arXiv subjects

N. F. Martin

Publications and source records attributed to N. F. Martin.

54 records · Page 3Linked to original sources

arXiv2012

Slicing The Monoceros Overdensity with Suprime-Cam

We derive distance, density and metallicity distribution of the stellar Monoceros Overdensity (MO) in the outer Milky Way, based on deep imaging with the Subaru Telescope. We applied CMD fitting techniques in three stripes at galactic longitudes: l=130 deg, 150 deg, 170 deg; and galactic latitudes: +15 < b [deg] < +25 . The MO appears as a wall of stars at a heliocentric distance of ~ 10.1\pm0.5 kpc across the observed longitude range with no distance change. The MO stars are more metal rich ([Fe/H] ~ -1.0) than the nearby stars at the same latitude. These data are used to test three different models for the origin of the MO: a perturbed disc model, which predicts a significant drop in density adjacent to the MO that is not seen; a basic flared disc model, which can give a good match to the density profile but the MO metallicity implies the disc is too metal rich to source the MO stars; and a tidal stream model, which bracket the distances and densities we derive for the MO, suggesting that a model can be found that would fully fit the MO data. Further data and modeling will be required to confirm or rule out the MO feature as a stream or as a flaring of the disc.

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arXiv2012

The structure of star clusters in the outer halo of M31

We present a structural analysis of halo star clusters in M31 based on deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. The clusters in our sample span a range in galactocentric projected distance from 13 to 100 kpc and thus reside in rather remote environments. Ten of the clusters are classical globulars, while four are from the Huxor et al. (2005, 2008) population of extended, old clusters. For most clusters, contamination by M31 halo stars is slight, and so the profiles can be mapped reliably to large radial distances from their centres. We find that the extended clusters are well fit by analytic King (1962) profiles with ~20 parsec core radii and ~100 parsec photometric tidal radii, or by Sersic profiles of index ~1 (i.e. approximately exponential). Most of the classical globulars also have large photometric tidal radii in the range 50-100 parsec, however the King profile is a less good fit in some cases, particularly at small radii. We find 60% of the classical globular clusters exhibit cuspy cores which are reasonably well described by Sersic profiles of index ~2-6. Our analysis also reinforces the finding that luminous classical globulars, with half-light radii <10 parsec, are present out to radii of at least 100 kpc in M31, which is in contrast to the situation in the Milky Way where such clusters (other than the unusual object NGC 2419) are absent beyond 40 kpc.

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arXiv2011

A Bayesian Approach to Locating the Red Giant Branch Tip Magnitude (Part I)

We present a new approach for identifying the Tip of the Red Giant Branch (TRGB) which, as we show, works robustly even on sparsely populated targets. Moreover, the approach is highly adaptable to the available data for the stellar population under study, with prior information readily incorporable into the algorithm. The uncertainty in the derived distances is also made tangible and easily calculable from posterior probability distributions. We provide an outline of the development of the algorithm and present the results of tests designed to characterize its capabilities and limitations. We then apply the new algorithm to three M31 satellites: Andromeda I, Andromeda II and the fainter Andromeda XXIII, using data from the Pan-Andromeda Archaeological Survey (PAndAS), and derive their distances as $731^{(+ 5) + 18}_{(- 4) - 17}$ kpc, $634^{(+ 2) + 15}_{(- 2) - 14}$ kpc and $733^{(+ 13)+ 23}_{(- 11) - 22}$ kpc respectively, where the errors appearing in parentheses are the components intrinsic to the method, while the larger values give the errors after accounting for additional sources of error. These results agree well with the best distance determinations in the literature and provide the smallest uncertainties to date. This paper is an introduction to the workings and capabilities of our new approach in its basic form, while a follow-up paper shall make full use of the method's ability to incorporate priors and use the resulting algorithm to systematically obtain distances to all of M31's satellites identifiable in the PAndAS survey area.

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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).

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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 spectroscopic survey of EC4, an Extended Cluster in Andromeda's halo

We present a spectroscopic survey of candidate red giant branch stars in the extended star cluster, EC4, discovered in the halo of M31 from our CFHT/MegaCam survey, overlapping the tidal streams, Stream Cp and Stream Cr. These observations used the DEep Imaging Multi-Object Spectrograph (DEIMOS) mounted on the Keck II telescope to obtain spectra around the CaII triplet region with ~1.3 Angstroms resolution. Six stars lying on the red giant branch within 2 core-radii of the centre of EC4 are found to have an average vr=-287.9^{+1.9}_{-2.4}km/s and velocity dispersion of 2.7^{+4.2}_{-2.7}km/s, taking instrumental errors into account. The resulting mass-to-light ratio for EC4 is M/L=6.7^{+15}_{-6.7}Msun/Lsun, a value that is consistent with a globular cluster within the 1 sigma errors we derive. From the summed spectra of our member stars, we find EC4 to be metal-poor, with [Fe/H]=-1.6+/-0.15. We discuss several formation and evolution scenarios which could account for our kinematic and metallicity constraints on EC4, and conclude that EC4 is most comparable with an extended globular cluster. We also compare the kinematics and metallicity of EC4 with Stream Cp and Stream Cr, and find that EC4 bears a striking resemblance to Stream Cp in terms of velocity, and that the two structures are identical in terms of both their spectroscopic and photometric metallicities. From this we conclude that EC4 is likely related to Stream Cp.

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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)

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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.

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arXiv2008

The structural properties and star formation history of Leo T from deep LBT photometry

We present deep, wide-field g and r photometry of the transition type dwarf galaxy Leo T, obtained with the blue arm of the Large Binocular Telescope. The data confirm the presence of both very young (<1 Gyr) as well as much older (>5 Gyr) stars. We study the structural properties of the old and young stellar populations by preferentially selecting either population based on their color and magnitude. The young population is significantly more concentrated than the old population, with half-light radii of 104+-8 and 148+-16 pc respectively, and their centers are slightly offset. Approximately 10% of the total stellar mass is estimated to be represented by the young stellar population. Comparison of the color-magnitude diagram (CMD) with theoretical isochrones as well as numerical CMD-fitting suggest that star formation began over 10 Gyr ago and continued in recent times until at least a few hundred Myr ago. The CMD-fitting results are indicative of two distinct star formation bursts, with a quiescent period around 3 Gyr ago, albeit at low significance. The results are consistent with no metallicity evolution and [Fe/H] ~ -1.5 over the entire age of the system. Finally, the data show little if any sign of tidal distortion of Leo T.

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arXiv2008

Numerical Color-Magnitude Diagram Analysis of SDSS Data and Application to the New Milky Way satellites

We have tested the application to Sloan Digital Sky Survey data of the software package MATCH, which fits color-magnitude diagrams (CMDs) to estimate stellar population parameters and distances. These tests on a set of six globular clusters show that these techniques recover their known properties. New ways of using the CMD-fitting software enable us to deal with an extended distribution of stars along the line-of-sight, to constrain the overall properties of sparsely populated objects, and to detect the presence of stellar overdensities in wide-area surveys. We then also apply MATCH to CMDs for twelve recently discovered Milky Way satellites to derive in a uniform fashion their distances, ages and metallicities. While the majority of them appear consistent with a single stellar population, CVn I, UMa II, and Leo T exhibit (from SDSS data alone) a more complex history with multiple epochs of star formation.

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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.

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arXiv2007

The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action

We present a deep photometric survey of M31, conducted with the CFHT and INT, covering the inner 50 kpc of the galaxy, the Southern quadrant out to 150 kpc, and extending to M33. This is the first systematic panoramic study of this very outermost region of galaxies. We detect several streams and other large-scale structures, and two new dwarf galaxies: And XV and XVI. The discovery of substructure on the minor axis, together with the fact that the light profile between 0.5 < R < 1.3 follows the exponential ``extended disk'', is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Underlying the substructures lies a faint, metal-poor, smooth and extremely extended halo, reaching out to at least 150 kpc. The smooth halo component in M31 has a profile that can be fit with a Hernquist model of immense scale radius ~55 kpc, almost a factor of 4 larger than theoretical predictions. Alternatively a power-law with exponent -1.91 +/- 0.11 can be fit to the profile. The total luminosity of this structure is similar to that of the halo of the Milky Way. This vast, smooth, underlying halo is reminiscent of a classical monolithic model and completely unexpected from modern galaxy formation models. M33 is also found to have an extended metal-poor halo component, which can be fit with a Hernquist model also of scale radius ~55 kpc. These extended slowly-decreasing halos will provide a challenge and strong constraints for further modeling. [Abridged]

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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.

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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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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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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.

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