arXiv subjects

A. Koch

Publications and source records attributed to A. Koch.

At least 55 records · Page 3Linked to original sources

arXiv2009

A constant dark matter halo surface density in galaxies

We confirm and extend the recent finding that the central surface density r_0*rho_0 galaxy dark matter halos, where r_0 and rho_0 are the halo core radius and central density, is nearly constant and independent of galaxy luminosity. Based on the co-added rotation curves of about 1000 spiral galaxies, mass models of individual dwarf irregular and spiral galaxies of late and early types with high-quality rotation curves and, galaxy-galaxy weak lensing signals from a sample of spiral and elliptical galaxies, we find that log(r_0*rho_0) = 2.15 +- 0.2, in units of log(Msol/pc^2). We also show that the observed kinematics of Local Group dwarf spheroidal galaxies are consistent with this value. Our results are obtained for galactic systems spanning over 14 magnitudes, belonging to different Hubble Types, and whose mass profiles have been determined by several independent methods. In the same objects, the approximate constancy of rho_0*r_0 is in sharp contrast to the systematical variations, by several orders of magnitude, of galaxy properties, including rho_0 and central stellar surface density.

astro-ph.CO↗

arXiv2009

On the Reported Death of the MACHO Era

We present radial velocity measurements of four wide halo binary candidates from the sample in Chaname & Gould (2004; CG04) which, to date, is the only sample containing a large number of such candidates. The four candidates that we have observed have projected separations >0.1 pc, and include the two widest binaries from the sample, with separations of 0.45 and 1.1 pc. We confirm that three of the four CG04 candidates are genuine, including the one with the largest separation. The fourth candidate, however, is spurious at the 5-sigma level. In the light of these measurements we re-examine the implications for MACHO models of the Galactic halo. Our analysis casts doubt on what MACHO constraints can be drawn from the existing sample of wide halo binaries.

astro-ph.GA↗

arXiv2008

The Highly Unusual Chemical Composition of the Hercules Dwarf Spheroidal Galaxy

We report on the abundance analysis of two red giants in the faint Hercules dwarf spheroidal (dSph) galaxy. These stars show a remarkable deficiency in the neutron-capture elements, while the hydrostatic alpha-elements (O, Mg) are strongly enhanced. Our data indicate [Ba/Fe] and [Mg/Fe] abundance ratios of <-2 dex and ~+0.8 dex, respectively, with essentially no detection of other n-capture elements. In contrast to the only other dSph star with similar abundance patterns, Dra 119, which has a very low metallicity at [Fe/H]=-2.95 dex, our objects, at [Fe/H]~-2.0 dex, are only moderately metal poor. The measured ratio of hydrostatic/explosive alpha-elements indicates that high-mass (~35 M_sun) Type II supernovae progenitors are the main, if not only, contributors to the enrichment of this galaxy. This suggests that star formation and chemical enrichment in the ultrafaint dSphs proceeds stochastically and inhomogeneously on small scales, or that the IMF was strongly skewed to high mass stars. The neutron capture deficiencies and the [Co/Fe] and [Cr/Fe] abundance ratios in our stars are similar to those in the extremely low metallicity Galactic halo. This suggests that either our stars are composed mainly of the ejecta from the first, massive, population III stars (but at moderately high [Fe/H]), or that SN ejecta in the Hercules galaxy were diluted with ~30 times less hydrogen than typical for extreme metal-poor stars.

astro-ph↗

arXiv2008

A spectroscopic confirmation of the Bootes II dwarf spheroidal

We present a new suite of photometric and spectroscopic data for the faint Bootes II dwarf spheroidal galaxy candidate. Our deep photometry, obtained with the INT/WFC, suggests a distance of 46 kpc and a small half-light radius of 4.0 arcmin (56 pc), consistent with previous estimates. Follow-up spectroscopy obtained with the Gemini/GMOS instrument yielded radial velocities and metallicities. While the majority of our targets covers a broad range in velocities and metallicities, we find five stars which share very similar velocities and metallicities and which are all compatible with the colors and magnitudes of the galaxy's likely red giant branch. We interpret these as a spectroscopic detection of the Bootes II system. These stars have a mean velocity of -117 km/s, a velocity dispersion of (10.5+-7.4) km/s and a mean [Fe/H] of -1.79 dex, with a dispersion of 0.14 dex. At this metallicity, Boo II is not consistent with the stellar-mass-metallicity relation for the more luminous dwarf galaxies. Coupled with our distance estimate, its high negative systemic velocity rules out any physical connection with its projected neighbor, the Bootes I dwarf spheroidal, which has a velocity of ~+100 km/s. The velocity and distance of Bootes II coincide with those of the leading arm of Sagittarius, which passes through this region of the sky, so that it is possible that Bootes II may be a stellar system associated with the Sagittarius stream. Finally, we note that the properties of Bootes II are consistent with being the surviving remnant of a previously larger and more luminous dSph galaxy.

astro-ph↗

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.

astro-ph↗

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

Complexity on Small Scales III: Iron and alpha Element Abundances in the Carina Dwarf Spheroidal Galaxy

We have obtained high-resolution spectroscopy of ten red giants in the Carina dwarf spheroidal (dSph) with UVES at the ESO/VLT. Here we present the abundances of O,Na,Mg,Si,Ca,Ti and Fe. By comparing the iron abundances [Fe/H] with calcium triplet (CaT) metallicities we show that the empirical CaT technique yields good agreement with the high-resolution data for [Fe/H]>-2 dex, but tends to deviate at lower metallicities. We identify two metal poor stars with iron abundances of -2.72 and -2.50 dex. These stars are found to have enhanced [alpha/Fe] ratios similar to those of stars in the Milky Way halo. However, the bulk of the Carina red giants are depleted in the [alpha/Fe] abundance ratios with respect to the Galactic halo at a given metallicity. One of our targets, with a [Fe/H] of -1.5 dex, is considerably depleted in almost all of the alpha-elements by ~0.5 dex compared to the solar values. Such a low [alpha/Fe] can be produced by stochastical fluctuations in terms of an incomplete mixing of single Type Ia and II SNe events into the ISM. Our derived element ratios are consistent with the episodic and extended SF in Carina known from its color-magnitude diagram. We find a considerable star-to-star scatter in the abundance ratios. This suggests that Carina's SF history varies with position within the galaxy, with incomplete mixing. Alternatively, the SF rate is so low that the high-mass stellar IMF is sparsely populated, as statistically expected in low-mass star clusters, leading to real scatter in the resultant mass-integrated yields. Both ideas are consistent with slow stochastic SF in dissolving associations, so that one may not speak of a single SF history at a detailed level (Abridged).

astro-ph↗

arXiv2008

An Accurate Age Determination for the SMC Star Cluster NGC121 with HST/ACS

As first Paper of a series devoted to study the old stellar population in clusters and fields in the Small Magellanic Cloud, we present deep observations of NGC121 in the F555W and F814W filters, obtained with the Advanced Camera for Surveys on the Hubble Space Telescope. The resulting color-magnitude diagram reaches ~3.5 mag below the main-sequence turn-off; deeper than any previous data. We derive the age of NGC121 using both absolute and relative age-dating methods. Fitting isochrones in the ACS photometric system to the observed ridge line of NGC121, gives ages of 11.8 +- 0.5 Gyr (Teramo), 11.2 +- 0.5 Gyr (Padova) and 10.5 +- 0.5 Gyr (Dartmouth). The cluster ridge line is best approximated by the alpha-enhanced Dartmouth isochrones. Placing our relative ages on an absolute age scale, we find ages of 10.9 +- 0.5 Gyr (from the magnitude difference between the main-sequence turn-off and the horizontal branch) and 11.5 +- 0.5 Gyr (from the absolute magnitude of the horizontal branch), respectively. These five different age determinations are all lower by 2 - 3 Gyr than the ages of the oldest Galactic globular clusters of comparable metallicity. Therefore we confirm the earlier finding that the oldest globular cluster in the Small Magellanic Cloud, NGC121, is a few Gyr younger than its oldest counterparts in the Milky Way and in other nearby dwarf galaxies such as the Large Magellanic Cloud, Fornax, and Sagittarius. If it were accreted into the Galactic halo, NGC121 would resemble the ''young halo globulars'', although it is not as young as the youngest globular clusters associated with the Sagittarius dwarf. The young age of NGC121 could result from delayed cluster formation in the Small Magellanic Cloud or result from the random survival of only one example of an initially small number star clusters.

astro-ph↗

arXiv2007

Stellar kinematics in the remote Leo II dwarf spheroidal galaxy -- Another brick in the wall

We present the projected velocity dispersion profile for the remote (d=233kpc) Galactic dwarf spheroidal (dSph) galaxy Leo II, based on 171 discrete stellar radial velocities that were obtained from medium-resolution spectroscopy using the FLAMES/GIRAFFE spectrograph at the European Southern Observatory, Chile. The dispersion profile of those stars with good membership probabilities is essentially flat with an amplitude of 6.6+-0.7 km/s over the full radial extent of our data, which probe to the stellar boundary of this galaxy. We find no evidence of any significant apparent rotation or velocity asymmetry which suggests that tidal effects cannot be invoked to explain Leo II's properties. From basic mass modeling, employing Jeans' equation, we derive a mass out to the limiting radius of (2.7+-0.5) 10^7 Msun and a global mass to light ratio of 27-45 in solar units, depending on the adopted total luminosity. A cored halo profile and a mild amount of tangential velocity anisotropy is found to account well for Leo II's observed kinematics, although we cannot exclude the possibility of a cusped halo with radially varying velocity anisotropy. All in all, this galaxy exhibits dark matter properties which appear to be concordant with the other dSph satellites of the Milky Way, namely a halo mass profile which is consistent with a central core and a total mass which is similar to the common mass scale seen in other dSphs.

astro-ph↗

arXiv2006

Stellar kinematics and metallicities in the Leo I dwarf spheroidal galaxy -- wide field implications for galactic evolution

We present low-resolution spectroscopy of 120 red giants in the Galactic satellite dwarf spheroidal (dSph) Leo I, obtained with the GeminiN-GMOS and Keck-DEIMOS spectrographs. We find stars with velocities consistent with membership of Leo I out to 1.3 King tidal radii. By measuring accurate radial velocities with a median measurement error of 4.6 km/s we find a mean systemic velocity of 284.2 km/s with a global velocity dispersion of 9.9 km/s. The dispersion profile is consistent with being flat out to the last data point. We show that a marginally-significant rise in the radial dispersion profile at a radius of 3' is not associated with any real localized kinematical substructure. Given its large distance from the Galaxy, tides are not likely to have affected the velocity dispersion, a statement we support from a quantitative kinematical analysis, as we observationally reject the occurrence of a significant apparent rotational signal or an asymmetric velocity distribution. Mass determinations adopting both isotropic stellar velocity dispersions and more general models yield a M/L ratio of 24, which is consistent with the presence of a significant dark halo with a mass of about 3x10^7 M_sun, in which the luminous component is embedded. This suggests that Leo I exhibits dark matter properties similar to those of other dSphs in the Local Group. Our data allowed us also to determine metallicities for 58 of the targets. We find a mildly metal poor mean of -1.31 dex and a full spread covering 1 dex. In contrast to the majority of dSphs, Leo I appears to show no radial gradient in its metallicities, which points to a negligible role of external influences in this galaxy's evolution.

astro-ph↗

arXiv2006

Complexity on Small Scales II: Metallicities and Ages in the Leo II Dwarf Spheroidal Galaxy

We present metallicities and ages for 52 red giants in the remote Galactic dwarf spheroidal (dSph) galaxy Leo II. These stars cover the entire surface area of Leo II and are radial velocity members. We obtained medium-resolution multi-fiber spectroscopy with ESO/VLT's FLAMES spectrograph. The metallicities were determined based on the near-infrared Ca II triplet. The resulting metallicity distribution (MD) is asymmetric and peaks at [Fe/H]=-1.74 dex on the Carretta & Gratton scale. The full range in metallicities extends from -2.4 to -1.1 dex. As in other dSphs, no extremely metal-poor red giants were found. We compare Leo II's observed MD with model predictions for several other Galactic dSphs from the literature. Leo II clearly exhibits a lack of more metal poor stars, in analogy to the classical G-dwarf problem, which may indicate a comparable `K-giant problem'. Moreover, its evolution appears to have been affected by galactic winds. We use our inferred metallicities as an input parameter for isochrone fits to SDSS photometry and derive approximate ages. The resulting age-metallicity distribution covers the full age range from 2-15 Gyr on our adopted isochrone scale. During the first 7 Gyr relative to the oldest stars [Fe/H] appears to have remained almost constant. The almost constant metallicity at higher ages and a slight drop by about 0.3 dex thereafter may be indicative of rejuvenation by low metallicity gas. Overall, the age-metallicity relation appears to support the formation of Leo II from pre-enriched gas. Evidence for enrichment is seen during the recent 2-4 Gyr. Our findings support earlier photometric findings of Leo II as a galaxy with a prominent old and a dominant intermediate-age population. We do not find a significant radial metallicity gradient nor age gradient in our data.(Abridged)

astro-ph↗

arXiv2006

The Age-Metallicity Relation of the SMC

The Small Magellanic Cloud (SMC) is the only dwarf galaxy in the Local Group known to have formed and preserved populous star clusters continuously over the past 12 Gyr. These clusters provide a unique, closely spaced set of single-age, single-metallicity tracers for a detailed study of the SMC's age-metallicity relation. Spectroscopic metallicity measurements, however, exist only for 6 of its clusters. Here we present metallicities for 7 additional SMC clusters based on Ca II triplet observations. The total sample comprises clusters spanning an age range of 12 Gyr. Complemented by age estimates from literature these objects provide us with a well-sampled, well-defined age-metallicity relation. We compare our first results with the chemical evolution history of the SMC in terms of a simple closed box model. We find that the general trend is well reproduced by this model.

astro-ph↗

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.

astro-ph↗

arXiv2006

Further Evidence for a Merger Origin for the Thick Disk: Galactic Stars Along Lines-of-sight to Dwarf Spheroidal Galaxies

The history of the Milky Way Galaxy is written in the properties of its stellar populations. Here we analyse stars observed as part of surveys of local dwarf spheroidal galaxies, but which from their kinematics are highly probable to be non-members. The selection function -- designed to target metal-poor giants in the dwarf galaxies, at distances of ~100kpc -- includes F-M dwarfs in the Milky Way, at distances of up to several kpc. Thestars whose motions are analysed here lie in the cardinal directions of Galactic longitude l ~ 270 and l ~ 90, where the radial velocity is sensitive to the orbital rotational velocity. We demonstrate that the faint F/G stars contain a significant population with V_phi ~ 100km/s, similar to that found by a targeted, but limited in areal coverage, survey of thick-disk/halo stars by Gilmore, Wyse & Norris (2002). This value of mean orbital rotation does not match either the canonical thick disk or the stellar halo. We argue that this population, detected at both l ~ 270 and l ~ 90, has the expected properties of `satellite debris' in the thick-disk/halo interface, which we interpret as remnants of the merger that heated a pre-existing thin disk to form the thick disk.

astro-ph↗

arXiv2005

The Anisotropic Distribution of M 31 Satellite Galaxies: A Polar Great Plane of Early-Type Companions

The highly anisotropic distribution and apparent alignment of the Galactic satellites in polar great planes begs the question how common such distributions are. The satellite system of M31 is the only nearby system for which we currently have sufficiently accurate distances to study the three-dimensional satellite distribution. We present the spatial distribution of the 15 presently known M31 companions in a coordinate system centered on M31 and aligned with its disk. Through a detailed statistical analysis we show that the full satellite sample describes a plane that is inclined by -56 deg with respect to the poles of M31 and that has an r.m.s. height of 100 kpc. With 88% the statistical significance of this plane is low and it is unlikely to have a physical meaning. The great stellar stream found near Andromeda is inclined to this plane by 7 deg. There is little evidence for a Holmberg effect. If we confine our analysis to early-type dwarfs, we find a best-fit polar plane within 5 deg to 7 deg from the pole of M31. This polar great plane has a statistical significance of 99.3% and includes all dSphs (except for And II), M32, NGC 147, and PegDIG. The r.m.s. distance of these galaxies from the polar plane is 16 kpc. The nearby spiral M33 has a distance of only about 3 kpc from this plane, which points toward the M81 group. We discuss the anisotropic distribution of M31's early-type companions in the framework of three scenarios, namely as remnants of the break-up of a larger progenitor, as tracer of a prolate dark matter halo, and as tracer of collapse along large-scale filaments. (Abridged)

astro-ph↗

arXiv2005

Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy

The Carina dwarf spheroidal (dSph) galaxy is the only galaxy of this type that shows clearly episodic star formation separated by long pauses. Here we present metallicities for 437 radial velocity members of this Galactic satellite. We obtained medium-resolution spectroscopy with the multi-object spectrograph FLAMES at the ESO VLT. Our target red giants cover the entire projected surface area of Carina. Our spectra are centered at the near-infrared Ca triplet, which is a well-established metallicity indicator for old and intermediate-age red giants. The resulting data sample provides the largest collection of spectroscopically derived metallicities for a Local Group dSph to date. Four of our likely radial velocity members of Carina lie outside of this galaxy's nominal tidal radius, supporting earlier claims of the possible existence of such stars beyond the main body of Carina. We find a mean metallicity of [Fe/H]=-1.7 dex. The formal full width at half maximum of the metallicity distribution is 0.92 dex, while the full range of metallicities spans ~-3.0<[Fe/H]<0.0 dex. The metallicity distribution might be indicative of several subpopulations. There appears to be a mild radial gradient such that more metal-rich populations are more centrally concentrated, matching a similar trend for an increasing fraction of intermediate-age stars. This as well as the photometric colors of the more metal-rich red giants suggest that Carina exhibits an age-metallicity relation. Indeed the age-metallicity degeneracy seems to conspire to form a narrow red giant branch despite the considerable spread in metallicity and wide range of ages. The metallicity distribution is not well-matched by a simple closed-box model of chemical evolution, but requires models that take into account also infall and outflows. (Abridged)

astro-ph↗

arXiv2005

Andromeda IX: Properties of the Smallest M 31 Dwarf Satellite Galaxy

We present WIYN observations of the recently discovered And IX dwarf spheroidal (dSph) satellite galaxy of M 31. Our data, obtained at a natural seeing of 0.5'' and just reaching the horizontal branch level, confirm And IX as a dSph galaxy with a distance similar to M 31. A survey for carbon stars shows no evidence for an intermediate-age (1--10 Gyr) stellar population in And IX. From the red giant branch we estimate a metallicity of roughly -2 dex. Combined with the tip of the red giant branch luminosity, this results in a distance of 735 kpc, placing And IX approximately 45 kpc from M 31. This faint dSph follows the relations between luminosity and metallicity, and luminosity and surface brightness defined by other Local Group dSph galaxies. The core and tidal radii are found to be 1.35' and 5.9', respectively. We conclude that And IX -- despite its low luminosity -- might be an ordinary Local Group dSph and discuss implications for its formation from a once more massive, but stripped progenitor or from an intrinsically low-mass seed.

astro-ph↗

arXiv2004

Chemical Evolution in the Carina Dwarf Spheroidal

We present metallicities for 487 red giants in the Carina dwarf spheroidal (dSph) galaxy that were obtained from FLAMES low-resolution Ca triplet (CaT) spectroscopy. We find a mean [Fe/H] of -1.91 dex with an intrinsic dispersion of 0.25 dex, whereas the full spread in metallicities is at least one dex. The analysis of the radial distribution of metallicities reveals that an excess of metal poor stars resides in a region of larger axis distances. These results can constrain evolutionary models and are discussed in the context of chemical evolution in the Carina dSph.

astro-ph↗