Differences In Star Formation Histories Research Articles

Binaries drive high Type Ia supernova rates in dwarf galaxies

ABSTRACT The scaling of the specific Type Ia supernova (SN Ia) rate with host galaxy stellar mass $\dot{\text{N}}_\text{Ia} / \text{M}_\star \sim \text{M}_\star ^{-0.3}$ as measured in ASAS-SN and DES strongly suggests that the number of SNe Ia produced by a stellar population depends inversely on its metallicity. We estimate the strength of the required metallicity dependence by combining the average star formation histories (SFHs) of galaxies as a function of their stellar mass with the mass–metallicity relation (MZR) for galaxies and common parametrizations for the SN Ia delay-time distribution. The differences in SFHs can account for only ∼30 per cent of the increase in the specific SN Ia rate between stellar masses of M⋆ = 1010 and 107.2 M⊙. We find that an additional metallicity dependence of approximately ∼Z−0.5 is required to explain the observed scaling. This scaling matches the metallicity dependence of the close binary fraction observed in APOGEE, suggesting that the enhanced SN Ia rate in low-mass galaxies can be explained by a combination of their more extended SFHs and a higher binary fraction due to their lower metallicities. Due to the shape of the MZR, only galaxies below M⋆ ≈ 3 × 109 M⊙ are significantly affected by the metallicity-dependent SN Ia rates. The $\dot{\text{N}}_\text{Ia} / \text{M}_\star \sim \text{M}_\star ^{-0.3}$ scaling becomes shallower with increasing redshift, dropping by factor of ∼2 at 107.2 M⊙ between z = 0 and 1 with our ∼Z−0.5 scaling. With metallicity-independent rates, this decrease is a factor of ∼3. We discuss the implications of metallicity-dependent SN Ia rates for one-zone models of galactic chemical evolution.

Chaos and variance in galaxy formation

The evolution of galaxies is governed by equations with chaotic solutions: gravity and compressible hydrodynamics. While this micro-scale chaos and stochasticity has been well studied, it is poorly understood how it couples to macro-scale properties examined in simulations of galaxy formation. In this paper, we show how perturbations introduced by floating-point roundoff, random number generators, and seemingly trivial differences in algorithmic behaviour can produce non-trivial differences in star formation histories, circumgalactic medium (CGM) properties, and the distribution of stellar mass. We examine the importance of stochasticity due to discreteness noise, variations in merger timings and how self-regulation moderates the effects of this stochasticity. We show that chaotic variations in stellar mass can grow until halted by feedback-driven self-regulation or gas exhaustion. We also find that galaxy mergers are critical points from which large (as much as a factor of 2) variations in quantities such as the galaxy stellar mass can grow. These variations can grow and persist for more than a Gyr before regressing towards the mean. These results show that detailed comparisons of simulations require serious consideration of the magnitude of effects compared to run-to-run chaotic variation, and may significantly complicate interpreting the impact of different physical models. Understanding the results of simulations requires us to understand that the process of simulation is not a mapping of an infinitesimal point in configuration space to another, final infinitesimal point. Instead, simulations map a point in a space of possible initial conditions points to a volume of possible final states.

HST/ACS DIRECT AGES OF THE DWARF ELLIPTICAL GALAXIES NGC 147 AND NGC 185

We present the deepest optical photometry for any dwarf elliptical (dE) galaxy based on Hubble Space Telescope ACS observations of the Local Group dE galaxies NGC 147 and NGC 185. The resulting F606W and F814W color-magnitude diagrams are the first to reach below the main sequence turnoff in a dE galaxy, allowing us to determine full star formation histories in these systems. The ACS fields are located ~1.5 effective radii from the galaxy center to avoid photometric crowding. While our ACS pointings in both dEs show unambiguous evidence for old and intermediate age stars, the mean age in NGC 147 is ~ 4 Gyr younger as compared to NGC 185. In NGC 147, only 40% of stars were in place 12.5 Gyrs ago (z~5), with the bulk of the remaining stellar population forming between 5 to 7 Gyr. In contrast, 70% of stars were formed in NGC 185 field more than12.5 Gyr ago with the majority of the remaining population forming between 8 to 10 Gyr. Star formation ceased in both ACS fields at least 3 Gyr ago. Previous observations in the central regions of NGC 185 show evidence for star formation as recent as 100 Myr ago and a strong metallicity gradient with radius. We suggest that the orbit of NGC 185 has a larger pericenter as compared to NGC 147, allowing it to preserve radial gradients and maintain a small central reservoir of recycled gas. We interpret the inferred differences in star formation histories to imply an earlier infall time into the M31 environment for NGC 185 as compared to NGC 147.

Clues from star-formation histories - does the formation scenario of S0 galaxies depend on their luminosities?

AbstractLenticular (S0) galaxies are a morphological transition class between ellipticals and early-type spirals. Recent studies indicate that the formation scenario for lenticular galaxies depends on their near-IR luminosity (Barway et al. 2007;2009;2011). Low-luminosity lenticular galaxies likely formed by the stripping of gas from the disc of late-type spiral galaxies, which in turn formed their pseudo bulges through secular evolution processes, possibly involving multiple episodes of star formation. On the other hand, more luminous lenticulars likely formed at early epochs through a rapid collapse followed by rapid star formation, similar to the formation process of elliptical galaxies, and are characterized by a old coeval stellar population in the bulge. To investigate the imprint of these distinct formation scenarios on stellar populations, we combine GALEX/SDSS/2MASS photometry to construct color-color diagrams for a carefully defined sample of ~400 lenticular galaxies. We divide the sample into bright and faint groups as suggested by Barway et al. (2007). We find the (FUV-NUV) versus (NUV-K) color-color diagram shows larger scatter for faint lenticular galaxies while bright lenticular galaxies show a much tighter relation similar to that of ellipticals. Simple Stellar Population (SSP) analysis shows that ellipticals and bright lenticulars are dominated by a stellar population of age >109 years while faint lenticular galaxies are as young as 108 years, providing evidence for relatively recent star formation activity. Further, if we use the (NUV-r) < 5.4 color criterion to identify galaxies with recent (within 500 Myr) star formation, most faint lenticular galaxies fall into this category; bright lenticulars are mostly quiescent. Most faint lenticular galaxies generally have lower velocity dispersions. If we consider velocity as a virial mass indicator, majority of faint, recently star forming, lenticular galaxies have low virial masses compared to brighter lenticular galaxies. Such systematic differences in star formation history between bright and faint lenticular galaxies are consistent with the expectations from the luminosity dependent formation scenario described above.

Using the XMM-Newton Optical Monitor to Study Cluster Galaxy Evolution

ABSTRACTWe explore the application of XMM-Newton Optical Monitor (XMM-OM) ultraviolet (UV) data to study galaxy evolution. Our sample is constructed as the intersection of all Abell clusters with z < 0.05 and having archival XMM-OM data in either the UVM2 or UVW1 filters, plus optical and UV photometry from the Sloan Digital Sky Survey and GALEX, respectively. The 11 resulting clusters include 726 galaxies with measured redshifts, 520 of which have redshifts placing them within their parent Abell clusters. We develop procedures for manipulating the XMM-OM images and measuring galaxy photometry from them, and we confirm our results via comparison with published catalogs. Color-magnitude diagrams (CMDs) constructed using the XMM-OM data along with SDSS optical data show promise for evolutionary studies, with good separation between red and blue sequences and real variation in the width of the red sequence that is likely indicative of differences in star formation history. This is particularly true for UVW1 data, as the relative abundance of data collected using this filter and its depth make it an attractive choice. Available tools that use stellar synthesis libraries to fit the UV and optical photometric data may also be used, thereby better describing star formation history within the past billion years and providing estimates of total stellar mass that include contributions from young stars. Finally, color-color diagrams that include XMM-OM UV data appear useful to the photometric identification of both extragalactic and stellar sources.

Stellar population gradients in Fornax cluster S0 galaxies: connecting bulge and disc evolution

We present absorption-line index gradients for a sample of S0 galaxies in the Fornax Cluster. The sample has been selected to span a wide range in galaxy mass, and the deep VLT-FORS2 spectroscopy allows us to explore the stellar populations all the way to the outer disk-dominated regions of these galaxies. We find that globally, in both bulges and disks, star formation ceased earliest in the most massive systems, as a further manifestation of downsizing. However, within many galaxies, we find an age gradient which indicates that star formation ended first in the outermost regions. Metallicity gradients, when detected, are always negative such that the galaxy centres are more metal-rich. This finding fits with a picture in which star formation continued in the central regions, with enriched material, after it had stopped in the outskirts. Age and metallicity gradients are correlated, suggesting that large differences in star formation history between the inner and outer parts of S0 galaxies yield large differences in their chemical enrichment. In agreement with previous results, we conclude that the radial variations in the stellar populations of S0 galaxies are compatible with the hypothesis that these galaxies are the descendants of spiral galaxies whose star formation has ceased. With the addition of radial gradient information, we are able to show that this shutdown of star formation occurred from the outside inward, with the later star formation in the central regions offering a plausible mechanism for enhancing the bulge light in these systems, as the transformation to more bulge-dominated S0 galaxies requires.

Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters

We have observed 10 red giant stars in four old Large Magellanic Cloud globular clusters with the high-resolution spectrograph MIKE on the Magellan Landon Clay 6.5 m telescope. The stars in our sample have up to 20 elemental abundance determinations for the α-, iron peak, and neutron-capture element groups. We have also derived abundances for the light odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be more metal-rich than previous estimates from the Ca II triplet, and we derive [Fe/H] values closer to those obtained from the slope of the red giant branch. However, we confirm previous determinations for Hodge 11 and NGC 1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios are comparable to the values observed in old Galactic globular cluster stars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMC clusters do not share the low-Y behavior observed in some dwarf spheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however, are significantly lower than what is seen in the Galactic globular cluster system. Neither does the behavior of [Cu/Fe] as a function of [Fe/H] in our LMC clusters match the trend seen in the Galaxy, staying instead at a constant value of roughly -0.8. Because not all [α/Fe] ratios are suppressed, these abundance ratios cannot be attributed solely to the injection of Type Ia supernova material and instead reflect the differences in star formation history of the LMC versus the Milky Way. An extensive numerical experimental study was performed, varying both input parameters and stellar atmosphere models, to verify that the unusual abundance ratios derived in this study are not the result of the adopted atomic parameters, stellar atmospheres, or stellar parameters. We conclude that many of the abundances in the LMC globular clusters we observed are distinct from those observed in the Milky Way, and these differences are intrinsic to the stars in those systems.

Luminosity and Stellar Mass Functions of Local Star-forming Galaxies

We present the optical and near-infrared luminosity and mass functions of the local star-forming galaxies in the Universidad Complutense de Madrid (UCM) Survey. A bivariate method that explicitly deals with the Hα selection of the survey is used when estimating these functions. Total stellar masses have been calculated on a galaxy-by-galaxy basis taking into account differences in star formation histories. The main difference between the luminosity distributions of the UCM sample and the luminosity functions of the local galaxy population is a lower normalization (ϕ*), indicating a lower global volume density of UCM galaxies. The typical near-infrared luminosity (L*) of local star-forming galaxies is fainter than that of normal galaxies. This is a direct consequence of the lower stellar masses of our objects. However, at optical wavelengths (B and r), the luminosity enhancement arising from the young stars leads to M* values that are similar to those of normal galaxies. The fraction of the total optical and near-infrared luminosity density in the local universe associated with star-forming galaxies is 10%-20%. Fitting the total stellar mass function using a Schechter parameterization, we obtain α = -1.15 ± 0.15, log * = 10.82 ± 0.17 ☉, and log ϕ* = -3.04 ± 0.20 Mpc-3. This gives an integrated total stellar mass density of 107.83±0.07 ☉ Mpc-3 in local star-forming galaxies (H0 = 70 km s-1 Mpc-1, ΩM = 0.3, and Λ = 0.7). The volume-averaged burst strength of the UCM galaxies is b = 0.04 ± 0.01, defined as the ratio of the mass density of stars formed in recent bursts (with an age of <10 Myr) to the total stellar mass density in UCM galaxies. Finally, we derive that in the local universe, 13% ± 3% of the total baryon mass density in the form of stars is associated with star-forming galaxies.

Stellar Populations in High-z Galaxy Mergers

We investigate the nature of stellar populations of major galaxy mergers between late-type spirals considerably abundant in interstellar medium by performing numerical simulations designed to solve both the dynamical and chemical evolution in a self-consistent manner. We particularly consider that the star formation history of galaxy mergers is a crucial determinant for the nature of stellar populations of merger remnants, and therefore investigate how the difference in star formation history between galaxy mergers affects the chemical evolution of galaxy mergers.

Star Formation in Early‐Type Galaxies11The conference was held in Guanajuanto, Mexico, in 1998 July. Proceedings will be edited by P. Carral and J. Cepa and will be published in the ASP Conference Series.

The proceedings of this meeting will provide a great deal of information concerning current views on early-type galaxies, covering a broader perspective than their star formation history (SFH). Throughout the meeting, three questions were continuously posed by the speakers: (1) Do we need a new definition of early-type galaxies? (2) Are normal E galaxies those in clusters or in the field? and (3) Where do all these data fit in? Contrary to early beliefs, the notion that early-type galaxies may belong to different families is gaining increasing support. Observational evidence indicates that there are wide ranges of values allowed for most properties observed in early-type galaxies (e.g., luminosity, radius, density, isophotal shape, brightness profile, central vs. global properties, surface brightness variations, stellar populations, merger and interaction history, etc.). Both age and metallicity seem to vary along the early-type galaxy sequence, with most luminous ellipticals being the oldest and most metal-rich systems. The observed points to a nonhomologous 0.12 0.15 M/L ∝ M –M formation process, with differences in SFH among mergers. More luminous ellipticals may form from mergers of more massive spirals, resulting in higher star formation rates (SFRs) as well as stronger color and metallicity gradients in these systems. Solar scale abundance ratios are inconsistent with the observations: Mg and other a elements are enhanced with respect to solar values. The presence of dark matter, less concentrated than the luminous matter, seems well established in most ellipticals. However, NGC 404 seems to have no dark matter! Early-type galaxies have an ISM that is reminiscent of later types. Hot, warm, cool, and cold gas and dust have been detected in these galaxies. In general, the amount of gas detected is less than expected from stellar evolution during . S0 and pE galaxies show a 45% detection rate of H i 1 H0 (accreted disk?), whereas normal E galaxies show only a 5% detection rate. CO has been detected in 40% of E galaxies, including pE galaxies. The amount of molecular gas in some dE galaxies is higher than expected from their H i content. H i is centrally concentrated in low-mass E-type galaxies but not in gE galaxies. Star formation has been detected in some galaxies with an unusually high central density of H i. In most

Stellar Populations in Gas‐rich Galaxy Mergers. I. Dependence on Star Formation History

We investigate the nature of stellar populations of major galaxy mergers between late-type spirals with considerably abundant interstellar medium by performing numerical simulations designed to solve both the dynamical and chemical evolution of the mergers in a self-consistent manner. We particularly consider that the star formation history of galaxy mergers is a crucial determinant of the nature of stellar populations of merger remnants, and therefore we investigate how the difference in star formation history between galaxy mergers affects the chemical evolution of galaxy mergers. We found that the rapidity of star formation, which is defined as the ratio of the dynamical timescale to the timescale of gas consumption by star formation, is the most important determinant for a number of fundamental characteristics of stellar populations of merger remnants. The main results obtained in this study are the following. 1. A galaxy merger with more rapid star formation becomes elliptical with larger mean metallicity. This is primarily because, in the merger with more rapid star formation, a smaller amount of metal-enriched gas is tidally stripped away during merging, and, consequently, a larger amount of the gas can be converted to stellar components. This demonstrates that the cause of the color-magnitude relation of elliptical galaxies can be closely associated with the details of merging dynamics that depend on the rapidity of star formation in galaxy mergers. 2. A negative metallicity gradient fitted reasonably well by a power law can be reproduced by a dissipative galaxy merger with star formation. The magnitude of the metallicity gradient is larger for an elliptical galaxy formed by a galaxy merger with less rapid star formation. 3. The absolute magnitude of the metallicity gradient correlates with that of the age gradient in galaxy mergers in the sense that a merger remnant with a steeper negative metallicity gradient is more likely to show a steeper age gradient. 4. The outer part of a stellar population is both older and less metal-enriched than the nucleus in an elliptical galaxy formed by a galaxy merger with less rapid star formation. Moreover, the metallicity of the outer part of the gaseous component for some models with less rapid star formation is appreciably smaller than the stellar metallicity. This result implies that the origin of metal-poor hot gaseous X-ray halos in real elliptical galaxies can essentially be ascribed to the dynamics of dissipative galaxy merging. 5. Irrespective of the rapidity of star formation, the epoch of galaxy merging affects both the mean stellar metallicity and the mean stellar age of merger remnants: later galaxy mergers are more likely to become ellipticals with both younger and more metal-enriched stellar populations. This result reflects the fact that in the later mergers, a larger amount of more metal-enriched interstellar gas is preferentially converted into stars during the later period of star formation triggered by galaxy merging. These five results clearly demonstrate that even the chemical evolution of elliptical galaxies can be strongly affected by the details of dynamical evolution of galaxy merging, which are furthermore determined by the rapidity of star formation of galaxy mergers. In particular, tidal stripping of interstellar gas and the total amount of gaseous dissipation during galaxy merging are demonstrated to play vital roles in determining a number of chemical properties of merger remnants. Based on these results, we adopt a specific assumption of luminosity dependence on rapidity of star formation and thereby discuss how successfully the present merger model can reproduce a number of fundamental chemical, photometric, and spectroscopic characteristics of elliptical galaxies.

The Fundamental Plane and Merger Scenario. I. Star Formation History of Galaxy Mergers and Origin of the Fundamental Plane

We perform numerical simulations of galaxy mergers between star-forming and gas-rich spirals in order to explore the origin of the fundamental plane (FP) of elliptical galaxies. We consider, in particular, that the origin of the slope of the FP is essentially due to the nonhomology in the structure and kinematics of elliptical galaxies and, accordingly, investigate structural and kinematic properties of elliptical galaxies formed by dissipative galaxy merging with star formation. We found that the rapidity of star formation, which is defined as the ratio of dynamical timescale of a merger progenitor to the timescale of gas consumption by star formation, is a key determinant for nonhomology parameters, such as the density profile of the stellar component, the relative importance of global rotation in kinematics, and the ratio of total dynamical mass to luminous mass, in merger remnants. We furthermore found that this result does not depend so strongly on initial intrinsic spins of progenitor disks and orbital energy and angular momentum of mergers. These results strongly suggest that the structural and kinematic nonhomology observed in elliptical galaxies can be closely associated with the difference in star formation history between elliptical galaxies formed by dissipative galaxy merging. Based on these results, we discuss a close physical relation between the origin of the FP and the star formation history of elliptical galaxies.

Line-Strength Indices in Bright Spheroidal Galaxies: Evidence for a Stellar Population Dichotomy between Spheroidal and Elliptical Galaxies

We present new measurements of central line-strength indices (namely, Mg-2, (Fe), and H beta) and gradients for a sample of six bright spheroidal (Sph) galaxies in the Virgo Cluster. Comparison with similar measurements for elliptical (E) galaxies, galactic globular clusters (GGCs), and stellar population models yield the following results: (1) in contrast with bright E galaxies, bright Sph galaxies are consistent with solar abundance [Mg/Fe] ratios; (2) bright Sph galaxies exhibit metallicities ranging from values typical for metal-rich GGCs to those for E galaxies; (3) although absolute mean ages are quite model dependent, we find evidence that the stellar populations of some (if not all) Sph galaxies look significantly younger than GCCs; and (4) Mg-2 gradients of bright Sph galaxies are significantly shallower than those of E galaxies. We conclude that the dichotomy found in the structural properties of Sph and E galaxies is also observed in their stellar populations. A tentative interpretation in terms of differences in star formation histories is suggested.