[show abstract][hide abstract] ABSTRACT: Non-axisymmetries in the Galactic potential (spiral arms and bar) induce
kinematic groups such as the Hercules stream. Assuming that Hercules is caused
by the effects of the Outer Lindblad Resonance of the Galactic bar, we model
analytically its properties as a function of position in the Galaxy and its
dependence on the bar's pattern speed and orientation. Using data from the RAVE
survey we find that the azimuthal velocity of the Hercules structure decreases
as a function of Galactocentric radius, in a manner consistent with our
analytical model. This allows us to obtain new estimates of the parameters of
the Milky Way's bar. The combined likelihood function of the bar's pattern
speed and angle has its maximum for a pattern speed of Omega_b=(1.89 +- 0.08) x
Omega_0 where Omega_0 is the local circular frequency. Assuming a Solar radius
of 8.05 kpc and a local circular velocity of 238 km/s, this corresponds to
Omega_b=56 +- 2 km/s/kpc. On the other hand, the bar's orientation phi_b cannot
be constrained with the available data. In fact, the likelihood function shows
that a tight correlation exists between the pattern speed and the orientation,
implying that a better description of our best fit results is given by the
linear relation Omega_b/Omega_0=1.905 + 0.0044[phi_b(deg)-47.7], with standard
deviation of 0.02. For example, for an angle of phi_b=30deg the pattern speed
is 54.0+-0.5km/s/kpc. These results are not very sensitive to the other
Galactic parameters such as the circular velocity curve or the peculiar motion
of the Sun, and are robust to biases in distance.
[show abstract][hide abstract] ABSTRACT: Non-axisymmetric components, such as spirals and central bars, play a major
role in shaping galactic discs. An important aspect of the disc secular
evolution driven by these perturbers is the radial migration of stars. It has
been suggested recently that migration can populate a thick-disc component from
inner-disc stars with high vertical energies. Since this has never been
demonstrated in simulations, we study in detail the effect of radial migration
on the disc velocity dispersion and disc thickness, by separating simulated
stars into migrators and non-migrators. We apply this method to three isolated
barred Tree-SPH N-body galaxies with strong radial migration. Contrary to
expectations, we find that as stellar samples migrate, on the average, their
velocity dispersion change (by as much as 50%) in such a way as to
approximately match the non-migrating population at the radius at which they
arrive. We show that, in fact, migrators suppress heating in parts of the disc.
To confirm the validity of our findings, we also apply our technique to three
cosmological re-simulations, which use a completely different simulation scheme
and, remarkably, find very similar results. We believe the inability of
migration to thicken discs is a fundamental property of internal disc
evolution, irrespective of the migration mechanism at work. We explain this
with the approximate conservation of the (average) vertical and radial actions
rather than the energy. This "action mixing" can be used to constrain the
migration rate in the Milky Way: estimates of the average vertical action in
observations for different populations of stars should reveal flattening with
radius for older groups of stars.
Astronomy and Astrophysics 05/2012; · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: Disc non-axisymmetrc components, such as spirals and central bars, are
nowadays known to play an important role in shaping galactic discs. Here we use
Tree-SPH N-body simulations to examine the effect of these perturbers on two
aspects: the occurrence of multiple patterns in discs and the effects of radial
migration on disc thickening. We find that, in addition to a central bar,
multiple spiral patterns and lopsided modes develop in all models. Interaction
among these asymmetric features results in a large scale stellar migration.
However, we show that, despite the strong radial mixing, discs cannot be
thickened sufficiently to match observed thick discs. We relate this to the
adiabatic cooling as stars migrate radially outwards. We also find that the
bulge contribution to a thick-disc component for an Sa-type galaxy at ~2.5 disc
scale-lengths is less than 1% and zero in the case of a Milky Way-like,
Sb-type. Our findings cast doubt on the plausibility of thick disc formation
via stellar radial migration.
[show abstract][hide abstract] ABSTRACT: We follow the dynamical and chemical evolution of isolated systems tuned in their properties on selfgravitating spherical three component (gas+stars+dark matter) dwarf spheroidal galaxies (dSph). The system starts as a strongly dark matter dominated cuspy density profile. It is shown that after few Gyr of evolution the star formation processes are naturally able to reshape the cuspy dark matter profile into a flatter one. The same family of models is then evolved by orbiting in a Milky Way external potential in order to investigate the history of the Carina dwarf galaxy. Structural parameters and the star formation history of Carina are self-consistently determined in good agreement with the observations.
[show abstract][hide abstract] ABSTRACT: The chemodynamical evolution of spherical multi-component self-gravitating models for isolated dwarf galaxies is studied. We compared their evolution with and without feedback effects from star formation processes. We found that initially cuspy dark matter profiles flatten with time without any special tuning conditions as a result of star formation. Thus the seemingly flattened profiles found in many dwarfs do not contradict the cuspy profiles predicted by cosmological models. We also calculated the chemical evolution of stars and gas, to permit comparisons with observational data.
Astronomy and Astrophysics 01/2010; · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report the discovery of two well-defined tidal tails emerging from the sparse remote globular cluster Palomar 5. These tails stretch out symmetrically to both sides of the cluster in the direction of constant Galactic latitude and subtend an angle of 26 on the sky. The tails have been detected in commissioning data of the Sloan Digital Sky Survey, providing deep five-color photometry in a 25-wide band along the equator. The stars in the tails make up a substantial part (~) of the current total population of cluster stars in the magnitude interval 19.5 ≤ i* ≤ 22.0. This reveals that the cluster is subject to heavy mass loss. The orientation of the tails provides an important key for the determination of the cluster's Galactic orbit.
The Astrophysical Journal 12/2008; 548(2):L165. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the second data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities (RVs) and stellar atmosphere parameters of up to one million stars using the 6dF multi-object spectrograph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO). It is obtaining medium resolution spectra (median R=7,500) in the Ca-triplet region (8,410--8,795 \AA) for southern hemisphere stars in the magnitude range 9<I<12. Following the first data release (Steinmetz et al. 2006) the current release doubles the sample of published RVs, now containing 51,829 RVs for 49,327 individual stars observed on 141 nights between April 11 2003 and March 31 2005. Comparison with external data sets shows that the new data collected since April 3 2004 show a standard deviation of 1.3 km/s, about twice better than for the first data release. For the first time this data release contains values of stellar parameters from 22,407 spectra of 21,121 individual stars. They were derived by a penalized \chi^2 method using an extensive grid of synthetic spectra calculated from the latest version of Kurucz models. From comparison with external data sets, our conservative estimates of errors of the stellar parameters (for a spectrum with S/N=40) are 400 K in temperature, 0.5 dex in gravity, and 0.2 dex in metallicity. We note however that the internal errors estimated from repeat RAVE observations of 822 stars are at least a factor 2 smaller. We demonstrate that the results show no systematic offsets if compared to values derived from photometry or complementary spectroscopic analyses. The data release includes proper motion and photometric measurements. It can be accessed via the RAVE webpage: http://www.rave-survey.org and through CDS. Comment: 85 pages, 23 figures, 14 tables, accepted for publication in the Astronomical Journal
The Astronomical Journal 06/2008; · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: The tidal tails of the globular cluster Palomar 5 are analyzed over a 41 deg2 area of the Sloan Digital Sky Survey photometric catalogs. The matched filter algorithm provides the maximum possible signal-to-noise detection of the cluster stars over the measured background, and the expected and actual effectiveness of the technique in the context of this data set is discussed. The stellar background is examined in some detail for systematic variation as a function of Galactic position in order to assess its effect on the detection efficiency. Of the total number of Pal 5 stars detected, 45% are out in the tails. The tails are found as the only additional 3 σ overdensity of cluster stars over the entire 41 deg2 area studied. The annular-averaged density of stars along the tails is fitted to a power law in radius with best-fit index -1.58 ± 0.07, significantly steeper than that predicted from a constant orbit-averaged mass-loss rate.
The Astronomical Journal 12/2007; 124(1):349. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have investigated the spatial extent and structure of the Draco dwarf spheroidal galaxy by using deep wide-field multicolor CCD photometry from the Sloan Digital Sky Survey (SDSS). Our study covers an area of 27 deg2 around the center of the Draco dwarf and reaches 2 mag below the level of Draco's horizontal branch. The SDSS photometry allows very effective filtering in color-magnitude space. With such filtering the density of the foreground of Galactic field stars is decreased by more than an order of magnitude, and the stellar population of the Draco dwarf galaxy thus stands out with much higher contrast than in former investigations. We show that the spatial distribution of Draco's red giants, red horizontal-branch stars, and subgiants down to i* = 21.7 mag does not provide evidence for the existence of tidally induced tails or a halo of unbound stars. The projected surface density of the dwarf galaxy is flattened with a nearly constant ellipticity of 0.29 ± 0.02 at position angle 88° ± 3°. The radial profile can be fitted by King models, as well as by a generalized exponential. Using the empirical King profile, the core radius and the limiting (or tidal) radius along the major axis are rc = 77 and rt = 401, respectively; the latter means that the size of the Draco dwarf galaxy is 40% larger than previously measured. Fitting the profile of King's theoretical models yields a still larger limiting radius of rt = 495. There is no clear indication of a taillike extension of the Draco population beyond this radius. A break in the radial surface density profile, which might indicate a halo of extratidal stars, is also not found in our Draco data. We conclude that down to the above magnitude limit tidal effects can exist only at a level of 10-3 of the central surface density of Draco or below. The regular structure of Draco found from the new data argues against its being a portion of an unbound tidal stream and lends support to the assumption of dynamical equilibrium, which is the basis for mass estimation. The changes in the values for the core radius and limiting radius imply that the total mass of Draco is higher by more than a factor of 2. Using a King spherical model of equivalent size as a reference and adopting a line-of-sight velocity dispersion of either 10.7 or 8.5 km s-1, we derive estimates of the total mass within radius rt of (3.5 ± 0.7) × 107 M and (2.2 ± 0.5) × 107 M, respectively. From the combined i-band flux of all possible Draco members that lie within major-axis radius rt we determine the total luminosity of the Draco dwarf galaxy as (L/L)i = (2.4 ± 0.5) × 105. This includes corrections for the flux of the foreground stars and the unseen fainter part of the Draco population. We thus obtain overall mass-to-light ratios M/Li of 146 ± 42, or 92 ± 28 in solar units. In summary, our results strengthen the case for a strongly dark matter–dominated, bound stellar system.
The Astronomical Journal 12/2007; 122(5):2538. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Using wide-field photometric data from the Sloan Digital Sky Survey (SDSS), we recently showed that the Galactic globular cluster Palomar 5 is in the process of being tidally disrupted. Its tidal tails were initially detected in a 25 wide band along the celestial equator. A new analysis of SDSS data for a larger field now reveals that the tails of Pal 5 have a much larger spatial extent and can be traced over an arc of 10° on the sky, corresponding to a projected length of 4 kpc at the distance of the cluster. The tail that trails behind the Galactic motion of the cluster fades into the field at an angular distance of 65 from the cluster center but shows a pronounced density maximum between 2° and 4° from the center. The leading tail, of length 35, extends down to the border of the available field and thus presumably continues beyond it. The projected width of these tails is small and almost constant (FWHM ~ 120 pc), which implies that they form a dynamically cold and hence long-lived structure. The number of former cluster stars found in the tails adds up to about 1.2 times the number of stars in the cluster, that is, the tails are more massive than the cluster in its present state. The radial profile of stellar surface density in the tails approximately follows a power law rγ with -1.5 ≤ γ ≤ -1.2. The stream of debris from Pal 5 is significantly curved, which demonstrates its acceleration by the Galactic potential. The stream sets tight constraints on the geometry of the cluster's Galactic orbit. We conclude that the cluster is presently near the apocenter but has repeatedly undergone disk crossings in the inner part of the Galaxy leading to strong tidal shocks. Using the spatial offset between the tails and the cluster's orbit, we estimate the mean drift rate of the tidal debris and thus the mean mass-loss rate of the cluster. Our results suggest that the observed debris originates mostly from mass loss within the last 2 Gyr. The cluster is likely to be destroyed after the next disk crossing, which will happen in about 100 Myr. There is strong evidence against the suggestion that Pal 5 might be associated with the Sagittarius dwarf galaxy.
The Astronomical Journal 12/2007; 126(5):2385. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report new constraints on the local escape speed of our Galaxy. Our analysis is based on a sample of high-velocity stars from the RAVE survey and two previously published data sets. We use cosmological simulations of disc galaxy formation to motivate our assumptions on the shape of the velocity distribution, allowing for a significantly more precise measurement of the escape velocity compared to previous studies. We find that the escape velocity lies within the range 498 < vesc < 608 km s−1 (90 per cent confidence), with a median likelihood of 544 km s−1. The fact that v2esc is significantly greater than 2v2circ (where vcirc= 220 km s−1 is the local circular velocity) implies that there must be a significant amount of mass exterior to the solar circle, that is, this convincingly demonstrates the presence of a dark halo in the Galaxy. We use our constraints on vesc to determine the mass of the Milky Way halo for three halo profiles. For example, an adiabatically contracted NFW halo model results in a virial mass of 1.42+1.14−0.54× 1012 M⊙ and virial radius of (90 per cent confidence). For this model the circular velocity at the virial radius is 142+31−21 km s−1. Although our halo masses are model dependent, we find that they are in good agreement with each other.
Monthly Notices of the Royal Astronomical Society 07/2007; 379(2):755 - 772. · 5.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the first data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, surface gravity) of up to one million stars using the 6dF multi-object spectrograph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO). The RAVE program started in 2003, obtaining medium resolution spectra (median R=7,500) in the Ca-triplet region ($\lambda\lambda$ 8,410--8,795 \AA) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9<I<12. The first data release is described in this paper and contains radial velocities for 24,748 individual stars (25,274 measurements when including re-observations). Those data were obtained on 67 nights between 11 April 2003 to 03 April 2004. The total sky coverage within this data release is $\sim$4,760 square degrees. The average signal to noise ratio of the observed spectra is 29.5, and 80% of the radial velocities have uncertainties better than 3.4 km/s. Combining internal errors and zero-point errors, the mode is found to be 2 km/s. Repeat observations are used to assess the stability of our radial velocity solution, resulting in a variance of 2.8 km/s. We demonstrate that the radial velocities derived for the first data set do not show any systematic trend with color or signal to noise. The RAVE radial velocities are complemented in the data release with proper motions from Starnet 2.0, Tycho-2 and SuperCOSMOS, in addition to photometric data from the major optical and infrared catalogs (Tycho-2, USNO-B, DENIS and 2MASS). The data release can be accessed via the RAVE webpage: http://www.rave-survey.org.
The Astronomical Journal 07/2006; · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: The globular cluster Palomar 5 is remarkable not only because of its extended massive tidal tails, but also for its very low mass and velocity dispersion, and its size, which is much larger than its theoretical tidal radius. In order to understand these extreme properties, we performed more than 1000 N-body simulations of clusters traversing the Milky Way on the orbit of Pal 5. Tidal shocks at disk crossings near perigalacticon dominate the evolution of extended low-concentration clusters, resulting in massive tidal tails and often in a quick destruction of the cluster. The very large size of Pal 5 can be explained as the result of an expansion following the heating induced by the last strong disk shock ~150 Myr ago. Some of the models can reproduce the low observed velocity dispersion and the relative fractions of stars in the tails and between the inner and outer parts of the tails. Our simulations illustrate to what extent the observable tidal tails trace out the orbit of the parent object. The tidal tails of Pal 5 show substantial structure not seen in our simulations. We argue that this structure is probably caused by Galactic substructure, such as giant molecular clouds, spiral arms, and dark matter clumps, which was ignored in our modeling. Clusters initially larger than their theoretical tidal limit remain so, because after being shocked, they settle into a new equilibrium near apogalacticon where they are unaffected by the perigalactic tidal field. This implies that, contrary to previous wisdom, globular clusters on eccentric orbits may well remain supertidally limited and hence vulnerable to strong disk shocks, which dominate their evolution until destruction. Our simulations unambiguously predict the destruction of Pal 5 at its next disk crossing in ~110 Myr. This corresponds to only 1% of the cluster lifetime, suggesting that many more similar systems could once have populated the inner parts of the Milky Way but have been transformed into debris streams by the Galactic tidal field.
The Astronomical Journal 01/2004; · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We detected extended, curved stellar tidal tails emanating from the sparse, disrupting halo globular cluster Pal 5, which cover 10 degrees on the sky. These streams allow us to infer the orbit of Pal 5 and to ultimately constrain the Galactic potential at its location.
[show abstract][hide abstract] ABSTRACT: Sloan Digital Sky Survey data for the field of the globular cluster Pal 5 reveal the existence of a long massive stream of tidal debris spanning an arc of 10 degrees on the sky. Pal 5 thus provides an outstanding example for tidal disruption of globular clusters in the Milky Way. Radial velocities from VLT spectra show that Pal 5 has an extremely low velocity dispersion, in accordance with the very low mass derived from its total luminosity.
[show abstract][hide abstract] ABSTRACT: Wide-field photometric data from the Sloan Digital Sky Survey have recently revealed that the Galactic globular cluster Palomar 5 is in the process of being tidally disrupted (Odenkirchen et al. 2001). Here we investigate the kinematics of this sparse remote star cluster using high resolution spectra from the Very Large Telescope (VLT). Twenty candidate cluster giants located within 6 arcmin of the cluster center have been observed with the UV-Visual Echelle Spectrograph (UVES) on VLT-UT2. The spectra provide radial velocities with a typical accuracy of 0.15 km/s. We find that the sample contains 17 certain cluster members with very coherent kinematics, two unrelated field dwarfs, and one giant with a deviant velocity, which is most likely a cluster binary showing fast orbital motion. From the confirmed members we determine the heliocentric velocity of the cluster as -58.7 +- 0.2 km/s. The total line-of-sight velocity dispersion of the cluster stars is 1.1 +- 0.2 km/s (all members) or 0.9 +- 0.2 km/s (stars on the red giant branch only). This is the lowest velocity dispersion that has so far been measured for a stellar system classified as a globular cluster. The shape of the velocity distribution suggests that there is a significant contribution from orbital motions of binaries and that the dynamical part of the velocity dispersion is therefore still substantially smaller than the total dispersion. ... (abridged) Comment: 29 pages including 10 figures, accepted for publication in the Astronomical Journal
The Astronomical Journal 06/2002; · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have analysed commissioning data of the Sloan Digital Sky Survey (SDSS) in a 2.5o wide equatorial band in order to search for tidal debris around the distant halo globular cluster Palomar 5. SDSS provides deep multi-color photometry in the u',g',r',i' and z' bands. These data have been used to locate the cluster population in multi-color-magnitude space and to efficiently separate extratidal cluster stars from the field population by means of a cell-count algorithm. The tidal extension of the cluster is traced by stars from about 0.5 mag above to 2 mag below the cluster main-sequence turn-off. We find clear evidence for a symmetrical two-sided tail which extends at least 1.5o in north-eastern and south-western direction and is likely to continue further on beyond the edges of the currently available field. The orientation of the tail which is along constant galactic latitude provides an efficient constraint on the direction of the clusters tangential space motion. The spatial structure of the tail closely resembles the characteristic structures seen in numerically simulated cluster tails. Integrating the overdensity of stars in the region of the tail we find that the extra-tidal part of the cluster population equals about 50% of the current stellar content inside the cluster in the same magnitude range. We conclude that the cluster has experienced very heavy mass-loss during the last passages through the inner Galaxy.
[show abstract][hide abstract] ABSTRACT: The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of ~106 galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg2 of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux- and wavelength-calibrated, with 4096 pixels from 3800 to 9200 Å at R~1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.
The Astronomical Journal, v.123, 485-548 (2002). 01/2002;