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DOE OSTI2011

THE STELLAR ARCHEOLOGY OF THE M33 DISK: RECENT STAR-FORMING HISTORY AND CONSTRAINTS ON THE TIMING OF AN INTERACTION WITH M31

Images recorded with MegaCam are used to investigate the recent (t {<=} 0.25 Gyr) star-forming history (SFH) of the Local Group Sc galaxy M33. The data sample the entire star-forming disk, as well as areas immediately to the north and south of the galaxy. The properties of the stellar disk change near R{sub GC} = 8 kpc. Within this radius the luminosity function of main-sequence stars indicates that the star formation rate (SFR) has been constant with time during at least the past 250 Myr, while at larger radii the SFR has declined during this same time period. That the recent SFR in the inner disk has been constant suggests that M33 has evolved in isolation for at least the past {approx}0.5 Gyr, thereby providing a constraint on the timing of any recent interaction with M31. The color of the main-sequence ridgeline changes with radius, suggesting a gradient in extinction of size {Delta}A{sub V} /{Delta}R{sub GC} = -0.05 mag kpc{sup -1}. The fractional contribution that young stars make to the total mass of the stellar disk changes with radius, peaking near 8 kpc. Evidence is also presented of systematic spatial variations in the SFH of the disk, such that the SFR during the past 100 Myr in the southern half of the galaxy has been {approx}0.4 dex higher than in the northern half. Finally, structures with sizes spanning many kpc that contain blue objects-presumably main-sequence stars that formed during intermediate epochs-are identified near the disk boundary. It is argued that these are tidal features that were pulled from the main body of M33 and-in some cases-are the fossil remnants of star formation that occurred in an extended disk during intermediate epochs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE ACS NEARBY GALAXY SURVEY TREASURY. VIII. THE GLOBAL STAR FORMATION HISTORIES OF 60 DWARF GALAXIES IN THE LOCAL VOLUME

We present uniformly measured star formation histories (SFHs) of 60 nearby (D {approx}< 4 Mpc) dwarf galaxies based on color-magnitude diagrams of resolved stellar populations from images taken with the Hubble Space Telescope and analyzed as part of the ACS Nearby Galaxy Survey Treasury program (ANGST). This volume-limited sample contains 12 dwarf spheroidal (dSph)/dwarf elliptical (dE), 5 dwarf spiral, 28 dwarf irregular (dI), 12 dSph/dI (transition), and 3 tidal dwarf galaxies. The sample spans a range of {approx}10 mag in M{sub B} and covers a wide range of environments, from highly interacting to truly isolated. From the best-fit SFHs, we find three significant results for dwarf galaxies in the ANGST volume: (1) the majority of dwarf galaxies formed the bulk of their mass prior to z {approx} 1, regardless of current morphological type; (2) the mean SFHs of dIs, transition dwarf galaxies (dTrans), and dSphs are similar over most of cosmic time, and only begin to diverge a few Gyr ago, with the clearest differences between the three appearing during the most recent 1 Gyr; and (3) the SFHs are complex and the mean values are inconsistent with simple SFH models, e.g., single bursts, constant star formation rates (SFRs), or smooth, exponentially declining SFRs. The mean SFHs show clear divergence from the cosmic SFH at z {approx}< 0.7, which could be evidence that low-mass systems have experienced delayed star formation relative to more massive galaxies. The sample shows a strong density-morphology relationship, such that the dSphs in the sample are less isolated than the 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 stripping and tidal forces. In terms of their environments, SFHs, and gas fractions, the majority of the dTrans appear to be low-mass dIs that simply lack H{alpha} emission, similar to Local Group (LG) dTrans DDO 210. However, a handful of dTrans have remarkably low gas fractions, suggesting that they have nearly exhausted their gas supply, analogous to LG dTrans such as Phoenix. Finally, we have also included extensive exploration of uncertainties in the SFH recovery method, including the optimization of time resolution, the effects of photometric depth, and impact of systematic uncertainties due to the limitations in current stellar evolution models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE STAR FORMATION HISTORY OF THE MILKY WAY'S NUCLEAR STAR CLUSTER

We present spatially resolved imaging and integral field spectroscopy data for 450 cool giant stars within 1 pc from Sgr A*. We use the prominent CO bandheads to derive effective temperatures of individual giants. Additionally we present the deepest spectroscopic observation of the Galactic center (GC) so far, probing the number of B9/A0 main-sequence stars (2.2-2.8 M{sub sun}) in two deep fields. From spectrophotometry we construct a Hertzsprung-Russell diagram of the red giant population and fit the observed diagram with model populations to derive the star formation history of the nuclear cluster. We find (1) that the average nuclear star formation rate dropped from an initial maximum {approx}10 Gyr ago to a deep minimum 1-2 Gyr ago and increased again during the last few hundred Myrs, (2) that roughly 80% of the stellar mass formed more than 5 Gyr ago, and (3) that mass estimates within R {approx} 1 pc from Sgr A* favor a dominant star formation mode with a 'normal' Chabrier/Kroupa initial mass function for the majority of the past star formation in the GC. The bulk stellar mass seems to have formed under conditions significantly different from the young stellar disks, perhaps because at the time of the formation of the nuclear cluster the massive black hole and its sphere of influence were much smaller than today.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE ASSEMBLY HISTORY OF DISK GALAXIES. I. THE TULLY-FISHER RELATION TO z {approx_equal} 1.3 FROM DEEP EXPOSURES WITH DEIMOS

We present new measures of the evolving scaling relations between stellar mass, luminosity and rotational velocity for a morphologically inclusive sample of 129 disk-like galaxies with z{sub AB} < 22.5 in the redshift range 0.2 <z < 1.3, based on spectra from DEep Imaging Multi-Object Spectrograph on the Keck II telescope, multi-color Hubble Space Telescope (HST) Advanced Camera for Surveys photometry, and ground-based K{sub s} -band imaging. A unique feature of our survey is the extended spectroscopic integration times, leading to significant improvements in determining characteristic rotational velocities for each galaxy. Rotation curves are reliably traced to the radius where they begin to flatten for {approx}90% of our sample, and we model the HST-resolved bulge and disk components in order to accurately de-project our measured velocities, accounting for seeing and dispersion. We demonstrate the merit of these advances by recovering an intrinsic scatter on the stellar mass Tully-Fisher relation a factor of two to three less than in previous studies at intermediate redshift and comparable to that of locally determined relations. With our increased precision, we find that the relation is well established by (z) {approx} 1, with no significant evolution to (z) {approx} 0.3, {Delta}M{sub *} {approx} 0.04 {+-} 0.07 dex. A clearer trend of evolution is seen in the B-band Tully-Fisher relation corresponding to a decline in luminosity of {Delta}M{sub B} {approx} 0.85 {+-} 0.28 magnitudes at fixed velocity over the same redshift range, reflecting the changes in star formation over this period. As an illustration of the opportunities possible when gas masses are available for a sample such as ours, we show how our dynamical and stellar mass data can be used to evaluate the likely contributions of baryons and dark matter to the assembly history of spiral galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

PROBING THE STAR FORMATION HISTORY AND INITIAL MASS FUNCTION OF THE z {approx} 2.5 LENSED GALAXY SMM J163554.2+661225 WITH HERSCHEL

We present the analysis of Herschel Spectral and Photometric Imaging Receiver far-infrared (FIR) observations of the z = 2.515 lensed galaxy SMM J163554.2+661225. Combining new 250, 350, and 500 {mu}m observations with existing data, we make an improved fit to the FIR spectral energy distribution of this galaxy. We find a total infrared (IR) luminosity of L(8-1000 {mu}m) = 6.9 {+-} 0.6 Multiplication-Sign 10{sup 11} L{sub Sun }, a factor of three more precise over previous L{sub IR} estimates for this galaxy, and one of the most accurate measurements for any galaxy at these redshifts. This FIR luminosity implies an unlensed star formation rate (SFR) for this galaxy of 119 {+-} 10 M{sub Sun} yr{sup -1}, which is a factor of 1.9 {+-} 0.35 lower than the SFR derived from the nebular Pa{alpha} emission line (a 2.5{sigma} discrepancy). Both SFR indicators assume an identical Salpeter initial mass function (IMF) with slope {Gamma} = 2.35 over a mass range of 0.1-100 M{sub Sun }; thus this discrepancy suggests that more ionizing photons may be necessary to account for the higher Pa{alpha}-derived SFR. We examine a number of scenarios and find that the observations can be explained with a varying star formation history (SFH) due to an increasing SFR, paired with a slight flattening of the IMF. If the SFR is constant in time, then larger changes need to be made to the IMF by either increasing the upper mass cutoff to {approx}200 M{sub Sun }, or a flattening of the IMF slope to 1.9 {+-} 0.15, or a combination of the two. These scenarios result in up to double the number of stars with masses above 20 M{sub Sun }, which produce the requisite increase in ionizing photons over a Salpeter IMF with a constant SFH.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MODELING THE EFFECTS OF STAR FORMATION HISTORIES ON H{alpha} AND ULTRAVIOLET FLUXES IN NEARBY DWARF GALAXIES

We consider the effects of non-constant star formation histories (SFHs) on H{alpha} and GALEX far-ultraviolet (FUV) star formation rate (SFR) indicators. Under the assumption of a fully populated Chabrier initial mass function (IMF), we compare the distribution of H{alpha}-to-FUV flux ratios from {approx}1500 simple, periodic model SFHs with observations of 185 galaxies from the Spitzer Local Volume Legacy survey. We find a set of SFH models that are well matched to the data, such that more massive galaxies are best characterized by nearly constant SFHs, while low-mass systems experience burst amplitudes of {approx}30 (i.e., an increase in the SFR by a factor of 30 over the SFR during the inter-burst period), burst durations of tens of Myr, and periods of {approx}250 Myr; these SFHs are broadly consistent with the increased stochastic star formation expected in systems with lower SFRs. We analyze the predicted temporal evolution of galaxy stellar mass, R-band surface brightness, H{alpha}-derived SFR, and blue luminosity, and find that they provide a reasonable match to observed flux distributions. We find that our model SFHs are generally able to reproduce both the observed systematic decline and increased scatter in H{alpha}-to-FUV ratios toward low-mass systems, without invoking other physical mechanisms. We also compare our predictions with those from the Integrated Galactic IMF theory with a constant SFR. We find that while both predict a systematic decline in the observed ratios, only the time variable SFH models are capable of producing the observed population of low-mass galaxies (M{sub *} {approx}< 10{sup 7} M{sub Sun }) with normal H{alpha}-to-FUV ratios. These results demonstrate that a variable IMF alone has difficulty explaining the observed scatter in the H{alpha}-to-FUV ratios. We conclude by considering the limitations of the model SFHs and discuss the use of additional empirical constraints to improve future SFH modeling efforts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

RECONSTRUCTING THE PHOTOMETRIC LIGHT CURVES OF EARTH AS A PLANET ALONG ITS HISTORY

By utilizing satellite-based estimations of the distribution of clouds, we have studied Earth's large-scale cloudiness behavior according to latitude and surface types (ice, water, vegetation, and desert). These empirical relationships are used here to reconstruct the possible cloud distribution of historical epochs of Earth's history such as the Late Cretaceous (90 Ma ago), the Late Triassic (230 Ma ago), the Mississippian (340 Ma ago), and the Late Cambrian (500 Ma ago), when the landmass distributions were different from today's. With this information, we have been able to simulate the globally integrated photometric variability of the planet at these epochs. We find that our simple model reproduces well the observed cloud distribution and albedo variability of the modern Earth. Moreover, the model suggests that the photometric variability of the Earth was probably much larger in past epochs. This enhanced photometric variability could improve the chances for the difficult determination of the rotational period and the identification of continental landmasses for a distant planets.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2012

THE STAR FORMATION HISTORY OF M32

We use deep Hubble Space Telescope Advanced Camera for Surveys/High Resolution Channel observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5 Gyr old stars contribute {approx}40% {+-} 17% of M32's mass, while {approx}55% {+-} 21% of M32's mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are (Age) = 6.8 {+-} 1.5 Gyr and ([M/H]) = -0.01 {+-} 0.08 dex. The SFH additionally indicates the presence of young (<2 Gyr old), metal-poor ([M/H] {approx} -0.7) stars, suggesting that blue straggler stars contribute {approx}2% of the mass at F1; the remaining {approx}3% of the mass is in young metal-rich stars. Line-strength indices computed from the SFH imply a light-weighted mean age and metallicity of 4.9 Gyr and [M/H] = -0.12 dex, and single stellar population-equivalent parameters of 2.9 {+-} 0.2 Gyr and [M/H] = 0.02 {+-} 0.01 dex at F1 ({approx}2.7 r{sub e} ). This contradicts spectroscopic studies that show a steep age gradient from M32's center to 1 r{sub e} . The inferred SFH of the M31 background field F2 reveals that the majority of its stars are old, with {approx}95% of its mass already acquired 5-14 Gyr ago. It is composed of two dominant populations; {approx}30% {+-} 7.5% of its mass is in a 5-8 Gyr old population, and {approx}65% {+-} 9% of the mass is in an 8-14 Gyr old population. The mass-weighted mean age and metallicity of F2 are (Age) = 9.2 {+-} 1.2 Gyr and ([M/H]) = -0.10 {+-} 0.10 dex, respectively. Our results suggest that the inner disk and spheroid populations of M31 are indistinguishable from those of the outer disk and spheroid. Assuming the mean age of M31's disk at F2 ({approx}1 disk scale length) to be {approx}5-9 Gyr, our results agree with an inside-out disk formation scenario for M31's disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A REVISED AGE FOR UPPER SCORPIUS AND THE STAR FORMATION HISTORY AMONG THE F-TYPE MEMBERS OF THE SCORPIUS-CENTAURUS OB ASSOCIATION

We present an analysis of the ages and star formation history of the F-type stars in the Upper Scorpius (US), Upper Centaurus-Lupus (UCL), and Lower Centaurus-Crux (LCC) subgroups of Scorpius-Centaurus (Sco-Cen), the nearest OB association. Our parent sample is the kinematically selected Hipparcos sample of de Zeeuw et al., restricted to the 138 F-type members. We have obtained classification-resolution optical spectra and have also determined the spectroscopic accretion disk fraction. With Hipparcos and 2MASS photometry, we estimate the reddening and extinction for each star and place the candidate members on a theoretical H-R diagram. For each subgroup we construct empirical isochrones and compare to published evolutionary tracks. We find that (1) our empirical isochrones are consistent with the previously published age-rank of the Sco-Cen subgroups; (2) subgroups LCC and UCL appear to reach the main-sequence turn-on at spectral types {approx}F4 and {approx}F2, respectively. An analysis of the A-type stars shows US reaching the main sequence at about spectral type {approx}A3. (3) The median ages for the pre-main-sequence members of UCL and LCC are 16 Myr and 17 Myr, respectively, in agreement with previous studies, however we find that (4) Upper Sco is much older than previously thought. The luminosities of the F-type stars in US are typically a factor of {approx}2.5 less luminous than predicted for a 5 Myr old population for four sets of evolutionary tracks. We re-examine the evolutionary state and isochronal ages for the B-, A-, and G-type Upper Sco members, as well as the evolved M supergiant Antares, and estimate a revised mean age for Upper Sco of 11 {+-} 1 {+-} 2 Myr (statistical, systematic). Using radial velocities and Hipparcos parallaxes we calculate a lower limit on the kinematic expansion age for Upper Sco of >10.5 Myr (99% confidence). However, the data are statistically consistent with no expansion. We reevaluate the inferred masses for the known substellar companions in Upper Sco using the revised age and find that the inferred masses are typically {approx}20%-70% higher than the original estimates which had assumed a much younger age; specifically, we estimate the mass of 1RXS J1609-2105b to be 14{sup +2}{sub -3} M{sub Jup}, suggesting that it is a brown dwarf rather than a planet. Finally, we find the fraction of F-type stars exhibiting H{alpha} emission and/or a K-band excess consistent with accretion to be 0/17 (<19%; 95% CL) in US at {approx}11 Myr, while UCL has 1/41 (2{sup +5}{sub -1}%; 68% CL) accretors and LCC has 1/50 (2{sup +4}{sub -1}%; 68% CL) accretors at {approx}16 Myr and {approx}17 Myr, respectively.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A CHANDRA STUDY OF THE RADIO GALAXY NGC 326: WINGS, OUTBURST HISTORY, AND ACTIVE GALACTIC NUCLEUS FEEDBACK

NGC 326 is one of the most prominent 'X'- or 'Z'-shaped radio galaxies (XRGs/ZRGs) and has been the subject of several studies attempting to explain its morphology through either fluid motions or reorientation of the jet axis. We examine a 100 ks Chandra X-Ray Observatory exposure and find several features associated with the radio galaxy: a high-temperature front that may indicate a shock, high-temperature knots around the rim of the radio emission, and a cavity associated with the eastern wing of the radio galaxy. A reasonable interpretation of these features in light of the radio data allows us to reconstruct the history of the active galactic nucleus (AGN) outbursts. The active outburst was likely once a powerful radio source which has since decayed, and circumstantial evidence favors reorientation as the means to produce the wings. Because of the obvious interaction between the radio galaxy and the intracluster medium and the wide separation between the active lobes and wings, we conclude that XRGs are excellent sources in which to study AGN feedback in galaxy groups by measuring the heating rates associated with both active and passive heating mechanisms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE STAR FORMATION HISTORY OF LEO T FROM HUBBLE SPACE TELESCOPE IMAGING

We present the star formation history (SFH) of the faintest known star-forming galaxy, Leo T, based on deep imaging taken with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2). The HST/WFPC2 color-magnitude diagram (CMD) of Leo T is exquisitely deep, extending {approx}2 mag below the oldest main-sequence turnoff, permitting excellent constraints on star formation at all ages. We use a maximum likelihood CMD fitting technique to measure the SFH of Leo T assuming three different sets of stellar evolution models: Padova (solar-scaled metallicity) and BaSTI (both solar-scaled and {alpha}-enhanced metallicities). The resulting SFHs are remarkably consistent at all ages, indicating that our derived SFH is robust to the choice of stellar evolution model. From the lifetime SFH of Leo T, we find that 50% of the total stellar mass formed prior to z {approx} 1 (7.6 Gyr ago). Subsequent to this epoch, the SFH of Leo T is roughly constant until the most recent {approx}25 Myr, where the SFH shows an abrupt drop. This decrease could be due to a cessation of star formation or stellar initial mass function sampling effects, but we are unable to distinguish between the two scenarios. Overall, our measured SFH is consistent with previously derived SFHs of Leo T. However, the HST-based solution provides improved age resolution and reduced uncertainties at all epochs. The SFH, baryonic gas fraction, and location of Leo T are unlike any of the other recently discovered faint dwarf galaxies in the Local Group, and instead bear strong resemblance to gas-rich dwarf galaxies (irregular or transition), suggesting that gas-rich dwarf galaxies may share common modes of star formation over a large range of stellar mass ({approx}10{sup 5}-10{sup 9} M{sub Sun }).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ON THE ESTIMATION OF SYSTEMATIC UNCERTAINTIES OF STAR FORMATION HISTORIES

In most star formation history (SFH) measurements, the reported uncertainties are those due to effects whose sizes can be readily measured: Poisson noise, adopted distance and extinction, and binning choices in the solution itself. However, the largest source of error, systematics in the adopted isochrones, is usually ignored and very rarely explicitly incorporated into the uncertainties. I propose a process by which estimates of the uncertainties due to evolutionary models can be incorporated into the SFH uncertainties. This process relies on application of shifts in temperature and luminosity, the sizes of which must be calibrated for the data being analyzed. While there are inherent limitations, the ability to estimate the effect of systematic errors and include them in the overall uncertainty is significant. The effects of this are most notable in the case of shallow photometry, with which SFH measurements rely on evolved stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SPATIALLY RESOLVED SPECTROSCOPIC STAR FORMATION HISTORIES OF NEARBY DISKS: HINTS OF STELLAR MIGRATION

We use the Mitchell Spectrograph (formerly VIRUS-P) to observe 12 nearby disk galaxies. We successfully measure ages in the outer disk in six systems. In three cases (NGC 2684, NGC 6155, and NGC 7437), we find that a downward break in the disk surface brightness profile corresponds with a change in the dominant stellar population with the interior being dominated by active star formation and the exterior having older stellar populations that are best fit with star formation histories that decline with time. The observed increase in average stellar ages beyond a profile break is similar to theoretical models that predict surface brightness breaks are caused by stellar migration, with the outer disk being populated from scattered old interior stars. In three more cases (IC 1132, NGC 4904, and NGC 6691), we find no significant change in the stellar population as one crosses the break radius. In these galaxies, both the inner and outer disks are dominated by active star formation and younger stellar populations. While radial migration can contribute to the stellar populations beyond the break, it appears that more than one mechanism is required to explain all of our observed stellar profile breaks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE ASSEMBLY HISTORY OF DISK GALAXIES. II. PROBING THE EMERGING TULLY-FISHER RELATION DURING 1 < z < 1.7

Through extended integrations using the recently installed deep depletion CCD on the red arm of the Keck I Low Resolution Imaging Spectrograph, we present new measurements of the resolved spectra of 70 morphologically selected star-forming galaxies with i{sub AB} < 24.1 in the redshift range 1 {approx}< z < 1.7. Using the formalism introduced in Paper I of this series and available Hubble Space Telescope (HST) Advanced Camera for Surveys images, we successfully recover rotation curves using the extended emission line distribution of [O II] 3727 A to 2.2 times the disk scale radius for a sample of 42 galaxies. Combining these measures with stellar masses derived from HST and ground-based near-infrared photometry enables us to construct the stellar mass Tully-Fisher relation (M{sub *}-TFR) in the time interval between the well-constructed relation defined at z {approx_equal} 1 in Paper I and the growing body of resolved dynamics probed with integral field unit spectrographs at z > 2. Remarkably, we find a well-defined TFR with up to 60% increase in scatter and zero-point shift constraint of {Delta}M{sub *} = 0.02 {+-} 0.02 dex since z {approx} 1.7, compared to the local relation. Although our sample is incomplete in terms of either a fixed stellar mass or star formation rate limit, we discuss the implications that typical star-forming disk galaxies evolve to arrive on a well-defined TFR within a surprisingly short period of cosmic history.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE CHARACTERISTIC STAR FORMATION HISTORIES OF GALAXIES AT REDSHIFTS z {approx} 2-7

A large sample of spectroscopically confirmed star-forming galaxies at redshifts 1.4 {<=} z{sub spec} {<=} 3.7, with complementary imaging in the near- and mid-IR from the ground and from the Hubble Space Telescope and Spitzer Space Telescope, is used to infer the average star formation histories (SFHs) of typical galaxies from z {approx} 2 to 7. For a subset of 302 galaxies at 1.5 {<=} z{sub spec} < 2.6, we perform a detailed comparison of star formation rates (SFRs) determined from spectral energy distribution (SED) modeling (SFRs[SED]) and those calculated from deep Keck UV and Spitzer/MIPS 24 {mu}m imaging (SFRs[IR+UV]). Exponentially declining SFHs yield SFRs[SED] that are 5-10 times lower on average than SFRs[IR+UV], indicating that declining SFHs may not be accurate for typical galaxies at z {approx}> 2. The SFRs of z {approx} 2-3 galaxies are directly proportional to their stellar masses (M{sub *}), with unity slope-a result that is confirmed with Spitzer/IRAC stacks of 1179 UV-faint (R>25.5) galaxies-for M{sub *} {approx}> 5 Multiplication-Sign 10{sup 8} M{sub Sun} and SFRs {approx}> 2 M{sub Sun} yr{sup -1}. We interpret this result in the context of several systematic biases that can affect determinations of the SFR-M{sub *} relation. The average specific SFRs at z {approx} 2-3 are remarkably similar within a factor of two to those measured at z {approx}> 4, implying that the average SFH is one where SFRs increase with time. A consequence of these rising SFHs is that (1) a substantial fraction of UV-bright z {approx} 2-3 galaxies had faint sub-L* progenitors at z {approx}> 4; and (2) gas masses must increase with time from z = 2 to 7, over which time the net cold gas accretion rate-as inferred from the specific SFR and the Kennicutt-Schmidt relation-is {approx}2-3 times larger than the SFR. However, if we evolve to higher redshift the SFHs and masses of the halos that are expected to host L* galaxies at z {approx} 2, then we find that {approx}< 10% of the baryons accreted onto typical halos at z {approx}> 4 actually contribute to star formation at those epochs. These results highlight the relative inefficiency of star formation even at early cosmic times when galaxies were first assembling.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

STAR FORMATION HISTORY IN TWO FIELDS OF THE SMALL MAGELLANIC CLOUD BAR

The Bar is the most productive region of the Small Magellanic Cloud in terms of star formation but also the least studied one. In this paper, we investigate the star formation history of two fields located in the SW and in the NE portion of the Bar using two independent and well-tested procedures applied to the color-magnitude diagrams of their stellar populations resolved by means of deep Hubble Space Telescope photometry. We find that the Bar experienced a negligible star formation activity in the first few Gyr, followed by a dramatic enhancement from 6 to 4 Gyr ago and a nearly constant activity since then. The two examined fields differ both in the rate of star formation and in the ratio of recent over past activity, but share the very low level of initial activity and its sudden increase around 5 Gyr ago. The striking similarity between the timing of the enhancement and the timing of the major episode in the Large Magellanic Cloud is suggestive of a close encounter triggering star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE STAR FORMATION HISTORY AND CHEMICAL EVOLUTION OF STAR-FORMING GALAXIES IN THE NEARBY UNIVERSE

We have determined the metallicity (O/H) and nitrogen abundance (N/O) of a sample of 122,751 star-forming galaxies (SFGs) from the Data Release 7 of the Sloan Digital Sky Survey. For all these galaxies we have also determined their morphology and obtained a comprehensive picture of their star formation history (SFH) using the spectral synthesis code STARLIGHT. The comparison of the chemical abundance with the SFH allows us to describe the chemical evolution of the SFGs in the nearby universe (z {<=} 0.25) in a manner consistent with the formation of their stellar populations and morphologies. A high fraction (45%) of the SFGs in our sample show an excess abundance of nitrogen relative to their metallicity. We also find this excess to be accompanied by a deficiency of oxygen, which suggests that this could be the result of effective starburst winds. However, we find no difference in the mode of star formation of the nitrogen-rich and nitrogen-poor SFGs. Our analysis suggests that they all form their stars through a succession of bursts of star formation extended over a period of few Gyr. What produces the chemical differences between these galaxies seems therefore to be the intensity of the bursts: the galaxies with an excess of nitrogen are those that are presently experiencing more intense bursts or have experienced more intense bursts in their past. We also find evidence relating the chemical evolution process to the formation of the galaxies: the galaxies with an excess of nitrogen are more massive, and have more massive bulges and earlier morphologies than those showing no excess. Contrary to expectation, we find no evidence that the starburst wind efficiency decreases with the mass of the galaxies. As a possible explanation we propose that the loss of metals consistent with starburst winds took place during the formation of the galaxies, when their potential wells were still building up, and consequently were weaker than today, making starburst winds more efficient and independent of the final mass of the galaxies. In good agreement with this interpretation, we also find evidence consistent with downsizing, according to which the more massive SFGs formed before the less massive ones.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE TWO-PHASE FORMATION HISTORY OF SPIRAL GALAXIES TRACED BY THE COSMIC EVOLUTION OF THE BAR FRACTION

We study the evolution of galactic bars and the link with disk and spheroid formation in a sample of zoom-in cosmological simulations. Our simulation sample focuses on galaxies with present-day stellar masses in the 10{sup 10}-10{sup 11} M{sub Sun} range, in field and loose group environments, with a broad variety of mass growth histories. In our models, bars are almost absent from the progenitors of present-day spirals at z > 1.5, and they remain rare and generally too weak to be observable down to z Almost-Equal-To 1. After this characteristic epoch, the fractions of observable and strong bars rise rapidly, bars being present in 80% of spiral galaxies and easily observable in two thirds of these at z {<=} 0.5. This is quantitatively consistent with the redshift evolution of the observed bar fraction, although the latter is presently known up to z Almost-Equal-To 0.8 because of band-shifting and resolution effects. Our models hence predict that the decrease in the bar fraction with increasing redshift should continue with a fraction of observable bars not larger than 10%-15% in disk galaxies at z > 1. Our models also predict later bar formation in lower-mass galaxies, in agreement with existing data. We find that the characteristic epoch of bar formation, namely redshift z Almost-Equal-To 0.8-1 in the studied mass range, corresponds to the epoch at which today's spirals acquire their disk-dominated morphology. At higher redshift, disks tend to be rapidly destroyed by mergers and gravitational instabilities and rarely develop significant bars. We hence suggest that the bar formation epoch corresponds to the transition between an early 'violent' phase of spiral galaxy formation at z {>=} 1 and a late 'secular' phase at z {<=} 0.8. In the secular phase, the presence of bars substantially contributes to the growth of the (pseudo-)bulge, but the bulge mass budget remains statistically dominated by the contribution of mergers, interactions, and disk instabilities at high redshift. Early bars at z > 1 are often short-lived, while most of the bars formed at z {<=} 1 persist down to z = 0, late cosmological gas infall being necessary to maintain some of them.

79 ASTRONOMY AND ASTROPHYSICS↗