-
[show abstract]
[hide abstract]
ABSTRACT: Context. Stellar collision rates in globular clusters (GCs) do not appear to
correlate with horizontal branch (HB) morphology, sug- gesting that dynamics
does not play a role in the second-parameter problem. However, core densities
and collision rates derived from surface-brightness may be significantly
underestimated as the surface-brightness profile of GCs is not necessarily a
good indicator of the dynamical state of GC cores. Core-collapse may go
unnoticed if high central densities of dark remnants are present. Aims. We test
whether GC HB morphology data supports a dynamical contribution to the
so-called second-parameter effect. Methods. To remove first-parameter
dependence we fitted the maximum effective temperature along the HB as a
function of metal- licity in a sample of 54 Milky Way GCs. We plotted the
residuals to the fit as a function of second-parameter candidates, namely
dynamical age and total luminosity. We considered dynamical age (i.e. the ratio
between age and half-light relaxation time) among possible second-parameters.
We used a set of direct N-body simulations, including ones with dark remnants
to illustrate how core density peaks, due to core collapse, in a dynamical-age
range similar to that in which blue HBs are overabundant with respect to the
metallicity expectation, especially for low-concentration initial conditions.
Results. GC total luminosity shows nonlinear behavior compatible with the
self-enrichment picture. However, the data are amenable to a different
interpretation based on a dynamical origin of the second-parameter effect.
Enhanced mass-stripping in the late red-giant- branch phase due to stellar
interactions in collapsing cores is a viable candidate mechanism. In this
picture, GCs with HBs bluer than expected based on metallicity are those
undergoing core-collapse.
05/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: The mean color of globular clusters (GCs) in early-type galaxies is in
general bluer than the integrated color of halo field stars in host galaxies.
Metal-rich GCs often appear more associated with field stars than metal-poor
GCs, yet show bluer colors than their host galaxy light. Motivated by the
discovery of multiple stellar populations in Milky Way GCs, we present a new
scenario in which the presence of second-generation (SG) stars in GCs is
responsible for the color discrepancy between metal-rich GCs and field stars.
The model assumes that the SG populations have enhanced helium abundance as
evidenced by observations, and it gives a good explanation of the bluer optical
colors of metal-rich GCs than field stars as well as strong Balmer lines and
blue UV colors of metal-rich GCs. Ours may be complementary to the recent
scenario suggesting the difference in stellar mass functions (MFs) as an origin
for the GC-to-star color offset. A quantitative comparison is given between the
SG and MF models.
04/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: We present wide-field multiband photometry of globular cluster (GC) systems
in NGC 1399, NGC 1404, and NGC 1387 located at the central region of the Fornax
galaxy cluster. Observation was carried out through U, B, V, and I bands, which
marks one of the widest and deepest U-band studies on extragalactic GC systems.
The present U-band photometry enables us to significantly reduce the
contamination by a factor of two for faint sources (Vo~23.5). The main results
based on some 2000 GC candidates around NGC 1399, NGC 1404, and NGC 1387 are as
follows: (1) the GC system in each galaxy exhibits bimodal color distributions
in all colors examined, but the shape of color histograms varies systematically
depending on colors; (2) NGC 1399 shows that the mean colors of both blue and
red GCs become bluer with increasing galactocentric radius; (3) NGC 1399 shows
overabundance of GCs in the directions of NGC 1404 and NGC 1387, indicating
their ongoing interactions; and (4) NGC 1399 also exhibits a ~0.5' offset
between the center of the inner GC distribution and the galaxy's optical
center, suggesting that NGC 1399 is not yet dynamically relaxed and may be
undergoing merger events.
01/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: There is increasing evidence that some massive globular clusters (GCs) host
multiple stellar populations having different heavy element abundances enriched
by supernovae. They usually accompany multiple red giant branches (RGBs) in the
color-magnitude diagrams (CMDs), and are distinguished from most of the other
GCs which display variations only in light element abundances. In order to
investigate the star formation histories of these peculiar GCs, we have
constructed synthetic CMDs for \omega\ Cen, M22, and NGC 1851. Our models are
based on the updated versions of Yonsei-Yale (Y^2) isochrones and horizontal
branch (HB) evolutionary tracks which include the cases of enhancements in both
helium and the total CNO abundances. To estimate ages and helium abundances of
subpopulations in each GC, we have compared our models with the observations on
the Hess diagram by employing a \chi^2 minimization technique. We find that
metal-rich subpopulations in each of these GCs are also enhanced in helium
abundance, and the age differences between the metal-rich and metal-poor
subpopulations are fairly small (~0.3-1.7 Gyr), even in the models with the
observed variations in the total CNO content. These are required to
simultaneously reproduce the observed extended HB and the splits on the main
sequence, subgiant branch, and RGB. Our results are consistent with the
hypothesis that these GCs are the relics of more massive primeval dwarf
galaxies that merged and disrupted to form the proto-Galaxy.
11/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: We present a series of papers on the year-2012 version of Yonsei Evolutionary
Population Synthesis (YEPS) model which is constructed on over 20 years of
heritage. This first paper delineates the spectroscopic aspect of integrated
light from stellar populations older than 1 Gyr. The standard YEPS is based on
the most up-to-date Yonsei-Yale stellar evolutionary tracks and BaSel 3.1 flux
libraries, and provides absorption line indices of the Lick/IDS system and
high-order Balmer lines for simple stellar populations as functions of stellar
parameters, such as metallicity, age and {\alpha}-element mixture. Special care
has been taken to incorporate systematic contribution from horizontal-branch
stars which alters the temperature-sensitive Balmer lines significantly,
resulting in up to 5 Gyr difference in age estimation of old, metal-poor
stellar populations. We also find that the horizontal branches exert an
appreciable effect not only on the Balmer lines but also on the
metallicity-sensitive lines including the magnesium index. This is critical to
explain the intriguing bimodality found in index distributions of globular
clusters in massive galaxies and to derive spectroscopic metallicities
accurately from various indices. A full set of the spectroscopic and
photometric YEPS model data of the entire parameter space is currently
downloadable at http://web.yonsei.ac.kr/cosmic/data/YEPS.htm
10/2012;
-
Ricardo P. Schiavon,
Emanuele Dalessandro,
Sangmo T. Sohn,
Robert T. Rood,
Robert W. O'Connell,
Francesco R. Ferraro,
Barbara Lanzoni,
Giacomo Beccari,
Soo-Chang Rey,
Jaehyon Rhee,
R. Michael Rich,
Suk-Jin Yoon, Young-Wook Lee
[show abstract]
[hide abstract]
ABSTRACT: We present GALEX data for 44 Galactic globular clusters obtained during 3
GALEX observing cycles between 2004 and 2008. This is the largest homogeneous
data set on the UV photometric properties of Galactic globular clusters ever
collected. The sample selection and photometric analysis are discussed, and
color-magnitude diagrams are presented. The blue and intermediate-blue
horizontal branch is the dominant feature of the UV color-magnitude diagrams of
old Galactic globular clusters. Our sample is large enough to display the
remarkable variety of horizontal branch shapes found in old stellar
populations. Other stellar types that are obviously detected are blue
stragglers and post core-He burning stars. The main features of UV
color-magnitude diagrams of Galactic globular clusters are briefly discussed.
We establish the locus of post-core He burning stars in the UV color-magnitude
diagram and present a catalog of candidate AGB-manqu \'e, post early-AGB, and
post-AGB stars within our cluster sample.
01/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: Recent observations and modeling of globular clusters with multiple
populations strongly indicate the presence of super helium-rich subpopulations
in old stellar systems. Motivated by this, we have constructed new population
synthesis models with and without helium-enhanced subpopulations to investigate
their impact on the UV-upturn phenomenon of quiescent early-type galaxies. We
find that our models with helium- enhanced subpopulations can naturally
reproduce the strong UV-upturns observed in giant elliptical galaxies assuming
an age similar to that of old globular clusters in the Milky Way. The major
source of far-UV (FUV) flux, in this model, is relatively metal- poor and
helium-enhanced hot horizontal branch stars and their progeny. The Burstein et
al. (1988) relation of the F U V - V color with metallicity is also explained
either by the variation of the fraction of helium-enhanced subpopulations or by
the spread in mean age of stellar populations in early-type galaxies.
09/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We report the presence of two distinct red giant branches (RGBs) in the globular cluster NGC 288 from the narrowband calcium and Strömgren b and y photometry obtained at the CTIO 4 m Blanco telescope. The RGB of NGC 288 is clearly split into two in the hk [=(Ca – b) – (b – y)] index, while the split is not shown in the b – y color. Unlike other globular clusters with multiple populations reported thus far, the horizontal branch of NGC 288 is only mildly extended. Our stellar population models show that this and the presence of two distinct RGBs in NGC 288 can be reproduced if slightly metal-rich (Δ[m/H] 0.16) second generation stars are also enhanced in helium by small amount (ΔY 0.03) and younger by ~1.5 Gyr. The RGB split in the hk index is most likely indicating that the second generation stars were affected by supernovae enrichment, together with the pollution of lighter elements by intermediate-mass asymptotic giant branch stars or fast-rotating massive stars. In order to confirm this, however, spectroscopy of stars in the two distinct RGB groups is urgently required.
The Astrophysical Journal Letters 05/2011; 733(2):L45. · 5.53 Impact Factor
-
James D. Neill,
Mark Sullivan,
Avishay Gal-Yam,
Robert Quimby,
Eran Ofek,
Ted K. Wyder,
D. Andrew Howell,
Peter Nugent,
Mark Seibert,
D. Christopher Martin, [......],
Susan G. Neff,
David Schiminovich,
Luciana Bianchi,
José Donas,
Timothy M. Heckman, Young-Wook Lee,
Barry F. Madore,
Bruno Milliard,
R. Michael Rich,
Alex S. Szalay
[show abstract]
[hide abstract]
ABSTRACT: We use GALEX ultraviolet (UV) and optical integrated photometry of the hosts of seventeen luminous supernovae (LSNe, having peak M_V < -21) and compare them to a sample of 26,000 galaxies from a cross-match between the SDSS DR4 spectral catalog and GALEX interim release 1.1. We place the LSNe hosts on the galaxy NUV-r versus M_r color magnitude diagram (CMD) with the larger sample to illustrate how extreme they are. The LSN hosts appear to favor low-density regions of the galaxy CMD falling on the blue edge of the blue cloud toward the low luminosity end. From the UV-optical photometry, we estimate the star formation history of the LSN hosts. The hosts have moderately low star formation rates (SFRs) and low stellar masses (M_*) resulting in high specific star formation rates (sSFR). Compared with the larger sample, the LSN hosts occupy low-density regions of a diagram plotting sSFR versus M_* in the area having higher sSFR and lower M_*. This preference for low M_*, high sSFR hosts implies the LSNe are produced by an effect having to do with their local environment. The correlation of mass with metallicity suggests that perhaps wind-driven mass loss is the factor that prevents LSNe from arising in higher-mass, higher-metallicity hosts. The massive progenitors of the LSNe (>100 M_sun), by appearing in low-SFR hosts, are potential tests for theories of the initial mass function that limit the maximum mass of a star based on the SFR. Comment: 8 pages, 3 figures, 2 tables, accepted to ApJ, amended references and updated SN designations
11/2010;
-
Yeong-Shang Loh,
R. Michael Rich,
Sébastien Heinis,
Ryan Scranton,
Ryan P. Mallery,
Samir Salim,
D. Christopher Martin,
Ted Wyder,
Stéphane Arnouts,
Tom A. Barlow, [......],
Mark Seibert,
Luciana Bianchi,
Jose Donas,
Timothy M. Heckman, Young-Wook Lee,
Barry F. Madore,
Bruno Milliard,
Alex S. Szalay,
Barry Y. Welsh,
Suk Young Yi
[show abstract]
[hide abstract]
ABSTRACT: We measure the UV-optical color dependence of galaxy clustering in the local universe. Using the clean separation of the red and blue sequences made possible by the NUV - r color-magnitude diagram, we segregate the galaxies into red, blue and intermediate "green" classes. We explore the clustering as a function of this segregation by removing the dependence on luminosity and by excluding edge-on galaxies as a means of a non-model dependent veto of highly extincted galaxies. We find that \xi (r_p, \pi) for both red and green galaxies shows strong redshift space distortion on small scales -- the "finger-of-God" effect, with green galaxies having a lower amplitude than is seen for the red sequence, and the blue sequence showing almost no distortion. On large scales, \xi (r_p, \pi) for all three samples show the effect of large-scale streaming from coherent infall. On scales 1 Mpc/h < r_p < 10 Mpc/h, the projected auto-correlation function w_p(r_p) for red and green galaxies fits a power-law with slope \gamma ~ 1.93 and amplitude r_0 ~ 7.5 and 5.3, compared with \gamma ~ 1.75 and r_0 ~ 3.9 Mpc/h for blue sequence galaxies. Compared to the clustering of a fiducial L* galaxy, the red, green, and blue have a relative bias of 1.5, 1.1, and 0.9 respectively. The w_p(r_p) for blue galaxies display an increase in convexity at ~ 1 Mpc/h, with an excess of large scale clustering. Our results suggest that the majority of blue galaxies are likely central galaxies in less massive halos, while red and green galaxies have larger satellite fractions, and preferentially reside in virialized structures. If blue sequence galaxies migrate to the red sequence via processes like mergers or quenching that take them through the green valley, such a transformation may be accompanied by a change in environment in addition to any change in luminosity and color. Comment: accepted by MNRAS
04/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: Wide-field deep g'r'i' images obtained with the Megacam of the Canada-France-Hawaii Telescope are used to investigate the spatial configuration of stars around five metal-poor globular clusters M15, M30, M53, NGC 5053, and NGC 5466, in a field-of-view ~3°. Applying a mask filtering algorithm to the color-magnitude diagrams of the observed stars, we sorted cluster's member star candidates that are used to examine the characteristics of the spatial stellar distribution surrounding the target clusters. The smoothed surface density maps and the overlaid isodensity contours indicate that all of the five metal-poor globular clusters exhibit strong evidence of extratidal overdensity features over their tidal radii, in the form of extended tidal tails around the clusters. The orientations of the observed extratidal features show signatures of tidal tails tracing the clusters' orbits, inferred from their proper motions, and effects of dynamical interactions with the Galaxy. Our findings include detections of a tidal bridge-like feature and an envelope structure around the pair of globular clusters M53 and NGC 5053. The observed radial surface density profiles of target clusters have a deviation from theoretical King models, for which the profiles show a break at 0.5-0.7rt , extending the overdensity features out to 1.5-2rt . Both radial surface density profiles for different angular sections and azimuthal number density profiles confirm the overdensity features of tidal tails around the five metal-poor globular clusters. Our results add further observational evidence that the observed metal-poor halo globular clusters originate from an accreted satellite system, indicative of the merging scenario of the formation of the Galactic halo.
The Astronomical Journal 01/2010; 139(2):606. · 4.03 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: There is a growing body of evidence for the presence of multiple stellar populations in some globular clusters, including NGC 1851. For most of these peculiar globular clusters, however, the evidence for the multiple red giant branches (RGBs) having different heavy elemental abundances as observed in ω Centauri is hitherto lacking, although spreads in some lighter elements are reported. It is therefore not clear whether they also share the suggested dwarf galaxy origin of ω Cen or not. Here we show from the CTIO 4 m UVI photometry of the globular cluster NGC 1851 that its RGB is clearly split into two in the U – I color. The two distinct RGB populations are also clearly separated in the abundance of heavy elements as traced by calcium, suggesting that the Type II supernovae enrichment is also responsible, in addition to the pollutions of lighter elements by intermediate-mass asymptotic giant branch stars or fast-rotating massive stars. The RGB split, however, is not shown in the V – I color, as indicated by previous observations. Our stellar population models show that this and the presence of bimodal horizontal-branch distribution in NGC 1851 can be naturally reproduced if the metal-rich second generation stars are also enhanced in helium.
The Astrophysical Journal 12/2009; 707(2):L190. · 6.02 Impact Factor
-
James D. Neill,
Mark Sullivan,
D. Andrew Howell,
Alex Conley,
Mark Seibert,
D. Christopher Martin,
Tom A. Barlow,
Karl Foster,
Peter G. Friedman,
Patrick Morrissey, [......],
David Schiminovich,
Ted K. Wyder,
Luciana Bianchi,
José Donas,
Timothy M. Heckman, Young-Wook Lee,
Barry F. Madore,
Bruno Milliard,
R. Michael Rich,
and Alex S. Szalay
[show abstract]
[hide abstract]
ABSTRACT: We use multi-wavelength, matched aperture, integrated photometry from the Galaxy Evolution Explorer (GALEX), the Sloan Digital Sky Survey, and the RC3 to estimate the physical properties of 166 nearby galaxies hosting 168 well-observed Type Ia supernovae (SNe Ia). The ultraviolet (UV) imaging of local SN Ia hosts from GALEX allows a direct comparison with higher-redshift hosts measured at optical wavelengths that correspond to the rest-frame UV. Our data corroborate well-known features that have been seen in other SN Ia samples. Specifically, hosts with active star formation produce brighter and slower SNe Ia on average, and hosts with luminosity-weighted ages older than 1 Gyr produce on average more faint, fast, and fewer bright, slow SNe Ia than younger hosts. New results include that in our sample, the faintest and fastest SNe Ia occur only in galaxies exceeding a stellar mass threshold of ~1010 M ☉, leading us to conclude that their progenitors must arise in populations that are older and/or more metal rich than the general SN Ia population. A low host extinction subsample hints at a residual trend in peak luminosity with host age, after correcting for light-curve shape, giving the appearance that older hosts produce less-extincted SNe Ia on average. This has implications for cosmological fitting of SNe Ia, and suggests that host age could be useful as a parameter in the fitting. Converting host mass to metallicity and computing 56Ni mass from the supernova light curves, we find that our local sample is consistent with a model that predicts a shallow trend between stellar metallicity and the 56Ni mass that powers the explosion, but we cannot rule out the absence of a trend. We measure a correlation between 56Ni mass and host age in the local universe that is shallower and not as significant as that seen at higher redshifts. The details of the age-56Ni mass correlations at low and higher redshift imply a luminosity-weighted age threshold of ~3 Gyr for SN Ia hosts, above which they are less likely to produce SNe Ia with 56Ni masses above ~0.5 M ☉.
The Astrophysical Journal 12/2009; 707(2):1449. · 6.02 Impact Factor
-
Yun-Kyeong Sheen,
Hyunjin Jeong,
Sukyoung K. Yi,
Ignacio Ferreras,
Jennifer M. Lotz,
Knut A. G. Olsen,
Mark Dickinson,
Sydney Barnes,
Jang-Hyun Park,
Chang H. Ree, [......],
Karl Foster,
Peter G. Friendman, Young-Wook Lee,
D. Christopher Martin,
Patrick Morrissey,
Susan G. Neff,
David Schiminovich,
Mark Seibert,
Todd Small,
and Ted K. Wyder
[show abstract]
[hide abstract]
ABSTRACT: One possible channel for the formation of dwarf galaxies involves birth in the tidal tails of interacting galaxies. We report the detection of a bright UV tidal tail and several young tidal dwarf galaxy (TDG) candidates in the post-merger galaxy NGC 4922 in the Coma cluster. Based on a two-component population model (combining young and old stellar populations), we find that the light of tidal tail predominantly comes from young stars (a few Myr old). The Galaxy Evolution Explorer ultraviolet data played a critical role in the parameter (age and mass) estimation. Our stellar mass estimates of the TDG candidates are ~106–7 M ☉, typical for dwarf galaxies.
The Astronomical Journal 11/2009; 138(6):1911. · 4.03 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The most massive globular cluster in the Milky Way, omega Centauri, is thought to be the remaining core of a disrupted dwarf galaxy, as expected within the model of hierarchical merging. It contains several stellar populations having different heavy elemental abundances supplied by supernovae-a process known as metal enrichment. Although M 22 appears to be similar to omega Cen, other peculiar globular clusters do not. Therefore omega Cen and M 22 are viewed as exceptional, and the presence of chemical inhomogeneities in other clusters is seen as 'pollution' from the intermediate-mass asymptotic-giant-branch stars expected in normal globular clusters. Here we report Ca abundances for seven globular clusters and compare them to omega Cen. Calcium and other heavy elements can only be supplied through numerous supernovae explosions of massive stars in these stellar systems, but the gravitational potentials of the present-day clusters cannot preserve most of the ejecta from such explosions. We conclude that these globular clusters, like omega Cen, are most probably the relics of more massive primeval dwarf galaxies that merged and disrupted to form the proto-Galaxy.
Nature 11/2009; 462(7272):480-2. · 36.28 Impact Factor
-
Antara R. Basu-Zych,
David Schiminovich,
Sebastien Heinis,
Roderik Overzier,
Tim Heckman,
Michel Zamojski,
Olivier Ilbert,
Anton M. Koekemoer,
Tom A. Barlow,
Luciana Bianchi, [......],
Bruno Milliard,
Patrick Morrissey,
Susan G. Neff,
R. Michael Rich,
Samir Salim,
Mark Seibert,
Todd A. Small,
Alex S. Szalay,
Ted K. Wyder,
and Sukyoung Yi
[show abstract]
[hide abstract]
ABSTRACT: Studying the environments of 0.4 < z < 1.2 ultraviolet (UV)-selected galaxies, as examples of extreme star-forming galaxies (with star formation rates (SFRs) in the range of 3-30 M ☉ yr–1), we explore the relationship between high rates of star formation, host halo mass, and pair fractions. We study the large- and small-scale environments of local ultraviolet luminous galaxies (UVLGs) by measuring angular correlation functions. We cross-correlate these systems with other galaxy samples: a volume-limited sample (ALL), a blue luminous galaxy sample, and a luminous red galaxy (LRG) sample. We determine the UVLG comoving correlation length to be r 0 = 4.8+11.6 –2.4 h –1 Mpc at z = 1.0, which is unable to constrain the halo mass for this sample. However, we find that UVLGs form close (separation <30 kpc) pairs with the ALL sample, but do not frequently form pairs with LRGs. A rare subset of UVLGs, those with the highest FUV surface brightnesses, are believed to be local analogs of high-redshift Lyman break galaxies (LBGs) and are called Lyman break analogs (LBAs). LBGs and LBAs share similar characteristics (i.e., color, size, surface brightness, specific SFRs, metallicities, and dust content). Recent Hubble Space Telescope images of z ~ 0.2 LBAs show disturbed morphologies, signs of mergers and interactions. UVLGs may be influenced by interactions with other galaxies and we discuss this result in terms of other high star-forming, merging systems.
The Astrophysical Journal 06/2009; 699(2):1307. · 6.02 Impact Factor
-
Soo-Chang Rey,
Sangmo T. Sohn,
Michael A. Beasley, Young-Wook Lee,
R. Michael Rich,
Suk-Jin Yoon,
Sukyoung K. Yi,
Luciana Bianch,
Yongbeom Kang,
Kyeongsook Lee, [......],
Susan G. Neff,
David Schiminovich,
Mark Seibert,
Ted K. Wyder,
Jose Donas,
Timothy M. Heckman,
Barry F. Madore,
Bruno Milliard,
Alex S. Szalay,
Barry Y. Welsh
[show abstract]
[hide abstract]
ABSTRACT: We explore the age distribution of the globular cluster (GC) system of the nearby elliptical galaxy NGC 5128 using ultraviolet (UV) photometry from Galaxy Evolution Explorer (GALEX) observations, with UV - optical colors used as the age indicator. Most GCs in NGC 5128 follow the general trends of GCs in M31 and Milky Way in UV - optical color-color diagram, which indicates that the majority of GCs in NGC 5128 are old similar to the age range of old GCs in M31 and Milky Way. A large fraction of spectroscopically identified intermediate-age GC (IAGC) candidates with ~ 3-8 Gyr are not detected in the FUV passband. Considering the nature of intermediate-age populations being faint in the far-UV (FUV) passband, we suggest that many of the spectroscopically identified IAGCs may be truly intermediate in age. This is in contrast to the case of M31 where a large fraction of spectroscopically suggested IAGCs are detected in FUV and therefore may not be genuine IAGCs but rather older GCs with developed blue horizontal branch stars. Our UV photometry strengthens the results previously suggesting the presence of GC and stellar subpopulation with intermediate age in NGC 5128. The existence of IAGCs strongly indicates the occurrence of at least one more major star formation episode after a starburst at high redshift. Comment: 8 pages, 3 figures, accepted for ApJ Letter
06/2009;
-
Sébastien Heinis,
Tamás Budavári,
Alex S. Szalay,
Stéphane Arnouts,
Miguel A. Aragón-Calvo,
Ted K. Wyder,
Tom A. Barlow,
Karl Foster,
Friedman G. Peter,
D. Christopher Martin, [......],
David Schiminovich,
Mark Seibert,
Luciana Bianchi,
José Donas,
Timothy M. Heckman, Young-Wook Lee,
Barry F. Madore,
Bruno Milliard,
R. Michael Rich,
and Sukyoung K. Yi
[show abstract]
[hide abstract]
ABSTRACT: We measure the projected spatial correlation function wp (rp ) from a large sample combining Galaxy Evolution Explorer ultraviolet imaging with the Sloan Digital Sky Survey spectroscopic sample. We study the dependence of the clustering strength for samples selected on (NUV – r)abs color, specific star formation rate (SSFR), and stellar mass. We find that there is a smooth transition in the clustering of galaxies as a function of this color from weak clustering among blue galaxies to stronger clustering for red galaxies. The clustering of galaxies within the "green valley" has an intermediate strength, and is consistent with that expected from galaxy groups. The results are robust to the correction for dust extinction. The comparison with simple analytical modeling suggests that the halo occupation number increases with older star formation epochs. When splitting according to SSFR, we find that the SSFR is a more sensitive tracer of environment than stellar mass.
The Astrophysical Journal 06/2009; 698(2):1838. · 6.02 Impact Factor
-
Ted K. Wyder,
D. Christopher Martin,
Tom A. Barlow,
Karl Foster,
Peter G. Friedman,
Patrick Morrissey,
Susan G. Neff,
James D. Neill,
David Schiminovich,
Mark Seibert,
Luciana Bianchi,
José Donas,
Timothy M. Heckman, Young-Wook Lee,
Barry F. Madore,
Bruno Milliard,
R. Michael Rich,
Alex S. Szalay,
and Sukyoung K. Yi
[show abstract]
[hide abstract]
ABSTRACT: We investigate the nature of the star formation law at low gas surface densities using a sample of 19 low surface brightness (LSB) galaxies with existing H I maps in the literature, UV imaging from the Galaxy Evolution Explorer satellite, and optical images from the Sloan Digital Sky Survey. All of the LSB galaxies have (NUV – r) colors similar to those for higher surface brightness star-forming galaxies of similar luminosity indicating that their average star formation histories are not very different. Based upon four LSB galaxies with both UV and far-infrared (FIR) data, we find FIR/UV ratios significantly less than 1, implying low amounts of internal UV extinction in LSB galaxies. We use the UV images and H I maps to measure the star formation rate (SFR) and hydrogen gas surface density within the same region for all the galaxies. The LSB galaxy star formation rate surface densities lie below the extrapolation of the power law fit to the SFR surface density as a function of the total gas density for higher surface brightness galaxies. Although there is more scatter, the LSB galaxies also lie below a second version of the star formation law in which the SFR surface density is correlated with the gas density divided by the orbital time in the disk. The downturn seen in both star formation laws is consistent with theoretical models that predict lower star formation efficiencies in LSB galaxies due to the declining molecular fraction with decreasing density.
The Astrophysical Journal 04/2009; 696(2):1834. · 6.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We investigate the red giant branch (RGB) subpopulations of NGC 1851 using Ca uvby photometry. Our color-magnitude diagrams show that the RGB stars have two subpopulations and the RGB stars in NGC 1851 appear to have distinct elemental abundance patterns with the [Ca/H] abundance. We discuss that the elemental abundance patterns can be explained by the contributions from the asymptotic giant stars, confirming the previous studies by others. The RR Lyrae variables in NGC 1851 appear to have a large metallicity spread and, perhaps, a bimodal metallicity distribution. Our period shift analysis of the RR Lyrae variables shows that the helium enhancement appears to be insignificant in the NGC 1851 RR Lyrae population. However, the helium enrichment scenario in the blue or the red parts of horizontal branch (HB) cannot be completely ruled out. Our results show that about 18% of the bright RGB stars have enhanced CNO abundances, sharply in contrast to previous estimates by others (38%), making it difficult to explain the double subgiant, RGB sequences, and the bimodal HB distribution by their number ratios.
The Astrophysical Journal 03/2009; 695(1):L78. · 6.02 Impact Factor
