Showing papers on "Star formation published in 2008"

The Star Formation Efficiency in Nearby Galaxies: Measuring Where Gas Forms Stars Effectively

Abstract: We measure the star formation efficiency (SFE), the star formation rate (SFR) per unit of gas, in 23 nearby galaxies and compare it with expectations from proposed star formation laws and thresholds. We use H I maps from The H I Nearby Galaxy Survey (THINGS) and derive H2 maps of CO measured by HERA CO-Line Extragalactic Survey and Berkeley-Illinois-Maryland Association Survey of Nearby Galaxies. We estimate the SFR by combining Galaxy Evolution Explorer (GALEX) far-ultraviolet maps and the Spitzer Infrared Nearby Galaxies Survey (SINGS) 24 ?m maps, infer stellar surface density profiles from SINGS 3.6 ?m data, and use kinematics from THINGS. We measure the SFE as a function of the free fall and orbital timescales, midplane gas pressure, stability of the gas disk to collapse (including the effects of stars), the ability of perturbations to grow despite shear, and the ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly constant at (5.25 ? 2.5) ? 10?10 yr?1 (equivalent to an H2 depletion time of 1.9 ? 109 yr) as a function of all of these variables at our 800 pc resolution. Where the interstellar medium (ISM) is mostly H I, however, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2r 25-0.25r 25. We interpret this decline as a strong dependence of giant molecular cloud (GMC) formation on environment. The ratio of molecular-to-atomic gas appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H2-dominated to H I-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe?phase balance in the H I, H2 formation and destruction, and stellar feedback?governs the formation of GMCs from H I.

The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales

Abstract: We present a comprehensive analysis of the relationship between star formation rate surface density, ΣSFR, and gas surface density, Σgas, at sub-kpc resolution in a sample of 18 nearby galaxies. We use high-resolution H I data from The H I Nearby Galaxy Survey, CO data from HERACLES and the BIMA Survey of Nearby Galaxies, 24 μm data from the Spitzer Space Telescope, and UV data from the Galaxy Evolution Explorer. We target seven spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H2 dominated centers of spiral galaxies, their H I dominated outskirts and the H I rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N = 1.0 ± 0.2 relates ΣSFR and ΣH2 across our sample of spiral galaxies, i.e., that H2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ~2 × 109 years. The range of ΣH2 over which we measure this relation is ~3-50 M ☉ pc–2, significantly lower than in starburst environments. We find the same results when performing a pixel-by-pixel analysis, averaging in radial bins, or when varying the star formation tracer used. We interpret the linear relation and constant depletion time as evidence that stars are forming in giant molecular clouds with approximately uniform properties and that ΣH2 may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas surface density (Σgas) and ΣSFR varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between ΣHI and ΣSFR. As a result, the star formation efficiency (SFE), ΣSFR/Σgas, varies strongly across our sample and within individual galaxies. We show that this variation is systematic and consistent with the SFE being set by local environmental factors: in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. We attribute the similarity to common environments (low density, low metallicity, H I dominated) and argue that shear (which is typically absent in dwarfs) cannot drive the SFE. In addition to a molecular Schmidt law, the other general feature of our sample is a sharp saturation of H I surface densities at ΣHI ≈ 9 M ☉ pc–2 in both the spiral and dwarf galaxies. In the case of the spirals, we observe gas in excess of this limit to be molecular.

Things:the hi nearby galaxy survey

Abstract: We present “The HI Nearby Galaxy Survey (THINGS)”, a high spectral (≤52kms −1 ) and spatial (∼ 6 ′′ ) resolution survey of HI emission in 34 nearby galaxies obtained using the NRAO Very Large Array (VLA) The overarching scientific goal of THINGS is to investigate fundamental characteristics of the interstellar medium (ISM) related to galaxy morphology, star formation and mass distribution across the Hubble sequence Unique characteristics of the THINGS database are the homogeneous sensitivity as well as spatial and velocity resolution of the HI data which is at the limit of what can be achieved with the VLA for a significant number of galaxies A sample of 34 objects at distances 2 < D <15 Mpc (resulting in linear resolutions of ∼100 to 500pc) are targeted in THINGS, covering a wide range of star formation rates (∼ 10 −3 to 6 M⊙ yr −1 ), total HI masses MHI (001 to 14×10 9 M⊙), absolute luminosities MB (–115 to –217mag) and metallicities (75 to 92 in units of 12+log[O/H]) We describe the setup of the VLA observations, the data reduction procedures and the creation of the final THINGS data products We present an atlas of the integrated HI maps, the velocity fields, the second moment (velocity dispersion) maps and individual channel maps of each THINGS galaxy The THINGS data products are made publicly available through a dedicated webpage Accompanying THINGS papers address issues such as the small–scale structure of the ISM, the (dark) matter distribution in THINGS galaxies, and the processes leading to star formation Subject headings: surveys — galaxies: structure — galaxies: ISM — ISM: general — ISM: atoms — radio lines: galaxies

The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies

Abstract: The stellar masses, mean ages, metallicities, and star formation histories of galaxies are now commonly estimated via stellar population synthesis (SPS) techniques. SPS relies on stellar evolution calculations from the main sequence to stellar death, stellar spectral libraries, phenomenological dust models, and stellar initial mass functions (IMFs). The present work is the first in a series that explores the impact of uncertainties in key phases of stellar evolution and the IMF on the derived physical properties of galaxies and the expected luminosity evolution for a passively evolving set of stars. A Monte-Carlo Markov-Chain approach is taken to fit near-UV through near-IR photometry of a representative sample of low- and high-redshift galaxies with this new SPS model. Significant results include the following: 1) including uncertainties in stellar evolution, stellar masses at z~0 carry errors of ~0.3 dex at 95% CL with little dependence on luminosity or color, while at z~2, the masses of bright red galaxies are uncertain at the ~0.6 dex level; 2) either current stellar evolution models, current observational stellar libraries, or both, do not adequately characterize the metallicity-dependence of the thermally-pulsating asymptotic giant branch phase; 3) conservative estimates on the uncertainty of the slope of the IMF in the solar neighborhood imply that luminosity evolution per unit redshift is uncertain at the ~0.4 mag level in the K-band, which is a substantial source of uncertainty for interpreting the evolution of galaxy populations across time; 4) The more plausible assumption of a distribution of stellar metallicities, rather than a fixed value as is usually assumed, can have significant effects on the interpretation of colors blueward of the V-band. (ABRIDGED)

A semi-analytic model for the co-evolution of galaxies, black holes and active galactic nuclei

Abstract: We present a new semi-analytic model that self-consistently traces the growth of supermassive black holes (BH) and their host galaxies within the context of the Lambda cold dark matter (� CDM) cosmological framework. In our model, the energy emitted by accreting black holes regulates the growth of the black holes themselves, drives galactic scale winds that can remove cold gas from galaxies, and produces powerful jets that heat the hot gas atmospheres surrounding groups and clusters. We present a comprehensive comparison of our model predictions with observational measurements of key physical properties of low-redshift galaxies, such as cold gas fractions, stellar metallicities and ages, and specific star formation rates. We find that our new models successfully reproduce the exponential cut-off in the stellar mass function and the stellar and cold gas mass densities at z ∼ 0, and predict that star formation should be largely, but not entirely, quenched in massive galaxies at the present day. We also find that our model of self-regulated BH growth naturally reproduces the observed relation between BH mass and bulge mass. We explore the global formation history of galaxies and black holes in our models, presenting predictions for the cosmic histories of star formation, stellar mass assembly, cold gas and metals. We find that models assuming the ‘concordance’ � CDM cosmology overproduce star formation and stellar mass at high redshift (z 2). A model with less small-scale power predicts less star formation at high redshift, and excellent agreement with the observed stellar mass assembly history, but may have difficulty accounting for the cold gas in quasar absorption systems at high redshift (z ∼ 3–4).

A simple model to interpret the ultraviolet, optical and infrared emission from galaxies

Abstract: We present a simple, largely empirical but physically motivated model to interpret the mid- and far-infrared spectral energy distributions of galaxies consistently with the emission at ultraviolet, optical and near-infrared wavelengths. Our model relies on an existing angle-averaged prescription to compute the absorption of starlight by dust in stellar birth clouds and in the ambient interstellar medium (ISM) in galaxies. We compute the spectral energy distribution of the power reradiated by dust in stellar birth clouds as the sum of three components: a component of polycyclic aromatic hydrocarbons (PAHs); a mid-infrared continuum characterising the emission from hot grains at temperatures in the range 130–250 K; and a component of grains in thermal equilibrium with adjustable temperature in the range 30–60 K. In the ambient ISM, we fix for simplicity the relative proportions of these three components to reproduce the spectral shape of diffuse cirrus emission in the Milky Way, and we include a component of cold grains in thermal equilibrium with adjustable temperature in the range 15–25 K. Our model is both simple and versatile enough that it can be used to derive statistical constraints on the star formation histories and dust contents of large samples of galaxies using a wide range of ultraviolet, optical and infrared observations. We illustrate this by deriving median-likelihood estimates of the star formation rates, stellar masses, effective dust optical depths, dust masses, and relative strengths of different dust components of 66 well-studied nearby star-forming galaxies from the Spitzer Infrared Nearby Galaxy Survey (SINGS). We explore how the constraints derived in this way depend on the available spectral information. From our analysis of the SINGS sample, we conclude that the mid- and far-infrared colours of galaxies correlate strongly with the specific star formation rate, as well as with other galaxywide quantities connected to this parameter, such as the ratio of infrared luminosity between stellar birth clouds and the ambient ISM, the contributions by PAHs and grains in thermal equilibrium to the total infrared emission, and the ratio of dust mass to stellar mass. Our model can be straightforwardly applied to interpret ultraviolet, optical and infrared spectral energy distributions from any galaxy sample.

AMAZE - I. The evolution of the mass–metallicity relation at z $>$ 3

Abstract: We present initial results of an ESO-VLT large programme (AMAZE) aimed at determining the evolution of the mass-metallicity relation at z > 3 by means of deep near-IR spectroscopy. Gas metallicities are measured, for an initial sample of nine star forming galaxies at z ∼ 3.5, by means of optical nebular lines redshifted into the near-IR. Stellar masses are accurately determined by using Spitzer-IRAC data, which sample the rest-frame near-IR stellar light in these distant galaxies. When compared with previous surveys, the mass-metallicity relation inferred at z ∼ 3.5 shows an evolution much stronger than observed at lower redshifts. The evolution is prominent even in massive galaxies, indicating that z ∼ 3 is an epoch of major action in terms of star formation and metal enrichment also for massive systems. There are also indications that the metallicity evolution of low mass galaxies is stronger relative to high mass systems, an effect which can be considered the chemical version of the galaxy downsizing. The mass-metallicity relation observed at z ∼ 3. 5i s difficult to reconcile with the predictions of some hierarchical evolutionary models. Such discrepancies suggest that at z > 3 galaxies are assembled mostly with relatively un-evolved sub-units, i.e. small galaxies with low star formation efficiency. The bulk of the star formation and metallicity evolution probably occurs once small galaxies are already assembled into bigger systems.

Galaxies in a simulated $\Lambda$CDM Universe I: cold mode and hot cores

Abstract: We study the formation of galaxies in a (50 Mpc/h)^3 cosmological simulation (2x288^3 particles), evolved using the entropy conserving SPH code Gadget-2. Most of the baryonic mass in galaxies of all masses is originally acquired through filamentary "cold mode" accretion of gas that was never shock heated to its halo virial temperature, confirming the key feature of our earlier results obtained with a different SPH code (Keres et al. 2005). Atmospheres of hot, virialized gas develop in halos above ~2.5e11 Msun, a transition mass that is nearly constant from z=3 to z=0. Cold accretion persists in halos above the transition mass, especially at z>=2. It dominates the growth of galaxies in low mass halos at all times, and it is the main driver of the cosmic star formation history. Satellite galaxies have accretion rates similar to central galaxies of the same baryonic mass at high redshifts, but they have less accretion than comparable central galaxies at low redshift. Relative to our earlier results, the Gadget-2 simulations predict much lower rates of "hot mode" accretion from the virialized gas component of massive halos. At z<=1, typical hot accretion rates in halos above 5e12 Msun are below 1 Msun/yr, even though our simulation does not include AGN heating or other forms of "preventive" feedback. The inner density profiles of hot gas in these halos are shallow, with long associated cooling times. The cooling recipes typically used in semi-analytic models can overestimate the accretion rates in these halos by orders of magnitude, so such models may overemphasize the role of preventive feedback in producing observed galaxy masses and colors. A fraction of the massive halos develop cuspy profiles and significant cooling rates between z=1 and z=0, a redshift trend similar to the observed trend in the frequency of cooling flow clusters.

From Rings to Bulges: Evidence for Rapid Secular Galaxy Evolution at z ~ 2 from Integral Field Spectroscopy in the SINS Survey

Abstract: We present Hα integral field spectroscopy of well-resolved, UV/optically selected z ~ 2 star-forming galaxies as part of the SINS survey with SINFONI on the ESO VLT. Our laser guide star adaptive optics and good seeing data show the presence of turbulent rotating star-forming outer rings/disks, plus central bulge/inner disk components, whose mass fractions relative to the total dynamical mass appear to scale with the [N II]/Hα flux ratio and the star formation age. We propose that the buildup of the central disks and bulges of massive galaxies at z ~ 2 can be driven by the early secular evolution of gas-rich proto-disks. High-redshift disks exhibit large random motions. This turbulence may in part be stirred up by the release of gravitational energy in the rapid "cold" accretion flows along the filaments of the cosmic web. As a result, dynamical friction and viscous processes proceed on a timescale of <1 Gyr, at least an order of magnitude faster than in z ~ 0 disk galaxies. Early secular evolution thus drives gas and stars into the central regions and can build up exponential disks and massive bulges, even without major mergers. Secular evolution along with increased efficiency of star formation at high surface densities may also help to account for the short timescales of the stellar buildup observed in massive galaxies at z ~ 2.

MAMBO Mapping Of Spitzer c2d Small Clouds And Cores

Abstract: Aims. To study the structure of nearby (<500 pc) dense starless and star-forming cores with the particular goal to identify and understand evolutionary trends in core properties, and to explore the nature of Very Low Luminosity Objects (<0.1 L ⊙ ; VeLLOs). Methods. Using the MAMBO bolometer array, we create maps unusually sensitive to faint (few mJy per beam) extended (≈5') thermal dust continuum emission at 1.2 mm wavelength. Complementary information on embedded stars is obtained from Spitzer, IRAS, and 2MASS. Results. Our maps are very rich in structure, and we characterize extended emission features ("subcores") and compact intensity peaks in our data separately to pay attention to this complexity. We derive, e.g., sizes, masses, and aspect ratios for the subcores, as well as column densities and related properties for the peaks. Combination with archival infrared data then enables the derivation of bolometric luminosities and temperatures, as well as envelope masses, for the young embedded stars. Conclusions. Starless and star-forming cores occupy the same parameter space in many core properties; a picture of dense core evolution in which any dense core begins to actively form stars once it exceeds some fixed limit in, e.g., mass, density, or both, is inconsistent with our data. A concept of necessary conditions for star formation appears to provide a better description: dense cores fulfilling certain conditions can form stars, but they do not need to, respectively have not done so yet. Comparison of various evolutionary indicators for young stellar objects in our sample (e.g., bolometric temperatures) reveals inconsistencies between some of them, possibly suggesting a revision of some of these indicators. Finally, we challenge the notion that VeLLOs form in cores not expected to actively form stars, and we present a first systematic study revealing evidence for structural differences between starless and candidate VeLLO cores.

Spectroscopic parameters for 451 stars in the HARPS GTO planet search program - Stellar [Fe/H] and the frequency of exo-Neptunes

Abstract: To understand the formation and evolution of solar-type stars in the solar neighborhood, we need to measure their stellar parameters to high accuracy. We present a catalogue of accurate stellar parameters for 451 stars that represent the HARPS Guaranteed Time Observations (GTO) "high precision" sample. Spectroscopic stellar parameters were measured using high signal-to-noise (S/N) spectra acquired with the HARPS spectrograph. The spectroscopic analysis was completed assuming LTE with a grid of Kurucz atmosphere models and the recent ARES code for measuring line equivalent widths. We show that our results agree well with those ones presented in the literature (for stars in common). We present a useful calibration for the effective temperature as a function of the index color B - V and [Fe/H]. We use our results to study the metallicity-planet correlation, namely for very low mass planets. The results presented here suggest that in contrast to their jovian couterparts, neptune-like planets do not form preferentially around metal-rich stars. The ratio ofjupiter-to-neptunes is also an increasing function of stellar metallicity. These results are discussed in the context of the core-accretion model for planet formation.

Analytical Theory for the Initial Mass Function: CO Clumps and Prestellar Cores

Abstract: We derive an analytical theory of the prestellar core initial mass function (IMF) based on an extension of the Press-Schechter statistical formalism. Our approach relies on the general concept of the gravothermal and gravoturbulent collapse of a molecular cloud, with a selection criterion based on the thermal or turbulent Jeans mass, which yields the derivation of the mass spectrum of self-gravitating objects in a quiescent or a turbulent environment. The same formalism also yields the mass spectrum of non-self-gravitating clumps produced in supersonic flows. The mass spectrum of the self-gravitating cores reproduces well the observed IMF. The theory predicts that the shape of the IMF results from two competing contributions, namely, a power law at large scales and an exponential cutoff (lognormal form) centered around the characteristic mass for gravitational collapse. The cutoff exists both in the case of thermal or turbulent collapse, provided that the underlying density field has a lognormal distribution. Whereas pure thermal collapse produces a power-law tail steeper than the Salpeter value, -->dN/dlog M M−x with -->x 1.35, the latter is recovered exactly for the (three-dimensional) value of the spectral index of the velocity power spectrum, -->n 3.8, found in observations and in numerical simulations of isothermal supersonic turbulence. Indeed, the theory predicts that -->x = (n + 1)/(2n − 4) for self-gravitating structures and -->x = 2 − n'/3 for non-self-gravitating structures, where -->n' is the power spectrum index of -->log ρ . We show that, whereas supersonic turbulence promotes the formation of both massive stars and brown dwarfs, it has an overall negative impact on star formation, decreasing the star formation efficiency. This theory provides a novel theoretical foundation to understand the origin of the IMF and provides useful guidance to numerical simulations exploring star formation, while making testable predictions.

The Resolved Properties of Extragalactic Giant Molecular Clouds

Abstract: We use high spatial resolution observations of CO to systematically measure the resolved size-line width, luminosity-line width, luminosity-size, and mass-luminosity relations of GMCs in a variety of extragalactic systems. Although the data are heterogeneous, we analyze them in a consistent manner to remove the biases introduced by limited sensitivity and resolution, thus obtaining reliable sizes, velocity dispersions, and luminosities. We compare the results obtained in dwarf galaxies with those from the Local Group spiral galaxies. We find that extragalactic GMC properties measured across a wide range of environments are very much compatible with those in the Galaxy. The property that shows the largest variability is their resolved brightness temperature, although even that is similar to the average Galactic value in most sources. We use these results to investigate metallicity trends in the cloud average column density and virial CO-to-H2 factor. We find that these measurements do not accord with simple predictions from photoionization-regulated star formation theory, although this could be due to the fact that we do not sample small enough spatial scales or the full gravitational potential of the molecular cloud. We also find that the virial CO-to-H2 conversion factor in CO-bright GMCs is very similar to Galactic and that the excursions do not show a measurable metallicity trend. We contrast these results with estimates of molecular mass based on far-infrared measurements obtained for the Small Magellanic Cloud, which systematically yield larger masses, and interpret this discrepancy as arising from large H2 envelopes that surround the CO-bright cores. We conclude that GMCs identified on the basis of their CO emission are a unique class of objects that exhibit a remarkably uniform set of properties from galaxy to galaxy.

Mass, metal, and energy feedback in cosmological simulations

Abstract: Using GADGET-2 cosmological hydrodynamic simulations including an observationally constrained model for galactic outflows, we investigate how feedback from star formation distributes mass, metals, and energy on cosmic scales from z = 6 → 0. We include instantaneous enrichment from Type II supernovae (SNe), as well as delayed enrichment from Type Ia SNe and stellar [asymptotic giant branch (AGB)] mass loss, and we individually track carbon, oxygen, silicon and iron using the latest yields. Following on the success of the momentum-driven wind scalings, we improve our implementation by using an on-the-fly galaxy finder to derive wind properties based on host galaxy masses. By tracking wind particles in a suite of simulations, we find: (1) wind material re-accretes on to a galaxy (usually the same one it left) on a recycling time-scale that varies inversely with galaxy mass (e.g. <1 Gyr for L ∗ galaxies at

Cold gas accretion in galaxies

Abstract: Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: (1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. This suggests ongoing minor mergers and recent arrival of external gas. It may be regarded, therefore, as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M 31. (2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is undoubtedly produced by galactic fountains, it is likely that a part of it is of extragalactic origin. Also the Milky Way has extra-planar gas complexes: the Intermediate- and High-Velocity Clouds (IVCs and HVCs). (3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. (4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The new gas could be added to the halo or be deposited in the outer parts of galaxies and form reservoirs for replenishing the inner parts and feeding star formation. The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean “visible” accretion rate of cold gas in galaxies of at least $${0.2\, M_{\odot} year^{-1}}$$ . In order to reach the accretion rates needed to sustain the observed star formation ( $${\approx 1 M_{\odot} year^{-1}}$$ ), additional infall of large amounts of gas from the IGM seems to be required.

Planet formation around stars of various masses : the snow line and the frequency of giant planets

Abstract: We use a semianalytic circumstellar disk model that considers movement of the snow line through evolution of accretion and the central star to investigate how gas giant frequency changes with stellar mass. The snow line distance changes weakly with stellar mass; thus, giant planets form over a wide range of spectral types. The probability that a given star has at least one gas giant increases linearly with stellar mass from 0.4 to 3 M☉. Stars more massive than 3 M☉ evolve quickly to the main sequence, which pushes the snow line to 10-15 AU before protoplanets form and limits the range of disk masses that form giant planet cores. If the frequency of gas giants around solar mass stars is 6%, we predict occurrence rates of 1% for 0.4 M☉ stars and 10% for 1.5 M☉ stars. This result is largely insensitive to our assumed model parameters. Finally, the movement of the snow line as stars 2.5 M☉ move to the main sequence may allow the ocean planets suggested by Leger et al. to form without migration.

An Hα Imaging Survey of Galaxies in the Local 11 Mpc Volume

Abstract: As part of a broader effort to characterize the population of star-forming galaxies in the local universe, we have carried out an H α + [N II] imaging survey for an essentially volume-limited sample of galaxies within 11 Mpc of the Milky Way. This first paper describes the design of the survey, the observation, data processing, and calibration procedures, and the characteristics of the galaxy sample. The main product of the paper is a catalog of integrated Hα fluxes, luminosities, and equivalent widths for the galaxies in the sample. We briefly discuss the completeness properties of the survey and compare the distribution of the sample and its star formation properties to other large Hα imaging surveys. These data form the foundation for a series of follow-up studies of the star formation properties of the local volume, and the properties and duty cycles of star formation bursts in dwarf galaxies.

The effect of galaxy mass ratio on merger-driven starbursts

Abstract: We employ numerical simulations of galaxy mergers to explore the effect of galaxy mass ratio on merger-driven starbursts. Our numerical simulations include radiative cooling of gas, star formation, and stellar feedback to follow the interaction and merger of four disc galaxies. The galaxy models span a factor of 23 in total mass and are designed to be representative of typical galaxies in the local universe. We find that the merger-driven star formation is a strong function of merger mass ratio, with very little, if any, induced star formation for large mass ratio mergers. We define a burst efficiency that is useful to characterize the merger-driven star formation and test that it is insensitive to uncertainties in the feedback parametrization. In accord with previous work we find that the burst efficiency depends on the structure of the primary galaxy. In particular, the presence of a massive stellar bulge stabilizes the disc and suppresses merger-driven star formation for large mass ratio mergers. Direct, coplanar merging orbits produce the largest tidal disturbance and yield the most intense burst of star formation. Contrary to naive expectations, a more compact distribution of gas or an increased gas fraction both decrease the burst efficiency. Owing to the efficient feedback model and the newer version of smoothed particle hydrodynamics employed here, the burst efficiencies of the mergers presented here are smaller than in previous studies.

Large-scale structure of the molecular gas in Taurus revealed by high linear dynamic range spectral line mapping

Abstract: We report the results of a 100 deg2 survey of the Taurus molecular cloud region in 12CO and 13CO -->J = 1→ 0. The image of the cloud in each velocity channel includes -->3 × 106 Nyquist-sampled pixels on a 20 -->'' grid. The high sensitivity and large spatial dynamic range of the maps reveal a very complex, highly structured cloud morphology, including filaments, cavities, and rings. The axes of the striations seen in the 12CO emission from relatively diffuse gas are aligned with the direction of the magnetic field. We have developed a statistical method for analyzing the pixels in which 12CO but not 13CO is detected, which allows us to determine the CO column in the diffuse portion of the cloud, as well as in the denser regions in which we detect both isotopologues. Using a column-density-dependent model for the CO fractional abundance, we derive the mass of the region mapped to be -->2.4 × 104 M☉, more than twice as large as would be obtained using a canonical fixed fractional abundance of 13CO, and a factor of 3 greater than would be obtained considering only the high column density regions. We determine that half the mass of the cloud is in regions having column density below -->2.1 × 1021 cm−2. The distribution of young stars in the region covered is highly nonuniform, with the probability of finding a star in a pixel with a specified column density rising sharply for -->N(H2) = 6 × 1021 cm−2. We determine a relatively low star formation efficiency (mass of young stars/mass of molecular gas), between 0.3% and 1.2%, and an average star formation rate during the past 3 Myr of -->8 × 10−5 stars yr−1.

Riding the Spiral Waves: Implications of Stellar Migration for the Properties of Galactic Disks

Abstract: Stars in disks of spiral galaxies are usually assumed to remain roughly at their birth radii. This assumption is built into decades of modeling of the evolution of stellar populations in our own Galaxy and in external systems. We present results from self-consistent high-resolution N-body + smooth particle hydrodynamics simulations of disk formation, in which stars migrate across significant galactocentric distances due to resonant scattering with transient spiral arms, while preserving their circular orbits. We investigate the implications of such migrations for observed stellar populations. Radial migration provides an explanation for the observed flatness and spread in the age-metallicity relation and the relative lack of metal-poor stars in the solar neighborhood. The presence of radial migration also prompts rethinking of interpretations of extragalactic stellar population data, especially for determinations of star formation histories.

Cold gas accretion in galaxies

Abstract: Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: 1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. It may be regarded as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is produced by galactic fountains, it is likely that a part of it is of extragalactic origin. 3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. 4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates needed to sustain the observed star formation (~1 Msol/yr), additional infall of large amounts of gas from the IGM seems to be required.

An H-alpha Imaging Survey of Galaxies in the Local 11 Mpc Volume

Abstract: As part of a broader effort to characterize the population of star-forming galaxies in the local universe, we have carried out an H-alpha+[NII] imaging survey for an essentially volume-limited sample of galaxies within 11 Mpc of the Milky Way. This paper describes the design of the survey, the observation, data processing, and calibration procedures, and the characteristics of the galaxy sample. The main product of the paper is a catalog of integrated H-alpha fluxes, luminosities, and equivalent widths for the galaxies in the sample. We briefly discuss the completeness properties of the survey and compare the distribution of the sample and its star formation properties to other large H-alpha imaging surveys. These data form the foundation for a series of follow-up studies of the star formation properties of the local volume, and the properties and duty cycles of star formation bursts in dwarf galaxies.

The origin of the galaxy mass-metallicity relation and implications for galactic outflows

Abstract: Using cosmological hydrodynamic simulations that dynamically incorporate enriched galactic outflows together with analytical modelling, we study the origin of the stellar mass-gas-phase metallicity relation (MZR). We find that metallicities are driven by an equilibrium between the rate of enrichment owing to star formation and the rate of dilution owing to infall of unenriched gas. This equilibrium is in turn governed by the outflow strength. As such, the MZR provides valuable insights and strong constraints on galactic outflow properties across cosmic time. We compare three outflow models: no outflows, a 'constant-wind model that emulates the popular Dekel & Silk scenario, and a 'momentum-driven wind' model that best reproduces z ≥ 2 intergalactic medium metallicity data. Only the momentum-driven wind scaling simulation is able to reproduce the observed z ∼ 2 MZR's slope, amplitude, and scatter. In order to understand why, we construct a one-zone chemical evolution model guided by simulations. This model shows that the MZR in our outflow simulations can be understood in terms of three parameters: (i) the equilibrium metallicity Z g,eq = YS FR / ACC (where y = net yield), reflecting the enrichment balance between star formation rate S FR and gas accretion rate ACC ; (ii) the dilution time t d = M g /M SFR , representing the time-scale for a galaxy to return to Z g,eq after a metallicity-perturbing interaction; and (iii) the blowout mass M blowout , which is the galaxy stellar mass above which winds can escape its halo. Without outflows, galaxy metallicities exceed observations by approximately two to three times, although the slope of the MZR is roughly correct owing to greater star formation efficiencies in larger galaxies. When outflows with mass-loading factor η w are present, galaxies below M blowout obey Z g,eq ≈ y/(1 + η w ), while above M blowout , Z g,eq → y. Our constant-wind model has M blowout ∼ 10 10 M ⊙ , which yields a sharp upturn in the MZR above this scale and a flat MZR with large scatter below it, in strong disagreement with observations. Our momentum-driven wind model naturally reproduces the observed Z g ∞ M 0.3 * because Z g,eq ∞ η -1 w ∞ M 1/3 * when η w » 1 (i.e. at low masses). The flattening of the MZR at M* ≥ 10 10.5 M ⊙ observed by Tremonti et al. is reflective of the mass-scale where η w ∼ 1 rather than a characteristic outflow speed; in fact, the outflow speed plays little role in the MZR except through M blowout . The tight observed MZR scatter is ensured when t d ≤ dynamical time, which is only satisfied at all masses in our momentum-driven wind model. We also discuss secondary effects on the MZR, such as baryonic stripping from neighbouring galaxies' outflows.

Structural properties of pseudo-bulges, classical bulges and elliptical galaxies: an SDSS perspective

Abstract: We have performed 2D bulge/bar/disc decompositions using g, r and i-band images of a representative sample of nearly 1000 galaxies from the Sloan Digital Sky Survey. We show that the Petrosian concentration index is a better proxy for bulge-to-total ratio than the global Sersic index. We show that pseudo-bulges can be distinguished from classical bulges as outliers in the Kormendy relation. We provide the structural parameters and distributions of stellar masses of ellipticals, classical bulges, pseudo- bulges, discs and bars, and find that 32 per cent of the total stellar mass in massive galaxies in the local universe is contained in ellipticals, 36 per cent in discs, 25 per cent in classical bulges, 3 per cent in pseudo-bulges and 4 per cent in bars. Pseudo-bulges are currently undergoing intense star formation activity and populate the blue cloud of the colour-magnitude diagram. Most (though not all) classical bulges are quiescent and populate the red sequence of the diagram. Classical bulges follow a correlation between bulge Sersic index and bulge-to-total ratio, while pseudo-bulges do not. In addition, for a fixed bulge-to-total ratio, pseudo-bulges are less concentrated than classical bulges. Pseudo-bulges follow a mass-size relation similar to that followed by bars, and different from that followed by classical bulges. In the fundamental plane, pseudo-bulges occupy the same locus as discs. While these results point out different formation processes for classical and pseudo-bulges, we also find a significant overlap in their properties, indicating that the different processes might happen concomitantly. Finally, classical bulges and ellipticals follow offset mass-size relations, suggesting that high-mass bulges might not be simply high-mass ellipticals surrounded by discs.

Galaxy Pairs in the Sloan Digital Sky Survey I: Star Formation, AGN Fraction, and the Luminosity/Mass-Metallicity Relation

Abstract: (Abridged). We present a sample of 1716 galaxies with companions within Delta v < 500 km/s, r_p < 80 kpc and stellar mass ratio 0.1 < M_1/M_2 < 10 from the Sloan Digital Sky Survey (SDSS) Data Release 4 (DR4). In agreement with previous studies, we find an enhancement in the star formation rate (SFR) of galaxy pairs at projected separations < 30--40 kpc. In addition, we find that this enhancement is highest (and extends to the greatest separations) for galaxies of approximately equal mass, the so-called `major' pairs. However, SFR enhancement can still be detected for a sample of galaxy pairs whose masses are within a factor of 10 of each other. In agreement with the one previous study of the luminosity-metallicity (LZ) relation in paired galaxies, we find an offset to lower metallicities (by ~ 0.1 dex) for a given luminosity for galaxies in pairs compared to the control sample. We also present the first mass-metallicity (MZ) relation comparison between paired galaxies and the field, and again find an offset to lower metallicities (by ~ 0.05 dex) for a given mass. The smaller offset in the MZ relation indicates that both higher luminosities and lower metallicities may contribute to the shift of pairs relative to the control in the LZ relation. We show that the offset in the LZ relation depends on galaxy half light radius, r_h. Galaxies with r_h < 3 kpc and with a close companion show a 0.05-0.1 dex downwards offset in metallicity compared to control galaxies of the same size. Finally, we study the AGN fraction in both the pair and control sample and find that whilst selecting galaxies in different cuts of color and asymmetry yields different AGN fractions, the fraction for pairs and the control sample are consistent for a given set of selection criteria.

The Evolution of Galaxy Mergers and Morphology at z<1.2 in the Extended Groth Strip

Abstract: We present the quantitative rest-frame B morphological evolution and galaxy merger fraction at 0.2 1011 L☉ are disk galaxies, and only ~15% are classified as major merger candidates. Edge-on and dusty disk galaxies (Sb-Ir) are almost a third of the red sequence at z ~ 1.1, while E/S0/Sa make up over 90% of the red sequence at z ~ 0.3. Approximately 2% of our full sample are red mergers. We conclude (1) the merger rate does not evolve strongly between 0.2 < z < 1.2; (2) the decrease in the volume-averaged star formation rate density since z ~ 1 is a result of declining star formation in disk galaxies rather than a disappearing population of major mergers; (3) the build-up of the red sequence at z < 1 can be explained by a doubling in the number of spheroidal galaxies since z ~ 1.2.

Mid-Infrared Spectral Diagnosis of Submillimeter Galaxies

Abstract: We present deep mid-IR spectroscopy with Spitzer of 13 SMGs in the GOODS-N field. We find strong PAH emission in all of our targets, which allows us to measure mid-IR spectroscopic redshifts and place constraints on the contribution from star formation and AGN activity to the mid-IR emission. In the high-S/N composite spectrum, we find that the hot dust continuum from an AGN contributes at most 30% of the mid-IR luminosity. Individually, only 2/13 SMGs have continuum emission dominating the mid-IR luminosity; one of these SMGs, C1, remains undetected in the deep X-ray images but shows a steeply rising continuum in the mid-IR indicative of a Compton-thick AGN. We find that the mid-IR properties of SMGs are distinct from those of 24 μm-selected ULIRGs at z ~ 2; the former are predominantly dominated by star formation, while the latter are a more heterogeneous sample with many showing significant AGN activity. We fit the IRS spectrum and the mid-IR to radio photometry of SMGs with template SEDs to determine the best estimate of the total IR luminosity from star formation. While many SMGs contain an AGN as evinced by their X-ray properties, our multiwavelength analysis shows that the total IR luminosity, LIR, in SMGs is dominated by star formation. We find that high-redshift SMGs lie on the relation between LIR and LPAH ,6.2 (or LPAH ,7.7 or LPAH ,11.3) that has been established for local starburst galaxies. This suggests that PAH luminosity can be used as a proxy for the SFR in SMGs. SMGs are consistent with being a short-lived cool phase in a massive merger where the AGN does not appear to have become strong enough to heat the dust and dominate the mid- or far-IR emission.

Simulating galactic outflows with kinetic supernova feedback

Abstract: Feedback from star formation is thought to play a key role in the formation and evolution of galaxies, but its implementation in cosmological simulations is currently hampered by a lack of numerical resolution. We present and test a subgrid recipe to model feedback from massive stars in cosmological smoothed particle hydrodynamics simulations. The energy is distributed in kinetic form among the gas particles surrounding recently formed stars. The impact of the feedback is studied using a suite of high-resolution simulations of isolated disc galaxies embedded in dark haloes with total mass 10 10 and 10 12 h −1 M� . We focus, in particular, on the effect of pressure forces on wind particles within the disc, which we turn off temporarily in some of our runs to mimic a recipe that has been widely used in the literature. We find that this popular recipe gives dramatically different results because (ram) pressure forces on expanding superbubbles determine both the structure of the disc and the development of largescale outflows. Pressure forces exerted by expanding superbubbles puff up the disc, giving the dwarf galaxy an irregular morphology and creating a galactic fountain in the massive galaxy. Hydrodynamic drag within the disc results in a strong increase in the effective mass loading of the wind for the dwarf galaxy, but quenches much of the outflow in the case of the high-mass galaxy.

The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities

Abstract: Gas in galactic disks is collected by gravitational instabilities into giant atomic-molecular complexes, but only the inner, molecular parts of these structures are able to collapse to form stars. Determining what controls the ratio of atomic to molecular hydrogen in complexes is therefore a significant problem in star formation and galactic evolution. In this paper we use the model of H_2 formation, dissociation, and shielding developed in the previous paper in this series to make theoretical predictions for atomic to molecular ratios as a function of galactic properties. We find that the molecular fraction in a galaxy is determined primarily by its column density and secondarily by its metallicity, and is to good approximation independent of the strength of the interstellar radiation field. We show that the column of atomic hydrogen required to shield a molecular region against dissociation is ~10 Msun pc^-2 at solar metallicity. We compare our model to data from recent surveys of the Milky Way and of nearby galaxies, and show that the both the primary dependence of molecular fraction on column density and the secondary dependence on metallicity that we predict are in good agreement with observed galaxy properties.

Short-duration gamma-ray bursts with extended emission from protomagnetar spin-down

Abstract: Evidence is growing for a class of gamma-ray bursts (GRBs) characterized by an initial ∼0.1-1 s spike of hard radiation followed, after a ∼3-10 s lull in emission, by a softer period of extended emission lasting ∼10-100 s. In a few well-studied cases, these 'short GRBs with extended emission' show no evidence for a bright associated supernova (SN). We propose that these events are produced by the formation and early evolution of a highly magnetized, rapidly rotating neutron star (a 'protomagnetar') which is formed from the accretion-induced collapse (AIC) of a white dwarf (WD), the merger and collapse of a WD-WD binary or perhaps, the merger of a double neutron star binary. The initial emission spike is powered by accretion on to the protomagnetar from a small disc that is formed during the AIC or merger event. The extended emission is produced by a relativistic wind that extracts the rotational energy of the protomagnetar on a time-scale ∼ 10-100 s. The ∼ 10 s delay between the prompt and extended emission is the time required for the newly formed protomagnetar to cool sufficiently that the neutrino-heated wind from its surface becomes ultrarelativistic. Because a protomagnetar ejects little or no 56 Ni (< 10 -3 M ⊙ ), these events should not produce a bright SN-like transient. We model the extended emission from GRB060614 using spin-down calculations of a cooling protomagnetar, finding reasonable agreement with observations for a magnetar with an initial rotation period of ∼1 ms and a surface dipole field of ∼3 x 10 15 G. If GRBs are indeed produced by AIC or WD-WD mergers, they should occur within a mixture of both early- and late-type galaxies and should not produce strong gravitational wave emission. An additional consequence of our model is the existence of X-ray flashes unaccompanied by a bright SN and not associated with massive star formation.