Jay Strader

Extragalactic Globular Clusters and Galaxy Formation

Globular cluster (GC) systems have now been studied in galaxies ranging from dwarfs to giants and spanning the full Hubble sequence of morphological types. Imaging and spectroscopy with the Hubble Space Telescope and large ground-based telescopes have together established that most galaxies have bimodal color distributions that reflect two subpopulations of old GCs: metal-poor and metal-rich. The characteristics of both subpopulations are correlated with those of their parent galaxies. We argue that metal-poor GCs formed in low-mass dark matter halos in the early universe and that their properties reflect biased galaxy assembly. The metal-rich GCs were born in the subsequent dissipational buildup of their parent galaxies and their ages and abundances indicate that most massive early-type galaxies formed the bulk of their stars at early times. Detailed studies of both subpopulations offer some of the strongest constraints on hierarchical galaxy formation that can be obtained in the near-field.

The N2K Consortium. II. A Transiting Hot Saturn around HD 149026 with a Large Dense Core

Doppler measurements from Subaru and Keck have revealed radial velocity variations in the V = 8.15, G0 IV star HD 149026 consistent with a Saturn-mass planet in a 2.8766 day orbit. Photometric observations at Fairborn Observatory have detected three complete transit events with depths of 0.003 mag at the predicted times of conjunction. HD 149026 is now the second-brightest star with a transiting extrasolar planet. The mass of the star, based on interpolation of stellar evolutionary models, is 1.3 ± 0.1 M_☉; together with the Doppler amplitude K_1 = 43.3 m s^(-1), we derive a planet mass M sin i = 0.36M_J and orbital radius 0.042 AU. HD 149026 is chromospherically inactive and metal-rich with spectroscopically derived [Fe/H] = +0.36, T_(eff) = 6147 K, log g = 4.26, and v sin i = 6.0 km s^(-1). Based on T_(eff) and the stellar luminosity of 2.72 L_☉, we derive a stellar radius of 1.45 R_☉. Modeling of the three photometric transits provides an orbital inclination of 85o.3 ± 1o.0 and (including the uncertainty in the stellar radius) a planet radius of (0.725 ± 0.05)R_J. Models for this planet mass and radius suggest the presence of a ~67 M_⊕ core composed of elements heavier than hydrogen and helium. This substantial planet core would be difficult to construct by gravitational instability.

The least-luminous galaxy: Spectroscopy of the milky way satellite Segue 1

We present Keck/DEIMOS spectroscopy of Segue 1, an ultra-low-luminosity (M_V = –1.5^(+0.6)_(–0.8)) Milky Way satellite companion. While the combined size and luminosity of Segue 1 are consistent with either a globular cluster or a dwarf galaxy, we present spectroscopic evidence that this object is a dark matter-dominated dwarf galaxy. We identify 24 stars as members of Segue 1 with a mean heliocentric recession velocity of 206 ± 1.3 km s^(–1). Although Segue 1 spatially overlaps the leading arm of the Sagittarius stream, its velocity is 100 km s^(–1) different from that predicted for recent Sagittarius tidal debris at this position. We measure an internal velocity dispersion of 4.3 ± 1.2 km s^(–1). Under the assumption that these stars are gravitationally bound and in dynamical equilibrium, we infer a total mass of 4.5^(+4.7)_(–2.5) × 10^5 M_☉ in the mass-follow-light case; using a two-component maximum-likelihood model, we determine a mass within 50 pc of 8.7^(+13)_(–5.2) × 10^5 M_☉ . These imply mass-to-light (M/L) ratios of ln(M/L_V ) = 7.2^(+1.1)_(–1.2) (M/L_V = 1320^(+2680)_(–940)) and M/L_V = 2440^(+1580)_(–1775), respectively. The error distribution of the M/L is nearly lognormal, thus Segue 1 is dark matter-dominated at a high significance. Although we cannot rule out the possibility that Segue 1 has been tidally disrupted, we do not find kinematic evidence supporting tidal effects. Using spectral synthesis modeling, we derive a metallicity for the single red giant branch star in our sample of [Fe/H] = –3.3 ± 0.2 dex. Finally, we discuss the prospects for detecting gamma rays from annihilation of dark matter particles and show that Segue 1 is the most promising satellite for indirect dark matter detection. We conclude that Segue 1 is the least luminous of the ultra-faint galaxies recently discovered around the Milky Way, and is thus the least-luminous known galaxy.

The N2K Consortium. IV. New Temperatures and Metallicities for More than 100,000 FGK Dwarfs

We have created specialized target lists for radial velocity surveys that are biased toward stars that (1) possess planets and (2) are easiest to observe with current detection techniques. We use a procedure that uniformly estimates fundamental stellar properties of Tycho 2 stars, with errors, using spline functions of broadband photometry and proper motion found in Hipparcos/Tycho 2 and 2MASS. We provide estimates of Teff and distance for 2.4 × 106 Tycho 2 stars that lack trigonometric distances. For stars that appear to be FGK dwarfs, we also derive [Fe/H] and identify unresolved binary systems with mass ratios 1.25 < M1/M2 < 3.0. For FGK dwarfs with photometric error σV < 0.05, or V < 9, our temperature model gives a 1 σ error of σT = +58.7/ - 65.9 K and our metallicity model gives a 1 σ error of σ[Fe/H] = +0.13/ - 0.14 dex. The binarity model can be used to remove 70% of doubles with 1.25 < M1/M2 < 3.0 from a magnitude-limited sample of dwarfs at a cost of cutting 20% of the sample. Our estimates of distance and spectral type enable us to isolate 354,822 Tycho 2 dwarfs, 321,996 absent from Hipparcos, with giant contamination of 2.6% and 7.2%, respectively. Roughly 100,000 of these stars, not in Hipparcos, have sufficiently low photometric errors to retain 0.13-0.3 dex [Fe/H] accuracy and 80-100 K temperature accuracy (1 σ). Our metallicity estimates have been used to identify targets for N2K, a large-scale radial velocity search for hot jupiters, which has verified the errors presented here. The catalogs that we publish can be used to further large-scale studies of Galactic structure and chemical evolution and to provide potential reference stars for narrow-angle astrometry programs such as the Space Interferometry Mission and large-aperture optical interferometry.

A PANCHROMATIC VIEW OF THE RESTLESS SN 2009ip REVEALS THE EXPLOSIVE EJECTION OF A MASSIVE STAR ENVELOPE

The double explosion of SN 2009ip in 2012 raises questions about our understanding of the late stages of massive star evolution. Here we present a comprehensive study of SN 2009ip during its remarkable rebrightenings. High-cadence photometric and spectroscopic observations from the GeV to the radio band obtained from a variety of ground-based and space facilities (including the Very Large Array, Swift, Fermi, Hubble Space Telescope, and XMM) constrain SN 2009ip to be a low energy (E similar to 1050 erg for an ejecta mass similar to 0.5 M-circle dot) and asymmetric explosion in a complex medium shaped by multiple eruptions of the restless progenitor star. Most of the energy is radiated as a result of the shock breaking out through a dense shell of material located at similar to 5 x 10(14) cm with M similar to 0.1 M-circle dot, ejected by the precursor outburst similar to 40 days before the major explosion. We interpret the NIR excess of emission as signature of material located further out, the origin of which has to be connected with documented mass-loss episodes in the previous years. Our modeling predicts bright neutrino emission associated with the shock break-out if the cosmic-ray energy is comparable to the radiated energy. We connect this phenomenology with the explosive ejection of the outer layers of the massive progenitor star, which later interacted with material deposited in the surroundings by previous eruptions. Future observations will reveal if the massive luminous progenitor star survived. Irrespective of whether the explosion was terminal, SN 2009ip brought to light the existence of new channels for sustained episodic mass loss, the physical origin of which has yet to be identified.

The N2K Consortium. IV. New temperatures and metallicities for 100,000+ FGK dwarfs

We have created specialized target lists for radial velocity surveys that are biased toward stars that (1) possess planets and (2) are easiest to observe with current detection techniques. We use a procedure that uniformly estimates fundamental stellar properties of Tycho 2 stars, with errors, using spline functions of broadband photometry and proper motion found in Hipparcos/Tycho 2 and 2MASS. We provide estimates of Teff and distance for 2.4 × 106 Tycho 2 stars that lack trigonometric distances. For stars that appear to be FGK dwarfs, we also derive [Fe/H] and identify unresolved binary systems with mass ratios 1.25 < M1/M2 < 3.0. For FGK dwarfs with photometric error σV < 0.05, or V < 9, our temperature model gives a 1 σ error of σT = +58.7/ - 65.9 K and our metallicity model gives a 1 σ error of σ[Fe/H] = +0.13/ - 0.14 dex. The binarity model can be used to remove 70% of doubles with 1.25 < M1/M2 < 3.0 from a magnitude-limited sample of dwarfs at a cost of cutting 20% of the sample. Our estimates of distance and spectral type enable us to isolate 354,822 Tycho 2 dwarfs, 321,996 absent from Hipparcos, with giant contamination of 2.6% and 7.2%, respectively. Roughly 100,000 of these stars, not in Hipparcos, have sufficiently low photometric errors to retain 0.13-0.3 dex [Fe/H] accuracy and 80-100 K temperature accuracy (1 σ). Our metallicity estimates have been used to identify targets for N2K, a large-scale radial velocity search for hot jupiters, which has verified the errors presented here. The catalogs that we publish can be used to further large-scale studies of Galactic structure and chemical evolution and to provide potential reference stars for narrow-angle astrometry programs such as the Space Interferometry Mission and large-aperture optical interferometry.

Spatially resolved spectroscopy of early-type galaxies over a range in mass

Long-slit spectra have been obtained with the Keck telescope for a sample of 11 early-type galaxies covering a wide range in luminosity and hence mass. Rotation velocity and velocity dispersions, together with 19 Lick line-strength gradients have been measured, to, on average, two effective radii. Stellar population models taking into account the effect of the non-solar chemical composition have been used to derive ages, metallicities and α/Fe abundances along the radius. We find that line-strength gradients are due mainly to variations of the total metallicity with the radius. One galaxy out of 11 shows very strong age gradients, with a young central component, while the age gradient for the rest of the sample is very shallow or consistent with zero. We also find small variations in the [c /Fe] ratio with radius. Contrary to what is expected in simple collapse models, galaxies show both positive and negative [α/Fe] profiles. This rules out a solely inside-out, or outside-in, formation mechanism for all early-type galaxies. We do not find a correlation between the metallicity and the [a/Fe] gradients, and the local metallicity is not correlated with the local velocity dispersion for all the galaxies of our sample, which rules out scenarios where the delay in the onset of the galactic winds is the only mechanism producing the metallicity gradients. We found that metallicity gradients are correlated with the shape of the isophotes and the central mean age and metallicity of the galaxies, for galaxies younger than ∼10 Gyr. We show that the correlation between the gradients and the central values is not due to the correlation of the errors and indicates that the same process that shaped the gradient, also modified the structural parameters of the galaxies and triggered star formation in their centres. This strongly supports the merger scenario for the formation of these systems, where the degree of dissipation during those mergers increases as the mass of the progenitor galaxies decreases. Finally, we also find a dichotomy in the plane grad [a/Fe]-[a/Fe] between galaxies with velocity dispersions below and above ∼200 km s -1 , which requires confirmation with larger samples.

Globular clusters in Virgo ellipticals: Unexpected results for giants and dwarfs from Advanced Camera for Surveys imaging

We have analyzed archival Hubble Space Telescope Advanced Camera for Surveys images in g and z of the globular cluster (GC) systems of 53 ellipticals in the Virgo Cluster, spanning massive galaxies to dwarf ellipticals (dEs). Several new results emerged. (1) In the giant ellipticals (gEs) M87 and NGC 4649, there is a correlation between luminosity and color for individual blue (metal-poor) GCs, such that more massive GCs are more red (metal-rich). A plausible interpretation of this result is self-enrichment, and a speculative suggestion is that these GCs once possessed dark matter halos. (2) The dispersion in color is nearly twice as large for the metal-rich GCs as for the metal-poor GCs. However, there is evidence for a nonlinear relation between g - z and metallicity, and the dispersion in metallicity may be the same for both subpopulations. (3) Very luminous, intermediate-color GCs are common in gEs. These objects may be remnants of many stripped dwarfs, analogs of ω Cen in the Galaxy. (4) There is a continuity of GC system colors from gEs to some dEs; in particular, many dEs have metal-rich GC subpopulations. We also confirm the GC color-galaxy luminosity relations found previously for both metal-poor and metal-rich GC subpopulations. (5) There are large differences in GC specific frequency among dEs, independent of the presence of a nucleus and the fraction of metal-rich GCs. Over -15 < MB < -18 we find little correlation between specific frequency and MB. But we do find evidence for two separate SN classes of dEs: those with B-band SN ~ 2 and those with populous GC systems that have SN ranging from ~5 to 20 with median SN ~ 10. Together, these points suggest multiple formation channels for dEs in the Virgo Cluster. (6) The peak of the GC luminosity function (GCLF) is the same for both gEs and dEs. This is contrary to expectations of dynamical friction on massive GCs, unless the primordial GCLF varies between gEs and dEs. Among gEs the GCLF turnover varies by a surprisingly small 0.05 mag, an encouraging result for its use as an accurate standard candle.

THE RELATIONSHIPS AMONG COMPACT STELLAR SYSTEMS: A FRESH VIEW OF ULTRACOMPACT DWARFS

We use a combined imaging and spectroscopic survey of the nearby central cluster galaxy, M87, to assemble a sample of 34 confirmed ultracompact dwarfs (UCDs) with half-light radii of 210 pc measured from Hubble Space Telescope images. This doubles the existing sample in M87, making it the largest such sample for any galaxy, while extending the detection of UCDs to unprecedentedly low luminosities (MV =− 9). With this expanded sample, we find no correlation between size and luminosity, in contrast to previous suggestions, and no general correlation between size and galactocentric distance. We explore the relationships between UCDs, less luminous extended clusters (including faint fuzzies), globular clusters (GCs), as well as early-type galaxies and their nuclei, assembling an extensive new catalog of sizes and luminosities for stellar systems. Most of the M87 UCDs follow a tight color–magnitude relation, offset from the metal-poor GCs. This, along with kinematical differences, demonstrates that most UCDs are a distinct population from normal GCs, and not simply a continuation to larger sizes and higher luminosities. The UCD color–magnitude trend couples closely with that for Virgo dwarf elliptical nuclei. We conclude that the M87 UCDs are predominantly stripped nuclei. The brightest and reddest UCDs may be the remnant nuclei of more massive galaxies while a subset of the faintest UCDs may be tidally limited and related to more compact star clusters. In the broader context of galaxy assembly, blue UCDs may trace halo build-up by accretion of low-mass satellites, while red UCDs may be markers of metal-rich bulge formation in larger galaxies.

The SLUGGS Survey : kinematics for over 2500 globular clusters in 12 early-type galaxies

We present a spectro-photometric survey of 2522 extragalactic globular clusters (GCs) around twelve early-type galaxies, nine of which have not been published previously. Combining space‐based and multi‐colour wide field ground‐based imagi ng, with spectra from the Keck DEIMOS instrument, we obtain an average of 160 GC radial velocities per galaxy, with a high velocity precision of � 15 km s 1 per GC. After studying the photometric properties of the GC systems, such as their spatial and colour distribut ions, we focus on the kinematics of metal-poor (blue) and metal-rich (red) GC subpopulations to an average distance of � 8 effective radii from the galaxy centre. Our results show that for some systems the bimodality in GC colour is also present in GC kinematics. The kinematics of the red GC subpopulations are strongly coupled with the host galaxy stellar kinematics. The blue GC subpopulations are more dominated by random motions, especially in the outer regions, and decoupled fro m the red GCs. Peculiar GC kinematic profiles are seen in some galaxies: the blue GCs in NGC 821 rotate along the galaxy minor axis, whereas the GC system of the lenticular galaxy NGC 7457 appears to be strongly rotation supported in the outer region. We supplement our galaxy sample with data from the literature and carry out a number of tests to study the kinematic differences between the two GC subpopulations. We confirm that the GC kinematics are coupled with the host galaxy properties and find that the velocity kurtosis and the slope of their velocity dispersion profiles is different between the two GC subpopulations in more massive galaxies.