arXiv subjects

Ricardo R. Munoz

Publications and source records attributed to Ricardo R. Munoz.

At least 37 records · Page 2Linked to original sources

arXiv2014

The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era

We present new constraints on the star formation histories of six ultra-faint dwarf galaxies: Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I. Our analysis employs a combination of high-precision photometry obtained with the Advanced Camera for Surveys on the Hubble Space Telescope, medium-resolution spectroscopy obtained with the DEep Imaging Multi-Object Spectrograph on the W.M. Keck Observatory, and updated Victoria-Regina isochrones tailored to the abundance patterns appropriate for these galaxies. The data for five of these Milky Way satellites are best fit by a star formation history where at least 75% of the stars formed by z~10 (13.3 Gyr ago). All of the galaxies are consistent with 80% of the stars forming by z~6 (12.8 Gyr ago) and 100% of the stars forming by z~3 (11.6 Gyr ago). The similarly ancient populations of these galaxies support the hypothesis that star formation in the smallest dark matter sub-halos was suppressed by a global outside influence, such as the reionization of the universe.

astro-ph.GA↗

arXiv2014

A search for stellar tidal debris of defunct dwarf galaxies around globular clusters in the inner Galactic halo

In the hierarchical formation scenario in which the outer halo of the Milky Way is the result of the continuous accretion of low-mass galaxies, a fraction of the Galactic globular cluster system might have originated in and been accreted with already extinct dwarf galaxies. In this context, we expect that the remnants of these progenitor galaxies might be still populating the surroundings of those accreted globulars. In this work, we present wide-field photometry of a sample of 23 globular clusters in the Galactocentric distance range 10 < Rg < 40kpc, which we use to search for remnants of their hypothetical progenitor systems. Our deep photometry reveals the presence of underlying stellar populations along the line-of-sight of about half of the globulars included in our sample. Among the detections lying in the footprint of the Sagittarius tidal stream, which we identify via the comparison with its orbit derived from numerical simulations, only Whiting1 and NGC7492 seem to be inmersed in that remnant at a compatible heliocentric distance. We also confirm the existence of a subjacent Main-Sequence feature in the surroundings of NGC1851. A tentative detection of the vast Hercules-Aquila cloud is unveiled in the background of NGC7006.

astro-ph.GA↗

arXiv2013

A Tidally-Stripped Stellar Component of the Magellanic Bridge

Deep photometry of the Small Magellanic Cloud (SMC) stellar periphery (R=4 deg, 4.2 kpc) is used to study its line-of-sight depth with red clump (RC) stars. The RC luminosity function is affected little by young (<1 Gyr) blue-loop stars in these regions because their main-sequence counterparts are not observed in the color magnitude diagrams. The SMC's eastern side is found to have a large line-of-sight depth (~23 kpc) while the western side has a much shallower depth (~10 kpc), consistent with previous photographic plate photometry results. We use a model SMC RC luminosity function to deconvolve the observed RC magnitudes and construct the density function in distance for our fields. Three of the eastern fields show a distance bimodality with one component at the "systemic" ~67 kpc SMC distance and a second component at ~55 kpc. Our data are not reproduced well by the various extant Magellanic Cloud and Stream simulations. However, the models predict that the known HI Magellanic Bridge (stretching from the SMC eastward towards the LMC) has a decreasing distance with angle from the SMC and should be seen in both the gaseous and stellar components. From comparison with these models we conclude that the most likely explanation for our newly identified ~55 kpc stellar structure in the eastern SMC is a stellar counterpart of the HI Magellanic Bridge that was tidally stripped from the SMC ~200 Myr ago during a close encounter with the LMC. This discovery has important implications for microlensing surveys of the SMC.

astro-ph.GA↗

arXiv2013

The Formation History of the Ultra-Faint Dwarf Galaxies

We present early results from a Hubble Space Telescope survey of the ultra-faint dwarf galaxies. These Milky Way satellites were discovered in the Sloan Digital Sky Survey, and appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in the efforts to understand the missing satellite problem. Because they are the least luminous, most dark matter dominated, and least chemically evolved galaxies known, the ultra-faint dwarfs are the best candidate fossils from the early universe. The primary goal of the survey is to measure the star-formation histories of these galaxies and discern any synchronization due to the reionization of the universe. We find that the six galaxies of our survey have very similar star-formation histories, and that each is dominated by stars older than 12 Gyr.

astro-ph.CO↗

arXiv2013

A Megacam Survey of Outer Halo Satellites. II. Blue Stragglers in the Lowest Stellar Density Systems

We present a homogeneous study of blue straggler stars across ten outer halo globular clusters, three classical dwarf spheroidal and nine ultra-faint galaxies based on deep and wide-field photometric data taken with MegaCam on the Canada-France-Hawaii Telescope. We find blue straggler stars to be ubiquitous among these Milky Way satellites. Based on these data, we can test the importance of primordial binaries or multiple systems on blue straggler star formation in low density environments. For the outer halo globular clusters we find an anti-correlation between the specific frequency of blue straggler and absolute magnitude, similar to that previously observed for inner halo clusters. When plotted against density and encounter rate, the frequency of blue stragglers are well fitted by single trends with smooth transitions between dwarf galaxies and globular clusters, which points to a common origin for their blue stragglers. The fraction of blue stragglers stays constant and high in the low encounter rate regime spanned by our dwarf galaxies, and decreases with density and encounter rate in the range spanned by our globular clusters. We find that young stars can mimic blue stragglers in dwarf galaxies only if their ages are 2.5+/-0.5 Gyr and they represent ~1-7% of the total number of stars, which we deem highly unlikely. These results point to mass-transfer or mergers of primordial binaries or multiple systems as the dominant blue straggler formation mechanism in low density systems.

astro-ph.GA↗

arXiv2012

Stellar Kinematics of the Andromeda II Dwarf Spheroidal Galaxy

We present kinematical profiles and metallicity for the M31 dwarf spheroidal (dSph) satellite galaxy Andromeda II (And II) based on Keck DEIMOS spectroscopy of 531 red giant branch stars. Our kinematical sample is among the largest for any M31 satellite and extends out to two effective radii (r_eff = 5.3' = 1.1 kpc). We find a mean systemic velocity of -192.4+-0.5 km/s and an average velocity dispersion of sigma_v = 7.8+-1.1 km/s. While the rotation velocity along the major axis of And II is nearly zero (<1 km/s), the rotation along the minor axis is significant with a maximum rotational velocity of v_max=8.6+-1.8 km/s. We find a kinematical major axis, with a maximum rotational velocity of v_max=10.9+-2.4 km/s, misaligned by 67 degrees to the isophotal major axis. And II is thus the first dwarf galaxy with evidence for nearly prolate rotation with a v_max/sigma_v = 1.1, although given its ellipticity of epsilon = 0.10, this object may be triaxial. We measured metallicities for a subsample of our data, finding a mean metallicity of [Fe/H] = -1.39+- 0.03 dex and an internal metallicity dispersion of 0.72+-0.03 dex. We find a radial metallicity gradient with metal-rich stars more centrally concentrated, but do not observe a significant difference in the dynamics of two metallicity populations. And II is the only known dwarf galaxy to show minor axis rotation making it a unique system whose existence offers important clues on the processes responsible for the formation of dSphs.

astro-ph.CO↗

arXiv2012

A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: VII. Kinematics of the Main Body of the Sagittarius dSph

We have assembled a large-area spectroscopic survey of giant stars in the Sagittarius (Sgr) dwarf galaxy core. Using medium resolution (R ~15,000), multifiber spectroscopy we have measured velocities of these stars, which extend up to 12 degrees from the galaxy's center (3.7 core radii or 0.4 times the King limiting radius). From these high quality spectra we identify 1310 Sgr members out of 2296 stars surveyed distributed across 24 different fields across the Sgr core. Additional slit spectra were obtained of stars bridging from the Sgr core to its trailing tail. Our systematic, large area sample shows no evidence for significant rotation, a result at odds with the ~20 km/s rotation required as an explanation for the bifurcation seen in the Sgr tidal stream; the observed small (<= 4 km/s) velocity trend along primarily the major axis is consistent with models of the projected motion of an extended body on the sky with no need for intrinsic rotation. The Sgr core is found to have a flat velocity dispersion (except for a kinematically colder center point) across its surveyed extent and into its tidal tails, a property that matches the velocity dispersion profiles measured for other Milky Way dwarf spheroidal (dSph) galaxies. We comment on the possible significance of this observed kinematical similarity for the dynamical state of the other classical Milky Way dSphs in light of the fact that Sgr is clearly a strongly tidally disrupted system.

astro-ph.GA↗

arXiv2012

The Primeval Populations of the Ultra-Faint Dwarf Galaxies

We present new constraints on the star formation histories of the ultra-faint dwarf (UFD) galaxies, using deep photometry obtained with the Hubble Space Telescope (HST). A galaxy class recently discovered in the Sloan Digital Sky Survey, the UFDs appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in efforts to understand the missing satellite problem. They are the least luminous, most dark-matter dominated, and least chemically-evolved galaxies known. Our HST survey of six UFDs seeks to determine if these galaxies are true fossils from the early universe. We present here the preliminary analysis of three UFD galaxies: Hercules, Leo IV, and Ursa Major I. Classical dwarf spheroidals of the Local Group exhibit extended star formation histories, but these three Milky Way satellites are at least as old as the ancient globular cluster M92, with no evidence for intermediate-age populations. Their ages also appear to be synchronized to within ~1 Gyr of each other, as might be expected if their star formation was truncated by a global event, such as reionization.

astro-ph.CO↗

arXiv2012

The Discovery of an Ultra-Faint Star Cluster in the Constellation of Ursa Minor

We report the discovery of a new ultra-faint globular cluster in the constellation of Ursa Minor, based on stellar photometry from the MegaCam imager at the Canada-France-Hawaii Telescope (CFHT). We find that this cluster, Munoz 1, is located at a distance of 45 +/- 5 kpc and at a projected distance of only 45 arcmin from the center of the Ursa Minor dSph galaxy. Using a Maximum Likelihood technique we measure a half-light radius of 0.5 arcmin, or equivalently 7 pc and an ellipticity consistent with being zero. We estimate its absolute magnitude to be M_V=-0.4 +/- 0.9, which corresponds to L_V=120 (+160, -65) L_sun and we measure a heliocentric radial velocity of -137 +/- 4 km/s based on Keck/DEIMOS spectroscopy. This new satellite is separate from Ursa Minor by ~30 kpc and 110 km/s suggesting the cluster is not obviously associated with the dSph, despite the very close angular separation. Based on its photometric properties and structural parameters we conclude that Munoz 1 is a new ultra-faint stellar cluster. Along with Segue 3 this is one of the faintest stellar clusters known to date.

astro-ph.GA↗

arXiv2011

Segue 3: An Old, Extremely Low luminosity Star Cluster in the Milky Way's Halo

We investigate the kinematic and photometric properties of the Segue 3 Milky Way companion using Keck/DEIMOS spectroscopy and Magellan/IMACS g and r-band imaging. Using maximum likelihood methods to analyze the photometry, we study the structure and stellar population of Segue 3. We find the half-light radius of Segue 3 is 26" +/- 5" (2.1 +/- 0.4 pc, for a distance of 17 kpc) and the absolute magnitude is a mere M_V = 0.0 +/- 0.8 mag, making Segue 3 the least luminous old stellar system known. We find Segue 3 to be consistent with a single stellar population, with an age of 12.0 +1.5/-0.4 Gyr and an [Fe/H] of -1.7 +0.07/-0.27. Line-of-sight velocities from the spectra are combined with the photometry to determine a sample of 32 stars which are likely associated with Segue 3. The member stars within three half-light radii have a velocity dispersion of 1.2 +/- 2.6 km/s. Photometry of the members indicates the stellar population has a spread in [Fe/H] of <0.3 dex. These facts, together with the small physical size of Segue 3, imply the object is likely an old, faint stellar cluster which contains no significant dark matter. We find tentative evidence for stellar mass loss in Segue 3 through the eleven candidate member stars outside of three half-light radii, as expected from dynamical arguments. Interpretation of the data outside of three half-light radii, is complicated by the object's spatial coincidence with a previously known halo substructure, which may enhance contamination of our member sample.

astro-ph.GA↗

arXiv2011

Discovery of a Large Stellar Periphery Around the Small Magellanic Cloud

The Magellanic Clouds are a local laboratory for understanding the evolution and properties of dwarf irregular galaxies. To reveal the extended structure and interaction history of the Magellanic Clouds we have undertaken a large-scale photometric and spectroscopic study of their stellar periphery (the MAgellanic Periphery Survey, MAPS). We present first MAPS results for the Small Magellanic Cloud (SMC): Washington M, T2 + DDO51 photometry reveals metal-poor red giant branch stars in the SMC that extend to large radii (~11 kpc), are distributed nearly azimuthally symmetrically (ellipticity=0.1), and are well-fitted by an exponential profile (out to R~7.5 deg). An ~6 Gyr old, [Fe/H] -1.3 main-sequence turnoff is also evident to at least R=7.3 deg, and as far as 8.4 deg in some directions. We find evidence for a "break" population beyond ~8 radial scalelengths having a very shallow radial density profile that could be either a bound stellar halo or a population of extratidal stars. The distribution of the intermediate stellar component (3<R<7.5 deg) contrasts with that of the inner stellar component (R<3 deg), which is both more elliptical (ellipticity~0.3) and offset from the center of the intermediate component by 0.59 deg, although both components share a similar radial exponential scale length. This offset is likely due to a perspective effect because stars on the eastern side of the SMC are closer on average than stars on the western side. This mapping of its outer stellar structures indicates that the SMC is more complex than previously thought.

astro-ph.GA↗

arXiv2010

Accurate masses for dispersion-supported galaxies

We derive an accurate mass estimator for dispersion-supported stellar systems and demonstrate its validity by analyzing resolved line-of-sight velocity data for globular clusters, dwarf galaxies, and elliptical galaxies. Specifically, by manipulating the spherical Jeans equation we show that the dynamical mass enclosed within the 3D deprojected half-light radius r_1/2 can be determined with only mild assumptions about the spatial variation of the stellar velocity dispersion anisotropy. We find M_1/2 = 3 σ_los^2 r_1/2 / G ~ 4 σ_los^2 R_eff / G, where σ_los^2 is the luminosity-weighted square of the line-of-sight velocity dispersion and R_eff is the 2D projected half-light radius. While deceptively familiar in form, this formula is not the virial theorem, which cannot be used to determine accurate masses unless the radial profile of the total mass is known a priori. We utilize this finding to show that all of the Milky Way dwarf spheroidal galaxies (MW dSphs) are consistent with having formed within a halo of mass approximately 3 x 10^9 M_sun in Lambda CDM cosmology. The faintest MW dSphs seem to have formed in dark matter halos that are at least as massive as those of the brightest MW dSphs, despite the almost five orders of magnitude spread in luminosity. We expand our analysis to the full range of observed dispersion-supported stellar systems and examine their I-band mass-to-light ratios (M/L). The M/L vs. M_1/2 relation for dispersion-supported galaxies follows a U-shape, with a broad minimum near M/L ~ 3 that spans dwarf elliptical galaxies to normal ellipticals, a steep rise to M/L ~ 3,200 for ultra-faint dSphs, and a more shallow rise to M/L ~ 800 for galaxy cluster spheroids.

astro-ph.CO↗

arXiv2009

Turning the Tides on the Ultra-Faint Dwarf Spheroidal Galaxies: Coma Berenices and Ursa Major II

We present deep CFHT/MegaCam photometry of the ultra-faint Milky Way satellite galaxies Coma Berenices (ComBer) and Ursa Major II (UMa II). These data extend to r~25, corresponding to three magnitudes below the main sequence turn-offs in these galaxies. We robustly calculate a total luminosity of M_V=-3.8 +/- 0.6 for ComBer and M_V=-3.9 +/- 0.5 for UMa II, in agreement with previous results. ComBer shows a fairly regular morphology with no signs of active tidal stripping down to a surface brightness limit of 32.4 magarcsec^-2. Using a maximum likelihood analysis, we calculate the half-light radius of ComBer to be r_half=74 +/- 4 pc (5.8 +/- 0.3 arcmin) and its ellipticity e=0.36 +/- 0.04. In contrast, UMa II shows signs of on-going disruption. We map its morphology down to mu_V=32.6 mag arcsec^-2 and found that UMa II is larger than previously determined, extending at least ~700 pc (1.2 deg on the sky) and it is also quite elongated with an ellipticity of e=0.50 +/- 0.2. However, our estimate for the half-light radius, 123 +/- 3 pc (14.1 +/- 0.3 arcmin) is similar to previous results. We discuss the implications of these findings in the context of potential indirect dark matter detections and galaxy formation. We conclude that while ComBer appears to be a stable dwarf galaxy, UMa II shows signs of on-going tidal interaction.

astro-ph.GA↗

arXiv2009

Kinematics and Metallicities in the Bootes III Stellar Overdensity: a Disrupted Dwarf Galaxy?

We report the results of a spectroscopic study of the Bootes III (BooIII) stellar overdensity carried out with the Hectospec multifiber spectrograph on the MMT telescope. Radial velocities have been measured for 193 BooIII candidate stars selected to have magnitudes and colors consistent with its upper main sequence and lower red giant branch, as well as a number of horizontal branch candidates. From 20 identified candidate BooIII members, we measure a systemic velocity of V_sun=197.5+-3.8 km/s and a velocity dispersion of sigma_o=14.0+-3.2 km/s. We use the somewhat large velocity dispersion and the implied highly radial orbit, along with morphological evidence from Grillmair (2009) and stellar abundances, to argue that BooIII is likely the first known object observed in a transitional state between being a bound dwarf galaxy and a completely unbound tidal stream.

astro-ph.GA↗

arXiv2007

Modeling The Structure And Dynamics of Dwarf Spheroidal Galaxies with Dark Matter And Tides

We report the results of N-body simulations of disrupting satellites aimed at exploring whether the observed features of dSphs can be accounted for with simple, mass-follows-light (MFL) models including tidal disruption. As a test case, we focus on the Carina dwarf Spheroidal (dSph), which presently has the most extensive data at large radius. We find that previous N-body, MFL simulations of dSphs did not sufficiently explore the parameter space of satellite mass, density and orbital shape to find adequate matches to Galactic dSph systems, whereas with a systematic survey of parameter space we are able to find tidally disrupting, MFL satellite models that rather faithfully reproduce Carina's velocity profile, velocity dispersion profile and projected density distribution over its entire sampled radius. The successful MFL model satellites have very eccentric orbits, currently favored by Cold Dark Matter (CDM) models, and central velocity dispersions that still yield an accurate representation of the bound mass and observed central M/L~40 of Carina, despite inflation of the velocity dispersion outside the dSph core by unbound debris. Our survey of parameter space also allows us to address a number of commonly held misperceptions of tidal disruption and its observable effects on dSph structure and dynamics. The simulations suggest that even modest tidal disruption can have a profound effect on the observed dynamics of dSph stars at large radii. Satellites that are well-described by tidally disrupting MFL models could still be fully compatible with L-CDM if for example they represent a later stage in the evolution of luminous subhalos.

astro-ph↗

arXiv2007

A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: Variation of the Metallicity Distribution Function Along the Sagittarius Stream

We present reliable measurements of the metallicity distribution function (MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy, based on high resolution, echelle spectroscopy of candidate M giant members of the Sgr system. The Sgr MDF is found to evolve significantly from a median [Fe/H] ~-0.4 in the core to ~-1.1 dex over a Sgr leading arm length representing ~2.5-3.0 Gyr of dynamical (i.e. tidal stripping) age. This is direct evidence that there can be significant chemical differences between current dSph satellites and the bulk of the stars they have contributed to the halo. Our results suggest that Sgr experienced a significant change in binding energy over the past several Gyr, which has substantially decreased its tidal boundary across a radial range over which there must have been a significant metallicity gradient in the progenitor galaxy. By accounting for MDF variation along the debris arms, we approximate the MDF Sgr would have had several Gyr ago. We also analyze the MDF of a moving group of M giants we previously discovered towards the North Galactic Cap having opposite radial velocities to the infalling Sgr leading arm stars there and propose that most of these represent Sgr trailing arm stars overlapping the Sgr leading arm in this part of the sky. If so, these trailing arm stars further demonstrate the strong MDF evolution within the Sgr stream.

astro-ph↗

arXiv2006

Exploring Halo Substructure with Giant Stars: The Dynamics and Metallicity of the Dwarf Spheroidal in Bootes

We report the results of a spectroscopic study of the Bootes (Boo) dwarf spheroidal (dSph) galaxy carried out with the WIYN telescope and the Hydra multifiber spectrograph. Radial velocities have been measured for 58 Boo candidate stars selected to have magnitudes and colors consistent with its red and asymptotic giant branches. Within the 13' half-light radius, seven members of Boo yield a systemic velocity of V_r=95.6+-3.4 km/s and a velocity dispersion of 6.6+-2.3 km/s. This implies a mass on the order of 1 x 10^7 M_sun, similar to the inferred masses of other Galactic dSphs. Adopting a total Boo luminosity of L=1.8 x 10^4 L_sun to 8.6 x 10^4 L_sun implies M/L ~ 610 to 130, making Boo, the most distorted known Milky Way dwarf galaxy, potentially also the darkest. From the spectra of Boo member stars we estimate its metallicity to be [Fe/H] ~ -2.5, which would make it the most metal poor dSph known to date.

astro-ph↗

arXiv2006

Exploring Halo Substructure with Giant Stars XI: The Tidal Tails of the Carina Dwarf Spheroidal and the Discovery of Magellanic Cloud Stars in the Carina Foreground

A new large-area Washington M,T_2+DDO51 filter survey of more than 10 deg^2 around the Carina dSph galaxy reveals a spectroscopically confirmed power law radial density "break" population of Carina giant stars extending several degrees beyond the central King profile. Magellan telescope MIKE spectroscopy establishes the existence of Carina stars to at least 4.5 times its central King limiting radius, r_lim and primarily along Carina's major axis. To keep these stars bound to the dSph would require a global Carina mass-to-light ratio of M/L > 6,300 M/L_sun. The MIKE velocities, supplemented with ~950 additional Carina field velocities from archived VLT+GIRAFFE spectra with r<=r_lim, demonstrate a nearly constant Carina velocity dispersion to just beyond r = r_lim, and both a rising velocity dispersion and a velocity shear at still larger radii. Together, the observational evidence suggests that the discovered extended Carina population represents tidal debris from the dSph. Of 65 giant candidates at large angular radii from the Carina center for which MIKE spectra have been obtained 94% are associated either with Carina or a second, newly discovered diffuse, but strongly radial velocity-coherent (velocity dispersion of 9.8 km s^-1), foreground halo system. The fifteen stars in this second, retrograde velocity population have (1) a mean metallicity ~1 dex higher than that of Carina, and (2) colors and magnitudes consistent with the red clump of the Large Magellanic Cloud (LMC). Additional spectroscopy of giant star candidates in fields linking Carina and the LMC shows a smooth velocity gradient between the LMC and the retrograde Carina moving group. We conclude that we have found Magellanic stars almost twice as far (22 deg) from the LMC center than previously known.

astro-ph↗