arXiv subjects

Andrew E. Dolphin

Publications and source records attributed to Andrew E. Dolphin.

At least 109 records · Page 6Linked to original sources

arXiv2014

The Panchromatic Hubble Andromeda Treasury. Progression of Large-Scale Star Formation across Space and Time in M31

We investigate the clustering of early-type stars younger than 300 Myr on galactic scales in M31. Based on the stellar photometric catalogs of the Panchromatic Hubble Andromeda Treasury program that also provides stellar parameters derived from the individual energy distributions, our analysis is focused on the young stars in three star-forming regions, located at galactocentric distances of about 5, 10, and 15 kpc, corresponding to the inner spiral arms, the ring structure, and the outer arm, respectively. We apply the two-point correlation function to our selected sample to investigate the clustering behavior of these stars across different time- and length-scales. We find that young stellar structure survives across the whole extent of M31 longer than 300 Myr. Stellar distribution in all regions appears to be self-similar, with younger stars being systematically more strongly clustered than the older, which are more dispersed. The observed clustering is interpreted as being induced by turbulence, the driving source for which is probably gravitational instabilities driven by the spiral arms, which are stronger closer to the galactic centre.

astro-ph.GA↗

arXiv2014

Constraints for the Progenitor Masses of 17 Historic Core-Collapse Supernovae

Using resolved stellar photometry measured from archival HST imaging, we generate color-magnitude diagrams of the stars within 50 pc of the locations of historic core-collapse supernovae that took place in galaxies within 8 Mpc. We fit these color-magnitude distributions with stellar evolution models to determine the best-fit age distribution of the young population. We then translate these age distributions into probability distributions for the progenitor mass of each SNe. The measurements are anchored by the main-sequence stars surrounding the event, making them less sensitive to assumptions about binarity, post-main-sequence evolution, or circumstellar dust. We demonstrate that, in cases where the literature contains masses that have been measured from direct imaging, our measurements are consistent with (but less precise than) these measurements. Using this technique, we constrain the progenitor masses of 17 historic SNe, 11 of which have no previous estimates from direct imaging. Our measurements still allow the possibility that all SNe progenitor masses are <20 M_sun. However, the large uncertainties for the highest-mass progenitors also allow the possibility of no upper-mass cutoff.

astro-ph.SR↗

arXiv2014

Comparing M31 and Milky Way Satellites: The Extended Star Formation Histories of Andromeda II and Andromeda XVI

We present the first comparison between the lifetime star formation histories (SFHs) of M31 and Milky Way (MW) satellites. Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we obtained deep optical imaging of Andromeda II (M$_{V} = -$12.0; log(M$_{\star}$/M$_{\odot}$) $\sim$ 6.7) and Andromeda XVI (M$_{V} = -$7.5; log(M$_{\star}$/M$_{\odot}$) $\sim$ 4.9) yielding color-magnitude diagrams that extend at least 1 magnitude below the oldest main sequence turnoff, and are similar in quality to those available for the MW companions. And II and And XVI show strikingly similar SFHs: both formed 50-70% of their total stellar mass between 12.5 and 5 Gyr ago (z$\sim$5-0.5) and both were abruptly quenched $\sim$ 5 Gyr ago (z$\sim$0.5). The predominance of intermediate age populations in And XVI makes it qualitatively different from faint companions of the MW and clearly not a pre-reionization fossil. Neither And II nor And XVI appears to have a clear analog among MW companions, and the degree of similarity in the SFHs of And II and And XVI is not seen among comparably faint-luminous pairs of MW satellites. These findings provide hints that satellite galaxy evolution may vary substantially among hosts of similar stellar mass. Although comparably deep observations of more M31 satellites are needed to further explore this hypothesis, our results underline the need for caution when interpreting satellite galaxies of an individual system in a broader cosmological context.

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arXiv2014

The Star Formation Histories of Local Group Dwarf Galaxies II. Searching For Signatures of Reionization

We search for signatures of reionization in the star formation histories (SFHs) of 38 Local Group dwarf galaxies (10$^4$ $<$ M$_{\star}$ $<$ 10$^9$ M$_{\odot}$). The SFHs are derived from color-magnitude diagrams using archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. Only five quenched galaxies (And V, And VI, And XIII, Leo IV, Hercules) are consistent with forming the bulk of their stars before reionization, when full uncertainties are considered. Observations of 13 of the predicted `true fossils' identified by Bovill & Ricotti show that only two (Hercules and Leo IV) indicate star formation quenched by reionization. However, both are within the virial radius of the Milky Way and evidence of tidal disturbance complicates this interpretation. We argue that the late-time gas capture scenario posited by Ricotti for the low mass, gas-rich, and star-forming fossil candidate Leo T is observationally indistinguishable from simple gas retention. Given the ambiguity between environmental effects and reionization, the best reionization fossil candidates are quenched low mass field galaxies (e.g., KKR 25).

astro-ph.GA↗

arXiv2014

The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations

We present uniformly measured star formation histories (SFHs) of 40 Local Group dwarf galaxies based on color-magnitude diagram (CMD) analysis from archival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can be recovered from CMDs that do not reach the oldest main sequence turn-off (MSTO), but emphasize that the oldest MSTO is critical for precisely constraining the earliest epochs of star formation. We find that: (1) the average lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an exponentially declining SFH with $τ$ $\sim$ 5 Gyr; (2) lower luminosity dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the average SFHs of dwarf irregulars (dIrrs), transition dwarfs (dTrans), and dwarf ellipticals (dEs) can be approximated by the combination of an exponentially declining SFH ($τ$ $\sim$ 3-4 Gyr) for lookback ages $>$ 10-12 Gyr ago and a constant SFH thereafter; (4) the observed fraction of stellar mass formed prior to z=2 ranges considerably (80\% for galaxies with M $<$ 10$^5$ M$_{\odot}$ to 30\% for galaxies with M$>$10$^7$ M$_{\odot}$) and is largely explained by environment; (5) the distinction between "ultra-faint" and "classical" dSphs is arbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass prior to z=2 than the SDSS galaxies from Leiter 2012 and the SFHs from the abundance matching models of Behroozi et al. 2013. This may indicate higher than expected star-formation efficiencies at early times in low mass galaxies. Finally, we provide all the SFHs in tabulated electronic format for use by the community.

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arXiv2014

The Panchromatic Hubble Andromeda Treasury. VI. The reliability of far-ultraviolet flux as a star formation tracer on sub-kpc scales

We have used optical observations of resolved stars from the Panchromatic Hubble Andromeda Treasury (PHAT) to measure the recent (< 500 Myr) star formation histories (SFHs) of 33 FUV-bright regions in M31. The region areas ranged from ~$10^4$ to $10^6$ pc$^2$, which allowed us to test the reliability of FUV flux as a tracer of recent star formation on sub-kpc scales. The star formation rates (SFRs) derived from the extinction-corrected observed FUV fluxes were, on average, consistent with the 100-Myr mean SFRs of the SFHs to within the 1$σ$ scatter. Overall, the scatter was larger than the uncertainties in the SFRs and particularly evident among the smallest regions. The scatter was consistent with an even combination of discrete sampling of the initial mass function and high variability in the SFHs. This result demonstrates the importance of satisfying both the full-IMF and the constant-SFR assumptions for obtaining precise SFR estimates from FUV flux. Assuming a robust FUV extinction correction, we estimate that a factor of 2.5 uncertainty can be expected in FUV-based SFRs for regions smaller than $10^5$ pc$^2$, or a few hundred pc. We also examined ages and masses derived from UV flux under the common assumption that the regions are simple stellar populations (SSPs). The SFHs showed that most of the regions are not SSPs, and the age and mass estimates were correspondingly discrepant from the SFHs. For those regions with SSP-like SFHs, we found mean discrepancies of 10 Myr in age and a factor of 3 to 4 in mass. It was not possible to distinguish the SSP-like regions from the others based on integrated FUV flux.

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arXiv2014

Infrared Tip of the Red Giant Branch and Distances to the Maffei/IC 342 Group

In this paper, we extend the use of the tip of the red giant branch (TRGB) method to near-infrared wavelengths from previously-used $I$-band, using the \textit{Hubble Space Telescope (HST)} Wide Field Camera 3 (WFC3). Upon calibration of a color dependency of the TRGB magnitude, the IR TRGB yields a random uncertainty of $\sim 5%$ in relative distance. The IR TRGB methodology has an advantage over the previously-used ACS $F606W$ and $F814W$ filter set for galaxies that suffer from severe extinction. Using the IR TRGB methodology, we obtain distances toward three principal galaxies in the Maffei/IC 342 complex, which are located at low Galactic latitudes. New distance estimates using the TRGB method are 3.45$^{+0.13}_{-0.13}$ Mpc for IC 342, 3.37$^{+0.32}_{-0.23}$ Mpc for Maffei 1 and 3.52$^{+0.32}_{-0.30}$ Mpc for Maffei 2. The uncertainties are dominated by uncertain extinction, especially for Maffei 1 and Maffei 2. Our IR calibration demonstrates the viability of the TRGB methodology for observations with the \textit{James Webb Space Telescope (JWST)}.

astro-ph.GA↗

arXiv2014

The ACS LCID project. X. The Star Formation History of IC 1613: Revisiting the Over-Cooling Problem

We present an analysis of the star formation history (SFH) of a field near the half light radius in the Local Group dwarf irregular galaxy IC 1613 based on deep Hubble Space Telescope Advanced Camera for Surveys imaging. Our observations reach the oldest main sequence turn-off, allowing a time resolution at the oldest ages of ~1 Gyr. Our analysis shows that the SFH of the observed field in IC 1613 is consistent with being constant over the entire lifetime of the galaxy. These observations rule out an early dominant episode of star formation in IC 1613. We compare the SFH of IC 1613 with expectations from cosmological models. Since most of the mass is in place at early times for low mass halos, a naive expectation is that most of the star formation should have taken place at early times. Models in which star formation follows mass accretion result in too many stars formed early and gas mass fractions which are too low today (the "over-cooling problem"). The depth of the present photometry of IC 1613 shows that, at a resolution of ~1 Gyr, the star formation rate is consistent with being constant, at even the earliest times, which is difficult to achieve in models where star formation follows mass assembly.

astro-ph.GA↗

arXiv2014

Distance Determinations to SHIELD Galaxies from HST Imaging

The Survey of HI in Extremely Low-mass Dwarf galaxies (SHIELD) is an on-going multi-wavelength program to characterize the gas, star formation, and evolution in gas-rich, very low-mass galaxies. The galaxies were selected from the first ~10% of the HI ALFALFA survey based on their inferred low HI mass and low baryonic mass, and all systems have recent star formation. Thus, the SHIELD sample probes the faint end of the galaxy luminosity function for star-forming galaxies. Here, we measure the distances to the 12 SHIELD galaxies to be between 5-12 Mpc by applying the tip of the red giant method to the resolved stellar populations imaged by the Hubble Space Telescope. Based on these distances, the HI masses in the sample range from $4\times10^6$ to $6\times10^7$ M$_{\odot}$, with a median HI mass of $1\times10^7$ M$_{\odot}$. The TRGB distances are up to 73% farther than flow-model estimates in the ALFALFA catalog. Because of the relatively large uncertainties of flow model distances, we are biased towards selecting galaxies from the ALFALFA catalog where the flow model underestimates the true distances. The measured distances allow for an assessment of the native environments around the sample members. Five of the galaxies are part of the NGC 672 and NGC 784 groups, which together constitute a single structure. One galaxy is part of a larger linear ensemble of 9 systems that stretches 1.6 Mpc from end to end. Two galaxies reside in regions with 1-4 neighbors, and four galaxies are truly isolated with no known system identified within a radius of 1 Mpc.

astro-ph.GA↗

arXiv2014

Infall of nearby galaxies into the Virgo cluster as traced with HST

We measured the Tip of the Red Giant Branch distances to nine galaxies in the direction to the Virgo cluster using the Advanced Camera for Surveys on the Hubble Space Telescope. These distances put seven galaxies: GR 34, UGC 7512, NGC 4517, IC 3583, NGC 4600, VCC 2037 and KDG 215 in front of the Virgo, and two galaxies: IC 3023, KDG 177 likely inside the cluster. Distances and radial velocities of the galaxies situated between us and the Virgo core clearly exhibit the infall phenomenon toward the cluster. In the case of spherically symmetric radial infall we estimate the radius of the "zero-velocity surface" to be (7.2+-0.7) Mpc that yields the total mass of the Virgo cluster to be (8.0+-2.3) X 10^{14} M_sun in good agreement with its virial mass estimates. We conclude that the Virgo outskirts does not contain significant amounts of dark matter beyond its virial radius.

astro-ph.GA↗

arXiv2013

Constraining the age of the NGC 4565 HI Disk Warp: Determining the Origin of Gas Warps

We have mapped the distribution of young and old stars in the gaseous HI warp of NGC 4565. We find a clear correlation of young stars (<600 Myr) with the warp, but no coincident old stars (>1 Gyr), which places an upper limit on the age of the structure. The formation rate of the young stars, which increased ~300 Myr ago relative to the surrounding regions, is (6.3 +2.5/-1.5) x 10^-5 M_sol/yr/kpc^2. This implies a ~60+/-20 Gyr depletion time of the HI warp, similar to the timescales calculated for the outer HI disks of nearby spiral galaxies. While some stars associated with the warp fall into the asymptotic giant branch (AGB) region of the color magnitude diagram, where stars could be as old as 1 Gyr, further investigation suggests that they may be interlopers rather than real AGB stars. We discuss the implications of these age constraints for the formation of HI warps, and the gas fueling of disk galaxies.

astro-ph.GA↗

arXiv2013

HST/WFC3 Observations of Low-Mass Globular Clusters AM 4 and Palomar 13: Physical Properties and Implications for Mass Loss

We investigate the loss of low-mass stars in two of the faintest globular clusters known, AM 4 and Palomar 13 (Pal 13), using HST/WFC3 F606W and F814W photometry. To determine the physical properties of each cluster --- age, mass, metallicity, extinction, present day mass function (MF) --- we use the maximum likelihood color-magnitude diagram (CMD) fitting program MATCH and the Dartmouth, Padova and BaSTI stellar evolution models. For AM 4, the Dartmouth models provide the best match to the CMD and yield an age of >13 Gyr, metallicity log Z/Z_solar = -1.68 +/- 0.08, a distance modulus (m-M)_V = 17.47 +/- 0.03 and reddening A_V = 0.19 +/- 0.02. For Pal 13 the Dartmouth models give an age of 13.4 +/- 0.5 Gyr, log Z/Z_solar = -1.55 +/- 0.06, (m-M)_V = 17.17 +/- 0.02 and A_V = 0.43 +/- 0.01. We find that the systematic uncertainties due to choice in assumed stellar model greatly exceed the random uncertainties, highlighting the importance of using multiple stellar models when analyzing stellar populations. Assuming a single-sloped power law MF, we find that AM 4 and Pal 13 have spectral indices alpha = +0.68 +/- 0.34 and alpha = -1.67 +/- 0.25 (where a Salpeter MF has alpha = +1.35), respectively. Comparing our derived slopes with literature measurements of cluster integrated magnitude (M_V) and MF slope indicates that AM 4 is an outlier. Its MF slope is substantially steeper than clusters of comparable luminosity, while Pal 13 has a MF in line with the general trend. We discuss both primordial and dynamical origins for the unusual MF slope of AM 4 and tentatively favor the dynamical scenario. However, MF slopes of more low luminosity clusters are needed to verify this hypothesis.

astro-ph.GA↗

arXiv2013

On the Estimation of Random Uncertainties of Star Formation Histories

The standard technique for measurement of random uncertainties of star formation histories (SFHs) is the bootstrap Monte Carlo, in which the color-magnitude diagram (CMD) is repeatedly resampled. The variation in SFHs measured from the resampled CMDs is assumed to represent the random uncertainty in the SFH measured from the original data. However, this technique systematically and significantly underestimates the uncertainties for times in which the measured star formation rate is low or zero, leading to overly (and incorrectly) high confidence in that measurement. This study proposes an alternative technique, the Markov Chain Monte Carlo (MCMC), which samples the probability distribution of the parameters used in the original solution to directly estimate confidence intervals. While the most commonly used MCMC algorithms are incapable of adequately sampling a probability distribution that can involve thousands of highly correlated dimensions, the Hybrid Monte Carlo algorithm is shown to be extremely effective and efficient for this particular task. Several implementation details, such as the handling of implicit priors created by parameterization of the SFH, are discussed in detail.

astro-ph.IM↗

arXiv2013

Cosmicflows-2: The Data

Cosmicflows-2 is a compilation of distances and peculiar velocities for over 8000 galaxies. Numerically the largest contributions come from the luminosity-linewidth correlation for spirals, the TFR, and the related Fundamental Plane relation for E/S0 systems, but over 1000 distances are contributed by methods that provide more accurate individual distances: Cepheid, Tip of the Red Giant Branch, Surface Brightness Fluctuation, SNIa, and several miscellaneous but accurate procedures. Our collaboration is making important contributions to two of these inputs: Tip of the Red Giant Branch and TFR. A large body of new distance material is presented. In addition, an effort is made to assure that all the contributions, our own and those from the literature, are on the same scale. Overall, the distances are found to be compatible with a Hubble Constant H_0 = 74.4 +-3.0 km/s/Mpc. The great interest going forward with this data set will be with velocity field studies. Cosmicflows-2 is characterized by a great density and high accuracy of distance measures locally, falling to sparse and coarse sampling extending to z=0.1.

astro-ph.CO↗

arXiv2013

Timescales on which Star Formation Affects the Neutral ISM

Turbulent neutral hydrogen (HI) line widths are often thought to be driven primarily by star formation (SF), but the timescale for converting SF energy to HI kinetic energy is unclear. As a complication, studies on the connection between Hi line widths and SF in external galaxies often use broadband tracers for the SF rate, which must implicitly assume that SF histories (SFHs) have been constant over the timescale of the tracer. In this paper, we compare measures of HI energy to time-resolved SFHs in a number of nearby dwarf galaxies. We find that HI energy surface density is strongly correlated only with SF that occurred 30-40 Myr ago. This timescale corresponds to the approximate lifetime of the lowest mass supernova progenitors (~ 8 Msun). This analysis suggests that the coupling between SF and the neutral ISM is strongest on this timescale, due either to an intrinsic delay between the release of the peak energy from SF or to the coherent effects of many SNe during this interval. At Σ_SFR > 10^-3 Msun yr^-1 kpc^-2, we find a mean coupling efficiency between SF energy and HI energy of ε= 0.11 +/- 0.04 using the 30-40 Myr timescale. However, unphysical efficiencies are required in lower Σ_SFR systems, implying that SF is not the primary driver of HI kinematics at Σ_SFR < 10^-3 Msun yr^-1 kpc^-2.

astro-ph.GA↗

arXiv2013

Measuring Galaxy Star Formation Rates From Integrated Photometry: Insights from Color-Magnitude Diagrams of Resolved Stars

We use empirical star formation histories (SFHs), measured from HST-based resolved star color-magnitude diagrams, as input into population synthesis codes to model the broadband spectral energy distributions (SEDs) of ~50 nearby dwarf galaxies (6.5 < log M/M_* < 8.5, with metallicities ~10% solar). In the presence of realistic SFHs, we compare the modeled and observed SEDs from the ultraviolet (UV) through near-infrared (NIR) and assess the reliability of widely used UV-based star formation rate (SFR) indicators. In the FUV through i bands, we find that the observed and modeled SEDs are in excellent agreement. In the Spitzer 3.6micron and 4.5micron bands, we find that modeled SEDs systematically over-predict observed luminosities by up to ~0.2 dex, depending on treatment of the TP-AGB stars in the synthesis models. We assess the reliability of UV luminosity as a SFR indicator, in light of independently constrained SFHs. We find that fluctuations in the SFHs alone can cause factor of ~2 variations in the UV luminosities relative to the assumption of a constant SFH over the past 100 Myr. These variations are not strongly correlated with UV-optical colors, implying that correcting UV-based SFRs for the effects of realistic SFHs is difficult using only the broadband SED. Additionally, for this diverse sample of galaxies, we find that stars older than 100 Myr can contribute from <5% to100% of the present day UV luminosity, highlighting the challenges in defining a characteristic star formation timescale associated with UV emission. We do find a relationship between UV emission timescale and broadband UV-optical color, though it is different than predictions based on exponentially declining SFH models. Our findings have significant implications for the comparison of UV-based SFRs across low-metallicity populations with diverse SFHs.

astro-ph.CO↗

arXiv2013

Comparing the Ancient Star Formation Histories of the Magellanic Clouds

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)

astro-ph.CO↗

arXiv2012

The Panchromatic Hubble Andromeda Treasury IV. A Probabilistic Approach to Inferring the High Mass Stellar Initial Mass Function and Other Power-law Functions

We present a probabilistic approach for inferring the parameters of the present day power-law stellar mass function (MF) of a resolved young star cluster. This technique (a) fully exploits the information content of a given dataset; (b) accounts for observational uncertainties in a straightforward way; (c) assigns meaningful uncertainties to the inferred parameters; (d) avoids the pitfalls associated with binning data; and (e) is applicable to virtually any resolved young cluster, laying the groundwork for a systematic study of the high mass stellar MF (M > 1 Msun). Using simulated clusters and Markov chain Monte Carlo sampling of the probability distribution functions, we show that estimates of the MF slope, α, are unbiased and that the uncertainty, Δα, depends primarily on the number of observed stars and stellar mass range they span, assuming that the uncertainties on individual masses and the completeness are well-characterized. Using idealized mock data, we compute the lower limit precision on α and provide an analytic approximation for Δα as a function of the observed number of stars and mass range. We find that ~ 3/4 of quoted literature uncertainties are smaller than the theoretical lower limit. By correcting these uncertainties to the theoretical lower limits, we find the literature studies yield <α>=2.46 with a 1-σ dispersion of 0.35 dex. We verify that it is impossible for a power-law MF to obtain meaningful constraints on the upper mass limit of the IMF. We show that avoiding substantial biases in the MF slope requires: (1) including the MF as a prior when deriving individual stellar mass estimates; (2) modeling the uncertainties in the individual stellar masses; and (3) fully characterizing and then explicitly modeling the completeness for stars of a given mass. (abridged)

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