arXiv subjects

Bruce G. Elmegreen

Publications and source records attributed to Bruce G. Elmegreen.

At least 145 records · Page 8Linked to original sources

arXiv2012

HI Power Spectra and the Turbulent ISM of Dwarf Irregular Galaxies

HI spatial power spectra (PS) were determined for a sample of 24 nearby dwarf irregular galaxies selected from the LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes - The HI Nearby Galaxy Survey) sample. The two-dimensional (2D) power spectral indices asymptotically become a constant for each galaxy when a significant part of the line profile is integrated. For narrow channel maps, the PS become shallower as the channel width decreases, and this shallowing trend continues to our single channel maps. This implies that even the highest velocity resolution of 1.8 km/s is not smaller than the thermal dispersion of the coolest, widespread HI component. The one-dimensional PS of azimuthal profiles at different radii suggest that the shallower PS for narrower channel width is mainly contributed by the inner disks, which indicates that the inner disks have proportionally more cooler HI than the outer disks. Galaxies with lower luminosity (M_B > -14.5 mag) and star formation rate (SFR, log(SFR (M\odot/yr)) < -2.1) tend to have steeper PS, which implies that the HI line-of-sight depths can be comparable with the radial length scales in low mass galaxies. A lack of a correlation between the inertial-range spectral indices and SFR surface density implies that either non-stellar power sources are playing a fundamental role in driving the interstellar medium (ISM) turbulent structure, or the nonlinear development of turbulent structures has little to do with the driving sources.

astro-ph.CO↗

arXiv2012

Age and Mass for 920 LMC Clusters Derived from 100 Million Monte Carlo Simulations

We present new age and mass estimates for 920 stellar clusters in the Large Magellanic Cloud (LMC) based on previously published broad-band photometry and the stellar cluster analysis package, MASSCLEANage. Expressed in the generic fitting formula, d^{2}N/dM dt ~ M^α t^β, the distribution of observed clusters is described by α= -1.5 to -1.6 and β= -2.1 to -2.2. For 288 of these clusters, ages have recently been determined based on stellar photometric color-magnitude diagrams, allowing us to gauge the confidence of our ages. The results look very promising, opening up the possibility that this sample of 920 clusters, with reliable and consistent age, mass and photometric measures, might be used to constrain important characteristics about the stellar cluster population in the LMC. We also investigate a traditional age determination method that uses a χ^2 minimization routine to fit observed cluster colors to standard infinite mass limit simple stellar population models. This reveals serious defects in the derived cluster age distribution using this method. The traditional χ^2 minimization method, due to the variation of U,B,V,R colors, will always produce an overdensity of younger and older clusters, with an underdensity of clusters in the log(age/yr)=[7.0,7.5] range. Finally, we present a unique simulation aimed at illustrating and constraining the fading limit in observed cluster distributions that includes the complex effects of stochastic variations in the observed properties of stellar clusters.

astro-ph.GA↗

arXiv2012

Local Tadpole Galaxies

Tadpole galaxies have a giant star-forming region at the end of an elongated intensity distribution. Here we use SDSS data to determine the ages, masses, and surface densities of the heads and tails in 14 local tadpoles selected from the Kiso and Michigan surveys of UV-bright galaxies, and we compare them to tadpoles previously studied in the Hubble Ultra Deep Field. The young stellar mass in the head scales linearly with restframe galaxy luminosity, ranging from ~10^5 M_solar at galaxy absolute magnitude U=-13 mag to 10^9 M_solar at U=-20 mag. The corresponding head surface density increases from several M_solar pc^{-2} locally to 10-100 M_solar pc^{-2} at high redshift, and the star formation rate per unit area in the head increases from ~0.01 M_solar yr^{-1} kpc^{-2} locally to ~1 M_solar yr^{-1} kpc^{-2} at high z. These local values are normal for star-forming regions, and the increases with redshift are consistent with other cosmological star formation rates, most likely reflecting an increase in gas abundance. The tails in the local sample look like bulge-free galaxy disks. Their photometric ages decrease from several Gyr to several hundred Myr with increasing z, and their surface densities are more constant than the surface densities of the heads. The far outer intensity profiles in the local sample are symmetric and exponential. We suggest that most local tadpoles are bulge-free galaxy disks with lopsided star formation, perhaps from environmental effects such as ram pressure or disk impacts, or from a Jeans length comparable to half the disk size.

astro-ph.CO↗

arXiv2012

What triggers star formation in galaxies?

Processes that promote the formation of dense cold clouds in the interstellar media of galaxies are reviewed. Those that involve background stellar mass include two-fluid instabilities, spiral density wave shocking, and bar accretion. Young stellar pressures trigger gas accumulation on the periphery of cleared cavities, which often take the form of rings by the time new stars form. Stellar pressures also trigger star formation in bright-rim structures, directly squeezing the pre-existing clumps in nearby clouds and clearing out the lower density gas between them. Observations of these processes are common. How they fit into the empirical star formation laws, which relate the star formation rate primarily to the gas density, is unclear. Most likely, star formation follows directly from the formation of cold dense gas, whatever the origin of that gas. If the average pressure from the weight of the gas layer is large enough to produce a high molecular fraction in the ambient medium, then star formation should follow from a variety of processes that combine and lose their distinctive origins. Pressurized triggering might have more influence on the star formation rate in regions with low average molecular fraction. This implies, for example, that the arm/interarm ratio of star formation efficiency should be higher in the outer regions of galaxies than in the main disks.

astro-ph.CO↗

arXiv2012

In-spiraling Clumps in Blue Compact Dwarf Galaxies

Giant star-formation clumps in dwarf irregular galaxies can have masses exceeding a few percent of the galaxy mass enclosed inside their orbital radii. They can produce sufficient torques on dark matter halo particles, halo stars, and the surrounding disk to lose their angular momentum and spiral into the central region in 1 Gyr. Pairs of giant clumps with similarly large relative masses can interact and exchange angular momentum to the same degree. The result of this angular momentum loss is a growing central concentration of old stars, gas, and star formation that can produce a long-lived starburst in the inner region, identified with the BCD phase. This central concentration is proposed to be analogous to the bulge in a young spiral galaxy. Observations of star complexes in five local BCDs confirm the relatively large clump masses that are expected for this process. The observed clumps also seem to contain old field stars, even after background light subtraction, in which case the clumps may be long-lived. The two examples with clumps closest to the center have the largest relative clump masses and the greatest contributions from old stars. An additional indication that the dense central regions of BCDs are like bulges is the high ratio of the inner disk scale height to the scale length, which is comparable to 1 for four of the galaxies.

astro-ph.CO↗

arXiv2011

Outside-in Shrinking of the Star-forming Disk of Dwarf Irregular Galaxies

We have studied multi-band surface brightness profiles of a representative sample of 34 nearby dwarf irregular galaxies (dIrrs). Our data include GALEX FUV/NUV, UBV, Hα, and Spitzer 3.6 \mum images. These galaxies constitute the majority of the LITTLE THINGS survey. By modeling the azimuthal averages of the spectral energy distributions with a complete library of star formation (SF) histories, we derived the stellar mass surface density distributions and the SF rate averaged over three different timescales: the recent 0.1 Gyr, 1 Gyr and a Hubble time. We find that, for \sim 80% (27 galaxies) of our sample galaxies, radial profiles (at least in the outer part) at shorter wavelengths have shorter disk scale lengths than those at longer wavelengths. This indicates that the star-forming disk has been shrinking. In addition, the radial distributions of the stellar mass surface density are well described as piece-wise exponential profiles, and \sim 80% of the galaxies have steeper mass profiles in the outer disk than in the inner region. The steep radial decline of SF rate in the outer parts compared to that in the inner disks gives a natural explanation for the down-bending stellar mass surface density profiles. Within the inner disks, our sample galaxies on average have constant ratios of recent SF rate to stellar mass with radius. Nevertheless, \sim 35% (12 galaxies, among which 7 have baryonic mass < 10^8 M\odot) of the sample exhibit negative slopes across the observed disk, which is in contrast with the "inside-out" disk growth scenario suggested for luminous spiral galaxies. The tendency of SF to become concentrated toward the inner disks in low mass dIrrs is interpreted as a result of their susceptibility to environmental effects and regulation through stellar feedback.

astro-ph.CO↗

arXiv2011

Reconstructing the stellar mass distributions of galaxies using S4G IRAC 3.6 and 4.5 micron images: I. Correcting for contamination by PAH, hot dust, and intermediate age stars

With the aim of constructing accurate 2D maps of the stellar mass distribution in nearby galaxies from S4G 3.6 and 4.5 micron images, we report on the separation of the light from old stars from the emission contributed by contaminants (e.g. hot dust and the 3.3 micron PAH feature). Results for a small sample of six disk galaxies (NGC 1566, NGC 2976, NGC 3031, NGC 3184, NGC 4321, and NGC 5194) with a range of morphological properties, dust contents and star formation histories are presented to demonstrate our approach. We use an Independent Component Analysis (ICA) technique designed to separate statistically independent source distributions, maximizing the distinction in the [3.6]-[4.5] colors of the sources. The technique also removes emission from intermediate-age evolved red objects with a low mass-to-light ratio, such as asymptotic giant branch (AGB) and red supergiant (RSG) stars, revealing maps of the underlying old distribution of light with [3.6]-[4.5] colors consistent with the colors of K and M giants. Contaminants are identified via comparison to the non-stellar emission imaged at 8 microns, which is dominated by the broad PAH feature. Using the measured 3.6/8 micron ratio to select the individual contaminants, we find that hot dust and PAH together contribute between ~5-15% to the integrated light at 3.6 microns, while light from regions dominated by intermediate-age stars accounts for only 1-5%. Locally, however, the contribution from either contaminant can reach much higher levels; dust contributes on average 22% to the emission in star-forming regions throughout the sample, while intermediate age-stars contribute upwards of 50% in localized knots. The removal of these contaminants with ICA leaves maps of the old stellar disk that retain a high degree of structural information and are ideally suited for tracing the stellar mass, as will be the focus in a companion paper.

astro-ph.CO↗

arXiv2011

Thick disks of edge-on galaxies seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G): Lair of missing baryons?

Most, if not all, disk galaxies have a thin (classical) disk and a thick disk. In most models thick disks are thought to be a necessary consequence of the disk formation and/or evolution of the galaxy. We present the results of a study of the thick disk properties in a sample of carefully selected edge-on galaxies with types ranging from T=3 to T=8. We fitted one-dimensional luminosity profiles with physically motivated functions - the solutions of two stellar and one gaseous isothermal coupled disks in equilibrium - which are likely to yield more accurate results than other functions used in previous studies. The images used for the fits come from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We found that thick disks are on average more massive than previously reported, mostly due to the selected fitting function. Typically, the thin and the thick disk have similar masses. We also found that thick disks do not flare significantly within the observed range in galactocentric radii and that the ratio of thick to thin disk scaleheights is higher for galaxies of earlier types. Our results tend to favor an in situ origin for most of the stars in the thick disk. In addition the thick disk may contain a significant amount of stars coming from satellites accreted after the initial build-up of the galaxy and an extra fraction of stars coming from the secular heating of the thin disk by its own overdensities. Assigning thick disk light to the thin disk component may lead to an underestimate of the overall stellar mass in galaxies, because of different mass to light ratios in the two disk components. On the basis of our new results, we estimate that disk stellar masses are between 10% and 50% higher than previously thought and we suggest that thick disks are a reservoir of "local missing baryons".

astro-ph.CO↗

arXiv2011

The unusual vertical mass distribution of NGC 4013 seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G)

NGC 4013 is a nearby Sb edge-on galaxy known for its "prodigious" HI warp and its "giant" tidal stream. Previous work on this unusual object shows that it cannot be fitted satisfactorily by a canonical thin+thick disk structure. We have produced a new decomposition of NGC 4013, considering three stellar flattened components (thin+thick disk plus an extra and more extended component) and one gaseous disk. All four components are considered to be gravitationally coupled and isothermal. To do so, we have used the 3.6micron images from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We find evidence for NGC4013 indeed having a thin and a thick disk and an extra flattened component. This smooth and extended component (scaleheight z_EC kpc) could be interpreted as a thick disk or as a squashed ellipsoidal halo and contains ~20% of the total mass of all three stellar components. We argue it is unlikely to be related to the ongoing merger or due to the off-plane stars from a warp in the other two disk components. Instead, we favor a scenario in which the thick disk and the extended component were formed in a two-stage process, in which an initially thick disk has been dynamically heated by a merger soon enough in the galaxy history to have a new thick disk formed within it.

astro-ph.CO↗

arXiv2011

The Outer Disks of Dwarf Irregular Galaxies

To explore the properties of extreme outer stellar disks, we obtained ultra-deep V and GALEX UV images of 4 dwarf irregular galaxies and one Blue Compact Dwarf galaxy and ultra-deep B images of 3 of these. Our V-band surface photometry extends to 29.5 magnitudes arcsec^-2. We convert the FUV and V-band photometry, along with Halpha photometry, into radial star formation rate profiles that are sensitive to timescales from 10 Myrs to the lifetime of the galaxy. We also compare the stellar distributions, surface brightness profiles, and star formation rate profiles to HI-line emission maps, gas surface density profiles, and gas kinematics. Our data lead us to two general observations: First, the exponential disks in these irregular galaxies are extraordinarily regular. The stellar disks continue to decline exponentially as far as our measurements extend. In spite of lumpiness in the distribution of young stars and HI distributions and kinematics that have significant unordered motions, sporadic processes that have built the disks-star formation, radial movement of stars, and perhaps even perturbations from the outside-have, nevertheless, conspired to produce standard disk profiles. Second, there is a remarkable continuity of star formation throughout these disks over time. In four out of five of our galaxies the star formation rate in the outer disk measured from the FUV tracks that determined from the V-band, to within factors of 5, requiring star formation at a fairly steady rate over the galaxy's lifetime. Yet, the HI surface density profiles generally decline with radius more shallowly than the stellar light, and the gas is marginally gravitationally stable against collapse into clouds. Outer stellar disks are challenging our concepts of star formation and disk growth and provide a critical environment in which to understand processes that mold galaxy disks.

astro-ph.CO↗

arXiv2011

VII Zw 403: HI Structure in a Blue Compact Dwarf Galaxy

We present optical (UBVJ), ultraviolet (FUV, NUV), and high resolution atomic hydrogen (HI) observations of the nearby blue compact dwarf (BCD), VII Zw 403. We find that VII Zw 403 has a relatively high HI mass-to-light ratio for a BCD. The rotation velocity is nominally 10-15 km/s, but rises to ~20 km/s after correction for the ~8-10 km/s random motions present in the gas. The velocity field is complex; including a variation in the position angle of the major axis going from the NE to the SW parts of the galaxy. Our high resolution HI maps reveal structure in the central gas, including a large, low-density HI depression or hole between the southern and northern halves of the galaxy, coincident with an unresolved x-ray source. Although interactions have been proposed as the triggering mechanism for the vigorous star formation occurring in BCDs, VII Zw 403 does not seem to have been tidally triggered by an external interaction, as we have found no nearby possible perturbers. It also doesn't appear to fall in the set of galaxies that exhibit a strong central mass density concentration, as its optical scale length is large in comparison to similar systems. However, there are some features that are compatible with an accretion event: optical/HI axis misalignment, a change in position angle of the kinematic axis, and a complex velocity field.

astro-ph.GA↗

arXiv2011

Gravitational Instabilities in Two-Component Galaxy Disks with Gas Dissipation

Growth rates for gravitational instabilities in a thick disk of gas and stars are determined for a turbulent gas that dissipates on the local crossing time. The scale heights are derived from vertical equilibrium. The accuracy of the usual thickness correction, 1/(1+kH), is better than 6% in the growth rate when compared to exact integrations for the gravitational acceleration in the disk. Gas dissipation extends the instability to small scales, removing the minimum Jeans length. This makes infinitesimally thin disks unstable for all Toomre-Q values, and reasonably thick disks stable at high Q primarily because of thickness effects. The conventional gas+star threshold, Qtot increases from ~1 without dissipation to 2 or 3 when dissipation has a rate equal to the crossing rate over a perturbation scale. Observations of Qtot~2-3 and the presence of supersonic turbulence suggest that disks are unstable over a wide range of scales. Such instabilities drive spiral structure if there is shear and clumpy structure if shear is weak; they may dominate the generation of turbulence. Feedback regulation of Qtot is complex because the stellar component does not cool; the range of spiral strengths from multiple arm to flocculent galaxies suggests that feedback is weak. Gravitational instabilities may have a connection to star formation even when the star formation rate scales directly with the molecular mass because the instabilities return dispersed gas to molecular clouds and complete the cycle of cloud formation and destruction. The mass flow to dense clouds by instabilities can be 10 times larger than the star formation rate.

astro-ph.GA↗

arXiv2011

On the Initial Conditions for Star Formation and the IMF

Density probability distribution functions (PDFs) for turbulent self-gravitating clouds should be convolutions of the local log-normal PDF, which depends on the local average density rho-ave and Mach number M, and the probability distribution functions for rho-ave and M, which depend on the overall cloud structure. When self-gravity drives a cloud to increased central density, the total PDF develops an extended tail. If there is a critical density or column density for star formation, then the fraction of the local mass exceeding this threshold becomes higher near the cloud center. These elements of cloud structure should be in place before significant star formation begins. Then the efficiency is high so that bound clusters form rapidly, and the stellar initial mass function (IMF) has an imprint in the gas before destructive radiation from young stars can erase it. The IMF could arise from a power-law distribution of mass for cloud structure. These structures should form stars down to the thermal Jeans mass MJ at each density in excess of a threshold. The high-density tail of the PDF, combined with additional fragmentation in each star-forming core, extends the IMF into the Brown Dwarf regime. The core fragmentation process is distinct from the cloud structuring process and introduces an independen core fragmentation mass function (CFMF). The CFMF would show up primarily below the IMF peak.

astro-ph.GA↗

arXiv2011

NGC 2207/IC 2163: A Grazing Encounter with Large Scale Shocks

Radio continuum, Spitzer infrared, optical and XMM-Newton X-ray and UVM2 observations are used to study large-scale shock fronts, young star complexes, and the galactic nuclei in the interacting galaxies NGC 2207/IC 2163. There are two types of large-scale shock fronts in this galaxy pair. The shock front along the rim of the ocular oval in IC 2163 has produced vigorous star formation in a dusty environment. In the outer part of the companion side of NGC 2207, a large-scale front attributed to disk or halo scraping is particularly bright in the radio continuum but not in any tracers of recent star formation or in X-rays. This radio continuum front may be mainly in the halo on the back side of NGC 2207 between the two galaxies. Values of the flux density ratio S(8 um)/S(6 cm) of kpc-sized, Spitzer IRAC star-forming clumps in NGC 2207/IC 2163 are compared with those of giant H II regions in M81. We find evidence that in 2001 a radio supernova was present in the core of feature i, a mini-starburst on an outer arm of NGC 2207. X-ray emission is detected from the NGC 2207 nucleus and from nine discrete sources, one of which corresponds to SN 1999ec, and another may be a radio supernova or a background quasar. The X-ray luminosity and X-ray spectrum of the NGC 2207 nucleus suggests it is a highly absorbed, low luminosity AGN.

astro-ph.CO↗

arXiv2011

Star Formation on Galactic Scales: Empirical Laws

Empirical star formation laws from the last 20 years are reviewed with a comparison to simulations. The current form in main galaxy disks has a linear relationship between the star formation rate per unit area and the molecular cloud mass per unit area with a timescale for molecular gas conversion of about 2 Gyr. The local ratio of molecular mass to atomic mass scales nearly linearly with pressure, as determined from the weight of the gas layer in the galaxy. In the outer parts of galaxies and in dwarf irregular galaxies, the disk can be dominated by atomic hydrogen and the star formation rate per unit area becomes directly proportional to the total gas mass per unit area, with a consumption time of about 100 Gyr. The importance of a threshold for gravitational instabilities is not clear. Observations suggest such a threshold is not always important, while simulations generally show that it is. The threshold is difficult to evaluate because it is sensitive to magnetic and viscous forces, the presence of spiral waves and other local effects, and the equation of state.

astro-ph.GA↗

arXiv2011

Star Formation in Spiral Arms

The origin and types of spiral arms are reviewed with an emphasis on the connections between these arms and star formation. Flocculent spiral arms are most likely the result of transient instabilities in the gas that promote dense cloud formation, star formation, and generate turbulence. Long irregular spiral arms are usually initiated by gravitational instabilities in the stars, with the gas contributing to and following these instabilities, and star formation in the gas. Global spiral arms triggered by global perturbations, such as a galaxy interaction, can be wavemodes with wave reflection in the inner regions. They might grow and dominate the disk for several rotations before degenerating into higher-order modes by non-linear effects. Interstellar gas flows through these global arms, and through the more transient stellar spiral arms as well, where it can reach a high density and low shear, thereby promoting self-gravitational instabilities. The result is the formation of giant spiral arm cloud complexes, in which dense molecular clouds form and turn into stars. The molecular envelops and debris from these clouds appear to survive and drift through the interarm regions for a long time, possibly 100 Myr or more, with lingering spontaneous star formation and triggered star formation in the pieces that are still at high-pressure edges near older HII regions.

astro-ph.GA↗

arXiv2011

Star Formation Patterns and Hierarchies

Star formation occurs in hierarchical patterns in both space and time. Galaxies form large regions on the scale of the interstellar Jeans length and these large regions apparently fragment into giant molecular clouds and cloud cores in a sequence of decreasing size and increasing density. Young stars follow this pattern, producing star complexes on the largest scales, OB associations on smaller scales, and so on down to star clusters and individual stars. Inside each scale and during the lifetime of the cloud on that scale, smaller regions come and go in a hierarchy of time. As a result, cluster positions are correlated with power law functions, and so are their ages. At the lowest level in the hierarchy, clusters are observed to form in pairs. For any hierarchy like this, the efficiency is automatically highest in the densest regions. This high efficiency promotes bound cluster formation. Also for any hierarchy, the mass function of the components is a power law with a slope of around -2, as observed for clusters.

astro-ph.GA↗

arXiv2011

Triggered Star Formation

Triggered star formation in bright rims and shells is reviewed. Shells are commonly observed in the Milky Way and other galaxies, but most diffuse shells seen in HI or the infrared do not have obvious triggered star formation. Dense molecular shells and pillars around HII regions often do have such triggering, although sometimes it is difficult to see what is triggered and what stars formed in the gas before the pressure disturbances. Pillar regions without clear age gradients could have their stars scattered by the gravity of the heads. Criteria and timescales for triggering are reviewed. The insensitivity of the average star formation rate in a galaxy to anything but the molecular mass suggests that triggering is one of many processes that lead to gravitational collapse and star formation.

astro-ph.GA↗