l ~ 225, to D_{sun} ~ 9.3 kpc at l ~ 265, in good agreement with the predictions of our more recent N-body simulation that models CMa as a dwarf galaxy being accreted in a planar orbit onto the disc of the Milky Way. We confirm that the Canis Major system has all the characteristics of the relic of a dwarf galaxy seen on top of a large-scale overdensity that we detect all over the third and fourth Galactic quadrants (180 -5) that is identified as the stellar component of the southern Galactic Warp... (Abridged)....

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arXiv2005

The Discovery of Tidal Tails Around NGC 5466

We report the discovery of tidal tails around the high-latitude Galactic globular cluster NGC 5466 in Sloan Digital Sky Survey (SDSS) data. Neural networks are used to reconstruct the probability distribution of cluster stars in u,g,r,i and z space. The tails are clearly visible, once extra-galactic contaminants and field stars have been eliminated. They extend roughly 4 degrees on the sky, corresponding to about 1 kpc in projected length. The orientation of the tails is in good agreement with the cluster's Galactic orbit, as judged from the proper motion data.

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arXiv2005

Spectroscopy of twelve Type Ia supernovae at intermediate redshift

We present spectra of twelve Type Ia supernovae obtained in 1999 at the William Herschel Telescope and the Nordic Optical Telescope during a search for Type Ia supernovae (SN Ia) at intermediate redshift. The spectra range from z=0.178 to z=0.493, including five high signal-to-noise ratio SN Ia spectra in the still largely unexplored range 0.15 < z < 0.3. Most of the spectra were obtained before or around restframe B-band maximum light. None of them shows the peculiar spectral features found in low-redshift over- or under-luminous SN Ia. Expansion velocities of characteristic spectral absorption features such as SiII at 6355 angs., SII at 5640 angs. and CaII at 3945 angs. are found consistent with their low-z SN Ia counterparts.

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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.

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arXiv2005

The INT Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS)

The INT Photometric Halpha Survey of the Northern Galactic Plane (IPHAS) is a 1800 square degrees CCD survey of the northern Milky Way spanning the latitude range -5 < b < +5 (degrees) and reaching down to r' ~ 20 (10-sigma). It may increase the number of known northern emission line sources by an order of magnitude. Representative observations and an assessment of point-source data from IPHAS, now underway, are presented. The data obtained are Wide Field Camera images in Halpha narrow-band, and Sloan r' and i' broad-band, filters. We simulate IPHAS (r' - Halpha, r' - i') point-source colours using a spectrophotometric library of stellar spectra and available filter transmission profiles: this gives expected colours for (i) solar-metallicity stars, without Halpha emission, and (ii) emission line stars. Comparisons with Aquila field observations show that simulated normal star colours reproduce the data well for spectral types earlier than M. Spectroscopic follow-up of a Cepheus field confirms that sources lying above the main stellar locus in the (r' - Halpha, r' - i') plane are emission line objects, with very few failures. Examples of Halpha deficit objects -- a white dwarf and a carbon star -- are shown to be readily distinguished by their IPHAS colours. The role IPHAS can play in studies of nebulae is discussed briefly, and illustrated by a continuum-subtracted mosaic image of the SNR, Shajn 147. The final catalogue of IPHAS point sources will contain photometry on ~80 million objects. (abridged)

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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.

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arXiv2005

The VLT-FLAMES Survey of Massive Stars: Observations in the Galactic Clusters NGC 3293, NGC 4755 and NGC 6611

We introduce a new survey of massive stars in the Galaxy and the Magellanic Clouds using the Fibre Large Array Multi-Element Spectrograph (FLAMES) instrument at the Very Large Telescope (VLT). Here we present observations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph (R ~ 25,000), in fields centered on the open clusters NGC 3293, NGC 4755 and NGC 6611. These data are supplemented by a further 50 targets observed with the Fibre-Fed Extended Range Optical Spectrograph (FEROS, R = 48,000). Following a description of our scientific motivations and target selection criteria, the data reduction methods are described; of critical importance the FLAMES reduction pipeline is found to yield spectra that are in excellent agreement with less automated methods. Spectral classifications and radial velocity measurements are presented for each star, with particular attention paid to morphological peculiarities and evidence of binarity. These observations represent a significant increase in the known spectral content of NGC 3293 and NGC 4755, and will serve as standards against which our subsequent FLAMES observations in the Magellanic Clouds will be compared.

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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.

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arXiv2005

A Study of the Type II-P Supernova 2003gd in M74

We present photometric and spectroscopic data of the type II-P supernova 2003gd, which was discovered in M74 close to the end of its plateau phase. SN 2003gd is the first type II supernova to have a directly confirmed red supergiant progenitor. We compare SN 2003gd with SN 1999em, a similar type II-P supernova, and estimate an explosion date of 18th March 2003. We determine a reddening towards the supernova of E(B-V) = 0.14+/-0.06, using three different methods. We also calculate three new distances to M74 of 9.6+/-2.8 Mpc, 7.7+/-1.7 Mpc and 9.6+/-2.2 Mpc. The former was estimated using the Standardised Candle Method (SCM), for type II supernovae, and the latter two using the Brightest Supergiants Method (BSM). When combined with existing kinematic and BSM distance estimates, we derive a mean value of 9.3+/-1.8 Mpc. SN 2003gd was found to have a lower tail luminosity compared to other ``normal'' type II-P SNe bringing into question the nature of this supernova. We present a discussion concluding that this is a ``normal'' type II-P supernova which is consistent with the observed progenitor mass of 8(+4/-2) Mo.

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arXiv2004

Two distinct ancient components in the Sculptor Dwarf Spheroidal Galaxy: First Results from DART

We have found evidence for the presence of two distinct ancient stellar components (both geq 10 Gyr old) in the Sculptor dwarf spheroidal galaxy. We used the ESO Wide Field Imager (WFI) in conjunction with the VLT/FLAMES spectrograph to study the properties of the resolved stellar population of Sculptor out to and beyond the tidal radius. We find that two components are discernible in the spatial distribution of Horizontal Branch stars in our imaging, and in the [Fe/H] and v_hel distributions for our large sample of spectroscopic measurements. They can be generally described as a ``metal-poor'' component ([Fe/H]< -1.7) and a ``metal-rich'' component ([Fe/H]> -1.7). The metal-poor stars are more spatially extended than the metal-rich stars, and they also appear to be kinematically distinct. These results provide an important insight into the formation processes of small systems in the early universe and the conditions found there. Even this simplest of galaxies appears to have had a surprisingly complex early evolution.

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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.

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