The Extended Environment of M17: A Star Formation History
Matthew S. Povich, Ed Churchwell, John H. Bieging, Miju Kang, Barbara A. Whitney, Crystal L. Brogan, Craig A. Kulesa, Martin Cohen, Brian L. Babler, Remy Indebe… 展开作者
Matthew S. Povich, Ed Churchwell, John H. Bieging, Miju Kang, Barbara A. Whitney, Crystal L. Brogan, Craig A. Kulesa, Martin Cohen, Brian L. Babler, Remy Indebetouw, Marilyn R. Meade, Thomas P. Robitaille
M17 is one of the youngest and most massive nearby star-formation regions in the Galaxy. It features a bright H II region erupting as a blister from the side of a giant molecular cloud (GMC). Combining photometry from the Spitzer GLIMPSE survey with complementary infrared (IR) surveys, we identify candidate young stellar objects (YSOs) throughout a 1.5 deg x 1 deg field that includes the M17 complex. The long sightline through the Galaxy behind M17 creates significant contamination in our YSO sample from unassociated sources with similar IR colors. Removing contaminants, we produce a highly-reliable catalog of 96 candidate YSOs with a high probability of association with the M17 complex. We fit model spectral energy distributions to these sources and constrain their physical properties. Extrapolating the mass function of 62 intermediate-mass YSOs (M >3 Msun), we estimate that >1000 stars are in the process of forming in the extended outer regions of M17.
From IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim of a large shell structure ~0.5 deg in diameter (~20 pc at 2.1 kpc). We present new maps of CO and 13CO (J=2-1) emission, which show that the shell is a coherent, kinematic structure associated with M17 at v = 19 km/s. The shell is an extended bubble outlining the photodissociation region of a faint, diffuse H II region several Myr old. We provide evidence that massive star formation has been triggered by the expansion of the bubble. The formation of the massive cluster ionizing the M17 H II region itself may have been similarly triggered. We conclude that the star formation history in the extended environment of M17 has been punctuated by successive waves of massive star formation propagating through a GMC complex.
We present a new method that incorporates the horizontal branch morphology into synthetic colour-magnitude diagram based star formation history determinations. This method, we call MORGOTH, self-consistently takes into account all the stellar evolution phases up to the early asymptothic giant branch, flexibly modelling red giant branch mass loss. We test MORGOTH on a range of synthetic populations, and find that the inclusion of the horizontal branch significantly increases the precision of the resulting star formation histories. When the main sequence turn-off is detected, MORGOTH can fit the…
The calibrations linking observed luminosities to the star formation rate depend on the assumed stellar population synthesis model, initial mass function, star formation and metal enrichment history, and whether reprocessing by dust and gas is included. Consequently the shape and normalisation of the inferred cosmic star formation history is sensitive to these assumptions. Using v2.2.1 of the Binary Population and Spectral Synthesis (\bpass) model we determine a new set of calibration coefficients for the ultraviolet, thermal-infrared, and, hydrogen recombination lines. These ultraviolet and t…
The complex history of NGC 1427A revealed by its star clusters and star formation history
University of Vienna（维也纳大学）；European Southern Observatory（欧洲南方天文台）；Leibniz-Institut für Astrophysik Potsdam (AIP)（波茨坦莱布尼茨天体物理研究所）；Pontificia Universidad Católica de Chile（智利天主教 Pontificia 大学）
We measure the cosmic star formation history out to z = 1.3 using a sample of 918 radio-selected star forming galaxies within the 2 square degree COSMOS field. To increase our sample size, we combine 1.4 GHz flux densities from the VLA-COSMOS catalogue with flux densities measured from the VLA-COSMOS radio continuum image at the positions of I < 26.5 galaxies, enabling us to detect 1.4 GHz sources as faint as 40 uJy. We find radio measurements of the cosmic star formation history are highly dependent on sample completeness and models used to extrapolate the faint end of the radio luminosity fu…
The primary method for inferring the stellar mass ($M_*$) of a galaxy is through spectral energy distribution (SED) modeling. However, the technique rests on assumptions such as the galaxy star formation history and dust attenuation law that can severely impact the accuracy of derived physical properties from SED modeling. Here, we examine the effect that the assumed star formation history (SFH) has on the stellar properties inferred from SED fitting by ground truthing them against mock observations of high-resolution cosmological hydrodynamic galaxy formation simulations. Classically, SFHs ar…
Linking the Metallicity Enrichment History to the Star Formation History: An SFH-regulated Chemical Evolution Model and Its Implications for the Gas Cycling Process
Halo Occupation Distribution (HOD) is a model giving the average number of galaxies in a dark matter halo, function of its mass and other intrinsic properties, like distance from halo center, luminosity and redshift of its constituting galaxies. It is believed that these parameters could also be related to the galaxy history of formation. We want to investigate more this relation in order to test and better refine this model. To do that, we extract HOD indicators from EUCLID mock catalogs for different luminosity cuts and for redshifts ranges going from 0.1 < z < 3.0. We study and interpret th…
Stationary Density Wave Theory predicts the existence of an age gradient across the spiral arms with a phase crossing at the co-rotation radius. Using star formation history (SFH) maps of 12 nearby spiral galaxies derived from \textsc{LIGHTNING} \citep{Eufracio:2017}, a spectral energy distribution (SED) fitting procedure, and by using \textsc{Spirality} \citep{Shield:2015} a \textsc{MATLAB}-based code which plots synthetic spiral arms over \textsc{FITS} images, we have found a gradual decrement in pitch angles with increasing age, thus providing us with evidence in favour of the Stationary De…
We present a new method to determine the star formation and metal enrichment histories of any resolved stellar system. This method is based on the fact that any observed star in a colour-magnitude diagram will have a certain probability of being associated with an isochrone characterised by an age t and metallicity [Fe/H] (i.e. to have formed at the time and with the metallicity of that isochrone). We formulate this as a maximum likelihood problem that is then solved with a genetic algorithm. We test the method with synthetic simple and complex stellar populations. We also present tests using …
We present the star formation history (SFH) of the Sextans dwarf spheroidal galaxy based on deep archive $B$,$I$ photometry taken with Suprime-Cam at Subaru telescope focusing our analysis on the inner region of the galaxy, fully located within the core radius. Within the errors of our SFH we have not detected any metallicity gradient along the considered radial distance interval. As a main result of this work we can state that the Sextans dwarf spheroidal stopped forming stars less than $\sim1.3$ Gyr after Big Bang in correspondance to the end of the reionization epoch. We have been able to c…
We explore the radial variation of star formation histories in dwarf galaxies simulated with Feedback In Realistic Environments (FIRE) physics. The sample contains 9 low-mass field dwarfs with M_ star = 10^5 - 10^7 M_sun from previous FIRE results, and a new suite of 17 higher mass field dwarfs with M_star = 10^7 - 10^9 M_sun introduced here. We find that age gradients are common in our dwarfs, with older stars dominant at large radii. The strength of the gradient correlates with overall galaxy age such that earlier star formation produces a more pronounced gradient. The relation between forma…
We present the star formation history (SFH) of the Sculptor dwarf spheroidal galaxy based on deep g,r photometry taken with DECam at the Blanco telescope, focusing our analysis on the central region of the galaxy extended up to $\sim 3$ core radii. We have investigated how the SFH changes radially, subdividing the sampled area into four regions, and have detected a clear trend of star formation. All the SFHs show a single episode of star formation, with the innermost region presenting a longer period of star formation of $\sim 1.5$ Gyr and for the outermost region the main period of star forma…
The Gaia DR2 catalog is the best source of stellar astrometric and photometric data available today. The history of the Milky Way galaxy is written in stone in this data set. Parallaxes and photometry tell us where the stars are today, when were they formed, and with what chemical content, i.e. their star formation history (SFH). We develop a Bayesian hierarchical model suited to reconstruct the SFH of a resolved stellar population. We study the stars brighter than $G\,=\,15$ within 100 pc of the Sun in Gaia DR2 and derive a SFH of the solar neighbourhood in agreement with previous determinati…
I present an algorithm for inverting the luminosity function for white dwarfs to obtain a maximum likelihood estimate of the star formation rate of the host stellar population. The algorithm is of the general class of Expectation Maximization, and involves iteratively improving an initial guess of the star formation rate. Tests show that the inversion results are quite sensitive to the assumed metallicity and initial mass function, but relatively insensitive to the initial-final mass relation and ratio of H/He atmosphere white dwarfs. Application to two independent determinations of the Solar …
We study the star formation history (SFH) of the ultra-diffuse galaxy (UDG) Dragonfly 44 (DF44) based on the simultaneous fit to near-ultraviolet to near-infrared photometry and high signal-to-noise optical spectroscopy. In fitting the observations we adopt an advanced physical model with a flexible SFH, and we discuss the results in the context of the degeneracies between stellar population parameters. Through reconstructing the mass-assembly history with a prior for extended star formation (akin to methods in the literature) we find that DF44 formed 90 per cent of its stellar mass by $z\sim …
Galaxy mass assembly is an end product of structure formation in the $Λ$CDM cosmology. As an extension of Lee \& Yi (2013), we investigate the assembly history of stellar components in galaxies as a function of halo environments and stellar mass using semi-analytic approaches. In our fiducial model, halo mass intrinsically determines the formation and assembly of the stellar mass. Overall, the ex situ fraction slowly increases in central galaxies with increasing halo mass but sharply increases for $\log M_{*}/M_{\odot}\gtrsim11$. A similar trend is also found in satellite galaxies, which impli…
We report here on the discovery of an extended far-infrared shell around the AGB star, R Cassiopeia, made by AKARI and Spitzer. The extended, cold circumstellar shell of R Cas spans nearly 3 arcmin and is probably shaped by interaction with the interstellar medium. This report is one of several studies of well-resolved mass loss histories of AGB stars under AKARI and Spitzer observing programs labeled "Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES)".
We present a detailed study of the Magellanic irregular galaxy NGC 4449 based on both archival and new photometric data from the Legacy Extragalactic UV Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys and Wide Field Camera 3. Thanks to its proximity ($D=3.82\pm 0.27$ Mpc) we reach stars 3 magnitudes fainter than the tip of the red giant branch in the F814W filter. The recovered star formation history spans the whole Hubble time, but due to the age-metallicity degeneracy of the red giant branch stars, it is robust only over the lookback time reached by our photometr…
It has recently been shown that the abundance of cold neutral gas may follow a similar evolution as the star formation history. This is physically motivated, since stars form out of this component of the neutral gas and if the case, would resolve the longstanding issue that there is a clear disparity between the total abundance of neutral gas and star forming activity over the history of the Universe. Radio-band 21-cm absorption traces the cold gas and comparison with the Lyman-alpha absorption, which traces all of the gas, provides a measure of the cold gas fraction or the spin temperature. T…
Taking advantage of the Gaia DR1, we combined TGAS parallaxes with the Tycho-2 and APASS photometry to calculate the star formation history (SFH) of the solar neighbourhood within 250 pc using the colour-magnitude diagram fitting technique. Our dynamically-evolved SFH is in excellent agreement with that calculated from the Hipparcos catalogue within 80 pc of the Sun, showing an enhanced star formation rate (SFR) in the past ~4 Gyr. We then correct the SFR for the disc thickening with age to obtain a SFR that is representative of the whole solar cylinder, and show that even with an extreme corr…
We present a numerical study of the impact of AGN accretion and feedback on the star formation history of barred disc galaxies. Our goal is to determine whether the effect of feedback is positive (enhanced star formation) or negative (quenched star formation), and to what extent. We performed a series of 12 hydrodynamical simulations of disc galaxies, 10 barred and 2 unbarred, with various initial gas fractions and AGN feedback prescriptions. In barred galaxies, gas is driven toward the centre of the galaxy and causes a starburst, followed by a slow decay, while in unbarred galaxies the SFR in…
We investigate the formation history of the stellar disk component in the Milky Way (MW) based on our new chemical evolution model. Our model considers several fundamental baryonic processes, including gas infall, re-accretion of outflowing gas, and radial migration of disk stars. Each of these baryonic processes in the disk evolution is characterized by model parameters, which are determined by fitting to various observational data of the stellar disk in the MW, including the radial dependence of the metallicity distribution function (MDF) of the disk stars, which has recently been derived in…
We develop a chemical evolution model in order to study the star formation history of the Milky Way. Our model assumes that the Milky Way is formed from a closed box-like system in the inner regions, while the outer parts of the disc experience some accretion. Unlike the usual procedure, we do not fix the star formation prescription (e.g. Kennicutt law) in order to reproduce the chemical abundance trends. Instead, we fit the abundance trends with age in order to recover the star formation history of the Galaxy. Our method enables one to recover with unprecedented accuracy the star formation hi…
We present deep photometry of the Carina dwarf Spheroidal galaxy in the B,V filters from CTIO/MOSAIC, out to and beyond the tidal radius. The accurately calibrated photometry is combined with spectroscopic metallicity distributions of Red Giant Branch stars to determine the detailed star formation and chemical evolution history. The star formation history confirms the episodic formation history of Carina and quantifies the duration and strength of each episode in great detail, as a function radius from the centre. Two main episodes of star formation occurred at old (>8 Gyr) and intermediate (2…
We present a detailed analysis of the Horizontal Branch of the Carina Dwarf Spheroidal Galaxy by means of synthetic modelling techniques, taking consistently into account the star formation history and metallicity evolution as determined from main sequence and red giant branch spectroscopic observations. We found that a range of integrated red giant branch mass loss values of 0.1-0.14 M, increasing with metallicity, is able to reproduce the colour extension of the old Horizontal Branch. However, leaving the mass loss as the only free parameter is not enough to match the detailed morphology of …
We investigate the relation between the star formation history and the evolution of the double-degenerate (DD) population in the thin disc of the Galaxy, which we assume to have formed 10 Gyr before the present. We introduce the use of star-formation contribution functions as a device for evaluating the birth rates, total number and merger rates of DDs. These contribution functions help to demonstrate the relation between star-formation history and the current DD population and, in particular, show how the numbers of different types of DD are sensitive to different epochs of star formation. We…
We present an updated model for the extragalactic background light (EBL) from stars and dust, over wavelengths approximately 0.1 to 1000 $μ$m. This model uses accurate theoretical stellar spectra, and tracks the evolution of star formation, stellar mass density, metallicity, and interstellar dust extinction and emission in the universe with redshift. Dust emission components are treated self-consistently, with stellar light absorbed by dust reradiated in the infrared as three blackbody components. We fit our model, with free parameters associated with star formation rate and dust extinction an…
We use the SMASH survey to obtain unprecedented deep photometry reaching down to the oldest main sequence turn-offs in the colour-magnitude diagrams (CMDs) of the Small Magellanic Cloud (SMC) and quantitatively derive its star formation history (SFH) using CMD fitting techniques. We identify five distinctive peaks of star formation in the last 3.5 Gyr, at $\sim $3, $\sim$2, $\sim$1.1, $\sim $0.45 Gyr ago, and one presently. We compare these to the SFH of the Large Magellanic Cloud (LMC) finding unequivocal synchronicity, with both galaxies displaying similar periods of enhanced star formation …
We present a combination of the Schwarzschild orbit-superposition dynamical modelling technique with the spatially-resolved mean stellar age and metallicity maps to uncover the formation history of galaxies. We apply this new approach to a remarkable 5-pointing mosaic of VLT/MUSE observations obtained by Guérou et al. (2016) extending to a maximum galactocentric distance of $120''$ ($\sim 5.6\ {\rm kpc}$) along the major axis, corresponding to $\sim 2.5\ {\rm R}_e$. Our method first identifies `families' of orbits from the dynamical model that represent dynamically-distinct structures of the g…
The star-formation histories of the main stellar components of the Milky Way constrain critical aspects of galaxy formation and evolution. I discuss recent determinations of such histories, together with their interpretation in terms of theories of disk galaxy evolution.
The expected gravitational wave (GW) signal due to double degenerates (DDs) in the thin Galactic disc is calculated using a Monte Carlo simulation. The number of young close DDs that will contribute observable discrete signals in the frequency range $1.58 - 15.8$ mHz is estimated by comparison with the sensitivity of proposed GW observatories. The present-day DD population is examined as a function of Galactic star-formation history alone. It is shown that the frequency distribution, in particular, is a sensitive function of the Galactic star formation history and could be used to measure the …
We carefully analyze how the abundance of Nitrogen over Oxygen evolves when dependent on metallicity stellar yields with a primary component of N proceeding from AGBs stars are used. We show the results obtained with a chemical evolution models grid, calculated with variable star formation efficiencies, which produce different star formation histories. Finally we see how the N/O abundance is related on the evolutionary history.
We show that the star formation history, the reionization history and the present luminosity function of galaxies are reproduced in a simple gravitational collapse model within the $Λ$CDM regime to almost a quantitative accuracy, when the physical conditions, the Jeans criterion and the cooling process, are taken into account. Taking a reasonable set of the model parameters, the reionisation takes place sharply at around redshift $1+z\simeq 7.5$, and the resulting luminosity function turns off at $L\simeq 10^{10.7}L_\odot$, showing the consistency between the star formation history and the rei…
We introduce a near-IR monitoring campaign of the Local Group spiral galaxy M33, carried out with the UK IR Telescope (UKIRT). The pulsating giant stars are identified and their distributions are used to derive the star formation rate as a function of age. We here present the star formation history for the central square kiloparsec. These stars are also important dust factories; we measure their dust production rates from a combination of our data with Spitzer Space Telescope mid-IR photometry.
We present a quantitative star formation history of the nearby dwarf galaxy UGCA 92. This irregular dwarf is situated in the vicinity of the Local Group of galaxies in a zone of strong Galactic extinction (IC 342 group of galaxies). The galaxy was resolved into stars with HST/ACS including old red giant branch. We have constructed a model of the resolved stellar populations and measured the star formation rate and metallicity as function of time. The main star formation activity period occurred about 8 - 14 Gyr ago. These stars are mostly metal-poor, with a mean metallicity [Fe/H] ~ -1.5 -- -2…
Young stellar populations provide a record of past star formation, and by establishing their members' dynamics and ages, it is possible to reconstruct the full history of star formation events. Gaia has greatly expanded the number of accessible stellar populations, with one of the most notable recently-discovered associations being Cepheus Far North (CFN), a population containing hundreds of members spanning over 100 pc. With its proximity (d $\lesssim$ 200 pc), apparent substructure, and relatively small population, CFN represents a manageable population to study in depth, with enough evidenc…
The aim of this work is to test to what extent the star cluster population of a galaxy can be utilised to constrain or estimate the star formation history, with the Large Magellanic Cloud as our testbed. We follow two methods to extract information about the star formation rate from star clusters, either using only the most massive clusters (Maschberger & Kroupa 2007) or using the whole cluster population, albeit this is only possible for a shorter age span. We compare these results with the star formation history derived from colour-magnitude diagrams and find good overall agreement for the m…
We present the first-ever global, spatially-resolved reconstruction of the star formation history (SFH) of the Large Magellanic Cloud (LMC), based on the application of our StarFISH analysis software to the multiband photometry of twenty million of its stars from the Magellanic Clouds Photometric Survey. The general outlines of our results are consistent with previously published results: following an initial burst of star formation, there was a quiescent epoch from approximately 12 to 5 Gyr ago. Star formation then resumed and has proceeded until the current time at an average rate of roughly…
By employing a simple semi-analytical star formation model where the formation rates of Population (Pop) I/II and III stars can be calculated, respectively, we account for the number distribution of gamma-ray bursts (GRBs) with high pseudo-redshifts that was derived from an empirical luminosity-indictor relationship. It is suggested that a considerable number of Pop III GRBs could exist in the present sample of Swift GRBs. By further combining the implication for the star formation history from the optical depth of the CMB photons, it is also suggested that only a very small fraction 0.6% of P…
We compare the impacts of uncertainties in both binary population synthesis models and the cosmic star formation history on the predicted rates of Gravitational Wave compact binary merger (GW) events. These uncertainties cause the predicted rates of GW events to vary by up to an order of magnitude. Varying the volume-averaged star formation rate density history of the Universe causes the weakest change to our predictions, while varying the metallicity evolution has the strongest effect. Double neutron-star merger rates are more sensitive to assumed neutron-star kick velocity than the cosmic st…
We use the semi-analytic model ChemTreeN, coupled to cosmological N-body simulations, to explore how different galaxy formation histories can affect observational properties of Milky Way-like galaxies' stellar haloes and their satellite populations. Gaussian processes are used to generate model emulators that allow one to statistically estimate a desired set of model outputs at any location of a p-dimensional input parameter space. This enables one to explore the full input parameter space orders of magnitude faster than could be done otherwise. Using mock observational data sets generated by …
Radio wavelengths offer the unique possibility of tracing the total star-formation rate in galaxies, both obscured and unobscured. As such, they may provide the most robust measurement of the star-formation history of the Universe. In this chapter we highlight the constraints that the SKA can place on the evolution of the star-formation history of the Universe, the survey area required to overcome sample variance, the spatial resolution requirements, along with the multi-wavelength ancillary data that will play a major role in maximising the scientific promise of the SKA. The required combinat…
We construct a parameterized model to explore the main properties of the star formation history of M33. We assume that the disk originates and grows by the primordial gas infall and adopt the simple form of gas accretion rate with one free parameter, the infall time-scale. We also include the contribution of gas outflow process. A major update of the model is that we adopt a molecular hydrogen correlated star formation law and calculate the evolution of the atomic and molecular gas separately. Comparisons between the model predictions and the observational data show that the model predictions …
The Taurus-Auriga complex is the prototypical low-mass star forming region, and provides a unique testbed of the star formation process, which left observable imprints on the spatial, kinematic, and temporal structure of its stellar population. Taurus's rich observational history has uncovered peculiarities that suggest a complicated star forming event, such as members at large distances from the molecular clouds and evidence of an age spread. With Gaia, an in-depth study of the Taurus census is possible to confirm membership, identify substructure, and reconstruct its star formation history. …
In this paper we propose a novel technique to constrain the progenitor binary black hole (BBH) formation history using the remnant masses and spins of merged black holes (BHs). Exploring different models, we found that dynamically formed BBHs are distributed differently in the mass-spin plane than those that have formed in isolation. Stellar evolution recipes crucially affect the remnant mass distribution, suggesting that future efforts should be devoted to finding a common way of modelling the evolutionary phases of single and binary stars. Our simple approach has allowed us to place weak con…
We report a new star formation history for the Tucana dwarf spheroidal galaxy, obtained from a new look at a deep HST/ACS colour-magnitude diagram. We combined information from the main sequence turn-off and the horizontal branch to resolve the ancient star formation rates on a finer temporal scale than previously possible. We show that Tucana experienced three major phases of star formation, two very close together at ancient times and the last one ending between 6 and 8 Gyr ago. We show that the three discrete clumps of stars on the horizontal branch are linked to the distinct episodes of st…
Determining the star formation history (SFH) is key to understand the formation and evolution of dwarf galaxies. Recovering the SFH in resolved galaxies is mostly based on deep colour--magnitude diagrams (CMDs), which trace the signatures of multiple evolutionary stages of their stellar populations. In distant and unresolved galaxies, the integrated light of the galaxy can be decomposed, albeit made difficult by an age--metallicity degeneracy. Another solution to determine the SFH of resolved galaxies is based on evolved stars; these luminous stars are the most accessible tracers of the underl…
We have obtained the time and space-resolved star formation history (SFH) of M51a (NGC 5194) by fitting GALEX, SDSS, and near infrared pixel-by-pixel photometry to a comprehensive library of stellar population synthesis models drawn from the Synthetic Spectral Atlas of Galaxies (SSAG). We fit for each space-resolved element (pixel) an independent model where the SFH is averaged in 137 age bins, each one 100 Myr wide. We used the Bayesian Successive Priors (BSP) algorithm to mitigate the bias in the present-day spatial mass distribution. We test BSP with different prior probability distribution…
Galaxy interactions are known to trigger starbursts. Young massive star clusters formed in interacting galaxies and mergers may become young globular clusters. The ages of these clusters can provide clues about the timing of interaction-triggered events, and thus provide an important way to reconstruct the star formation history of merging galaxies. Numerical simulations of galaxy mergers can implement different star formation rules. For instance, star formation dependent on gas density or triggered by shocks, predicts significantly different star formation histories. To test the validity of t…
We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group spiral galaxy M 33 (Triangulum). A new method has been developed by us to use pulsating giant stars to reconstruct the star formation history of galaxies over cosmological time as well as using them to map the dust production across their host galaxies. In first Instance the central square kiloparsec of M33 was monitored and long period variable stars (LPVs) were identified. We give evidence of two epochs of a star formation rate enhanced by a factor of a few. These stars are also imp…
If we are to develop a comprehensive and predictive theory of galaxy formation and evolution, it is essential that we obtain an accurate assessment of how and when galaxies assemble their stellar populations, and how this assembly varies with environment. There is strong observational support for the hierarchical assembly of galaxies, but by definition the dwarf galaxies we see today are not the same as the dwarf galaxies and proto-galaxies that were disrupted during the assembly. Our only insight into those disrupted building blocks comes from sifting through the resolved field populations of…
I briefly review what has been recently learned from determinations of mean stellar ages and abundances from integrated light studies of early-type galaxies, and discuss some new questions posed by recent data. A short discussion of spectroscopic ages is presented, but the main focus of this review is on the abundances of Fe, Mg, Ca, N, and C, obtained from comparisons of measurements taken in integrated spectra of galaxies with predictions from stellar population synthesis models.
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 HST 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. Th…
