Decreasing Star Formation Rate Research Articles

MIGHTEE-H i: the MH i – M* relation over the last billion years

ABSTRACT We study the $M_{\rm H\, {\small {I}}}-M_{\star }$ relation over the last billion years using the MIGHTEE-H i sample. We first model the upper envelope of the $M_{\rm H\, {\small {I}}}-M_{\star }$ relation with a Bayesian technique applied to a total number of 249 H i-selected galaxies, without binning the data sets, while taking account of the intrinsic scatter. We fit the envelope with both linear and non-linear models, and find that the non-linear model is preferred over the linear one with a measured transition stellar mass of log10(M⋆/M⊙) = 9.15 ± 0.87, beyond which the slope flattens. This finding supports the view that the lack of H i gas is ultimately responsible for the decreasing star formation rate observed in the massive main-sequence galaxies. For spirals alone, which are biased towards the massive galaxies in our sample, the slope beyond the transition mass is shallower than for the full sample, indicative of distinct gas processes ongoing for the spirals/high-mass galaxies from other types with lower stellar masses. We then create mock catalogues for the MIGHTEE-H i detections and non-detections with two main galaxy populations of late- and early-type galaxies to measure the underlying $M_{\rm H\, {\small {I}}}-M_{\star }$ relation. We find that the turnover in this relation persists whether considering the two galaxy populations as a whole or separately. We note that an underlying linear relation could mimic this turnover in the observed scaling relation, but a model with a turnover is strongly preferred. Measurements on the logarithmic average of H i masses against the stellar mass are provided as a benchmark for future studies.

VALES

We characterize the ionized gas outflows in 15 low-redshift star-forming galaxies, a Valparaíso ALMA/APEX Line Emission Survey (VALES) subsample, using MUSE integral field spectroscopy and GAMA photometric broadband data. We measure the emission-line spectra by fitting a double-component profile, with the second and broader component related to the outflowing gas. This interpretation is in agreement with the correlation between the observed star-formation rate (SFR) surface density (ΣSFR) and the second-component velocity dispersion (σ2nd), expected when tracing the feedback component. By modeling the broadband spectra with spectral energy distribution fitting and obtaining the star-formation histories of the sample, we observe a small decrease in SFR between 100 and 10 Myr in galaxies when the outflow Hα luminosity contribution is increased, indicating that the feedback somewhat inhibits the star formation within these timescales. The observed emission-line ratios are best reproduced by photoionization models when compared to shock ionization, indicating that radiation from the young stellar population is dominant and seems to be a consequence of a continuous star-formation activity instead of a “bursty” event. The outflow properties, such as the mass outflow rate (∼0.1 M⊙ yr−1), the outflow kinetic power (∼5.2 × 10−4%Lbol), and the mass loading factor (∼0.12), point toward a scenario where the measured feedback is not strong and has a low impact on the evolution of galaxies in general.

SDSS-IV MaNGA: The kinematic-morphology of galaxies on the mass versus star-formation relation in different environments

ABSTRACT We study the link between the kinematic-morphology of galaxies, as inferred from integral-field stellar kinematics, and their relation between mass and star formation rate. Our sample consists of ∼3200 galaxies with integral-field spectroscopic data from the MaNGA survey (Mapping Nearby Galaxies at Apache Point Observatory) with available determinations of their effective stellar angular momentum within the half-light radius $\lambda _{R_e}$. We find that for star-forming galaxies, namely along the star formation main sequence (SFMS), the $\lambda _{R_e}$ values remain large and almost unchanged over about two orders of magnitude in stellar mass, with the exception of the lowest masses $\mathcal {M}_{\star }\lesssim 2\times 10^{9} \, \mathcal {M}_{\odot }$, where $\lambda _{R_e}$ slightly decreases. The SFMS is dominated by spiral galaxies with small bulges. Below the SFMS, but above the characteristic stellar mass $\mathcal {M}_{\rm crit}\approx 2\times 10^{11} \, \mathcal {M}_{\odot }$, there is a sharp decrease in $\lambda _{R_e}$ with decreasing star formation rate (SFR): massive galaxies well below the SFMS are mainly slow-rotator early-type galaxies, namely genuinely spheroidal galaxies without discs. Below the SFMS and below $\mathcal {M}_{\rm crit}$ the decrease of $\lambda _{R_e}$ with decreasing SFR becomes modest or nearly absent: low-mass galaxies well below the SFMS, are fast-rotator early-type galaxies, and contain fast-rotating stellar discs like their star-forming counterparts. We also find a small but clear environmental dependence for the massive galaxies: in the mass range $10^{10.9}\!-\!10^{11.5} \, \mathcal {M}_{\odot }$, galaxies in rich groups or denser regions or classified as central galaxies have lower values of $\lambda _{R_e}$. While no environmental dependence is found for galaxies of lower mass. We discuss how the above results can be understood as due to the different star formation and mass assembly histories of galaxies with varying mass.

The SAMI Galaxy Survey: satellite galaxies undergo little structural change during their quenching phase

At fixed stellar mass, satellite galaxies show higher passive fractions than centrals, suggesting that environment is directly quenching their star formation. Here, we investigate whether satellite quenching is accompanied by changes in stellar spin (quantified by the ratio of the rotational to dispersion velocity V/$\sigma$) for a sample of massive ($M_{*}>$10$^{10}$ M$_{\odot}$) satellite galaxies extracted from the SAMI Galaxy Survey. These systems are carefully matched to a control sample of main sequence, high $V/\sigma$ central galaxies. As expected, at fixed stellar mass and ellipticity, satellites have lower star formation rate (SFR) and spin than the control centrals. However, most of the difference is in SFR, whereas the spin decreases significantly only for satellites that have already reached the red sequence. We perform a similar analysis for galaxies in the EAGLE hydro-dynamical simulation and recover differences in both SFR and spin similar to those observed in SAMI. However, when EAGLE satellites are matched to their `true' central progenitors, the change in spin is further reduced and galaxies mainly show a decrease in SFR during their satellite phase. The difference in spin observed between satellites and centrals at $z\sim$0 is primarily due to the fact that satellites do not grow their angular momentum as fast as centrals after accreting into bigger halos, not to a reduction of $V/\sigma$ due to environmental effects. Our findings highlight the effect of progenitor bias in our understanding of galaxy transformation and they suggest that satellites undergo little structural change before and during their quenching phase.

Delayed Stellar Mass Assembly in the Low Surface Brightness Dwarf Galaxy KDG 215∗

Abstract We present H i spectral line and optical broadband images of the nearby low surface brightness dwarf galaxy KDG 215. The H i images, acquired with the Karl G. Jansky Very Large Array, reveal a dispersion-dominated interstellar medium with only weak signatures of coherent rotation. The H i gas reaches a peak mass surface density of 6 M ⊙ pc−2 at the location of the peak surface brightness in the optical and the ultraviolet. Although KDG 215 is gas-rich, the Hα non-detection implies a very low current massive star formation rate. In order to investigate the recent evolution of this system, we have derived the recent and lifetime star formation histories from archival Hubble Space Telescope images. The recent star formation history shows a peak star formation rate ∼1 Gyr ago, followed by a decreasing star formation rate to the present day quiescent state. The cumulative star formation history indicates that a significant fraction of the stellar mass assembly in KDG 215 has occurred within the last 1.25 Gyr. KDG 215 is one of only a few known galaxies that demonstrates such a delayed star formation history. While the ancient stellar population (predominantly red giants) is prominent, the look-back time by which 50% of the mass of all stars ever formed had been created is among the youngest of any known galaxy.

From voids to filaments: environmental transformations of galaxies in the SDSS

We investigate the impact of filament and void environments on galaxies, looking for residual effects beyond the known relations with environment density. We quantified the host environment of galaxies as the distance to the spine of the nearest filament, and compared various galaxy properties within 12 bins of this distance. We considered galaxies up to 10 $h^{-1}$Mpc from filaments, i.e. deep inside voids. The filaments were defined by a point process (the Bisous model) from the Sloan Digital Sky Survey data release 10. In order to remove the dependence of galaxy properties on the environment density and redshift, we applied weighting to normalise the corresponding distributions of galaxy populations in each bin. After the normalisation with respect to environment density and redshift, several residual dependencies of galaxy properties still remain. Most notable is the trend of morphology transformations, resulting in a higher elliptical-to-spiral ratio while moving from voids towards filament spines, bringing along a corresponding increase in the $g-i$ colour index and a decrease in star formation rate. After separating elliptical and spiral subsamples, some of the colour index and star formation rate evolution still remains. The mentioned trends are characteristic only for galaxies brighter than about $M_{r} = -20$ mag. Unlike some other recent studies, we do not witness an increase in the galaxy stellar mass while approaching filaments. The detected transformations can be explained by an increase in the galaxy-galaxy merger rate and/or the cut-off of extragalactic gas supplies (starvation) near and inside filaments. Unlike voids, large-scale galaxy filaments are not a mere density enhancement, but have their own specific impact on the constituent galaxies, reducing the star formation rate and raising the chances of elliptical morphology also at a fixed environment density level.

The high-redshift star formation rate derived from gamma-ray bursts: possible origin and cosmic reionization

The collapsar model of long gamma-ray bursts (GRBs) indicates that they may trace the star formation history. So long GRBs may be a useful tool of measuring the high-redshift star formation rate (SFR). The collapsar model explains GRB formation via the collapse of a rapidly rotating massive star with $M> 30M_\odot$ into a black hole, which may imply a decrease of SFR at high redshift. However, we find that the \emph{Swift} GRBs during 2005-2012 are biased tracing the SFR, including a factor about $(1+z)^{0.5}$, which is in agreement with recent results. After taking this factor, the SFR derived from GRBs does not show steep drop up to $z\sim 9.4$. We consider the GRBs produced by rapidly rotating metal-poor stars with low masses to explain the high-redshift GRB rate excess. The chemically homogeneous evolution scenario (CHES) of rapidly rotating stars with mass larger than $12M_\odot$ is recognized as a promising path towards collapsars in connection with long GRBs. Our results indicate that the stars in the mass range $12M_\odot<M<30M_\odot$ for low enough metallicity $Z\leq 0.004$ with the GRB efficiency factor $10^{-5}$ can fit the derived SFR with good accuracy. Combining these two factors, we find that the conversion efficiency from massive stars to GRBs is enhanced by a factor of 10, which may be able to explain the excess of the high-redshift GRB rate. We also investigate the cosmic reionization history using the derived SFR. The GRB-inferred SFR would be sufficient to maintain cosmic reionization over $6<z<10$ and reproduce the observed optical depth of Thomson scattering to the cosmic microwave background.

THE SNAPSHOT HUBBLE U-BAND CLUSTER SURVEY (SHUCS). I. SURVEY DESCRIPTION AND FIRST APPLICATION TO THE MIXED STAR CLUSTER POPULATION OF NGC 4041

We present the Snapshot Hubble U-band Cluster Survey (SHUCS), a project aimed at characterizing the star cluster populations of ten nearby galaxies (d<23 Mpc, half within 12 Mpc) through new F336W (U band equivalent) imaging from WFC3, and archival BVI-equivalent data with HST. Completing the UBVI baseline reduces the age-extinction degeneracy of optical colours, thus enabling the measurement of reliable ages and masses for the thousands of clusters covered by our survey. The sample consists chiefly of face-on spiral galaxies at low inclination, in various degrees of isolation (isolated, in group, merging), and includes two AGN hosts. This first paper outlines the survey itself, the observational datasets, the analysis methods, and presents a proof-of-concept study of the large-scale properties and star cluster population of NGC 4041, a massive SAbc galaxy at a distance of 23 Mpc, and part of a small grouping of six giant members. We resolve two structural components with distinct stellar populations, a morphology more akin to merging and interacting systems. We also find strong evidence of a truncated, Schechter-type mass function, and a similarly segmented luminosity function. These results indicate that binning must erase much of the substructure present in the mass and luminosity functions, and might account for the conflicting reports on the intrinsic shape of these functions in the literature. We also note a tidal feature in the outskirts of the galaxy in GALEX UV imaging, and follow it up with a comprehensive multi-wavelength study of NGC 4041 and its parent group. We deduce a minor merger as a likely cause of its segmented structure and the observed pattern of a radially decreasing star formation rate. We propose that combining the study of star cluster populations with broad-band metrics is not only advantageous, but often easily achievable through archival datasets.

Recent star formation in the Lupus clouds as seen byHerschel

We present a study of the star formation histories of the Lupus I, III, and IV clouds using the Herschel 70-500 micron maps obtained by the Herschel Gould Belt Survey Key Project. By combining the new Herschel data with the existing Spitzer catalog we obtained an unprecedented census of prestellar sources and young stellar objects in the Lupus clouds, which allowed us to study the overall star formation rate (SFR) and efficiency (SFE). The high SFE of Lupus III, its decreasing SFR, and its large number of pre-main sequence stars with respect to proto- and prestellar sources, suggest that Lupus III is the most evolved cloud, and after having experienced a major star formation event in the past, is now approaching the end of its current star-forming cycle. Lupus I is currently undergoing a large star formation event, apparent by the increasing SFR, the large number of prestellar objects with respect to more evolved objects, and the high percentage of material at high extinction (e.g., above A_V=8 mag). Also Lupus IV has an increasing SFR; however, the relative number of prestellar sources is much lower, suggesting that its star formation has not yet reached its peak.

Multiwavelength study of nearly face-on low surface brightness disk galaxies

We study the ages of a large sample (1802) of nearly face-on disk low surface brightness galaxies (LSBGs) using the evolutionary population synthesis (EPS) model PEGASE with an exponentially decreasing star formation rate to fit their multiwavelength spectral energy distributions (SEDs) from far-ultraviolet (FUV) to near-infrared (NIR). The derived ages of LSBGs are 1–5 Gyr for most of the sample no matter if constant or varying dust extinction is adopted, which are similar to most of the previous studies on smaller samples. This means that these LSBGs formed the majority of their stars quite recently. However, a small part of the sample (∼2%–3%) has larger ages of 5–8 Gyr, meaning their major star forming process may have occurred earlier. At the same time, a large sample (5886) of high surface brightness galaxies (HSBGs) are selected and studied using the same method for comparisons. The derived ages are 1–5 Gyr for most of the sample (97%) as well. These results probably mean that these LSBGs have not much different star formation histories from their HSBGs counterparts. However, we should notice that the HSBGs are generally about 0.2 Gyr younger, which could mean that the HSBGs may have undergone more recent star forming activities than the LSBGs.

THE FUNDAMENTAL GAS DEPLETION AND STELLAR-MASS BUILDUP TIMES OF STAR-FORMING GALAXIES

Stars do not form continuously distributed over star-forming galaxies. They form in star clusters of different masses. This nature of clustered star formation is taken into account in the theory of the integrated galactic stellar initial mass function (IGIMF) in which the galaxy-wide initial mass function (IMF) on galaxy-wide scales is calculated by adding all IMFs of young star clusters. For massive stars, the IGIMF is steeper than the universal IMF in star clusters and steepens with decreasing star formation rate (SFR) which is called the IGIMF effect. The current SFR and the total Hα luminosity of galaxies therefore scale nonlinearly in the IGIMF theory compared to the classical case in which the galaxy-wide IMF is assumed to be constant and identical to the IMF in star clusters. Here we apply for the first time the revised SFR–LHα relation on a sample of local volume star-forming galaxies with measured Hα luminosities. The fundamental results are: (1) the SFRs of galaxies scale linearly with the total galaxy neutral gas mass, (2) the gas depletion timescales of dwarf irregular and large disk galaxies are about 3 Gyr, implying that dwarf galaxies do not have lower star formation efficiencies than large disk galaxies, and (3) the stellar-mass buildup times of dwarf and large galaxies are only in agreement with downsizing in the IGIMF context, but contradict downsizing within the traditional framework that assumes a constant galaxy-wide IMF.

Evolution of M81 with Exponentially Decreasing Star Formation Rate of PEGASE

Based on the large field multicolor observations of Beijing-Arizona-Taiwan-Connecticut (BATC) program, we obtain the spectral energy distribution (SED) for individual regions of M81. We study the structure and evolution of M81 with an evolutionary population synthesis (EPS) model, PEGASE. We find that the exponentially deceasing star formation rate (SFR) with star formation scale 3 Gyr (hereafter Exp, τ = 3 Gyr) gives the best agreement between the model predictions and the observed SEDs. We then obtain the structure, age distribution and evolutionary history of M81. There is a clear age gradient between the central and outer regions. The populations in the central regions are older than 7 Gyr, those in the outer regions are younger, at about 4.5 Gyr. The youngest components in the spiral arms have ages of about 2.5 Gyr or less.

Galaxy Star Formation as a Function of Environment in the Early Data Release of the Sloan Digital Sky Survey

We present in this paper a detailed analysis of the effect of environment on the star formation activity of galaxies within the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS). We have used the Halpha emission line to derive the star formation rate (SFR) for each galaxy within a volume-limited sample of 8598 galaxies with 0.05 less than or equal to z less than or equal to 0.095 and M (r*) less than or equal to 20.45. We find that the SFR of galaxies is strongly correlated with the local ( projected) galaxy density, and thus we present here a density-SFR relation that is analogous to the density-morphology relation. The effect of density on the SFR of galaxies is seen in three ways. First, the overall distribution of SFRs is shifted to lower values in dense environments compared with the field population. Second, the effect is most noticeable for the strongly star-forming galaxies (Halpha EW > 5 Angstrom) in the 75th percentile of the SFR distribution. Third, there is a break ( or characteristic density) in the density-SFR relation at a local galaxy density of similar to1 h(75)(-2) Mpc(-2). To understand this break further, we have studied the SFR of galaxies as a function of clustercentric radius from 17 clusters and groups objectively selected from the SDSS EDR data. The distribution of SFRs of cluster galaxies begins to change, compared with the field population, at a clustercentric radius of 3-4 virial radii (at the >1sigma statistical significance), which is consistent with the characteristic break in density that we observe in the density-SFR relation. This effect with clustercentric radius is again most noticeable for the most strongly star-forming galaxies. Our tests suggest that the density-morphology relation alone is unlikely to explain the density-SFR relation we observe. For example, we have used the ( inverse) concentration index of SDSS galaxies to classify late-type galaxies and show that the distribution of the star-forming (EW Halpha > 5Angstrom) late-type galaxies is different in dense regions ( within 2 virial radii) compared with similar galaxies in the field. However, at present, we are unable to make definitive statements about the independence of the density-morphology and density-SFR relation. We have tested our work against potential systematic uncertainties including stellar absorption, reddening, SDSS survey strategy, SDSS analysis pipelines, and aperture bias. Our observations are in qualitative agreement with recent simulations of hierarchical galaxy formation that predict a decrease in the SFR of galaxies within the virial radius. Our results are in agreement with recent 2dF Galaxy Redshift Survey results as well as consistent with previous observations of a decrease in the SFR of galaxies in the cores of distant clusters. Taken together, these works demonstrate that the decrease in SFR of galaxies in dense environments is a universal phenomenon over a wide range in density (from 0.08 to 10 h(75)(-2) Mpc(-2)) and redshift (out to z similar or equal to 0.5).

DDO 187: Do Dwarf Galaxies Have Extended, Old Halos?

If dwarf galaxies are primeval objects in the universe, as hierarchical galaxy formation scenarios predict, they should show traces of their old stellar populations, perhaps distributed in extended, differentiated structures. The working hypothesis that such a structure could exist is tested for the case of DDO 187, a field, dwarf irregular galaxy showing a high gas fraction and low metallicity. For this purpose, the structure, star formation history, and other properties of the galaxy are analyzed using the spatial distribution of stars, the color-magnitude diagrams of about 1500 resolved stars, and the fluxes of H II regions, together with data about the gas distribution. From the I magnitude of the tip of the red giant branch (ITRGB), the distance of DDO 187 to the Milky Way is estimated to be 2.5 ± 0.2 Mpc. The distance to several neighbor galaxies and groups has been computed, showing that DDO 187 is probably an isolated, field galaxy. The distance of DDO 187 to the Milky Way is almost 3 times smaller than that obtained from Cepheid light curves. Considering that this is the third case in which such a large disagreement is detected, it seems clear that Cepheid distance estimates based on a few stars, as usually happens in dwarf galaxies, must be accepted with caution. The star formation history of DDO 187 has been analyzed. The central region of DDO 187 shows an overall time decreasing star formation rate with a strong burst in its central region that happened between 20 and 100 Myr ago and a present-day star formation activity 3 times smaller than the maximum one. Besides this, a spatially extended stellar component has been found that has no young stars and exceeds the size of the gas component. In short, several results suggest that DDO 187 has a two-component, halo/disk-like structure: (1) differentiated morphologies for the inner (flat) and outer (spheroidal) stellar components, (2) a gas component less extended than the outer stellar component, and (3) an outer component lacking young stars, which are abundant in the inner component. The working hypothesis that a real halo/disk structure could be present is discussed. The conclusion is reached that the two-component hypothesis is not unrealistic, but nothing can be definitely stated until more detailed data, ideally including kinematics, are available.

A Study of Nine High-Redshift Clusters of Galaxies. II. Photometry, Spectra, and Ages of Clusters 0023+0423 and 1604+4304

We present an extensive photometric and spectroscopic study of two high-redshift clusters of galaxies based on data obtained from the Keck 10 m telescopes and the Hubble Space Telescope. The clusters Cl 0023+0423 (z = 0.84) and Cl 1604+4304 (z = 0.90) are part of a multiwavelength program of Oke, Postman & Lubin to study nine candidate clusters at z 0.6. Based on these observations, we study in detail both the field and cluster populations. From the confirmed cluster members, we find that Cl 0023+0423 actually consists of two components separated by ~2900 km s-1. A kinematic analysis indicates that the two components are a poor cluster with ~3 ? 1014 M? and a less massive group with ~1013 M?. Cl 1604+4304 is a centrally concentrated, rich cluster at z = 0.8967 with a velocity dispersion of 1226 km s-1 and a mass of ~3 ? 1015 M?. A large percentage of the cluster members show high levels of star formation activity. Approximately 57% and 50% of the galaxies are active in Cl 0023+0423 and Cl 1604+4304, respectively. These numbers are significantly larger than those found in intermediate-redshift clusters. We also observe many old, red galaxies. Found mainly in Cl 1604+4304, they have spectra consistent with passive stellar evolution, typical of the populations of early-type galaxies in low- and intermediate-redshift clusters. We have calculated their ages by comparing their spectral energy distributions to standard Bruzual & Charlot evolutionary models. We find that their colors are consistent with models having an exponentially decreasing star formation rate with a time constant of 0.6 Gyr. We also observe a significant luminosity brightening in our brightest cluster galaxies. Compared with brightest cluster galaxies at z ~ 0.1, we find a luminosity increase of ~1 mag in the rest MB and ~0.8 mag in the rest MV. In the field, we find that ~76% of the galaxies with z > 0.4 show emission-line activity. These numbers are consistent with previous studies. We find that an exponentially decaying star formation rate is required to produce the observed amount of star formation for the majority of the galaxies in our sample. A time constant of ? = 0.6 Gyr appears to be optimal. We also detect several interesting galaxies at z > 1. Two of these galaxies are extremely luminous, with strong Mg II ?2800 absorption and Fe II resonance-line absorption. These lines are so strong that we conclude that they must be generated within the atmospheres of a large population of young, hot stars.

The Resolved Stellar Population of Leo A

New observations of the resolved stellar population of the extremely metal-poor Magellanic dwarf irregular galaxy Leo A in Thuan-Gunn r, g, i, and narrowband Ha filters are presented. Using the recent Cepheid variable star distance determination to Leo A by Hoessel et al., we are able to create an accurate color-magnitude diagram (CMD). We have used the Bavesian inference method described by Tolstoy & Saha to calculate the likelihood of a Monte Carlo simulation of the stellar population of Leo A being a good match to the data within the well understood errors in the data. The magnitude limits on our data are sensitive enough to look back at similar to 1 Gyr of star formation history at the distance of Leo A. To explain the observed ratio of red to blue stars in the observed CMD, it is necessary to invoke either a steadily decreasing star formation rate toward the present lime or gaps in the star formation history. We also compare the properties of the observed stellar population with the known spatial distribution of the H I gas and H II regions to support the conclusions from CMD modeling. We consider the possibility that currently there is a period of diminished star formation in Leo A, as evidenced by the lack of very young stars in the CMD and the faint H Ir regions. How the chaotic II I distribution, with no observable rotation, fits into our picture of the evolution of Leo A is as yet unclear.

On the galactic evolution of D and He-3

The determined abundances of primordial $^4He$ and $^7Li$ provide a basis with which to test the standard model of big bang nucleosynthesis in conjunction with the other two light element isotopes $D$ and $^3He$, also produced in the big bang. Overall, consistency in the standard big bang nucleosynthesis model is best achieved for a baryon-to-photon ratio of typically $3 \times 10^{-10}$ for which the primordial value of $D$ is five times greater than the present observed abundance and about three times greater than the pre-solar value. We consider various models for the chemical evolution of the Galaxy to test the feasibility for the destruction of D without the overproduction of $^3He$ and overall metallicity. Models which are capable of achieving this goal include ones with a star formation rate proportional to the gas mass fraction or an exponentially decreasing star formation rate. We discuss the effect of parameters that govern the initial mass function and of surviving fractions of $^3He$ in stars between one and three solar masses.

The evolution of the interstellar medium in elliptical galaxies. I - The early wind phase

view Abstract Citations (97) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolution of the Interstellar Medium in Elliptical Galaxies. I. The Early Wind Phase David, L. P. ; Forman, W. ; Jones, C. Abstract We investigate the evolution of elliptical galaxies with the goals of understanding the properties of their interstellar medium as well as their effects on the environment. In particular, we study the effects of early supernova-driven winds on the galaxies themselves and on the clusters and compact groups in which these galaxies are the predominant population. Our simulation of the evolution of the interstellar medium in elliptical galaxies includes evolving stellar mass loss rates from Type I and II supernovae, stellar winds, and planetary nebulae. During the first 10^8^- 10^9^ yr, the high frequency of Type II supernovae is sufficient to drive early galactic winds in even the most luminous ellipticals. Depending on the stellar birthrate function, the galaxies can eject between 6% to 43% of their initial luminous mass during this early wind phase. The ejected gas is very hot (T ~ 10^8^ K) and highly enriched with newly synthesized metals with an oxygen to iron ratio approximately twice the solar value. Early winds from elliptical galaxies can exhibit a wide range of iron abundances depending on the birthrate function of the stellar population. We discuss the effects of the hot galactic winds on the energy content and elemental abundance of the interstellar medium. We show that early winds can produce an iron abundance of approximately one-half the solar value in a cluster which presently has an equal mass fraction in gas and stars, if the stellar population formed with a Salpeter initial mass function. These early winds can also beat the intracluster medium to temperatures greater than that produced by gravitational heating alone (i.e., the temperature associated with the velocity dispersion of the galaxies). We also present the results of two numerical simulations concerning the evolution of the interstellar medium in an elliptical galaxy model with a present optical luminosity of L_B_ = 10^11^ L_sun_. The galaxy model consists of a modified King profile for the luminous portion of the galaxy along with an isothermal dark halo which comprises 90% of the total galaxy mass. The stellar population in these models is assumed to form either with an exponentially decreasing star formation rate or instantaneously. After the early galactic wind phase, the models evolve from a pure wind to a pure accretion flow as the rate of Type II supernovae diminishes, and a cooling flow develops which endures until the present time. We allow mass deposition from cooling gas to be distributed over a wide range of radii in contrast with some earlier calculations. The peak X-ray luminosity of the interstellar medium during the early wind phase varies from 10^42^ - 10^44^ ergs s^-1^, depending on the formation time of the stellar system. At the present time, the X-ray luminosity of the hot coronae in our models is decreasing at a rate given by L_X_ is proportional to t^-1.3^. We show that the total gas mass of the interstellar medium, the X-ray luminosity, and the X-ray surface brightness after a Hubble time in our models are in good agreement with observations of giant elliptical galaxies. Publication: The Astrophysical Journal Pub Date: August 1990 DOI: 10.1086/169030 Bibcode: 1990ApJ...359...29D Keywords: Computerized Simulation; Elliptical Galaxies; Galactic Evolution; Interstellar Matter; Stellar Luminosity; Stellar Winds; Galactic Clusters; Galactic Mass; Planetary Nebulae; Supernovae; X Ray Sources; Astrophysics; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: INTERSTELLAR MATTER; GALAXIES: X-RAYS full text sources ADS | data products SIMBAD (1)

Neutron stars and white dwarfs in galactic halos?

The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.

Numerical simulations of galactic structure and helium abundance evolution in disk galaxies

view Abstract Citations (4) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Numerical Simulations of Galactic Structure and Helium Abundance Evolution in Disk Galaxies Balser, Dana S. ; Comins, Neil F. Abstract We have developed a three-dimensional, two gas-phase, stochastic self-propagating star formation (SSPSF) model of galaxies which allows gas to move from cell to cell, permanently removes some gas in the form of long-lived, low-mass stars, and follows the change in galactic chemistry, at least as far as to track changes from primordial abundances of hydrogen and helium to abundances of hydrogen and helium (plus metals) after many generations of star formation. The results of this model compare reasonably well with observation of the Milky Way: relative quantities of neutral and molecular gas, ionized gas, and stars are in excellent agreement with observations as is global helium plus metal formation history. Spiral structure is not sensitive to the parameters of the model and may be recovered in most cases. However, profiles of hot gas density increase from the center of the disk to a peak at 1 kpc and then decline with increasing radii whereas observations of our Galaxy indicate that the peak occurs at 5 kpc. Radial profiles of gas that is cold enough to undergo immediate star formation are consistently flat and therefore do not resemble gradients of the galaxy, although methods for determining densities of molecular hydrogen are still controversial. Parameters which determine the rate at which hydrogen is processed into helium may be set to match present radial helium density gradients. A strongly decreasing star formation rate must exist for the model to reproduce correct gas fractions of the galaxy. Global, galactic infall is plausible, assuming higher depletion rates. Publication: The Astrophysical Journal Pub Date: May 1988 DOI: 10.1086/166308 Bibcode: 1988ApJ...328..499B Keywords: Computerized Simulation; Disk Galaxies; Galactic Evolution; Galactic Structure; Helium; Star Formation; Abundance; Interstellar Gas; Ionized Gases; Stellar Mass; Vapor Phases; Astrophysics; STARS: FORMATION full text sources ADS |