arXiv subjects

R. A. Ibata

Publications and source records attributed to R. A. Ibata.

At least 37 records · Page 2Linked to original sources

arXiv2011

Exploring the Properties of the M31 Halo Globular Cluster System

Following on from our discovery of a significant population of M31 outer halo globular clusters (GCs), and updates to the Revised Bologna Catalogue of M31 GCs, we investigate the GC system of M31 out to an unprecedented radius (~120kpc). We derive various ensemble properties, including the magnitude, colour and metallicity distributions, as well as the GC number density profile. One of our most significant findings is evidence for a flattening in the radial GC number density profile in the outer halo. Intriguingly, this occurs at a galactocentric radius of ~2 degrees (~30 kpc) which is the radius at which the underlying stellar halo surface density has also been shown to flatten. The GCs which lie beyond this radius are remarkably uniform in terms of their blue (V-I)o colours, consistent with them belonging to an ancient population with little to no metallicity gradient. Structural parameters are also derived for a sample of 13 newly-discovered extended clusters (ECs) and we find the lowest luminosity ECs have magnitudes and sizes similar to Palomar-type GCs in the Milky Way halo. We argue that our findings provide strong support for a scenario in which a significant fraction of the outer halo GC population of M31 has been accreted.

astro-ph.CO↗

arXiv2010

The kinematic identification of a thick stellar disc in M31

We present the first characterization of a thick disc component in the Andromeda galaxy (M31) using kinematic data from the DEIMOS multi-object spectrograph instrument on Keck II. Using 21 fields in the South West of the galaxy, we measure the lag of this component with respect to the thin disc, as well as the dispersion, metallicity and scale length of the component. We find an average lag between the two components of =46.0+/-3.9km/s. The velocity dispersion of the thick disc is sigma_{thick}=50.8+/-1.9km/s, greater than the value of dispersion we determine for the thin disc, sigma_{thin}=35.7+/-1.0km/s. The thick disc is more metal poor than the thin disc, with [Fe/H]_{spec}=-1.0+/-0.1 compared to [Fe/H]_{spec}=-0.7+/-0.05 for the thin disc. We measure a radial scale length of the thin and thick discs of h_r=7.3+/-1.0 kpc and h_r=8.0+/-1.2 kpc. From this, we infer scale heights for both discs of 1.1+/-0.2 kpc and 2.8+/-0.6 kpc, both of which are ~2--3 times larger than those observed in the Milky Way. We estimate a mass range for the thick disc component of 2.4x10^{10}Msun< M_{*,thick} <4.1x10^{10}Msun. This value provides a useful constraint on possible formation mechanisms, as any proposed method for forming a thick disc must be able to heat (or deposit) at least this amount of material.

astro-ph.CO↗

arXiv2010

The Northern wraps of the Sagittarius Stream as traced by Red Clump stars: distances, intrinsic widths and stellar densities

We trace the tidal Stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) using Red Clump stars from the catalog of the Sloan Digital Sky Survey - Data Release 6, in the range 150° < RA < 220°, corresponding to the range of orbital azimuth 220° < Lambda < 290°. Substructures along the line of sight are identified as significant peaks in the differential star count profiles (SCP) of candidate Red Clump stars. A proper modeling of the SCPs allows us to obtain: (a) <10% accurate, purely differential distances with respect to the main body of Sgr, (b) estimates of the FWHM along the line of sight, and (c) estimates of the local density, for each detected substructure. In the range 255° < Lambda < 290° we cleanly and continuously trace various coherent structures that can be ascribed to the Stream, in particular: the well known northern portion of the leading arm, running from d~43 kpc at Lambda ~ 290° to d ~ 30 kpc at Lambda ~ 255°, and a more nearby coherent series of detections lying at constant distance d ~ 25 kpc, that can be identified with a wrap of the trailing arm. The latter structure, predicted by several models of the disruption of Sgr dSph, was never traced before; comparison with existing models indicates that the difference in distance between these portions of the leading and trailing arms may provide a powerful tool to discriminate between theoretical models assuming different shapes of the Galactic potential. A further, more distant wrap in the same portion of the sky is detected only along a couple of lines of sight.[abridged]

astro-ph.GA↗

arXiv2010

A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And IX, And XI, And XII, and And XIII

We present the first spectroscopic analysis of the faint M31 satellite galaxies, AndXI and AndXIII, and a reanalysis of existing spectroscopic data for two further faint companions, And IX and AndXII. By combining data obtained using the DEIMOS spectrograph mounted on the Keck II telescope with deep photometry from the Suprime-Cam instrument on Subaru, we have calculated global properties for the dwarfs, such as systemic velocities, metallicites and half-light radii.We find each dwarf to be very metal poor ([Fe/H] -2 both photometrically and spectroscopically, from their stacked spectrum), and as such, they continue to follow the luminosity-metallicity relationship established with brighter dwarfs. We are unable to resolve a dispersion for And XI due to small sample size and low S/N, but we set a one sigma upper limit of sigma-v <5 km/s. For And IX, And XII and And XIII we resolve velocity dispersions of v=4.5 (+3.4,-3.2), 2.6(+5.1,-2.6) and 9.7(+8.9,-4.5) km/s, and derive masses within the half light radii of 6.2(+5.3,-5.1)x10^6 Msun, 2.4 (+6.5,-2.4)x10^6 Msun and 1.1(+1.4,-0.7)x10^7 Msun respectively. We discuss each satellite in the context of the Mateo relations for dwarf spheroidal galaxies, and the Universal halo profiles established for Milky Way dwarfs (Walker et al. 2009). For both galaxies, this sees them fall below the Universal halo profiles of Walker et al. (2009). When combined with the findings of McConnachie & Irwin (2006a), which reveal that the M31 satellites are twice as extended (in terms of both half-light and tidal radii) as their Milky Way counterparts, these results suggest that the satellite population of the Andromeda system could inhabit halos that are significantly different from those of the Milky Way in terms of their central densities (abridged).

astro-ph.CO↗

arXiv2009

Deep Gemini/GMOS imaging of an extremely isolated globular cluster in the Local Group

We report on deep imaging of a remote M31 globular cluster, MGC1, obtained with Gemini/GMOS. Our colour-magnitude diagram for this object extends ~5 magnitudes below the tip of the red giant branch and exhibits features consistent with an ancient metal-poor stellar population, including a long, well-populated horizontal branch. The red giant branch locus suggests MGC1 has a metal abundance [M/H] ~ -2.3. We measure the distance to MGC1 and find that it lies ~160 kpc in front of M31 with a distance modulus of 23.95 +/- 0.06. Combined with its large projected separation of 117 kpc from M31 this implies a deprojected radius of Rgc = 200 +/- 20 kpc, rendering it the most isolated known globular cluster in the Local Group by some considerable margin. We construct a radial brightness profile for MGC1 and show that it is both centrally compact and rather luminous, with Mv = -9.2. Remarkably, the cluster profile shows no evidence for a tidal limit and we are able to trace it to a radius of at least 450 pc, and possibly as far as ~900 pc. The profile exhibits a power-law fall-off with exponent -2.5, breaking to -3.5 in its outermost parts. This core-halo structure is broadly consistent with expectations derived from numerical models, and suggests that MGC1 has spent many gigayears in isolation.

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arXiv2009

A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And XV and And XVI

We present the results of a spectroscopic survey of the recently discovered M31 satellites And XV and And XVI, lying at projected distances from the centre of M31 of 93 and 130 kpc respectively. These satellites lie to the South of M31, in regions of the stellar halo which wide field imaging has revealed as relative voids (compared to the degree-scale coherent stream-like structures). Using the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have defined probable members of these satellites, for which we derive radial velocities as precise as ~6 km/s down to i~21.5. While the distance to And XVI remains the same as previously reported (525pm50 kpc), we have demonstrated that the brightest three stars previously used to define the tip of the red giant branch (TRGB) in And XV are in fact Galactic, and And XV is actually likely to be much more distant at 770pm70 kpc (compared to the previous 630 kpc), increasing the luminosity from MV -9.4 to MV~-9.8. The And XV velocity dispersion is resolved with vr =-339+7-6 km/s and sigma-v = 11+7-5 km/s. The And XVI dispersion is not quite resolved at 1sigma with vr =-385+5-6 km/s and sigma-v = 0+10-indef km/s. Using the photometry of the confirmed member stars, we find metallicities of And XV (median [Fe/H]=-1.58, interquar- tile range +-0.08), and And XVI (median [Fe/H]=-2.23, interquartile range +-0.12). Stacking the spectra of the member stars, we find spectroscopic [Fe/H]=-1.8 (-2.1) for And XV (And XVI), with a uncertainty of ~0.2 dex in both cases. Our measure- ments of And XV reasonably resolve its mass (~10^8 Msun) and suggest a polar orbit, while the velocity of And XVI suggests it is approaching the M31 escape velocity given its large M31-centric distance.

astro-ph.SR↗

arXiv2009

An HST/ACS View of the Inhomogeneous Outer Halo of M31

We present a high precision photometric view of the stellar populations in the outer halo of M31, using data taken with the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). We analyse the field populations adjacent to 11 luminous globular clusters which sample the galactocentric radial range 18 < R < 100 kpc and reach a photometric depth of ~2.5 magnitudes below the horizontal branch (m_F814W ~27 mag). The colour-magnitude diagrams (CMDs) are well populated out to ~60 kpc and exhibit relatively metal-rich red giant branches, with the densest fields also showing evidence for prominent red clumps. We use the Dartmouth isochrones to construct metallicity distribution functions (MDFs) which confirm the presence of dominant populations with <[Fe/H]> = -0.6 to -1.0 dex and considerable metallicity dispersions of 0.2 to 0.3 dex (assuming a 10 Gyr population and scaled-Solar abundances). The average metallicity over the range 30 - 60 kpc is [Fe/H] = -0.8 +/- 0.14 dex, with no evidence for a significant radial gradient. Metal-poor stars ([Fe/H] <= -1.3) typically account for < 10-20 % of the population in each field, irrespective of radius. Assuming our fields are unbiased probes of the dominant stellar populations in these parts, we find that the M31 outer halo remains considerably more metal-rich than that of the Milky Way out to at least 60 kpc.

astro-ph.GA↗

arXiv2008

Kinematic and Chemical constraints on the formation of M31's inner and outer halo

The halo of M31 shows a wealth of substructures that are consistent with satellite accretion. Here we report on kinematic and abundance results from Keck/DEIMOS spectroscopy in the calcium triplet region of over 3500 red giant star candidates along the minor axis and in off-axis spheroid fields of M31. Our data reach out to large radial distances of 160 kpc. The derived velocity distributions show a kinematically cold substructure at 17 kpc that has been reported before. We devise an improved method to measure accurate metallicities from the calcium triplet in low signal-to-noise spectra using a coaddition of the individual lines. The resulting distribution leads us to note an even stronger gradient in the abundance distribution along M31's minor axis than previously detected. The mean metallicity in the outer halo reaches below -2 dex, with individual values as low as -2.6 dex. In the inner spheroid, at 17-19 kpc, we find a sharp decline of ~0.5 dex in metallicity, which roughly coincides with the edge of an extended disk, previously detected from star count maps. A large fraction of red giants in the most distant fields are likely members of M33's overlapping halo. A comparison of our velocities with those predicted by new N-body simulations argues that the event responsible for the giant Stream is most likely not responsible for the full population of the inner halo; we show further that the abundance distribution of the Stream is different from that of the inner halo, from which it becomes evident that the merger event that formed the outer halo cannot have contributed any significant material to the inner spheroid. All this evidence of severe structure changes in the halo suggests a high degree of infall and stochastic abundance accretion governing the build-up of M31's halo. (Abridged)

astro-ph↗

arXiv2008

The nucleus of the Sagittarius dSph galaxy and M54: a window on the process of galaxy nucleation

We present the results of a thorough study of the nucleus of the Sgr dwarf spheroidal galaxy and of the bright globular cluster M54 that resides within the same nucleus (Sgr,N). We have obtained accurate radial velocities and metallicity estimates for 1152 candidate Red Giant Branch stars of Sgr and M54 lying within ~ 9 arcmin from the center of the galaxy, from Keck/DEIMOS and VLT/FLAMES spectra of the infrared Calcium II triplet. Using both velocity and metallicity information we selected two samples of 425 and 321 very-likely members of M54 and of Sgr,N, respectively. The two considered systems display significantly different velocity dispersion profiles: M54 has a steeply decreasing profile from r=0, where sigma= 14.2 km/s, to r=3.5 arcmin where it reaches sigma=5.3 km/s, then it appears to rise again to sigma= 10 km/s at r=7 arcmin. In contrast Sgr,N has a uniformly flat profile at sigma=9.6 km/s over the whole 0 < r < 9 arcmin range. Using data from the literature we show that the velocity dispersion of Sgr remains constant at least out to r ~ 100 arcmin and there is no sign of the transition between the outer flat-luminosity-profile core and the inner nucleus in the velocity profile. These results - together with a re-analysis of the surface brightness profile of Sgr,N and a suite of dedicated N-body simulations - provide very strong support for the hypothesis that the nucleus of Sgr formed independently of M54, which probably plunged to its present position, coincident with Sgr,N, because of significant decay of the original orbit due to dynamical friction.

astro-ph↗

arXiv2008

The Nature and Origin of Substructure in the Outskirts of M31. I. Surveying the Stellar Content with HST/ACS

We present the largest and most detailed survey to date of the stellar populations in the outskirts of M31 based on the analysis of 14 deep HST/ACS pointings spanning the range 11.5-45.0 kpc. We conduct a quantitative comparison of the resolved stellar populations in these fields and identify several striking trends. The color-magnitude diagrams (CMDs), which reach ~3 magnitudes below the red clump, can be classified into two main categories based on their morphologies. `Stream-like' fields, so named for their similarity to the CMD of the giant stellar stream, are characterized by a red clump that slants bluewards at fainter magnitudes and an extended horizontal branch. On the other hand, `disk-like' fields exhibit rounder red clumps with significant luminosity width, lack an obvious horizontal branch and show evidence for recent star formation (~0.25 - 2.0 Gyr ago). We compare the spatial and line-of-sight distribution of stream-like fields with a recent simulation of the giant stream progenitor orbit and find an excellent agreement. These fields, found across the face of M31, attest to the high degree of pollution caused by this event. Disk-like material resides in the extended disk structure of M31 and is detected out to 44 kpc (projected); the uniform populations in these fields, including the ubiquitous presence of young populations, and the strong rotation reported elsewhere are most consistent with a scenario in which this structure has formed through heating and disruption of the existing thin disk, perhaps due to the impact of the giant stream progenitor. Our comparative analysis sheds new light on the likely composition of two of the ultra-deep pointings formerly presented as pure outer disk and pure halo in the literature.

astro-ph↗

arXiv2007

A Keck/DEIMOS spectroscopic survey of faint Galactic satellites: searching for the least massive dwarf galaxies

[abridged] We present the results of a spectroscopic survey of the recently discovered faint Milky Way satellites Boo, UMaI, UMaII and Wil1. Using the DEIMOS spectrograph on Keck, we have obtained samples that contain from 15 to 85 probable members of these satellites for which we derive radial velocities precise to a few km/s down to i~21-22. About half of these stars are observed with a high enough S/N to estimate their metallicity to within \pm0.2 dex. From this dataset, we show that UMaII is the only object that does not show a clear radial velocity peak. However, the measured systemic radial velocity (v_r=115\pm5 km/s) is in good agreement with recent simulations in which this object is the progenitor of the recently discovered Orphan Stream. The three other satellites show velocity dispersions that make them highly dark-matter dominated systems. In particular the Willman 1 object is not a globular cluster given its metallicity scatter over -2.0<[Fe/H]<-1.0 and is therefore almost certainly a dwarf galaxy or dwarf galaxy remnant. We measure a radial velocity dispersion of only 4.3_{-1.3}^{+2.3} km/s around a systemic velocity of -12.3\pm2.3 km/s which implies a mass-to-light ratio of ~700 and a total mass of ~5x10^5 Msun for this satellite, making it the least massive satellite galaxy known to date. Such a low mass could mean that the 10^7 Msun limit that had until now never been crossed for Milky Way and Andromeda satellite galaxies may only be an observational limit and that fainter, less massive systems exist within the Local Group. However, more modeling and an extended search for potential extra-tidal stars are required to rule out the possibility that these systems have not been significantly heated by tidal interaction.

astro-ph↗

arXiv2006

Inside the whale: the structure and dynamics of the isolated Cetus dwarf spheroidal

This paper presents a study of the Cetus dwarf, an isolated dwarf galaxy within the Local Group. A matched-filter analysis of the INT/WFC imaging of this system reveals no evidence for significant tidal debris that could have been torn from the galaxy, bolstering the hypothesis that Cetus has never significantly interacted with either the Milky Way or M31. Additionally, Keck/Deimos spectroscopic observations identify this galaxy as a distinct kinematic population possessing a systematic velocity of $-87\pm2{\rm km\ s^{-1}}$ and with a velocity dispersion of $17\pm2{\rm km s^{-1}}$; while tentative, these data also suggest that Cetus possesses a moderate rotational velocity of $\sim8{\rm km s^{-1}}$. The population is confirmed to be relatively metal-poor, consistent with ${\rm [Fe/H]\sim-1.9}$, and, assuming virial equilibrium, implies that the Cetus dwarf galaxy possesses a $M/L\sim70$. It appears, therefore, that Cetus may represent a primordial dwarf galaxy, retaining the kinematic and structural properties lost by other members of the dwarf population of the Local Group in their interactions with the large galaxies. An analysis of Cetus's orbit through the Local Group indicates that it is at apocentre; taken in conjunction with the general dwarf population, this shows the mass of the Local Group to be $\gta2\times10^{12}M_\odot$.

astro-ph↗

arXiv2006

Discovery and analysis of three faint dwarf galaxies and a globular cluster in the outer halo of the Andromeda galaxy

We present the discovery of three faint dwarf galaxies and a globular cluster in the halo of the Andromeda galaxy (M31), found in our MegaCam survey that spans the southern quadrant of M31, from a projected distance of ~50 kpc to \~150 kpc. Though the survey covers 57 sq. degrees, the four satellites lie within 2°of one another. We estimate that the globular cluster lies at a total distance of 175+/-55 kpc from M31, making it the farthest M31 globular cluster known. It also shows the typical characteristics of a bright globular cluster, with a half-light radius of 2.3+/-0.2 pc and an absolute magnitude in the V band of M_{V,0}=-8.5+/-0.3 and contains a very old population with a metallicity of [Fe/H]~-1.3. The three dwarf galaxies are all very faint, with absolute magnitudes in the range -7.3<M_{V,0}<-6.4, and show strikingly similar characteristics with metallicities of [Fe/H]~-1.4 and half-light radii of \~120+/-45 pc, making these dwarf galaxies two to three times smaller than the smallest previously known satellites of M31. We estimate them to be between 740 and 955 kpc from the Sun. Extrapolating from the coverage of the survey, we estimate that up to 45+/-20 satellites brighter than M_V~-6.5 should be orbiting M31. Hence faint dwarf galaxies cannot alone account for the missing satellites that are predicted by $Λ$CDM models, unless they reside in dark matter mini-halos that are more massive than the typical masse of 10^7 solar masses currently inferred from their central radial velocity dispersion. (abridged)

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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

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↗

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.

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