arXiv subjects

R. Ibata

Publications and source records attributed to R. Ibata.

At least 73 records · Page 4Linked to original sources

arXiv2009

Globular Cluster Candidates in NGC 891

We use deep images taken with the Advanced Camera for Surveys on board the Hubble Space Telescope of the disk galaxy NGC 891, to search for globular cluster candidates. This galaxy has long been considered to be a close analog in size and structure to the Milky Way and is nearly edge-on, facilitating studies of its halo population. These extraplanar ACS images, originally intended to study the halo field-star populations, reach deep enough to reveal even the faintest globular clusters that would be similar to those in the Milky Way. From the three pointings we have identified a total of 43 candidates after culling by object morphology, magnitude, and colour. We present (V,I) photometry for all of these, along with measurements of their effective radius and ellipticity. The 16 highest-rank candidates within the whole sample are found to fall in very much the same regions of parameter space occupied by the classic Milky Way globular clusters. Our provisional conclusion from this survey is that the total globular cluster population in NGC 891 as a whole may be almost as large as that of the Milky Way.

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arXiv2008

Do submillimeter galaxies really trace the most massive dark matter halos? Discovery of a high-z cluster in a highly active phase of evolution

We present detailed observations of a z~1.99 cluster of submillimeter galaxies (SMGs), discovered as the strongest redshift spike in our entire survey of ~100 SMGs across 800 square arcmin. It is the largest blank-field SMG concentration currently known and has <0.01% chance of being drawn from the underlying selection function for SMGs. We have compared UV observations of galaxies at this redshift, where we find a much less dramatic overdensity, having an 11% chance of being drawn from its selection function. We use this z~1.99 overdensity to compare the biasing of UV- and submm-selected galaxies, and test whether SMGs could reside in less overdense environments, with their apparent clustering signal being dominated by highly active merger periods in modest mass structures. This impressively active formation phase in a low mass cluster is not something seen in simulations, although we propose a toy model using merger bias which could account for the bias seen in the SMGs. While enhanced buildup of stellar mass appears characteristic of other high-z galaxy clusters, neither the UV- nor submm-galaxies in this structure exhibit larger stellar masses than their field galaxy counterparts (although the excess of SMGs in the structure represents a larger volume-averaged stellar mass than the field). Our findings have strong implications for future surveys for high-z galaxies at long wavelengths such as SCUBA2 and Herschel. We suggest that since these surveys will select galaxies during their episodes of peak starbursts, they could probe a much wider range of environments than just the progenitors of rich clusters, revealing more completely the key events and stages in galaxy formation and assembly.

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arXiv2008

The kinematic footprints of five stellar streams in Andromeda's halo

(abridged) We present a spectroscopic analysis of five stellar streams (`A', `B', `Cr', `Cp' and `D') as well as the extended star cluster, EC4, which lies within streamC, all discovered in the halo of M31 from our CFHT/MegaCam survey. These spectroscopic results were initially serendipitous, making use of our existing observations from the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, and thereby emphasizing the ubiquity of tidal streams that account for ~70% of the M31 halo stars in the targeted fields. Subsequent spectroscopy was then procured in streamCr/p and streamD to trace the velocity gradient along the streams. For the cluster EC4, candidate member stars with average [Fe/H]~-1.4 (Fe/H_spec=-1.6), are found at v_{hel}=-285 km/s suggesting it could be related to streamCp. No similarly obvious cold kinematic candidate is found for streamD, although candidates are proposed in both of two spectroscopic pointings along the stream (both at -400 km/s). Spectroscopy near the edge of streamB suggests a likely kinematic detection, while a candidate kinematic detection of streamA is found (plausibly associated to M33 rather than M31). The low dispersion of the streams in kinematics, physical thickness, and metallicity makes it hard to reconcile with a scenario whereby these stream structures as an ensemble are related to the giant southern stream. We conclude that the M31 stellar halo is largely made up of multiple kinematically cold streams.

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arXiv2008

AGB stars as tracers of metallicity and mean age across M33

Wide-field JHKs near-infrared observations covering an area of 1.8x1.8 sq. deg. centred on M33 were obtained using WFCAM at UKIRT. These data show a large population of intermediate-age asymptotic giant branch stars (AGB). We have used both C-type and M-type AGB stars to determine spatial variations in metallicity and mean age across the galaxy. We distinguished between C-type and M-type AGB stars from their location in the colour-magnitude diagram (J-Ks, Ks). The distribution of these stars is supported by a cross-identification between our sample and a catalogue of optically confirmed, long-period variable stars, as well as with the list of sources detected by Spitzer in the mid-infrared. We calculated the C/M ratio and the orientation of the galaxy in the sky, and compared the Ks magnitude distribution with theoretical distributions spanning a range of metallicities and star formation rates (SFRs). The C/M ratio surface map confirms a metallicity gradient in the galaxy corresponding to a spread in [Fe/H]=0.6 dex with substructures in the inner and outer galaxy. Magnitude and colour variations suggest orientation and extinction effects on the galaxy disc. Maps showing the distribution of mean age and metallicity obtained from the Ks method suggest that: the outer galaxy disc/halo is metal poorer than the nuclear region and metal-rich clumps in the inner galaxy change location with time. The average outer ring and nuclear stellar population is ~6 Gyr old while central regions are a few Gyr younger.

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arXiv2007

Globular clusters in the outer halo of M31: the survey

We report the discovery of 40 new globular clusters (GCs) that have been found in surveys of the halo of M31 based on INT/WFC and CHFT/Megacam imagery. A subset of these these new GCs are of an extended, diffuse nature, and include those already found in Huxor et al. (2005). The search strategy is described and basic positional and V and I photometric data are presented for each cluster. For a subset of these clusters, K-band photometry is also given. The new clusters continue to be found to the limit of the survey area (~100 kpc), revealing that the GC system of M31 is much more extended than previously realised. The new clusters increase the total number of confirmed GCs in M31 by approximately 10% and the number of confirmed GCs beyond 1 degree (~14 kpc) by more than 75%. We have also used the survey imagery as well recent HST archival data to update the Revised Bologna Catalogue (RBC) of M31 globular clusters.

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arXiv2007

Metallicity and mean age across M33

New wide-field near-infrared (NIR) imaging observations of M33 were obtained from UKIRT. These show a large population of intermediate-age stars considerably improving on previous NIR data. The spatial distribution of super giant stars, carbon-rich (C-rich or C stars) and oxygen-rich (O-rich or M stars) asymptotic giant branch (AGB) stars distinguished from the NIR colour-magnitude diagram (CMD) have been studied as well as the C/M ratio. The Ks magnitude distribution has been interpreted using theoretical models to derive the mean age and the mean metallicity across M33.

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arXiv2007

Strangers in the night: Discovery of a dwarf spheroidal galaxy on its first Local Group infall

We present spectroscopic observations of the AndXII dwarf spheroidal galaxy using DEIMOS/Keck-II, showing it to be moving rapidly through the Local Group (-556 km/s heliocentric velocity, -281 km/s relative to Andromeda from the MW), falling into the Local Group from ~115 kpc beyond Andromeda's nucleus. AndXII therefore represents a dwarf galaxy plausibly falling into the Local Group for the first time, and never having experienced a dense galactic environment. From Green Bank Telescope observations, a limit on the H{I} gas mass of <3000 Msun suggests that AndXII's gas could have been removed prior to experiencing the tides of the Local Group galaxies. Orbit models suggest the dwarf is close to the escape velocity of M31 for published mass models. AndXII is our best direct evidence for the late infall of satellite galaxies, a prediction of cosmological simulations.

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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

ACS photometry of newly-discovered globular clusters in the outer halo of M31

We report the first results from deep ACS imaging of ten classical globular clusters in the far outer regions (15 < R_p < 100 kpc) of M31. Eight of the clusters, including two of the most remote M31 globular clusters presently known, are described for the first time. Our F606W, F814W colour-magnitude diagrams extend ~ 3 magnitudes below the horizontal branch and clearly demonstrate that the majority of these objects are old (> 10 Gyr), metal-poor clusters. Five have [Fe/H] ~ -2.1, while an additional four have -1.9 < [Fe/H] < -1.5. The remaining object is more metal-rich, with [Fe/H] ~ -0.70. Several clusters exhibit the second parameter effect. Using aperture photometry, we estimate integrated luminosities and structural parameters for all clusters. Many, including all four clusters with projected radii greater than 45 kpc, are compact and very luminous, with -8.9 < M_V < -8.3. These four outermost clusters are thus quite unlike their Milky Way counterparts, which are typically diffuse, sub-luminous (-6.0 < M_V < -4.7) and more metal-rich (-1.8 < [Fe/H] < -1.3).

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arXiv2006

ACS photometry of extended, luminous globular clusters in the outskirts of M31

A new population of extended, luminous globular clusters has recently been discovered in the outskirts of M31. These objects have luminosities typical of classical globular clusters, but much larger half-light radii. We report the first results from deep ACS imaging of four such clusters, one of which is a newly-discovered example lying at a projected distance of ~60 kpc from M31. Our F606W, F814W colour-magnitude diagrams extend ~3 magnitudes below the horizontal branch level, and clearly demonstrate, for the first time, that all four clusters are composed of >10 Gyr old, metal-poor stellar populations. No evidence for multiple populations is observed. From a comparison with Galactic globular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H] < -1.8. The observed horizontal branch morphologies show a clear second parameter effect between the clusters. Preliminary radial luminosity profiles suggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the median value of the globular cluster luminosity function. Our results confirm that these four objects are bona fide old, metal-poor globular clusters, albeit with combined structures and luminosities unlike those observed for any other globular clusters in the Local Group or beyond.

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arXiv2006

A kinematically selected, metal-poor stellar halo in the outskirts of M31

We present evidence for a metal-poor, [Fe/H]$\sim-1.4$ $σ$=0.2 dex, stellar halo component detectable at radii from 10 kpc to 70 kpc, in our nearest giant spiral neighbor, the Andromeda galaxy. This metal-poor sample underlies the recently-discovered extended rotating component, and has no detected metallicity gradient. This discovery uses a large sample of 9861 radial velocities of Red Giant Branch (RGB) stars obtained with the Keck-II telescope and DEIMOS spectrograph, with 827 stars with robust radial velocity measurements isolated kinematically to lie in the halo component primarily by windowing out the extended rotating component which dominates the photometric profile of Andromeda out to $<$50 kpc (de-projected). The stars lie in 54 spectroscopic fields spread over an 8 square degree region, and are expected to fairly sample the halo to a radius of $\sim$70 kpc. The halo sample shows no significant evidence for rotation. Fitting a simple model in which the velocity dispersion of the component decreases with radius, we find a central velocity dispersion of $152\kms$ decreasing by $-0.90\kms/\kpc$. By fitting a cosmologically-motivated NFW halo model to the halo stars we constrain the virial mass of M31 to be greater than $9.0 \times 10^{11} \msun$ with 99% confidence. The properties of this halo component are very similar to that found in our Milky Way, revealing that these roughly equal mass galaxies may have led similar accretion and evolutionary paths in the early Universe.

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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

Spectroscopic classification of red high proper motion objects in the Southern Sky

We present the results of spectroscopic follow-up observations for a sample of 71 red objects with high proper motions in the range 0.08-1.14 arcsec/yr as detected using APM and SSS measurements of multi-epoch photographic Schmidt plates. Red objects were selected by combining the photographic BjRI magnitudes with 2MASS near-infrared JHKs magnitudes. Some 50 of the 71 spectroscopically classified objects turn out to be late-type (>M6) dwarfs and in more detail, the sample includes 35 ultracool dwarfs with spectral types between M8 and L2, some previously reported, as well as five M-type subdwarfs, including a cool esdM6 object, SSSPM J0500-5406. Distance estimates based on the spectral types and 2MASS J magnitudes place almost all of the late-type (>M6) dwarfs within 50 pc, with 25 objects located inside the 25 pc limit of the catalogue of nearby stars. Most of the early-type M dwarfs are located at larger distances of 100-200 pc, suggesting halo kinematics for some of them. All objects with Halpha equivalent widths larger than 10 Angstroms have relatively small tangential velocities (<50 km/s). Finally, some late-type but blue objects are candidate binaries.

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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

On the accretion origin of a vast extended stellar disk around the Andromeda galaxy

We present the discovery of an inhomogenous, low-surface brightness, extended disk-like structure around the Andromeda galaxy (M31) based on a large kinematic survey of more than 2800 stars with the Keck/DEIMOS spectrograph. The stellar structure spans radii from 15 kpc out to ~40 kpc, with detections out to R ~ 70 kpc. The constituent stars lag the expected velocity of circular orbits in the plane of the M31 disk by ~40 kms and have a velocity dispersion of ~30 kms. The color range on the upper RGB shows a large spread indicative of a population with a significant range of metallicity. The mean metallicity of the population, measured from Ca II equivalent widths, is [Fe/H] = -0.9 +/- 0.2. The morphology of the structure is irregular at large radii, and shows a wealth of substructures which must be transitory in nature, and are almost certainly tidal debris. The presence of these substructures indicates that the global entity was formed by accretion. This extended disk follows smoothly on from the central parts of M31 disk with an exponential density law of scale-length of 5.1 +/- 0.1 kpc, similar to that of the bright inner disk. The population possesses similar kinematic and abundance properties over the entire region where it is detected in the survey. We estimate that the structure accounts for approximately 10% of the total luminosity of the M31 disk, and given the huge scale, contains ~30% of the total disk angular momentum. This finding indicates that at least some galactic stellar disks are vastly larger than previously thought and are formed, at least in their outer regions, primarily by accretion. [abridged]

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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.

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arXiv2004

The tidal trail of NGC205?

Using data taken as part of the Isaac Newton Telescope Wide Field Camera (INT WFC) survey of M31, we have identified an arc-like overdensity of blue, presumably metal-poor, red giant branch stars in the north-west quadrant of M31. This feature is approximately 1 degree (15 kpc) in extent and has a surface brightness of Sigma_V' ~ 28.5 +/- 0.5 mags per sq. arcsec. The arc appears to emanate from the dwarf elliptical galaxy NGC205, and the colour of its red giant branch is significantly different to the M31 disk population but closely resembles that of NGC205. Further, using data taken with the DEep Imaging Multi-Object Spectrograph (DEIMOS) on Keck II, we identify the radial velocity signature of this arc. Its velocity dispersion is measured to be approximately 10km/s, similar to that of the central regions of NGC205 and typical of stellar streams. Based upon the spatial coincidence of these objects, the surface brightness, the velocity dispersions and the similarity in colour of the red giant branches, we postulate that the arc is part of a stellar stream, the progenitor of which is NGC205.

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arXiv2004

Taking measure of the Andromeda halo: a kinematic analysis of the giant stream surrounding M31

We present a spectroscopic survey of the giant stellar stream found in the halo of the Andromeda galaxy. Taken with the DEIMOS multi-object spectrograph on the Keck2 telescope, these data display a narrow velocity dispersion of 11+/-3 km/s, with a steady radial velocity gradient of 245 km/s over the 125 kpc radial extent of the stream studied so far. This implies that the Andromeda galaxy possesses a substantial dark matter halo. We fit the orbit of the stream in different galaxy potential models. In a simple model with a composite bulge, disk and halo, where the halo follows a ``universal'' profile that is compressed by the formation of the baryonic components, we find that the kinematics of the stream require a total mass inside 125 kpc of 7.5^(+2.5)_(-1.3) x 10^{11} M_Sun, or more than 5.4 x 10^{11} M_Sun at the 99% confidence level. This is the first galaxy in which it has been possible to measure the halo mass distribution by such direct dynamical means over such a large distance range. The resulting orbit shows that if M32 or NGC 205 are connected with the stream, they must either trail or lag the densest region of the stream by more than 100 kpc. Furthermore, according to the best-fit orbit, the stream passes very close to M31, causing its demise as a coherent structure and producing a fan of stars that will pollute the inner halo, thereby confusing efforts to measure the properties of genuine halo populations. Our data show that several recently identified planetary nebulae, which have been proposed as evidence for the existence of a new companion of M31, are likely members of the Andromeda Stream.

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