Browsing by Author "Lane, Richard R."

Now showing 1 - 20 of 32
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    An enquiry on the origins of N-rich stars in the inner Galaxy based on APOGEE chemical compositions
    (2021) Kisku, Shobhit; Schiavon, Ricardo P.; Horta, Danny; Mason, Andrew; Mackereth, J. Ted; Hasselquist, Sten; Garcia-Hernandez, D. A.; Bizyaev, Dmitry; Brownstein, Joel R.; Lane, Richard R.; Minniti, Dante; Pan, Kaike; Roman-Lopes, Alexandre
    Recent evidence based on APOGEE data for stars within a few kpc of the Galactic Centre suggests that dissolved globular clusters (GCs) contribute significantly to the stellar mass budget of the inner halo. In this paper, we enquire into the origins of tracers of GC dissolution, N-rich stars, that are located in the inner 4 kpc of the Milky Way. From an analysis of the chemical compositions of these stars, we establish that about 30 per cent of the N-rich stars previously identified in the inner Galaxy may have an accreted origin. This result is confirmed by an analysis of the kinematic properties of our sample. The specific frequency of N-rich stars is quite large in the accreted population, exceeding that of its in situ counterparts by near an order of magnitude, in disagreement with predictions from numerical simulations. We hope that our numbers provide a useful test to models of GC formation and destruction.
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    APOGEE chemical abundances of globular cluster giants in the inner Galaxy
    (2017) Schiavon, Ricardo P.; Johnson, Jennifer A.; Frinchaboy, Peter M.; Zasowski, Gail; Meszaros, Szabolcs; Garcia-Hernandez, D. A.; Cohen, Roger E.; Tang, Baitian; Villanova, Sandro; Geisler, Douglas; Beers, Timothy C.; Fernandez-Trincado, J. G.; Garcia Perez, Ana E.; Lucatello, Sara; Majewski, Steven R.; Martell, Sarah L.; O'Connell, Robert W.; Allende Prieto, Carlos; Bizyaev, Dmitry; Carrera, Ricardo; Lane, Richard R.; Malanushenko, Elena; Malanushenko, Viktor; Munoz, Ricardo R.; Nitschelm, Christian; Oravetz, Daniel; Pan, Kaike; Roman-Lopes, Alexandre; Schultheis, Matthias; Simmons, Audrey
    We report chemical abundances obtained by Sloan Digital Sky Survey (SDSS)-III/Apache Point Observatory Galactic Evolution Experiment for giant stars in five globular clusters located within 2.2 kpc of the Galactic Centre. We detect the presence of multiple stellar populations in four of those clusters (NGC 6553, NGC 6528, Terzan 5 and Palomar 6) and find strong evidence for their presence in NGC 6522. All clusters with a large enough sample present a significant spread in the abundances of N, C, Na and Al, with the usual correlations and anticorrelations between various abundances seen in other globular clusters. Our results provide important quantitative constraints on theoretical models for self-enrichment of globular clusters, by testing their predictions for the dependence of yields of elements such as Na, N, C and Al on metallicity. They also confirm that, under the assumption that field N-rich stars originate from globular cluster destruction, they can be used as tracers of their parental systems in the high-metallicity regime.
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    Chemical Cartography with APOGEE: Multi-element Abundance Ratios
    (2019) Weinberg, David H.; Holtzman, Jon A.; Hasselquist, Sten; Bird, Jonathan C.; Johnson, Jennifer A.; Shetrone, Matthew; Sobeck, Jennifer; Allende Prieto, Carlos; Bizyaev, Dmitry; Carrera, Ricardo; Cohen, Roger E.; Cunha, Katia; Ebelke, Garrett; Fernandez-Trincado, J. G.; Garcia-Hernandez, D. A.; Hayes, Christian R.; Jonsson, Henrik; Lane, Richard R.; Majewski, Steven R.; Malanushenko, Viktor; Meszaros, Szabolcs; Nidever, David L.; Nitschelm, Christian; Pan, Kaike; Rix, Hans-Walter; Rybizki, Jan; Schiavon, Ricardo P.; Schneider, Donald P.; Wilso, John C.; Zamora, Olga
    We map the trends of elemental abundance ratios across the Galactic disk, spanning R = 3-15 kpc and midplane distance vertical bar Z vertical bar = 0-2 kpc, for 15 elements in a sample of 20,485 stars measured by the SDSS/APOGEE survey (O, Na, Mg, Al, Si, P, S, K, Ca, V, Cr, Mn, Fe, Co, Ni). Adopting Mg rather than Fe as our reference element, and separating stars into two populations based on [Fe/Mg], we find that the median trends of [X/Mg] versus [Mg/H] in each population are nearly independent of location in the Galaxy. The full multi-element cartography can be summarized by combining these nearly universal median sequences with our measured metallicity distribution functions and the relative proportions of the low-[Fe/Mg] (high-alpha) and high-[Fe/Mg] (low-alpha) populations, which depend strongly on R and vertical bar Z vertical bar. We interpret the median sequences with a semi-empirical "two-process" model that describes both the ratio of core collapse and Type Ia supernova (SN Ia) contributions to each element and the metallicity dependence of the supernova yields. These observationally inferred trends can provide strong tests of supernova nucleosynthesis calculations. Our results lead to a relatively simple picture of abundance ratio variations in the Milky Way, in which the trends at any location can be described as the sum of two components with relative contributions that change systematically and smoothly across the Galaxy. Deviations from this picture and future extensions to other elements can provide further insights into the physics of stellar nucleosynthesis and unusual events in the Galaxy's history.
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    Disentangling the Galactic Halo with APOGEE. I. Chemical and Kinematical Investigation of Distinct Metal-poor Populations
    (2018) Hayes, Christian R.; Majewski, Steven R.; Shetrone, Matthew; Fernandez-Alvar, Emma; Allende Prieto, Carlos; Schuster, William J.; Carigi, Leticia; Cunha, Katia; Smith, Verne V.; Sobeck, Jennifer; Almeida, Andres; Beers, Timothy C.; Carrera, Ricardo; Fernandez-Trincado, J. G.; Garcia-Hernandez, D. A.; Geisler, Doug; Lane, Richard R.; Lucatello, Sara; Matthews, Allison M.; Minniti, Dante; Nitschelm, Christian; Tang, Baitian; Tissera, Patricia B.; Zamora, Olga
    We find two chemically distinct populations separated relatively cleanly in the [Fe/H]-[Mg/Fe] plane, but also distinguished in other chemical planes, among metal-poor stars (primarily with metallicities [Fe/H] < -0.9) observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and analyzed for Data Release 13 (DR13) of the Sloan Digital Sky Survey. These two stellar populations show the most significant differences in their [X/Fe] ratios for the alpha-elements, C+N, Al, and Ni. In addition to these populations having differing chemistry, the low metallicity high-Mg population (which we denote "the HMg population") exhibits a significant net Galactic rotation, whereas the low-Mg population (or "the LMg population") has halo-like kinematics with little to no net rotation. Based on its properties, the origin of the LMg population is likely an accreted population of stars. The HMg population shows chemistry (and to an extent kinematics) similar to the thick disk, and is likely associated with in situ formation. The distinction between the LMg and HMg populations mimics the differences between the populations of low-and high-a halo stars found in previous studies, suggesting that these are samples of the same two populations.
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    Disk-like Chemistry of the Triangulum-Andromeda Overdensity as Seen by APOGEE
    (IOP PUBLISHING LTD, 2018) Hayes, Christian R.; Majewski, Steven R.; Hasselquist, Sten; Beaton, Rachael L.; Cunha, Katia; Smith, Verne V.; Price Whelan, Adrian M.; Anguiano, Borja; Beers, Timothy C.; Carrera, Ricardo; Fernandez Trincado, J. G.; Frinchaboy, Peter M.; Garcia Hernandez, D. A.; Lane, Richard R.; Nidever, David L.; Nitschelm, Christian; Roman Lopes, Alexandre; Zamora, Olga
    The nature of the Triangulum-Andromeda (TriAnd) system has been debated since the discovery of this distant, low-latitude Milky Way (MW) overdensity more than a decade ago. Explanations for its origin are either as a halo substructure from the disruption of a dwarf galaxy, or a distant extension of the Galactic disk. We test these hypotheses using the chemical abundances of a dozen TriAnd members from the Sloan Digital Sky Survey-IV's (SDSS-IV's) 14th Data Release (DR14) of Apache Point Observatory Galactic Evolution Experiment (APOGEE) data to compare to APOGEE abundances of stars with similar metallicity from both the Sagittarius (Sgr) dSph and the outer MW disk. We find that TriAnd stars are chemically distinct from Sgr across a variety of elements, (C+N), Mg, K, Ca, Mn, and Ni, with a separation in [X/Fe] of about 0.1 to 0.4 dex depending on the element. Instead, the TriAnd stars, with a median metallicity of about -0.8, exhibit chemical abundance ratios similar to those of the lowest metallicity ([Fe/H] similar to-0.7)stars in the outer Galactic disk, and are consistent with expectations of extrapolated chemical gradients in the outer disk of the MW. These results suggest that TriAnd is associated with the MW disk, and, therefore, that the disk extends to this overdensity-i.e., past a Galactocentric radius of 24 kpc -albeit vertically perturbed about 7 kpc below the nominal disk midplane in this region of the Galaxy.
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    Do galaxy global relationships emerge from local ones? The SDSS IV MaNGA surface mass density-metallicity relation
    (OXFORD UNIV PRESS, 2016) Barrera Ballesteros, Jorge K.; Heckman, Timothy M.; Zhu, Guangtun B.; Zakamska, Nadia L.; Sanchez, Sebastian F.; Law, David; Wake, David; Green, Jenny E.; Bizyaev, Dmitry; Oravetz, Daniel; Simmons, Audrey; Malanushenko, Elena; Pan, Kaike; Roman Lopes, Alexandre; Lane, Richard R.
    We present the stellar surface mass density versus gas metallicity (Sigma(*)-Z) relation for more than 500 000 spatially resolved star-forming resolution elements (spaxels) from a sample of 653 disc galaxies included in the SDSS IV MaNGA survey. We find a tight relation between these local properties, with higher metallicities as the surface density increases. This relation extends over three orders of magnitude in the surface mass density and a factor of 4 in metallicity. We show that this local relationship can simultaneously reproduce two well-known properties of disc galaxies: their global mass-metallicity relationship and their radial metallicity gradients. We also find that the Sigma(*)-Z relation is largely independent of the galaxy's total stellar mass and specific star formation rate (sSFR), except at low stellar mass and high sSFR. These results suggest that in the present-day universe local properties play a key role in determining the gas-phase metallicity in typical disc galaxies.
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    Exploring the Galactic Warp through Asymmetries in the Kinematics of the Galactic Disk
    (2020) Cheng, Xinlun; Anguiano, Borja; Majewski, Steven R.; Hayes, Christian; Arras, Phil; Chiappini, Cristina; Hasselquist, Sten; Queiroz, Anna Barbara de Andrade; Nitschelm, Christian; Anibal Garcia-Hernandez, Domingo; Lane, Richard R.; Roman-Lopes, Alexandre; Frinchaboy, Peter M.
    Previous analyses of large databases of Milky Way stars have revealed the stellar disk of our Galaxy to be warped and that this imparts a strong signature on the kinematics of stars beyond the solar neighborhood. However, due to the limitation of accurate distance estimates, many attempts to explore the extent of these Galactic features have generally been restricted to a volume near the Sun. By combining the Gaia DR2 astrometric solution, StarHorse distances, and stellar abundances from the APOGEE survey, we present the most detailed and radially expansive study yet of the vertical and radial motions of stars in the Galactic disk. We map velocities of stars with respect to their Galactocentric radius, angular momentum, and azimuthal angle and assess their relation to the warp. A decrease in vertical velocity is discovered at Galactocentric radius R = 13 kpc and angular momentum L-z = 2800 kpc km s(-1). Smaller ripples in vertical and radial velocity are also discovered superposed on the main trend. We also discovered that trends in the vertical velocity with azimuthal angle are not symmetric about the peak, suggesting the warp is lopsided. To explain the global trend in vertical velocity, we built a simple analytical model of the Galactic warp. Our best fit yields a starting radius of 8.87(-0.09)(+0.08) kpc and precession rate of 13.57(-0.18)(+0.20) km s(-1) kpc(-1). These parameters remain consistent across stellar age groups, a result that supports the notion that the warp is the result of an external, gravitationally induced phenomenon.
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    Exploring the Stellar Age Distribution of the Milky Way Bulge Using APOGEE
    (2020) Hasselquist, Sten; Zasowski, Gail; Feuillet, Diane K.; Schultheis, Mathias; Nataf, David M.; Anguiano, Borja; Beaton, Rachael L.; Beers, Timothy C.; Cohen, Roger E.; Cunha, Katia; Fernandez-Trincado, Jose G.; Garcia-Hernandez, D. A.; Geisler, Doug; Holtzman, Jon A.; Johnson, Jennifer; Lane, Richard R.; Majewski, Steven R.; Bidin, Christian Moni; Nitschelm, Christian; Roman-Lopes, Alexandre; Schiavon, Ricardo; Smith, Verne V.; Sobeck, Jennifer
    We present stellar age distributions of the Milky Way bulge region using ages for similar to 6000 high-luminosity (log (g), metal-rich ([Fe/H] >= -0.5) bulge stars observed by the Apache Point Observatory Galactic Evolution Experiment. Ages are derived using The Cannon label-transfer method, trained on a sample of nearby luminous giants with precise parallaxes for which we obtain ages using a Bayesian isochrone-matching technique. We find that the metal-rich bulge is predominantly composed of old stars (>8 Gyr). We find evidence that the planar region of the bulge (vertical bar Z(GC)vertical bar <= 0.25 kpc) is enriched in metallicity, Z, at a faster rate (dZ/dt similar to 0.0034 Gyr(-1)) than regions farther from the plane (dZ/dt similar to 0.0013 Gyr(-1) at vertical bar Z(GC)vertical bar > 1.00 kpc). We identify a nonnegligible fraction of younger stars (age similar to 2-5 Gyr) at metallicities of +0.2 < [Fe/H] < +0.4. These stars are preferentially found in the plane (vertical bar Z(GC)vertical bar <= 0.25 kpc) and at R-cy approximate to 2-3 kpc, with kinematics that are more consistent with rotation than are the kinematics of older stars at the same metallicities. We do not measure a significant age difference between stars found inside and outside the bar. These findings show that the bulge experienced an initial starburst that was more intense close to the plane than far from the plane. Then, star formation continued at supersolar metallicities in a thin disk at 2 kpc less than or similar to R-cy less than or similar to 3 kpc until similar to 2 Gyr ago.
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    Identifying Sagittarius Stream Stars by Their APOGEE Chemical Abundance Signatures
    (2019) Hasselquist, Sten; Carlin, Jeffrey L.; Holtzman, Jon A.; Shetrone, Matthew; Hayes, Christian R.; Cunha, Katia; Smith, Verne; Beaton, Rachael L.; Sobeck, Jennifer; Allende Prieto, Carlos; Majewski, Steven R.; Anguiano, Borja; Bizyaev, Dmitry; Garcia-Hernandez, D. A.; Lane, Richard R.; Pan, Kaike; Nidever, David L.; Fernandez-Trincado, Jose. G.; Wilson, John C.; Zamora, Olga
    The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey provides precise chemical abundances of 18 chemical elements for similar to 176,000 red giant stars distributed over much of the Milky Way Galaxy (MW), and includes observations of the core of the Sagittarius dwarf spheroidal galaxy (Sgr). The APOGEE chemical abundance patterns of Sgr have revealed that it is chemically distinct from the MW in most chemical elements. We employ a k-means clustering algorithm to six-dimensional chemical space defined by [(C+N)/Fe], [O/Fe], [Mg/Fe], [Al/Fe], [Mn/Fe], and [Ni/Fe] to identify 62 MW stars in the APOGEE sample that have Sgr-like chemical abundances. Of the 62 stars, 35 have Gaia kinematics and positions consistent with those predicted by N-body simulations of the Sgr stream, and are likely stars that have been stripped from Sgr during the last two pericenter passages (<2 Gyr ago). Another 20 of the 62 stars exhibit chemical abundances indistinguishable from the Sgr stream stars, but are on highly eccentric orbits with median r(apo) similar to 25 kpc. These stars are likely the "accreted" halo population thought to be the result of a separate merger with the MW 8-11 Gyr ago. We also find one hypervelocity star candidate. We conclude that Sgr was enriched to [Fe/H] similar to -0.2 before its most recent pericenter passage. If the "accreted halo" population is from one major accretion event, then this progenitor galaxy was enriched to at least [Fe/H] similar to -0.6, and had a similar star formation history to Sgr before merging.
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    On the origin of the Monoceros Ring - I. Kinematics, proper motions, and the nature of the progenitor
    (2018) Guglielmo, Magda; Lane, Richard R.; Conn, Blair C.; Ho, Anna Y. Q.; Ibata, Rodrigo A.; Lewis, Geraint F.
    The Monoceros Ring (MRi) structure is an apparent stellar overdensity that has been postulated to entirely encircle the Galactic plane and has been variously described as being due to line-of-sight effects of the Galactic warp and flare or of extragalactic origin (via accretion). Despite being intensely scrutinized in the literature for more than a decade, no studies to date have been able to definitively uncover its origins. Here we use N-body simulations and a genetic algorithm to explore the parameter space for the initial position, orbital parameters, and, for the first time, the final location of a satellite progenitor. We fit our models to the latest Pan-STARRS data to determine whether an accretion scenario is capable of producing an in-plane ring-like structure matching the known parameters of the MRi. Our simulations produce streams that closely match the location, proper motion, and kinematics of the MRi structure. However, we are not able to reproduce the mass estimates from earlier studies based on Pan-STARRS data. Furthermore, in contrast to earlier studies, our best-fitting models are those for progenitors on retrograde orbits. If the MRi was produced by satellite accretion, we find that its progenitor has an initial mass upper limit of similar to 10(10) M-circle dot and the remnant is likely located behind the Galactic bulge, making it difficult to locate observationally. While our models produce realistic MRi-like structures, we cannot definitively conclude that the MRi was produced by the accretion of a satellite galaxy.
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    Quantifying radial migration in the Milky Way: inefficient over short time-scales but essential to the very outer disc beyond ∼15 kpc
    (2022) Lian, Jianhui; Zasowski, Gail; Hasselquist, Sten; Holtzman, Jon A.; Boardman, Nicholas; Cunha, Katia; Fernandez-Trincado, Jose G.; Frinchaboy, Peter M.; Garcia-Hernandez, D. A.; Nitschelm, Christian; Lane, Richard R.; Thomas, Daniel; Zhang, Kai
    Stellar radial migration plays an important role in reshaping a galaxy's structure and the radial distribution of stellar population properties. In this work, we revisit reported observational evidence for radial migration and quantify its strength using the age-[Fe/H] distribution of stars across the Milky Way with APOGEE data. We find a broken age-[Fe/H] relation in the Galactic disc at r > 6 kpc, with a more pronounced break at larger radii. To quantify the strength of radial migration, we assume stars born at each radius have a unique age and metallicity, and then decompose the metallicity distribution function (MDF) of mono-age young populations into different Gaussian components that originated from various birth radii at r(birth) < 13 kpc. We find that, at ages of 2 and 3 Gyr, roughly half the stars were formed within 1 kpc of their present radius, and very few stars (<5 per cent) were formed more than 4 kpc away from their present radius. These results suggest limited short-distance radial migration and inefficient long-distance migration in the Milky Way during the last 3 Gyr. In the very outer disc beyond 15 kpc, the observed age-[Fe/H] distribution is consistent with the prediction of pure radial migration from smaller radii, suggesting a migration origin of the very outer disc. We also estimate intrinsic metallicity gradients at ages of 2 and 3 Gyr of -0.061 and -0.063 dex kpc(-1), respectively.
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    SDSS-IV MaNGA: Cannibalism Caught in the Act-On the Frequency of Occurrence of Multiple Cores in Brightest Cluster Galaxies
    (2022) Hsu, Yun-Hsin; Lin, Yen-Ting; Huang, Song; Nelson, Dylan; Rodriguez-Gomez, Vicente; Lai, Hsuan-Ting; Greene, Jenny; Leauthaud, Alexie; Aragon-Salamanca, Alfonso; Bundy, Kevin; Emsellem, Eric; Merrifield, Michael; More, Surhud; Okabe, Nobuhiro; Rong, Yu; Brownstein, Joel R.; Lane, Richard R.; Pan, Kaike; Schneider, Donald P.
    Although it is generally accepted that massive galaxies form in a two-phased fashion, beginning with a rapid mass buildup through intense starburst activities followed by primarily dry mergers that mainly deposit stellar mass at outskirts, the late time stellar mass growth of brightest cluster galaxies (BCGs), the most massive galaxies in the universe, is still not well understood. Several independent measurements have indicated a slower mass growth rate than predictions from theoretical models. We attempt to resolve the discrepancy by measuring the frequency of BCGs with multiple cores, which serve as a proxy of the merger rates in the central region and facilitate a more direct comparison with theoretical predictions. Using 79 BCGs at z = 0.06-0.15 with integral field spectroscopic data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) project, we obtain a multiple-core fraction of 0.11 +/- 0.04 at z approximate to 0.1 within an 18 kpc radius from the center, which is comparable to the value of 0.08 +/- 0.04 derived from mock observations of 218 simulated BCGs from the cosmological hydrodynamical simulation IllustrisTNG. We find that most cores that appear close to the BCGs from imaging data turn out to be physically associated systems. Anchoring on the similarity in the multiple-core frequency between the MaNGA and IllustrisTNG, we discuss the mass growth rate of BCGs over the past 4.5 Gyr.
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    SDSS-IV MaNGA: environmental dependence of gas metallicity gradients in local star-forming galaxies
    (2019) Lian, Jianhui; Thomas, Daniel; Li, Cheng; Zheng, Zheng; Maraston, Claudia; Bizyaev, Dmitry; Lane, Richard R.; Yan, Renbin
    Within the standard model of hierarchical galaxy formation in a Lambda cold dark matter universe, the environment of galaxies is expected to play a key role in driving galaxy formation and evolution. In this paper, we investigate whether and how the gas metallicity and the star formation surface density (Sigma(SFR)) depend on galaxy environment. To this end, we analyse a sample of 1162 local, star-forming galaxies from the galaxy survey Mapping Nearby Galaxies at APO (MaNGA). Generally, both parameters do not show any significant dependence on environment. However, in agreement with previous studies, we find that low-mass satellite galaxies are an exception to this rule. The gas metallicity in these objects increases while their Sigma(SFR) decreases slightly with environmental density. The present analysis of MaNGA data allows us to extend this to spatially resolved properties. Our study reveals that the gas metallicity gradients of low-mass satellites flatten and their Sigma(SFR) gradients steepen with increasing environmental density. By extensively exploring a chemical evolution model, we identify two scenarios that are able to explain this pattern: metal-enriched gas accretion or pristine gas inflow with varying accretion time-scales. The latter scenario better matches the observed Sigma(SFR) gradients, and is therefore our preferred solution. In this model, a shorter gas accretion time-scale at larger radii is required. This suggests that 'outside-in quenching' governs the star formation processes of low-mass satellite galaxies in dense environments.
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    SDSS-IV MaNGA: full spectroscopic bulge-disc decomposition of MaNGA early-type galaxies
    (2019) Tabor, Martha; Merrifield, Michael; Aragon-Salamanca, Alfonso; Fraser-McKelvie, Amelia; Peterken, Thomas; Smethurst, Rebecca; Drory, Niv; Lane, Richard R.
    By applying spectroscopic decomposition methods to a sample of MaNGA early-type galaxies, we separate out spatially and kinematically distinct stellar populations, allowing us to explore the similarities and differences between galaxy bulges and discs, and how they affect the global properties of the galaxy. We find that the components have interesting variations in their stellar populations, and display different kinematics. Bulges tend to be consistently more metal rich than their disc counterparts, and while the ages of both components are comparable, there is an interesting tail of younger, more metal-poor discs. Bulges and discs follow their own distinct kinematic relationships, both on the plane of the stellar spin parameter, lambda(R), and ellipticity, epsilon, and in the relation between stellar mass, M-*, and specific angular momentum, j(*), with the location of the galaxy as a whole on these planes being determined by how much bulge and disc it contains. As a check of the physical significance of the kinematic decompositions, we also dynamically model the individual galaxy components within the global potential of the galaxy. The resulting components exhibit kinematic parameters consistent with those from the spectroscopic decomposition, and though the dynamical modelling suffers from some degeneracies, the bulges and discs display systematically different intrinsic dynamical properties. This work demonstrates the value in considering the individual components of galaxies rather than treating them as a single entity, which neglects information that may be crucial in understanding where, when, and how galaxies evolve into the systems we see today.
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    SDSS-IV MaNGA: stellar initial mass function variation inferred from Bayesian analysis of the integral field spectroscopy of early-type galaxies
    (2019) Zhou, Shuang; Mo, H. J.; Li, Cheng; Zheng, Zheng; Li, Niu; Du, Cheng; Mao, Shude; Parikh, Taniya; Lane, Richard R.; Thomas, Daniel
    We analyse the stellar initial mass functions (IMF) of a large sample of early-type galaxies (ETGs) provided by MaNGA (Mapping Nearby Galaxies at Apache Point Observatory). The large number of integral field unit (IFU) spectra of individual galaxies provide high signal-to-noise composite spectra that are essential for constraining IMF and to investigate possible radial gradients of the IMF within individual galaxies. The large sample of ETGs also make it possible to study how the IMF shape depends on various properties of galaxies. We adopt a novel approach to study IMF variations in ETGs, use Bayesian inferences based on full spectrum fitting. The Bayesian method provides a statistically rigorous way to explore potential degeneracy in spectrum fitting and to distinguish different IMFmodels with Bayesian evidence. We find that the IMF slope depends systematically on galaxy velocity dispersion, in that galaxies of higher velocity dispersion prefer amore bottom-heavy IMF, but the dependence is almost entirely due to the change of metallicity, Z, with velocity dispersion. The IMF shape also depends on stellar age, A, but the dependence is completely degenerate with that on metallicity through a combination AZ(-1.42). Using independent age and metallicity estimates we find that the IMF variation is produced by metallicity instead of age. The IMF near the centres of massive ETGs appears more bottom-heavy than that in the outer parts, while a weak opposite trend is seen for low-mass ETGs. Uncertainties produced by star formation history, dust extinction, a-element abundance enhancement, and noise in the spectra are tested.
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    SDSS-IV MaNGA: The MaNGA Dwarf Galaxy Sample Presentation
    (2022) Cano-Diaz, M.; Hernandez-Toledo, H. M.; Rodriguez-Puebla, A.; Ibarra-Medel, H. J.; Avila-Reese, V; Valenzuela, O.; Medellin-Hurtado, A. E.; Vazquez-Mata, J. A.; Weijmans, A.; Gonzalez, J. J.; Aquino-Ortiz, E.; Martinez-Vazquez, L. A.; Lane, Richard R.
    We present the MaNGA Dwarf galaxy (MaNDala) Value Added Catalog (VAC), from the final release of the Sloan Digital Sky Survey-IV program. MaNDala consists of 136 randomly selected bright dwarf galaxies with M (*) < 10(9.1) M (circle dot) and M ( g ) > -18.5, making it the largest integral field spectroscopy homogeneous sample of dwarf galaxies. We release a photometric analysis of the g, r, and z broadband imaging based on the DESI Legacy Imaging Surveys, as well as a spectroscopic analysis based on the Pipe3D SDSS-IV VAC. Our release includes the surface brightness (SB), geometric parameters, and color profiles, Sersic fits as well as stellar population properties (such as stellar ages, metallicities, and star formation histories), and emission lines' fluxes within the FOV and the effective radii of the galaxies. We find that the majority of the MaNDala galaxies are star-forming late-type galaxies with n (Sersic,r) similar to 1.6 that are centrals (central/satellite dichotomy). MaNDala covers a large range of SB values (we find 11 candidate ultra-diffuse galaxies and three compact ones), filling the gap between classical dwarfs and low-mass galaxies in the Kormendy Diagram and in the size-mass/luminosity relation, which seems to flatten at 10(8) < M (*)/M (circle dot) < 10(9) with R ( e,r ) similar to 2.7 kpc. A large fraction of MaNDala galaxies formed from an early low-metallicity burst of SF, but also from late SF events from more metal-enriched gas: half of the MaNDala galaxies assembled 50% of their mass at z > 2, while the last 20% was at z < 0.3. Finally, a bending of the sSFR-M (*) relation at M (*) similar to 10(9) M (circle dot) for the main-sequence galaxies seems to be supported by MaNDala.
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    Sloan digital sky survey IV: Mapping the Milky Way, nearby galaxies, and the distant universe
    (2017) Blanton, Michael R.; Bershady, Matthew A.; Abolfathi, Bela; Albareti, Franco D.; Allende Prieto, Carlos; Almeida, Andres; Alonso García, Javier; Anders, Friedrich; Zoccali, Manuela; Padilla, Nelson; Aquino Ortíz, Eril; Aragón Salamanca, Alfonso; Argudo Fernández, Maria; Armengaud, Eric; Aubourg, Eric; Avila Reese, Vladimir; Lacerna Zambrano, Iván Andrés; Lane, Richard R.; Andrews, Brett; Anderson, Scott F.
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    Spatial variations in the Milky Way disc metallicity-age relation
    (2019) Feuillet, Diane K.; Frankel, Neige; Lind, Karin; Frinchaboy, Peter M.; Garcia-Hernandez, D. A.; Lane, Richard R.; Nitschelm, Christian; Roman-Lopes, Alexandre
    Stellar ages are a crucial component to studying the evolution of the Milky Way. Using Gaia DR2 distance estimates, it is now possible to estimate stellar ages for a larger volume of evolved stars through isochrone matching. This work presents [M/H]-age and [alpha/M]-age relations derived for different spatial locations in the Milky Way disc. These relations are derived by hierarchically modelling the star formation history of stars within a given chemical abundance bin. For the first time, we directly observe that significant variation is apparent in the [M/H]-age relation as a function of both Galactocentric radius and distance from the disc midplane. The [M/H]-age relations support claims that radial migration has a significant effect in the plane of the disc. Using the [M/H] bin with the youngest mean age at each radial zone in the plane of the disc, the present-day metallicity gradient is measured to be -0.059 +/- 0.010 dex kpc(-1), in agreement with Cepheids and young field stars. We find a vertically flared distribution of young stars in the outer disc, confirming predictions of models and previous observations. The mean age of the [M/H]-[alpha/M] distribution of the solar neighbourhood suggests that the high-[M/H] stars are not an evolutionary extension of the low-alpha sequence. Our observational results are important constraints to Galactic simulations and models of chemical evolution.
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    Strong chemical tagging with APOGEE: 21 candidate star clusters that have dissolved across the Milky Way disc
    (2020) Price-Jones, Natalie; Bovy, Jo; Webb, Jeremy J.; Prieto, Carlos Allende; Beaton, Rachael; Brownstein, Joel R.; Cohen, Roger E.; Cunha, Katia; Donor, John; Frinchaboy, Peter M.; Garcia-Hernandez, D. A.; Lane, Richard R.; Majewski, Steven R.; Nidever, David L.; Roman-Lopes, Alexandre
    Chemically tagging groups of stars born in the same birth cluster is a major goal of spectroscopic surveys. To investigate the feasibility of such strong chemical tagging, we perform a blind chemical tagging experiment on abundances measured from APOGEE survey spectra. We apply a density-based clustering algorithm to the 8D chemical space defined by [Mg/Fe], [Al/Fe], [Si/Fe], [K/Fe], [Ti/Fe], [Mn/Fe], [Fe/H], and [Ni/Fe], abundances ratios which together span multiple nucleosynthetic channels. In a high-quality sample of 182 538 giant stars, we detect 21 candidate clusterswith more than 15 members. Our candidate clusters are more chemically homogeneous than a population of non-member stars with similar [Mg/Fe] and [Fe/H], even in abundances not used for tagging. Group members are consistent with having the same age and fall along a single stellar-population track in log g versus T-eff space. Each group's members are distributed over multiple kpc, and the spread in their radial and azimuthal actions increases with age. We qualitatively reproduce this increase using N-body simulations of cluster dissolution in Galactic potentials that include transient winding spiral arms. Observing our candidate birth clusters with high-resolution spectroscopy in other wavebands to investigate their chemical homogeneity in other nucleosynthetic groups will be essential to confirming the efficacy of strong chemical tagging. Our initially spatially compact but now widely dispersed candidate clusters will provide novel limits on chemical evolution and orbital diffusion in the Galactic disc, and constraints on star formation in loosely bound groups.
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    The 13th Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-IV Survey Mapping Nearby Galaxies at Apache Point Observatory
    (2017) Albareti, Franco D.; Allende Prieto, Carlos; Almeida, Andrés; Anders, Friedrich; Anderson, Scott; Lacerna Zambrano, Iván Andrés; Lane, Richard R.; Lin, Lihwai; Lin, Yen-Ting; Padilla, Nelson