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[show abstract]
[hide abstract]
ABSTRACT: We present preliminary results from a new HST archival program aimed at
tightly constraining the ancient (>4 Gyr ago) star formation histories (SFHs)
of the field populations of the SMC and LMC. We demonstrate the quality of the
archival data by constructing HST/WFPC2-based color-magnitude diagrams (CMDs;
M_{F555W} ~ +8) for 7 spatially diverse fields in the SMC and 8 fields in the
LMC. The HST-based CMDs are >2 magnitudes deeper than any from ground based
observations, and are particularly superior in high surface brightness regions,
e.g., the LMC bar, which contain a significant fraction of star formation and
are crowding limited from ground based observations. To minimize systematic
uncertainties, we derive the SFH of each field using an identical maximum
likelihood CMD fitting technique. We then compute an approximate mass weighted
average SFH for each galaxy. We find that both galaxies lack a dominant burst
of early star formation, which suggests either a suppression or an
under-fueling of early star formation. From 10-12 Gyr ago, the LMC experienced
a period of enhanced stellar mass growth relative to the SMC. Similar to some
previous studies, we find two notable peaks in the SFH of the SMC at ~4.5 and 9
Gyr ago, which could be due to repeated close passages with the LMC, implying
an interaction history that has persisted for at least 9 Gyr. We find little
evidence for strong periodic behavior in the lifetime SFHs of both MCs,
suggesting that repeated encounters with the Milky Way are unlikely. Beginning
~3.5 Gyr ago, both galaxies show increases in their SFHs, in agreement with
previous studies, and thereafter, track each other remarkably well. (abridged)
01/2013;
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Ana E. García Pérez,
Katia Cunha,
Matthew Shetrone,
Steven R. Majewski,
Jennifer A. Johnson,
Verne V. Smith,
Ricardo P. Schiavon, Jon Holtzman,
David Nidever,
Gail Zasowski, [......],
Szabolcs Meszaros,
Robert W. O'Connel,
Daniel Oravetz,
Kaike Pan,
Annie C. Robin,
Donald P. Schneider,
Mathias Schultheis,
Michael F. Skrutskie,
Audrey Simmonsand,
John C. Wilson
[show abstract]
[hide abstract]
ABSTRACT: Despite its importance for understanding the nature of early stellar
generations and for constraining Galactic bulge formation models, at present
little is known about the metal-poor stellar content of the central Milky Way.
This is a consequence of the great distances involved and intervening dust
obscuration, which challenge optical studies. However, the Apache Point
Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber,
high-resolution spectroscopic survey within Sloan Digital Sky Survey III
(SDSS-III), is exploring the chemistry of all Galactic stellar populations at
infrared wavelengths, with particular emphasis on the disk and the bulge. An
automated spectral analysis of data on 2,403 giant stars in twelve fields in
the bulge obtained during APOGEE commissioning yielded five stars with low
metallicity([Fe/H]$\le-1.7$), including two that are very metal-poor
[Fe/H]$\sim-2.1$ by bulge standards.
Luminosity-based distance estimates place the five stars within the outer
bulge, where other 1,246 of the analyzed stars may reside. A manual reanalysis
of the spectra verifies the low metallicities, and finds these stars to be
enhanced in the $\alpha$-elements O, Mg, and Si without significant
$\alpha$-pattern differences with other local halo or metal-weak thick-disk
stars of similar metallicity, or even with other more metal-rich bulge stars.
While neither the kinematics nor chemistry of these stars can yet definitively
determine which, if any, are truly bulge members, rather than denizens of other
populations co-located with the bulge, the newly-identified stars reveal that
the chemistry of metal-poor stars in the central Galaxy resembles that of
metal-weak thick-disk stars at similar metallicity.
01/2013;
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Verne V. Smith,
Katia Cunha,
Matthew D. Shetrone,
Szabolcs Meszaros,
Carlos Allende Prieto,
Dmitry Bizyaev,
Ana Garcia Perez,
Steven R. Majewski,
Ricardo Schiavon, Jon Holtzman,
Jennifer A. Johnson
[show abstract]
[hide abstract]
ABSTRACT: High-resolution H-band spectra of five bright field K, M, and MS giants,
obtained from the archives of the Kitt Peak National Observatory (KPNO) Fourier
Transform Spectrometer (FTS), are analyzed to determine chemical abundances of
16 elements. The abundances were derived via spectrum synthesis using the
detailed linelist prepared for the SDSS III Apache Point Galactic Evolution
Experiment (APOGEE), which is a high-resolution near-infrared spectroscopic
survey to derive detailed chemical abundance distributions and precise radial
velocities for 100,000 red giants sampling all Galactic stellar populations.
Measured chemical abundances include the cosmochemically important isotopes
12C, 13C, 14N, and 16O, along with Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co,
Ni, and Cu. A comparison of the abundances derived here with published values
for these stars reveals consistent results to ~0.1 dex. The APOGEE spectral
region and linelist is, thus, well-suited for probing both Galactic chemical
evolution, as well as internal nucleosynthesis and mixing in populations of red
giants using high-resolution spectroscopy.
12/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We map the spatial distribution of recent star formation over a few x 100 Myr
timescales in fifteen starburst dwarf galaxies using the location of young blue
helium burning stars identified from optically resolved stellar populations in
archival Hubble Space Telescope observations. By comparing the star formation
histories from both the high surface brightness central regions and the diffuse
outer regions, we measure the degree to which the star formation has been
centrally concentrated during the galaxies' starbursts, using three different
metrics for the spatial concentration. We find that the galaxies span a full
range in spatial concentration, from highly centralized to broadly distributed
star formation. Since most starbursts have historically been identified by
relatively short timescale star formation tracers (e.g., Halpha emission),
there could be a strong bias towards classifying only those galaxies with
recent, centralized star formation as starbursts, while missing starbursts that
are spatially distributed.
10/2012;
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Jo Bovy,
Carlos Allende Prieto,
Timothy C. Beers,
Dmitry Bizyaev,
Luiz N. da Costa,
Katia Cunha,
Garrett L. Ebelke,
Daniel J. Eisenstein,
Peter M. Frinchaboy,
Ana Elia García Pérez, [......],
Kaike Pan,
Helio J. Rocha-Pinto,
Ricardo P. Schiavon,
Donald P. Schneider,
Mathias Schultheis,
Michael Skrutskie,
Verne V. Smith,
David H. Weinberg,
John C. Wilson,
Gail Zasowski
[show abstract]
[hide abstract]
ABSTRACT: We measure the Milky Way's rotation curve over the Galactocentric range 4 kpc
<~ R <~ 14 kpc from the first year of data from the Apache Point Observatory
Galactic Evolution Experiment (APOGEE). We model the line-of-sight velocities
of 3,365 stars in fourteen fields with b = 0 deg between 30 deg < l < 210 deg
out to distances of 10 kpc using an axisymmetric kinematical model that
includes a correction for the asymmetric drift of the warm tracer population
(\sigma_R ~ 35 km/s). We determine the local value of the circular velocity to
be V_c(R_0) = 218 +/- 6 km/s and find that the rotation curve is approximately
flat with a local derivative between -3.0 km/s/kpc and 0.4 km/s/kpc. We also
measure the Sun's position and velocity in the Galactocentric rest frame,
finding the distance to the Galactic center to be 8 kpc < R_0 < 9 kpc, radial
velocity V_{R,sun} = -10 +/- 1 km/s, and rotational velocity V_{\phi,sun} =
242^{+10}_{-3} km/s, in good agreement with local measurements of the Sun's
radial velocity and with the observed proper motion of Sgr A*. We investigate
various systematic uncertainties and find that these are limited to offsets at
the percent level, ~2 km/s in V_c. Marginalizing over all the systematics that
we consider, we find that V_c(R_0) < 235 km/s at >99% confidence. We find an
offset between the Sun's rotational velocity and the local circular velocity of
26 +/- 3 km/s, which is larger than the locally-measured solar motion of 12
km/s. This larger offset reconciles our value for V_c with recent claims that
V_c >~ 240 km/s. Combining our results with other data, we find that the Milky
Way's dark-halo mass within the virial radius is ~8x10^{11} M_sun.
09/2012;
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David L. Nidever,
Gail Zasowski,
Steven R. Majewski,
Jonathan Bird,
Annie C. Robin,
Inma Martinez-Valpuesta,
Rachael L. Beaton,
Ralph Schoenrich,
Mathias Schultheis,
John C. Wilson, [......], Jon Holtzman,
Fred R. Hearty,
Elena Malanushenko,
Viktor Malanushenko,
Demitri Muna,
Daniel Oravetz,
Kaike Pan,
Audrey Simmons,
Stephanie Snedden,
Benjamin A. Weaver
[show abstract]
[hide abstract]
ABSTRACT: Commissioning observations with the Apache Point Observatory Galactic
Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey III, have
produced radial velocities (RVs) for ~4700 K/M-giant stars in the Milky Way
bulge. These high-resolution (R \sim 22,500), high-S/N (>100 per resolution
element), near-infrared (1.51-1.70 um; NIR) spectra provide accurate RVs
(epsilon_v~0.2 km/s) for the sample of stars in 18 Galactic bulge fields
spanning -1<l<20 deg, |b|<20 deg, and dec>-32 deg. This represents the largest
NIR high-resolution spectroscopic sample of giant stars ever assembled in this
region of the Galaxy. A cold (sigma_v~30 km/s), high-velocity peak (V_GSR \sim
+200 km/s) is found to comprise a significant fraction (~10%) of stars in many
of these fields. These high RVs have not been detected in previous MW surveys
and are not expected for a simple, circularly rotating disk. Preliminary
distance estimates rule out an origin from the background Sagittarius tidal
stream or a new stream in the MW disk. Comparison to various Galactic models
suggests that these high RVs are best explained by stars in orbits of the
Galactic bar potential, although some observational features remain
unexplained.
07/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We present follow-up observations and analysis of the recently discovered
short period low-mass eclipsing binary, SDSS J001641-000925. With an orbital
period of 0.19856 days, this system has one of the shortest known periods for
an M dwarf binary system. Medium-resolution spectroscopy and multi-band
photometry for the system are presented. Markov chain Monte Carlo modeling of
the light curves and radial velocities yields estimated masses for the stars of
M1 = 0.54 +/- 0.07 Msun and M2 = 0.34 +/- 0.04 Msun, and radii of R1 = 0.68 +/-
0.03 Rsun and R2 = 0.58 +/- 0.03 Rsun respectively. This solution places both
components above the critical Roche overfill limit, providing strong evidence
that SDSS J001641-000925 is the first verified M-dwarf contact binary system.
Within the follow-up spectroscopy we find signatures of non-solid body rotation
velocities, which we interpret as evidence for mass transfer or loss within the
system. In addition, our photometry samples the system over 9 years, and we
find strong evidence for period decay at the rate of dP/dt ~8 s/yr. Both of
these signatures raise the intriguing possibility that the system is in
over-contact, and actively losing angular momentum, likely through mass loss.
This places SDSS J001641-000925 as not just the first M-dwarf over-contact
binary, but one of the few systems of any spectral type known to be actively
undergoing coalescence. Further study SDSS J001641-000925 is on-going to verify
the nature of the system, which may prove to be a unique astrophysical
laboratory.
06/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Using star formation histories derived from optically resolved stellar
populations in nineteen nearby starburst dwarf galaxies observed with the
Hubble Space Telescope, we measure the stellar mass surface densities of stars
newly formed in the bursts. By assuming a star formation efficiency (SFE), we
then calculate the inferred gas surface densities present at the onset of the
starbursts. Assuming a SFE of 1%, as is often assumed in normal star-forming
galaxies, and assuming that the gas was purely atomic, translates to very high
HI surface densities (~10^2-10^3 Msun pc^-2), which are much higher than have
been observed in dwarf galaxies. This implies either higher values of SFE in
these dwarf starburst galaxies or the presence of significant amounts of H_2 in
dwarfs (or both). Raising the assumed SFEs to 10% or greater (in line with
observations of more massive starbursts associated with merging galaxies),
still results in HI surface densities higher than observed in 10 galaxies.
Thus, these observations appear to require that a significant fraction of the
gas in these dwarf starbursts galaxies was in the molecular form at the onset
of the bursts. Our results imply molecular gas column densities in the range
10^19-10^21 cm^-2 for the sample. In those galaxies where CO observations have
been made, these densities correspond to values of the CO-H_2 conversion factor
(X_CO) in the range >3-80x10^20 cm^-2 (K km s^-1)^-1, or up to 40x greater than
Galactic X_CO values.
05/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Using HST/COS/STIS and HIRES/Keck high-resolution spectra, we have studied a
remarkable HI absorbing complex at z=0.672 toward the quasar Q1317+277. The HI
absorption has a velocity spread of 1600 km/s, comprises 21 Voigt profile
components, and resides at an impact parameter of D=58 kpc from a bright, high
mass [log(M_vir/M_sun) ~ 13.7] elliptical galaxy that is deduced to have a 6
Gyr old, solar metallicity stellar population. Ionization models suggest the
majority of the structure is cold gas surrounding a shock heated cloud that is
kinematically adjacent to a multi-phase group of clouds with detected CIII, CIV
and OVI absorption, suggestive of a conductive interface near the shock. The
deduced metallicities are consistent with the moderate in situ enrichment
relative to the levels observed in the z ~ 3 Ly-alpha forest. We interpret the
HI complex as a metal-poor filamentary structure being shock heated as it
accretes into the halo of the galaxy. The data support the scenario of an early
formation period (z > 4) in which the galaxy was presumably fed by cold-mode
gas accretion that was later quenched via virial shocking by the hot halo such
that, by intermediate redshift, the cold filamentary accreting gas is
continuing to be disrupted by shock heating. Thus, continued filamentary
accretion is being mixed into the hot halo, indicating that the star formation
of the galaxy will likely remain quenched. To date, the galaxy and the HI
absorption complex provide some of the most compelling observational data
supporting the theoretical picture in which accretion is virial shocked in the
hot coronal halos of high mass galaxies.
05/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We derive the optical luminosity, colors, and ratios of the blue and red helium burning (HeB) stellar populations from archival Hubble Space Telescope observations of nineteen starburst dwarf galaxies and compare them with theoretical isochrones from Padova stellar evolution models across metallicities from Z = 0.001 to 0.009. We find that the observational data and the theoretical isochrones for both blue and red HeB populations overlap in optical luminosities and colors and the observed and predicted blue to red HeB ratios agree for stars older than 50 Myr over the time bins studied. These findings confirm the usefulness of applying isochrones to interpret observations of HeB populations. However, there are significant differences, especially for the red HeB population. Specifically, we find (1) offsets in color between the observations and theoretical isochrones of order 0.15 mag (0.5 mag) for the blue (red) HeB populations brighter than MV ~ –4 mag, which cannot be solely due to differential extinction; (2) blue HeB stars fainter than MV ~ –3 mag are bluer than predicted; (3) the slope of the red HeB sequence is shallower than predicted by a factor of ~3; and (4) the models overpredict the ratio of the most luminous blue to red HeB stars corresponding to ages 50 Myr. Additionally, we find that for the more metal-rich galaxies in our sample (Z 0.5 Z ☉), the red HeB stars overlap with the red giant branch stars in the color-magnitude diagrams, thus reducing their usefulness as indicators of star formation for ages 100 Myr.
The Astrophysical Journal 09/2011; 740(1):48. · 6.02 Impact Factor
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Benjamin F. Williams,
Julianne J. Dalcanton,
L. C. Johnson,
Daniel R. Weisz,
Anil C. Seth,
Andrew Dolphin,
Karoline M. Gilbert,
Evan Skillman,
Keith Rosema,
Stephanie M. Gogarten, Jon Holtzman,
and Roelof S. de Jong
[show abstract]
[hide abstract]
ABSTRACT: We present a measurement of the age distribution of stars residing in spiral disks and dwarf galaxies. We derive a complete star formation history of the ~140 Mpc3 covered by the volume-limited sample of galaxies in the Advanced Camera for Surveys (ACS) Nearby Galaxy Survey Treasury (ANGST). The total star formation rate density history (ρSFR(t)) is dominated by the large spirals in the volume, although the sample consists mainly of dwarf galaxies. Our ρSFR(t) shows a factor of ~3 drop at z ~ 2, in approximate agreement with results from other measurement techniques. While our results show that the overall ρSFR(t) has decreased since z ~ 1, the measured rates during this epoch are higher than those obtained from other measurement techniques. This enhanced recent star formation rate appears to be largely due to an increase in the fraction of star formation contained in low-mass disks at recent times. Finally, our results indicate that despite the differences at recent times, the epoch of formation of ~50% of the stellar mass in dwarf galaxies was similar to that of ~50% of the stellar mass in large spiral galaxies (z 2), despite the observed galaxy-to-galaxy diversity among the dwarfs.
The Astrophysical Journal Letters 05/2011; 734(1):L22. · 5.53 Impact Factor
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Benjamin F. Williams,
Julianne J. Dalcanton,
L. C. Johnson,
Daniel R. Weisz,
Anil C. Seth,
Andrew Dolphin,
Karoline M. Gilbert,
Evan Skillman,
Keith Rosema,
Stephanie M. Gogarten, Jon Holtzman,
Roelof S. de Jong
[show abstract]
[hide abstract]
ABSTRACT: We present a measurement of the age distribution of stars residing in spiral
disks and dwarf galaxies. We derive a complete star formation history of the
~140 Mpc^3 covered by the volume-limited sample of galaxies in the Advanced
Camera for Surveys (ACS) Nearby Galaxy Survey Treasury (ANGST). The total star
formation rate density history is dominated by the large spirals in the volume,
although the sample consists mainly of dwarf galaxies. Our measurement shows a
factor of ~3 drop at z~2, in approximate agreement with results from other
measurement techniques. While our results show that the overall star formation
rate density has decreased since z~1, the measured rates during this epoch are
higher than those obtained from other measurement techniques. This enhanced
recent star formation rate appears to be largely due to an increase in the
fraction of star formation contained in low-mass disks at recent times.
Finally, our results indicate that despite the differences at recent times, the
epoch of formation of ~50% of the stellar mass in dwarf galaxies was similar to
that of ~50% of the stellar mass in large spiral galaxies (z>~2), despite the
observed galaxy-to-galaxy diversity among the dwarfs.
05/2011;
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Eric Bullock,
Paula Szkody,
Anjum S. Mukadam,
Bernardo W. Borges,
Luciano Fraga,
Boris T. Gänsicke,
Thomas E. Harrison,
Arne Henden, Jon Holtzman,
Steve B. Howell,
Warrick A. Lawson,
Stephen Levine,
Richard M. Plotkin,
Mark Seibert,
Matthew Templeton,
Johanna Teske,
and Frederick J. Vrba
[show abstract]
[hide abstract]
ABSTRACT: The prototype of accreting, pulsating white dwarfs (GW Lib) underwent a large amplitude dwarf nova outburst in 2007. We used ultraviolet data from Galaxy Evolution Explorer and ground-based optical photometry and spectroscopy to follow GW Lib for three years following this outburst. Several variations are apparent during this interval. The optical shows a superhump modulation in the months following outburst, while a 19 minute quasi-periodic modulation lasting for several months is apparent in the year after outburst. A long timescale (about 4 hr) modulation first appears in the UV a year after outburst and increases in amplitude in the following years. This variation also appears in the optical two years after outburst but is not in phase with the UV. The pre-outburst pulsations are not yet visible after three years, likely indicating the white dwarf has not returned to its quiescent state.
The Astronomical Journal 02/2011; 141(3):84. · 4.03 Impact Factor
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Daniel R. Weisz,
Andrew E. Dolphin,
Julianne J. Dalcanton,
Evan D. Skillman, Jon Holtzman,
Benjamin F. Williams,
Karoline M. Gilbert,
Anil C. Seth,
Andrew Cole,
Stephanie M. Gogarten,
Keith Rosema,
Igor D. Karachentsev,
Kristen B. W. McQuinn,
Dennis Zaritsky
[show abstract]
[hide abstract]
ABSTRACT: We compare the cumulative star formation histories (SFHs) of Local Group (LG)
dwarf galaxies with those in the volume-limited ACS Nearby Galaxy Survey
Treasury (ANGST) sample (D < 4 Mpc), in order to understand how typical the LG
dwarf galaxies are relative to those in the nearby universe. The SFHs were
derived in a uniform manner from high quality optical color-magnitude diagrams
constructed from Hubble Space Telescope imaging. We find that the {\it mean}
cumulative SFHs of the LG dwarfs are comparable to the mean cumulative SFHs of
the ANGST sample for the three different morphological types (dwarf
spheroidals/ellipticals: dSph/dE; dwarf irregulars: dI; transition dwarfs:
dTrans). We also discuss effects such as population gradients and systematic
uncertainties in the stellar models that may influence the derived SFHs. Both
the ANGST and Local Group dwarf galaxies show a consistent and strong
morphology-density relationship, emphasizing the importance of environment in
the evolution of dwarf galaxies. Specifically, we confirm that dIs are found at
lower densities and higher luminosities than dSphs, within this large sample.
We also find that dTrans are located in similar environments to those occupied
by dwarf irregular galaxies, but have systematically lower luminosities that
are more comparable to those of dwarf spheroidals. The similarity of the SFHs
and morphology-density relationships of the LG and ANGST dwarf galaxies
suggests that the LG dwarfs are a good representation of dwarf galaxies in the
local universe.
01/2011;
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Daniel R. Weisz,
Julianne J. Dalcanton,
Benjamin F. Williams,
Karoline M. Gilbert,
Evan D. Skillman,
Anil C. Seth,
Andrew E. Dolphin,
Kristen B. W. McQuinn,
Stephanie M. Gogarten, Jon Holtzman,
Keith Rosema,
Andrew Cole,
Igor D. Karachentsev,
Dennis Zaritsky
[show abstract]
[hide abstract]
ABSTRACT: We present uniformly measured star formation histories (SFHs) of 60 nearby
(D~4Mpc) dwarf galaxies based on CMDs of resolved stellar populations from
images taken with HST as part of the ACS Nearby Galaxy Survey Treasury program
(ANGST). This volume-limited sample contains 12 dSph/dE, 5 dwarf spiral, 28 dI,
12 transition, and 3 tidal dwarf galaxies. From the best fit SFHs we find three
significant results: (1) the average dwarf galaxy formed >50% of its stars by
z~2 and 60% of its stars by z~1, regardless of current morphological type; (2)
the mean SFHs of dIs, dTrans, and dSphs are similar over most of cosmic time,
with the clearest differences appearing during the most recent 1 Gyr; and (3)
the mean values are inconsistent with simple SFH models, e.g., exponentially
declining SFRs. The mean SFHs are in general agreement with the cosmic SFH,
although we observe offsets near z~1 that could be evidence that low mass
systems experienced delayed star formation relative to more massive galaxies.
The sample shows a strong density-morphology relationship, such that the dSphs
are less isolated than dIs. We find that the transition from a gas-rich to
gas-poor galaxy cannot be solely due to internal mechanisms such as stellar
feedback, and instead is likely the result of external mechanisms, e.g., ram
pressure and tidal forces. The average transition dwarf is slightly less
isolated and less gas-rich than the typical dI. Further, the transition dwarfs
can be divided into two groups: interacting and gas-poor or isolated and
gas-rich, suggesting two possible evolutionary pathways.
01/2011;
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[show abstract]
[hide abstract]
ABSTRACT: The starburst phenomenon can shape the evolution of the host galaxy and the surrounding intergalactic medium. The extent of the evolutionary impact is partly determined by the duration of the starburst, which has a direct correlation with both the amount of stellar feedback and the development of galactic winds, particularly for smaller mass dwarf systems. We measure the duration of starbursts in twenty nearby, ongoing, and "fossil" starbursts in dwarf galaxies based on the recent star formation histories derived from resolved stellar population data obtained with the Hubble Space Telescope. Contrary to the shorter times of 3-10 Myr often cited, the starburst durations we measure range from 450to650 Myr in fifteen of the dwarf galaxies and up to 1.3 Gyr in four galaxies; these longer durations are comparable to or longer than the dynamical timescales for each system. The same feedback from massive stars that may quench the flickering star formation does not disrupt the overall burst event in our sample of galaxies. While five galaxies present fossil bursts, fifteen galaxies show ongoing bursts and thus the final durations may be longer than we report here for these systems. One galaxy shows a burst that has been ongoing for only 20 Myr; we are likely seeing the beginning of a burst event in this system. Using the duration of the starbursts, we calculate that the bursts deposited 1053.9-1057.2 erg of energy into the interstellar medium through stellar winds and supernovae, and produced 3%–26% of the host galaxy's mass.
The Astrophysical Journal 10/2010; 724(1):49. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We use archival HST observations of resolved stellar populations to derive the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In this first paper we present the observations, color-magnitude diagrams, and the SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data reduction, differential extinction, and treatment of photometric completeness. We adopt a star formation rate (SFR) threshold normalized to the average SFR of the individual system as a metric for classifying starbursts in SFHs derived from resolved stellar populations. This choice facilitates finding not only currently bursting galaxies but also "fossil" bursts increasing the sample size of starburst galaxies in the nearby (D<8 Mpc) universe. Thirteen of the eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr with variations on small timescales. A long >100 Myr temporal baseline is thus fundamental to any starburst definition or identification method. The longer lived bursts rule out rapid "self-quenching" of starbursts on global scales. The bursting galaxies' gas consumption timescales are shorter than the Hubble time for all but one galaxy confirming the short-lived nature of starbursts based on fuel limitations. Additionally, we find the strength of the H{\alpha} emission usually correlates with the CMD based SFR during the last 4-10 Myr. However, in four cases, the H{\alpha} emission is significantly less than what is expected for models of starbursts; the discrepancy is due to the SFR changing on timescales of a few Myr. The inherently short timescale of the H{\alpha} emission limits identifying galaxies as starbursts based on the current characteristics which may or may not be representative of the recent SFH of a galaxy. Comment: 53 pages, 11 figures
08/2010;
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Paula Szkody,
Anjum Mukadam,
Boris T. Gänsicke,
Ryan K. Campbell,
Thomas E. Harrison,
Steve B. Howell, Jon Holtzman,
Frederick M. Walter,
Arne Henden,
William Dillon,
Owen Boberg,
Shannon Dealaman,
and Christian S. Perone
[show abstract]
[hide abstract]
ABSTRACT: Time-resolved spectra throughout the orbit of EF Eri during its low accretion state were obtained with the solar blind channel on the Advanced Camera for Surveys on board the Hubble Space Telescope. The overall spectral distribution exhibits peaks at 1500 and 1700 Å, while the UV light curves display a quasi-sinusoidal modulation over the binary orbit. Models of white dwarfs (WDs) with a hot spot and cyclotron emission were attempted to fit the spectral variations throughout the orbit. A non-magnetic WD with a temperature of ~10,000 K and a hot spot with a central temperature of 15,000 K generally match the broad absorptions at 1400 and 1600 Å with those expected for the quasi-molecular H features H2 and H+ 2. However, the flux in the core of the Lyα absorption does not go to zero, implying an additional component, and the flux variations throughout the orbit are not well matched at long wavelengths. Alternatively, a 9500 K WD with a 100 MG cyclotron component can fit the lowest (phase 0.0) fluxes, but the highest fluxes (phase 0.5) require an additional source of magnetic field or temperature. The 100 MG field required for the UV fit is much higher than that which fits the optical/IR wavelengths, which would support previous suggestions of a complex field structure in polars.
The Astrophysical Journal 06/2010; 716(2):1531. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: GALEX near-ultraviolet (NUV) and far-ultraviolet (FUV) light curves of three extremely low accretion rate polars show distinct modulations in their UV light curves. While these three systems have a range of magnetic fields from 13 to 70 MG, and of late-type secondaries (including a likely brown dwarf in SDSSJ121209.31+013627.7), the accretion rates are similar, and the UV observations imply some mechanism is operating to create enhanced emission zones on the white dwarf. The UV variations match in phase to the two magnetic poles viewed in the optical in WX LMi and to the single poles evident in the optical in SDSSJ1212109.31+013627.7 and SDSSJ103100.55+202832.2. Simple spot models of the UV light curves show that if hot spots are responsible for the UV variations, the temperatures are on the order of 10,000-14,000 K. For the single pole systems, the size of the FUV spot must be smaller than the NUV and in all cases the geometry is likely more complicated than a simple circular spot.
The Astrophysical Journal 03/2010; 713(2):1183. · 6.02 Impact Factor
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Stephanie M. Gogarten,
Julianne J. Dalcanton,
Benjamin F. Williams,
Rok Roškar, Jon Holtzman,
Anil C. Seth,
Andrew Dolphin,
Daniel Weisz,
Andrew Cole,
Victor P. Debattista,
Karoline M. Gilbert,
Knut Olsen,
Evan Skillman,
Roelof S. de Jong,
Igor D. Karachentsev,
and Thomas R. Quinn
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ABSTRACT: We present new Hubble Space Telescope (HST) observations of NGC 300 taken as part of the Advanced Camera for Surveys Nearby Galaxy Survey Treasury (ANGST). Individual stars are resolved in these images down to an absolute magnitude of M F814W = 1.0 (below the red clump). We determine the star formation history of the galaxy in six radial bins by comparing our observed color-magnitude diagrams (CMDs) with synthetic CMDs based on theoretical isochrones. We find that the stellar disk out to 5.4 kpc is primarily old, in contrast with the outwardly similar galaxy M33. We determine the scale length as a function of age and find evidence for inside-out growth of the stellar disk: the scale length has increased from 1.1 ± 0.1 kpc 10 Gyr ago to 1.3 ± 0.1 kpc at present, indicating a buildup in the fraction of young stars at larger radii. As the scale length of M33 has recently been shown to have increased much more dramatically with time, our results demonstrate that two galaxies with similar sizes and morphologies can have very different histories. With an N-body simulation of a galaxy designed to be similar to NGC 300, we determine that the effects of radial migration should be minimal. We trace the metallicity gradient as a function of time and find a present-day metallicity gradient consistent with that seen in previous studies. Consistent results are obtained from archival images covering the same radial extent but differing in placement and filter combination.
The Astrophysical Journal 03/2010; 712(2):858. · 6.02 Impact Factor
