The STREGA survey – II. Globular cluster Palomar 12★ (original) (raw)
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Palomar 13: An Unusual Stellar System in the Galactic Halo
The Astrophysical Journal, 2002
We report the first results of a program to study the internal kinematics of globular clusters in the outer halo of the Milky Way. Using the Keck telescope and High Resolution Echelle Spectrometer, we have measured precise radial velocities for 30 candidate red giants in the direction of Palomar 13, an object traditionally cataloged as a compact, low-luminosity globular cluster. We have combined these radial velocities with published proper motion membership probabilities and new CCD photometry from the Keck and Canada-France-Hawaii telescopes, to isolate a sample of 21 probable members. We find a systemic velocity of v r s = 24.1±0.5 km s −1 and a projected, intrinsic velocity dispersion of σ p = 2.2±0.4 km s −1 . Although small, this dispersion is nevertheless several times larger than that expected for a globular cluster of this luminosity and central concentration. Taken at face value, this dispersion implies a mass-to-light ratio of Υ V = 40 +24 −17 based on the best-fit King-Michie model. The surface density profile of Palomar 13 also appears to be anomalous among Galactic globular clusters -depending upon the details of background subtraction and model-fitting, Palomar 13 either contains a substantial population of "extra-tidal" stars, or it is far more spatially extended than previously suspected. The full surface density profile is equally well-fit by a King-Michie model having a high concentration and large tidal radius, or by a NFW model. We examine -and tentatively reject -a number of possible explanations for the observed characteristics of Palomar 13 (e.g., velocity "jitter" among the red giants, spectroscopic binary stars, non-standard mass functions, modified Newtonian dynamics), and conclude that the two most plausible scenarios are either catastrophic heating during a recent perigalacticon passage, or the presence of a massive dark halo. Thus, the available evidence suggests that Palomar 13 is either a globular cluster which is now in the process of dissolving into the Galactic halo, or a faint, dark-matter-dominated stellar system.
Remnants of the Sagittarius Dwarf Spheroidal Galaxy around the Young Globular Cluster Palomar 12
Astrophysical Journal, 2002
Photometry of a large field around the young globular cluster Palomar 12 has revealed the main-sequence of a low surface-brightness stellar system. This main-sequence is indicative of a stellar population that varies significantly in metallicity and/or age, but in the mean is more metal poor than Pal 12. Under different assumptions for the properties of this population, we find distances from the Sun in the range 17-24 kpc, which encompasses the distance to Pal 12, 19.0pm0.919.0\pm0.919.0pm0.9 kpc. The stellar system is also detected in a field 2$\arcdeg$ North of Pal 12, which indicates it has a minimum diameter of sim0.9\sim0.9sim0.9 kpc. The orbit of Pal 12 (Dinescu et al. 2000), the color-magnitude diagram of the stellar system, their positions on the sky, and their distances suggest that they are debris from the tidal disruption of the Sgr dSph galaxy. We discuss briefly the implications for the evolution of Sgr and the Galactic halo.
Astronomical Journal, 2004
We present moderate resolution (~6 km/s) spectroscopy of 284 M giant candidates selected from the Two Micron All Sky Survey photometry. Radial velocities (RVs) are presented for stars mainly in the south, with a number having positions consistent with association to the trailing tidal tail of the Sagittarius (Sgr) dwarf galaxy. The latter show a clear RV trend with orbital longitude, as expected from models of the orbit and destruction of Sgr. A minimum 8 kpc width of the trailing stream about the Sgr orbital midplane is implied by verified RV members. The coldness of this stream (dispersion ~10 km/s) provides upper limits on the combined contributions of stream heating by a lumpy Galactic halo and the intrinsic dispersion of released stars, which is a function of the Sgr core mass. The Sgr trailing arm is consistent with a Galactic halo containing one dominant, LMC-like lump, however some lumpier halos are not ruled out. An upper limit to the total M/L of the Sgr core is 21 in solar units. A second structure that roughly mimics expectations for wrapped, leading Sgr arm debris crosses the trailing arm in the Southern Hemisphere; however, this may also be an unrelated tidal feature. Among the <13 kpc M giants toward the South Galactic Pole are some with large RVs that identify them as halo stars, perhaps part of the Sgr leading arm near the Sun. The positions and RVs of Southern Hemisphere M giants are compared with those of southern globular clusters potentially stripped from the Sgr system and support for association of Pal 2 and Pal 12 with Sgr debris is found. Our discussion includes description of a masked-filtered cross-correlation methodology that achieves better than 1/20 of a resolution element RVs in moderate resolution spectra.
Tracing the Sgr Stream with 2MASS. Detection of Stream stars around Outer Halo globular clusters
Astronomy & Astrophysics, 2003
We use infrared Color Magnitude Diagrams from the 2-Micron All-Sky Survey (2MASS) to search for stars belonging to the tidal stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) around selected Galactic globular clusters. Statistically significant detections are presented for the cases of Pal 12 and NGC 4147, strongly supporting the idea that these clusters are associated with the Sgr
GLOBULAR CLUSTER STREAMS AS GALACTIC HIGH-PRECISION SCALES—THE POSTER CHILD PALOMAR 5
The Astrophysical Journal, 2015
Using the example of the tidal stream of the Milky Way globular cluster Palomar 5 (Pal 5), we demonstrate how observational data on tidal streams can be efficiently reduced in dimensionality and modeled in a Bayesian framework. Our approach combines detection of stream overdensities by a Difference-of-Gaussians process with fast streakline models of globular cluster streams and a continuous likelihood function built from these models. Inference is performed with Markov chain Monte Carlo. By generating ≈ 10 7 model streams, we show that the unique geometry of the Pal 5 debris yields powerful constraints on the solar position and motion, the Milky Way and Pal 5 itself. All 10 model parameters were allowed to vary over large ranges without additional prior information. Using only readily-available SDSS data and a few radial velocities from the literature, we find that the distance of the Sun from the Galactic Center is 8.30 ± 0.25 kpc, and the transverse velocity is 253 ± 16 km s −1 . Both estimates are in excellent agreement with independent measurements of these two quantities. Assuming a standard disk and bulge model, we determine the Galactic mass within Pal 5's apogalactic radius of 19 kpc to be (2.1 ± 0.4) × 10 11 M . Moreover, we find the potential of the dark halo with a flattening of q z = 0.95 +0.16 −0.12 to be essentially spherical -at least within the radial range that is effectively probed by Pal 5. We also determine Pal 5's mass, distance and proper motion independently from other methods, which enables us to perform vital cross-checks. Our inferred heliocentric distance of Pal 5 is 23.6 +0.8 −0.7 kpc, in perfect agreement with, and more precise than estimates from isochrone fitting of deep HST imaging data. We conclude that finding and modeling more globular cluster streams is an efficient way for mapping out the structure of our Galaxy to high precision. With more observational data and by using additional prior information, the precision of this mapping can be significantly increased.
Tracing out the Northern Tidal Stream of the Sagittarius Dwarf Spheoridal Galaxy
2003
The main aim of this paper is to report two new detections of tidal debris in the northern stream of the Sagittarius dwarf galaxy located at 45 arcdeg and 55 arcdeg from the center of galaxy. Our observational approach is based on deep color-magnitude diagrams, that provides accurate distances, surface brightness and the properties of stellar population of the studied region of this tidal stream. The derived distances for these tidal debris wraps are 45 kpc and 54 kpc respectively.We also confirm these detections with numerical simulations of the Sagittarius dwarf plus the Milky Way. The model reproduces the present position and velocity of the Sagittarius main body and presents a long tidal stream formed by tidal interaction with the Milky Way potential. This model is also in good agreement with the available observations of the Sagittarius tidal stream. We also present a method for estimating the shape of the Milky Way halo potential using numerical simulations. From our simulations we obtain an oblateness of the Milky Way dark halo potential of 0.85, using the current database of distances and radial velocities of the Sagittarius tidal stream. The color-magnitude diagram of the apocenter of Sagittarius shows that this region of the stream shares the complex star formation history observed in the main body of the galaxy. We present the first evidence for a gradient in the stellar population along the stream, possibly correlated with its different pericenter passages. (abridged)
Astrophysical Journal, 2001
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 2.6 • on the sky. The tails have been detected in commissioning data of the Sloan Digital Sky Survey (SDSS), providing deep five-color photometry in a 2.5 • wide band along the equator. The stars in the tails make up a substantial part (∼ 1/3) 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, 2012
We present three-dimensional kinematics of Sagittarius (Sgr) trailing tidal debris in six fields located 70-130 • along the stream from the Sgr dwarf galaxy core. The data are from our proper-motion (PM) survey of Kapteyn's Selected Areas, in which we have measured accurate PMs to faint magnitudes in ∼ 40 ′ × 40 ′ fields evenly spaced across the sky. The radial velocity (RV) signature of Sgr has been identified among our followup spectroscopic data in four of the six fields and combined with mean PMs of spectroscopically-confirmed members to derive space motions of Sgr debris based on ∼15-64 confirmed stream members per field. These kinematics are compared to predictions of the Law & Majewski (2010a) model of Sgr disruption; we find reasonable agreement with model predictions in RVs and PMs along Galactic latitude. However, an upward adjustment of the Local Standard of Rest velocity (Θ LSR ) from its standard 220 km s −1 to at least 232 ± 14 km s −1 (and possibly as high as 264 ± 23 km s −1 ) is necessary to bring 3-D model debris kinematics and our measurements into agreement. Satisfactory model fits that simultaneously reproduce known position, distance, and radial velocity trends of the Sgr tidal streams, while significantly increasing Θ LSR , could only be achieved by increasing the Galactic bulge and disk mass while leaving the dark matter halo fixed to the best-fit values from Law & Majewski (2010a). We derive low-resolution spectroscopic abundances along this stretch of the Sgr stream and find a constant [Fe/H] ∼ -1.15 (with ∼ 0.5 dex scatter in each field -typical for dwarf galaxy populations) among the four fields with reliable measurements. A constant metallicity suggests that debris along the ∼ 60 • span of this study was all stripped from Sgr on the same orbital passage.
Unveiling palomar 2: The most obscure globular cluster in the outer halo
The Astronomical Journal, 1997
We present the first color-magnitude study for Palomar 2, a distant and heavily obscured globular cluster near the Galactic anticenter. Our (V, V − I) color-magnitude diagram (CMD), obtained with the UH8K camera at the CFHT, reaches V (lim) ≃ 24 and clearly shows the principal sequences of the cluster, though with substantial overall foreground absorption and differential reddening. The CMD morphology shows a well populated red horizontal branch with a sparser extension to the blue, similar to clusters such as NGC 1261, 1851, or 6229 with metallicities near [Fe/H] ≃ −1.3. From an average of several indicators, we estimate the foreground reddening at E(B − V ) = 1.24 ± 0.07 and obtain a true distance modulus (m − M) 0 = 17.1 ± 0.3, placing it ≃ 34 kpc from the Galactic center. We use starcounts of the bright stars to measure the core radius, half-mass radius, and central concentration of the cluster. Its integrated luminosity is M t V ≃ −7.9, making it clearly brighter and more massive than most other clusters in the outer halo. Very rough arguments based on its half-mass radius and radial velocity suggest that Palomar 2 is now moving in toward perigalacticon on a highly elliptical orbit (e ∼ > 0.7).