arXiv subjects

J. Wadsley

Publications and source records attributed to J. Wadsley.

At least 55 records · Page 3Linked to original sources

arXiv2010

Measuring the Clump Mass Function in the Age of SCUBA2, Herschel, and ALMA

We use simulated images of star-forming regions to explore the effects of various image acquisition techniques on the derived clump mass function. In particular, we focus on the effects of finite image angular resolution, the presence of noise, and spatial filtering. We find that, even when the image has been so heavily degraded with added noise and lowered angular resolution that the clumps it contains clearly no longer correspond to pre-stellar cores, still the clump mass function is typically consistent with the stellar initial mass function within their mutual uncertainties. We explain this result by suggesting that noise, source blending, and spatial filtering all randomly perturb the clump masses, biasing the mass function toward a lognormal form whose high-mass end mimics a Salpeter power law. We argue that this is a consequence of the central limit theorem and that it strongly limits our ability to accurately measure the true mass function of the clumps. We support this conclusion by showing that the characteristic mass scale of the clump mass function, represented by the ``break mass'', scales as a simple function of the angular resolution of the image from which the clump mass function is derived. This strongly constrains our ability to use the clump mass function to derive a star formation efficiency. We discuss the potential and limitations of the current and next generation of instruments for measuring the clump mass function.

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arXiv2010

Cosmological Galaxy Formation Simulations Using SPH

We present the McMaster Unbiased Galaxy Simulations (MUGS), the first 9 galaxies of an unbiased selection ranging in total mass from 5$\times10^{11}$ M$_\odot$ to 2$\times10^{12}$ M$_\odot$ simulated using n-body smoothed particle hydrodynamics (SPH) at high resolution. The simulations include a treatment of low temperature metal cooling, UV background radiation, star formation, and physically motivated stellar feedback. Mock images of the simulations show that the simulations lie within the observed range of relations such as that between color and magnitude and that between brightness and circular velocity (Tully-Fisher). The greatest discrepancy between the simulated galaxies and observed galaxies is the high concentration of material at the center of the galaxies as represented by the centrally peaked rotation curves and the high bulge-to-total ratios of the simulations determined both kinematically and photometrically. This central concentration represents the excess of low angular momentum material that long has plagued morphological studies of simulated galaxies and suggests that higher resolutions and a more accurate description of feedback will be required to simulate more realistic galaxies. Even with the excess central mass concentrations, the simulations suggest the important role merger history and halo spin play in the formation of disks.

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arXiv2009

Forming a Large Disc Galaxy from a z<1 Major Merger

Using high resolution SPH simulations in a fully cosmological Lambda CDM context we study the formation of a bright disk dominated galaxy that originates from a "wet" major merger at z=0.8. The progenitors of the disk galaxy are themselves disk galaxies that formed from early major mergers between galaxies with blue colors. A substantial thin stellar disk grows rapidly following the last major merger and the present day properties of the final remnant are typical of early type spiral galaxies, with an i band B/D ~0.65, a disk scale length of 7.2 kpc, g-r = 0.5 mag, an HI line width (W_{20}/2) of 238 km/sec and total magnitude i = -22.4. The key ingredients for the formation of a dominant stellar disk component after a major merger are: i) substantial and rapid accretion of gas through cold flows followed at late times by cooling of gas from the hot phase, ii) supernova feedback that is able to partially suppress star formation during mergers and iii) relative fading of the spheroidal component. The gas fraction of the progenitors' disks does not exceed 25% at z<3, emphasizing that the continuous supply of gas from the local environment plays a major role in the regrowth of disks and in keeping the galaxies blue. The results of this simulation alleviate the problem posed for the existence of disk galaxies by the high likelihood of interactions and mergers for galaxy sized halos at relatively low z.

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arXiv2009

Angular Momentum Transport in Extended Galactic Disks

We demonstrate a significant difference in the angular momentum transport properties of galactic disks between regions in which the interstellar medium is single phase or two phase. Our study is motivated by observations of HI in extended galactic disks which indicate velocity dispersions of nonthermal origin, suggesting that turbulence in the gas may be contributing significantly to the observed dispersion. To address this, we have implemented a shearing-box framework within the FLASH code. The new code was used to perform local simulations of galactic disks that incorporate differential rotation, self-gravity, vertical stratification, hydrodynamics and cooling. These simulations explore plausible mechanisms for driving turbulent motions via the thermal and self-gravitational instabilities coupling to differential rotation. Where a two-phase medium develops, gravitational angular momentum transporting stresses are much greater, creating a possible mechanism for transferring energy from galactic rotation to turbulence. In simulations where the disk conditions do not trigger the formation of a two-phase medium, it is found that perturbations to the flow damp without leading to a sustained mechanism for driving turbulence. The differing angular momentum transport properties of the single- and two-phase regimes of the disk suggest that a significant, dynamically motivated division can be drawn between the two, even when this division occurs far outside the star formation cutoff in a galactic disk.

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arXiv2008

The Role of Cold Flows in the Assembly of Galaxy Disks

We use high resolution cosmological hydrodynamical simulations to demonstrate that cold flow gas accretion, particularly along filaments, modifies the standard picture of gas accretion and cooling onto galaxy disks. In the standard picture, all gas is initially heated to the virial temperature of the galaxy as it enters the virial radius. Low mass galaxies are instead dominated by accretion of gas that stays well below the virial temperature, and even when a hot halo is able to develop in more massive galaxies there exist dense filaments that penetrate inside of the virial radius and deliver cold gas to the central galaxy. For galaxies up to ~L*, this cold accretion gas is responsible for the star formation in the disk at all times to the present. Even for galaxies at higher masses, cold flows dominate the growth of the disk at early times. Within this modified picture, galaxies are able to accrete a large mass of cold gas, with lower initial gas temperatures leading to shorter cooling times to reach the disk. Although star formation in the disk is mitigated by supernovae feedback, the short cooling times allow for the growth of stellar disks at higher redshifts than predicted by the standard model.

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arXiv2008

In situ formation of the massive stars around SgrA*

The formation of the massive young stars surrounding SgrA* is still an open question. In this paper, we simulate the infall of an isothermal, turbulent molecular cloud towards the Galactic Centre (GC). As it spirals towards the GC, the molecular cloud forms a small and dense disc around SgrA*. Efficient star formation (SF) is expected to take place in such a dense disc. We model SF by means of sink particles. At ~6x10^5 yr, ~6000 solar masses of stars have formed, and are confined within a thin disc with inner and outer radius of 0.06 and 0.5 pc, respectively. Thus, this preliminary study shows that the infall of a molecular cloud is a viable scenario for the formation of massive stars around SgrA*. Further studies with more realistic radiation physics and SF will be required to better constrain this intriguing scenario.

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arXiv2007

Breathing in Low Mass Galaxies: A Study of Episodic Star Formation

We simulate the collapse of isolated dwarf galaxies using SPH + N-Body simulations including a physically motivated description of the effects of supernova feedback. As the gas collapses and stars form, the supernova feedback disrupts enough gas to temporarily quench star formation. The gas flows outward into a hot halo, where it cools until star formation can continue once more and the cycle repeats. The star formation histories of isolated Local Group dwarf galaxies exhibit similar episodic bursts of star formation. We examine the mass dependence of the stellar velocity dispersions and find that they are no less than half the velocity of the halos measured at the virial radius.

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arXiv2006

The Origin and Evolution of the Mass-Metallicity Relationship for Galaxies: Results from Cosmological N-Body Simulations

We examine the origin and evolution of the mass-metallicity relationship (MZR, M-Z) for galaxies using high resolution cosmological SPH + N-Body simulations that include a physically motivated description of supernovae feedback and subsequent metal enrichment. We discriminate between two sources that may contribute to the origin of the MZR: 1) metal and baryon loss due to gas outflow, or 2) inefficient star formation at the lowest galaxy masses. Our simulated galaxies reproduce the observed MZR in shape and normalization both at z=0 and z=2. We find that baryon loss occurs due to UV heating before star formation turns on in galaxies with M_baryon < 10^8 M_sun, but that some gas loss due to supernovae induced winds is required to subsequently reproduce the low effective chemical yield observed in low mass galaxies. Despite this, we show that low star formation efficiencies, regulated by supernovae feedback, are primarily responsible for the lower metallicities of low mass galaxies and the overall M-Z trend. We find that the shape of the MZR is relatively constant with redshift, but that its normalization increases with time. Simulations with no energy feedback from supernovae overproduce metals at low galaxy masses by rapidly transforming a large fraction of their gas into stars. Despite the fact that our low mass galaxies have lost a majority of their baryons, they are still the most gas rich objects in our simulations due to their low star formation efficiencies.

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arXiv2006

Forming Disk Galaxies in Lambda CDM Simulations

We used fully cosmological, high resolution N-body + SPH simulations to follow the formation of disk galaxies with rotational velocities between 135 and 270 km/sec in a Lambda CDM universe. The simulations include gas cooling, star formation, the effects of a uniform UV background and a physically motivated description of feedback from supernovae. The host dark matter halos have a spin and last major merger redshift typical of galaxy sized halos as measured in recent large scale N--Body simulations. The simulated galaxies form rotationally supported disks with realistic exponential scale lengths and fall on both the I-band and baryonic Tully Fisher relations. An extended stellar disk forms inside the Milky Way sized halo immediately after the last major merger. The combination of UV background and SN feedback drastically reduces the number of visible satellites orbiting inside a Milky Way sized halo, bringing it in fair agreement with observations. Our simulations predict that the average age of a primary galaxy's stellar population decreases with mass, because feedback delays star formation in less massive galaxies. Galaxies have stellar masses and current star formation rates as a function of total mass that are in good agreement with observational data. We discuss how both high mass and force resolution and a realistic description of star formation and feedback are important ingredients to match the observed properties of galaxies.

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arXiv2005

Cosmological Simulations of Galaxy Formation I: Star Formation, Feedback, Resolution and Matching the Tully--Fisher Relation (among other things)

We used fully cosmological, high resolution N-body+SPH simulations to follow the formation of disk galaxies with a rotational velocity between 140 and 280 Km/sec in a Lambda CDM universe. The simulations include gas cooling, star formation (SF), the effects of a uniform UV background and a physically motivated description of feedback from supernovae (SN). Feedback parameters have been chosen to match the star formation rate and interstellar medium (ISM) properties of local galaxies. In cosmological simulations galaxies formed rotationally supported disks with realistic exponential scale lengths and fall on the I-band and baryonic Tully Fisher relations. The combination of UV background and SN feedback drastically reduced the number of visible satellites orbiting inside a Milky Way sized halo, bringing it in fair agreement with observations. Feedback delays SF in small galaxies and more massive ones contain older stellar populations. Here we focus on the SF and feedback implementations. We also briefly discuss how high mass and force resolution and a realistic description of SF and feedback are important ingredients to match the observed properties of galaxies.

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arXiv2004

Disk Galaxy Rotation Curves in Triaxial CDM Halos

We use N-body hydrodynamical simulations to study the structure of disks in triaxial potentials resembling CDM halos. Our analysis focuses on the accuracy of the dark mass distribution inferred from rotation curves derived from simulated long-slit spectra. We consider a massless disk embedded in a halo with axis ratios of 0.5:0.6:1.0 and with its rotation axis aligned with the minor axis of the halo. Closed orbits for the gaseous particles deviate from coplanar circular symmetry, resulting in a variety of long-slit rotation curve shapes, depending on the orientation of the disk relative to the line of sight. Rotation curves may thus differ significantly from the spherically-averaged circular velocity profile of the dark matter halo. "Solid-body" rotation curves--typically interpreted as a signature of a constant density core in the dark matter distribution--are obtained about 25% of the time for random orientations although the dark matter follows the cuspy density profile proposed by Navarro, Frenk & White (NFW). We conclude that the discrepancies reported between the shape of the rotation curve of low surface brightness galaxies and the structure of CDM halos may be resolved once the complex effects of halo triaxiality on the dynamics of the gas component is properly taken into account.

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arXiv2004

The Formation of a Realistic Disk Galaxy in Lambda Dominated Cosmologies

We simulate the formation of a realistic disk galaxy within the hierarchical scenario of structure formation and study its internal properties to the present epoch. We compare results from a LambdaCDM simulation with a LambdaWDM (2keV) simulation that forms significantly less small scale structure. We show how high mass and force resolution in both the gas and dark matter components play an important role in solving the angular momentum catastrophe claimed from previous simulations of galaxy formation within the hierarchical framework. The stellar material in the disk component has a final specific angular momentum equal to 40% and 90% of that of the dark halo in the LambdaCDM and LambdaWDM models respectively. The LambdaWDM galaxy has a drastically reduced satellite population and a negligible stellar spheroidal component. Encounters with satellites play only a minor role in disturbing the disk. Satellites possess a variety of star formation histories linked to mergers and pericentric passages along their orbit around the primary galaxy. In both cosmologies, the galactic halo retains most of the baryons accreted and builds up a hot gas phase with a substantial X-ray emission. Therefore, while we have been successful in creating a realistic stellar disk in a massive galaxy within the LambdaCDM scenario, energy injection emerges as necessary ingredient to reduce the baryon fraction in galactic halos, independent of the cosmology adopted. (abridged)

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arXiv2003

On the formation of the Magellanic Stream

We use high resolution N-Body/SPH simulations to study the hydro-dynamical and gravitational interaction between the Large Magellanic Cloud and the Milky Way. We model the dark and hot extended halo components as well as the stellar/gaseous disks of the two galaxies. Tidal forces distort the LMC's disk, forcing a bar and creating a diffuse stellar halo and a strong warp, although very few stars are unbound from the LMC. Ram-pressure from a low density ionised halo is then sufficient to remove $1.4 \times 10^8M_\odot$ of gas from the LMC's disk forming a great circle trailing stream around the Galaxy.

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arXiv2002

Star Formation: 3D Collapse of Turbulent Cloud Cores

We have performed fully 3D simulations of the collapse of molecular cloud cores which obey the logatropic equation of state. By following the collapse of these cores from states of near hydrodynamic equilibrium, we are able to produce accretion histories which closely resemble those of observed cores. The accretion proceeds in four distinct stages: an initial period of very slow accretion; a period of vigorous accretion following the development of a central density singularity, with a mass accretion rate proportional to t^3, as predicted by self-similar models; a period of relatively stable, vigorous accretion; and finally a gradual decrease in the accretion rate once about 50% of the available mass of the molecular cloud core has been accreted. These results may explain the accretion histories of cores as they pass through the pre-protostellar, Class 0, and Class I stages.

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arXiv2002

The effect of non--gravitational gas heating in groups and clusters of galaxies

We present a set of gas-dynamical simulations of galaxy groups and clusters aimed at exploring the effect of non-gravitational heating. We use GASOLINE, a parallel Tree+SPH code, to simulate the formation of four cosmic halos with temperature 0.5<T<8 keV. Non-gravitational heating is implemented in two different ways: (1) by imposing a minimum entropy floor at a given redshift, 1<z<5; (2) by gradually heating gas, proportionally to the SN rate expected from semi-analytical modeling of galaxy formation. Our main results are the following. (a) An extra heating energy of about 1 keV per gas particle is required to reproduce the observed Lx-T relation, independent of whether it is provided so as to create an entropy floor of 50-100 keV cm^2, or is modulated in redshift; our SN feedback recipe provides only 1/3 keV/part. (b) The M-T relation is almost unaffected by non-gravitational heating and follows the M T^{3/2} scaling, with a normalization ~40% higher than observed, independent of the heating scheme. The inclusion of cooling in a run of a small group has the effects of increasing T_ew by ~30%, possibly reconciling simulated and observed M-T relations, and of decreasing Lx by ~40%. In spite of the inclusion of SN feedback energy, almost 40% of the gas becomes cold, in excess of current observational estimates. (abridged)

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arXiv2002

3-D Simulations of the Gravitational Collapse of Logatropic Molecular Cloud Cores

We present the results of fully 3-D hydrodynamic simulations of the gravitational collapse of isolated, turbulent molecular cloud cores. Starting from initial states of hydrostatic equilibrium, we follow the collapse of both singular and nonsingular logatropic cores until the central protostar has accreted > 90% of the total available mass. We find that, in the collapse of a singular core with access to a finite mass reservoir, the mass of the central protostar increases as M_acc proportional to t^4 until it has accreted about 35% of the total available mass. For nonsingular cores of fiducial masses 1, 2.5, and 5 M_solar, we find that protostellar accretion proceeds slowly prior to the formation of a singular density profile. Immediately thereafter, the accretion rate in each case increases to about 10^{-6} M_solar/yr, for cores with central temperature T_c= 10 K and truncation pressure P_s = 1.3E5 k_B K/cm^3. It remains at that level until half the available mass has been accreted. After this point, the accretion rate falls steadily as the remaining material is accreted onto the growing protostellar core. We suggest that this general behaviour of the protostellar accretion rate may be indicative of evolution from the Class 0 to the Class I protostellar phase.

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arXiv2001

Pre-heating the ICM in high resolution simulations: the effect on the gas entropy

We present results from high-resolution Tree+SPH simulations of galaxy clusters and groups, aimed at studying the effect of non-gravitational heating on the entropy of the ICM. We simulate three systems, having emission-weighted temperature T=0.6,1 and 3 keV, with spatial resolution better than 1% of the virial radius. We consider the effect of different prescriptions for non-gravitational ICM heating, such as SN energy feedback, as predicted by semi-analytical models of galaxy formation, and two different minimum entropy floors, S_fl=50 and 100 keV cm^2, imposed at z=3. Simulations with only gravitational heating nicely reproduce predictions from self-similar ICM models, while extra heating is shown to break the self-similarity, by a degree which depends on total injected energy and on cluster mass. We use observational results on the excess entropy in central regions of galaxy systems, to constrain the amount of extra-heating required. We find that setting the entropy floor S_fl=50 keV cm^2, which corresponds to an extra heating energy of about 1 keV per particle, is able to reproduce the observed excess of ICM entropy.

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arXiv1998

Theoretical Tools for Large Scale Structure

We review the main theoretical aspects of the structure formation paradigm which impinge upon wide angle surveys: the early universe generation of gravitational metric fluctuations from quantum noise in scalar inflaton fields; the well understood and computed linear regime of CMB anisotropy and large scale structure (LSS) generation; the weakly nonlinear regime, where higher order perturbation theory works well, and where the cosmic web picture operates, describing an interconnected LSS of clusters bridged by filaments, with membranes as the intrafilament webbing. Current CMB+LSS data favour the simplest inflation-based $Λ$CDM models, with a primordial spectral index within about 5% of scale invariant and $Ω_Λ\approx 2/3$, similar to that inferred from SNIa observations, and with open CDM models strongly disfavoured. The attack on the nonlinear regime with a variety of N-body and gas codes is described, as are the excursion set and peak-patch semianalytic approaches to object collapse. The ingredients are mixed together in an illustrative gasdynamical simulation of dense supercluster formation.

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