Study Of Galaxy Formation Research Articles

Machine learning and cosmological simulations – I. Semi-analytical models

We present a new exploratory framework to model galaxy formation and evolution in a hierarchical universe by using machine learning (ML). Our motivations are two-fold: (1) presenting a new, promising technique to study galaxy formation, and (2) quantitatively analyzing the extent of the influence of dark matter halo properties on galaxies in the backdrop of semi-analytical models (SAMs). We use the influential Millennium Simulation and the corresponding Munich SAM to train and test various sophisticated machine learning algorithms (k-Nearest Neighbors, decision trees, random forests and extremely randomized trees). By using only essential dark matter halo physical properties for haloes of $M>10^{12} M_{\odot}$ and a partial merger tree, our model predicts the hot gas mass, cold gas mass, bulge mass, total stellar mass, black hole mass and cooling radius at z = 0 for each central galaxy in a dark matter halo for the Millennium run. Our results provide a unique and powerful phenomenological framework to explore the galaxy-halo connection that is built upon SAMs and demonstrably place ML as a promising and a computationally efficient tool to study small-scale structure formation.

On the nature of the barlens component in barred galaxies: what do boxy/peanut bulges look like when viewed face-on?

Barred galaxies have interesting morphological features whose presence and properties set constraints on galactic evolution. Here we examine barlenses, i.e. lens-like components whose extent along the bar major axis is shorter than that of the bar and whose outline is oval or circular. We identify and analyse barlenses in $N$-body plus SPH simulations, compare them extensively with those from the NIRS0S (Near-IR S0 galaxy survey) and the S$^4$G samples (Spitzer Survey of Stellar Structure in Galaxies) and find very good agreement. We observe barlenses in our simulations from different viewing angles. This reveals that barlenses are the vertically thick part of the bar seen face-on, i.e. a barlens seen edge-on is a boxy/peanut/X bulge. In morphological studies, and in the absence of kinematics or photometry, a barlens, or part of it, may be mistaken for a classical bulge. Thus the true importance of classical bulges, both in numbers and mass, is smaller than currently assumed, which has implications for galaxy formation studies. Finally, using the shape of the isodensity curves, we propose a rule of thumb for measuring the barlens extent along the bar major axis of moderately inclined galaxies, thus providing an estimate of which part of the bar is thicker.

Lensing measurements of the mass distribution in SDSS voids

We measure weak lensing mass profiles of voids from a volume-limited sample of SDSS Luminous Red Galaxies (LRGs). We find voids using an algorithm designed to maximize the lensing signal by dividing the survey volume into 2D slices, and then finding holes in this 2D distribution of LRGs. We perform a stacked shear measurement on about 20,000 voids with radii between 15-55 Mpc/h and redshifts between 0.16-0.37. We measure the characteristic radial shear signal of voids with a signal-to-noise of 7. The mass profile corresponds to a fractional underdensity of about -0.4 inside the void radius and a slow approach to the mean density indicating a partially compensated void structure. We compare our measured shape and amplitude with the predictions of Krause et al 2013. Voids in the galaxy distribution have been extensively modeled using simulations and measured in the SDSS. We discuss how the addition of void mass profiles can enable studies of galaxy formation and cosmology.

Extended halos and intracluster light using Planetary Nebulae as tracers in nearby clusters

AbstractSince the first detection of intracluster planetary nebulae in 1996, imaging and spectroscopic surveys identified such stars to trace the radial extent and the kinematics of diffuse light in clusters. This topic of research is tightly linked with the studies of galaxy formation and evolution in dense environment, as the spatial distribution and kinematics of planetary nebulae in the outermost regions of galaxies and in the cluster cores is relevant for setting constraints on cosmological simulations. In this sense, extragalactic planetary nebulae play a very important role in the near-field cosmology, in order to measure the integrated mass as function of radius and the orbital distribution of stars in structures placed in the densest regions of the nearby universe.

Extended halos and intracluster light using Planetary Nebulae as tracers in nearby clusters

AbstractSince the first detection of intracluster planetary nebulae in 1996, imaging and spectroscopic surveys identified such stars to trace the radial extent and the kinematics of diffuse light in clusters. This topic of research is tightly linked with the studies of galaxy formation and evolution in dense environment, as the spatial distribution and kinematics of planetary nebulae in the outermost regions of galaxies and in the cluster cores is relevant for setting constraints on cosmological simulations. In this sense, extragalactic planetary nebulae play a very important role in the near-field cosmology, in order to measure the integrated mass as function of radius and the orbital distribution of stars in structures placed in the densest regions of the nearby universe.

Dusty Starbursts within a z=3 Large Scale Structure revealed by ALMA

AbstractThe role of the large-scale structure is one of the most important theme in studying galaxy formation and evolution. However, it has been still mystery especially at z>2. On the basis of our ALMA 1.1 mm observations in a z ~ 3 protocluster field, it is suggested that submillimeter galaxies (SMGs) preferentially reside in the densest environment at z ~ 3. Furthermore we find a rich cluster of AGN-host SMGs at the core of the protocluster, combining with Chandra X-ray data. Our results indicate the vigorous star-formation and accelerated super massive black hole (SMBH) growth in the node of the cosmic web.

USING MEMBER GALAXY LUMINOSITIES AS HALO MASS PROXIES OF GALAXY GROUPS

Reliable halo mass estimation for a given galaxy system plays an important role both in cosmology and galaxy formation studies. Here we set out to find the way that can improve the halo mass estimation for those galaxy systems with limited brightest member galaxies been observed. Using four mock galaxy samples constructed from semi-analytical formation models, the subhalo abundance matching method and the conditional luminosity functions, respectively, we find that the luminosity gap between the brightest and the subsequent brightest member galaxies in a halo (group) can be used to significantly reduce the scatter in the halo mass estimation based on the luminosity of the brightest galaxy alone. Tests show that these corrections can significantly reduce the scatter in the halo mass estimations by $\sim 50\%$ to $\sim 70\%$ in massive halos depending on which member galaxies are considered. Comparing to the traditional ranking method, we find that this method works better for groups with less than five members, or in observations with very bright magnitude cut.

Local Group dSph radio survey with ATCA – I: observations and background sources

Dwarf spheroidal (dSph) galaxies are key objects in near-field cosmology, especially in connection to the study of galaxy formation and evolution at small scales. In addition, dSphs are optimal targets to investigate the nature of dark matter. However, while we begin to have deep optical photometric observations of the stellar population in these objects, little is known so far about their diffuse emission at any observing frequency, and hence on thermal and non-thermal plasma possibly residing within dSphs. In this paper, we present deep radio observations of six local dSphs performed with the Australia Telescope Compact Array at 16 cm wavelength. We mosaiced a region of radius of about one degree around three "classical" dSphs, Carina, Fornax, and Sculptor, and of about half of degree around three "ultra-faint" dSphs, BootesII, Segue2, and Hercules. The rms noise level is below 0.05 mJy for all the maps. The restoring beams FWHM ranged from 4.2 x 2.5 arcseconds to 30.0 x 2.1 arcseconds in the most elongated case. A catalogue including the 1392 sources detected in the six dSph fields is reported. The main properties of the background sources are discussed, with positions and fluxes of brightest objects compared with the FIRST, NVSS, and SUMSS observations of the same fields. The observed population of radio emitters in these fields is dominated by synchrotron sources. We compute the associated source number counts at 2 GHz down to fluxes of 0.25 mJy, which prove to be in agreement with AGN count models.

DISCOVERY OF A VERY LARGE STRUCTURE ATZ= 3.78

We report the discovery of a large-scale structure containing multiple protoclusters at z=3.78 in the Bo\"otes field. The spectroscopic discovery of five galaxies at z=3.783+/-0.002 lying within 1 Mpc of one another led us to undertake a deep narrow- and broad-band imaging survey of the surrounding field. Within a comoving volume of 72x72x25 Mpc^3, we have identified 65 Lyman alpha emitter (LAE) candidates at z=3.795+/-0.015, and four additional galaxies at z_spec=3.730,3.753,3.780,3.835. The galaxy distribution within the field is highly non-uniform, exhibiting three large (~3-5x) overdensities separated by 8-14 Mpc (physical) and possibly connected by filamentary structures traced by LAEs. The observed number of LAEs in the entire field is nearly twice the average expected in field environments, based on estimates of the Lya luminosity function at these redshifts. We estimate that by z=0 the largest overdensity will grow into a cluster of mass 10^15 Msun; the two smaller overdensities will grow into clusters of mass (2-6)x10^14 Msun. The highest concentration of galaxies is located at the southern end of the image, suggesting that the current imaging may not map the true extent of the large scale structure. Finding three large protocluster candidates within a single 0.3 deg^2 field is highly unusual; expectations from theory suggest that such alignments should occur less than 2% of the time. Searching for and characterizing such structures and accurately measuring their volume space density can therefore place constraints on the theory of structure formation. Such regions can also serve as laboratories for the study of galaxy formation in dense environments.

The EAGLE project: simulating the evolution and assembly of galaxies and their environments

We introduce the Virgo Consortium's EAGLE project, a suite of hydrodynamical simulations that follow the formation of galaxies and black holes in representative volumes. We discuss the limitations of such simulations in light of their finite resolution and poorly constrained subgrid physics, and how these affect their predictive power. One major improvement is our treatment of feedback from massive stars and AGN in which thermal energy is injected into the gas without the need to turn off cooling or hydrodynamical forces, allowing winds to develop without predetermined speed or mass loading factors. Because the feedback efficiencies cannot be predicted from first principles, we calibrate them to the z~0 galaxy stellar mass function and the amplitude of the galaxy-central black hole mass relation, also taking galaxy sizes into account. The observed galaxy mass function is reproduced to $\lesssim 0.2$ dex over the full mass range, $10^8 < M_*/M_\odot \lesssim 10^{11}$, a level of agreement close to that attained by semi-analytic models, and unprecedented for hydrodynamical simulations. We compare our results to a representative set of low-redshift observables not considered in the calibration, and find good agreement with the observed galaxy specific star formation rates, passive fractions, Tully-Fisher relation, total stellar luminosities of galaxy clusters, and column density distributions of intergalactic CIV and OVI. While the mass-metallicity relations for gas and stars are consistent with observations for $M_* \gtrsim 10^9 M_\odot$, they are insufficiently steep at lower masses. The gas fractions and temperatures are too high for clusters of galaxies, but for groups these discrepancies can be resolved by adopting a higher heating temperature in the subgrid prescription for AGN feedback. EAGLE constitutes a valuable new resource for studies of galaxy formation.

Discovery of a rich proto-cluster atz= 2.9 and associated diffuse cold gas in the VIMOS Ultra-Deep Survey (VUDS)

[Abridged] We characterise a massive proto-cluster at z=2.895 that we found in the COSMOS field using the spectroscopic sample of the VIMOS Ultra-Deep Survey (VUDS). This is one of the rare structures at z~3 not identified around AGNs or radio galaxies, so it is an ideal laboratory to study galaxy formation in dense environments. The structure comprises 12 galaxies with secure spectroscopic redshift in an area of 7'x8', in a z bin of Dz=0.016. The measured galaxy number overdensity is delta_g=12+/-2. This overdensity has total mass of M~8.1x10^(14)M_sun in a volume of 13x15x17 Mpc^3. Simulations indicate that such an overdensity at z~2.9 is a proto-cluster that will collapse in a cluster of total mass M~2.5x10^(15)M_sun at z=0. We compare the properties of the galaxies within the overdensity with a control sample at the same z but outside the overdensity. We did not find any statistically significant difference between the properties (stellar mass, SFR, sSFR, NUV-r, r-K) of the galaxies inside and outside the overdensity. The stacked spectrum of galaxies in the overdensity background shows a significant absorption feature at the wavelength of Lya redshifted at z=2.895 (lambda=4736 A), with a rest frame EW = 4+/- 1.4 A. Stacking only background galaxies without intervening sources at z~2.9 along their line of sight, we find that this absorption feature has a rest frame EW of 10.8+/-3.7 A, with a detection S/N of ~4. These EW values imply a high column density (N(HI)~3-20x10^(19)cm^(-2)), consistent with a scenario where such absorption is due to intervening cold gas streams, falling into the halo potential wells of the proto-cluster galaxies. However, we cannot exclude the hypothesis that this absorption is due to the diffuse gas within the overdensity.

Disk Galaxies in the Magneticum Pathfinder Simulations

AbstractWe presentMagneticum Pathfinder, a new set of hydrodynamical cosmological simulations covering a large range of cosmological scales. Among the important physical processes included in the simulations are the chemical and thermodynamical evolution of the diffuse gas as well as the evolution of stars and black holes and the corresponding feedback channels. In the high resolution boxes aimed at studies of galaxy formation and evolution, populations of both disk and spheroidal galaxies are self-consistently reproduced. These galaxy populations match the observed stellar mass function and show the same trends for disks and spheroids in the mass–size relation as observations from the SDSS. Additionally, we demonstrate that the simulated galaxies successfully reproduce the observed specific angular-momentum–mass relations for the two different morphological types of galaxies. In summary, theMagneticum Pathfindersimulations are a valuable tool for studying the assembly of cosmic and galactic structures in the universe.

The ALHAMBRA survey: reliable morphological catalogue of 22 051 early- and late-type galaxies

Advanced Large Homogeneous Area Medium Band Redshift Astronomical (ALHAMBRA) is photometric survey designed to trace the cosmic evolution and cosmic variance. It covers a large area of ∼4 deg2 in eight fields, where seven fields overlap with other surveys, allowing us to have complementary data in other wavelengths. All observations were carried out in 20 continuous, medium band (30 nm width) optical and 3 near-infrared (JHK) bands, providing the precise measurements of photometric redshifts. In addition, morphological classification of galaxies is crucial for any kind of galaxy formation and cosmic evolution studies, providing the information about star formation histories, their environment and interactions, internal perturbations, etc. We present a morphological classification of >40 000 galaxies in the ALHAMBRA survey. We associate to every galaxy a probability to be early type using the automated Bayesian code galsvm. Despite of the spatial resolution of the ALHAMBRA images (∼1 arcsec), for 22 051 galaxies, we obtained the contamination by other type of less than 10 per cent. Of those, 1640 and 10 322 galaxies are classified as early- (down to redshifts ∼0.5) and late-type (down to redshifts ∼1.0), respectively, with magnitudes F613W ≤ 22.0. In addition, for magnitude range 22.0 < F613W ≤ 23.0, we classified other 10 089 late-type galaxies with redshifts ≤1.3. We show that the classified objects populate the expected regions in the colour–mass and colour–magnitude planes. The presented data set is especially attractive given the homogeneous multiwavelength coverage available in the ALHAMBRA fields, and is intended to be used in a variety of scientific applications. The low-contamination catalogue (<10 per cent) is made publicly available with this paper.

Cool Gas in High-Redshift Galaxies

Over the past decade, observations of the cool interstellar medium (ISM) in distant galaxies via molecular and atomic fine structure line (FSL) emission have gone from a curious look into a few extreme, rare objects to a mainstream tool for studying galaxy formation out to the highest redshifts. Molecular gas has been observed in close to 200 galaxies at z &gt; 1, including numerous AGN host-galaxies out to z ∼ 7, highly star-forming submillimeter galaxies, and increasing samples of main-sequence color-selected star-forming galaxies at z ∼ 1.5 to 2.5. Studies have moved well beyond simple detections to dynamical imaging at kiloparsec-scale resolution and multiline, multispecies studies that determine the physical conditions in the ISM in early galaxies. Observations of the cool gas are the required complement to studies of the stellar density and star-formation history of the Universe as they reveal the phase of the ISM that immediately precedes star formation in galaxies. Current observations suggest that the order of magnitude increase in the cosmic star-formation rate density from z ∼ 0 to 2 is commensurate with a similar increase in the gas-to-stellar mass ratio in star-forming disk galaxies. Progress has been made in determining the CO luminosity to H2 mass conversion factor at high z, and the dichotomy between high versus low values for main-sequence versus starburst galaxies, respectively, appears to persist with increasing redshift, with a likely dependence on metallicity and other local physical conditions. There may also be two sequences in the relationship between star-formation rate and gas mass: one for starbursts, in which the gas consumption timescale is short (a few 107 years), and one for main sequence galaxies, with an order of magnitude longer gas consumption timescale. Studies of atomic FSL emission are rapidly progressing, with some tens of galaxies detected in the exceptionally bright [Cii] 158-μm line to date. The [Cii] line is proving to be a unique tracer of galaxy dynamics in the early Universe and, together with other atomic FSLs, has the potential to be the most direct means of obtaining spectroscopic redshifts for the first galaxies during cosmic reionization.

AN INTERACTING GALAXY SYSTEM ALONG A FILAMENT IN A VOID

Cosmological voids provide a unique environment for the study of galaxy formation and evolution. The galaxy population in their interior have significantly different properties than average field galaxies. As part of our Void Galaxy Survey (VGS), we have found a system of three interacting galaxies (VGS_31) inside a large void. VGS_31 is a small elongated group whose members are embedded in a common HI envelope. The HI picture suggests a filamentary structure with accretion of intergalactic cold gas from the filament onto the galaxies. We present deep optical and narrow band H_alpha data, optical spectroscopy, near-UV and far-UV GALEX and CO(1-0) data. We find that one of the galaxies, a Markarian object, has a ring-like structure and a tail evident both in optical and HI. While all three galaxies form stars in their central parts, the tail and the ring of the Markarian object are devoid of star formation. We discuss these findings in terms of a gravitational interaction and ongoing growth of galaxies out of a filament. VGS_31 is one of the first observed examples of a filamentary structure in a void. It is an important prototype for understanding the formation of substructure in a void. This system also shows that the galaxy evolution in voids can be as dynamic as in high density environments.

TRACING GALAXIES THROUGH COSMIC TIME WITH NUMBER DENSITY SELECTION

A central challenge in observational studies of galaxy formation is how to associate progenitor galaxies with their descendants at lower redshifts. One promising approach is to link galaxies at fixed number density, rather than fixed luminosity or mass. This method is effective if stellar mass rank order is broadly conserved through cosmic time. In this paper, we use the Guo et al. (2011) semi-analytical model to analyze under what circumstances this assumption is valid in the context of a cosmological simulation. Specifically, we select progenitor galaxies at a constant number density and compare the stellar mass evolution of their descendants to the evolution at a constant number density. The median stellar mass of the descendants increases by a factor of four (0.6 dex) from z = 3 to z = 0. Constant number density selection reproduces this to within 40% (0.15 dex) over a wide range of number densities. We show that the discrepancy primarily results from scatter in the stellar mass growth rates and merging. After applying simple, observationally-based corrections for these processes, the discrepancy is reduced to 12% (0.05 dex). We conclude that number density selection can be used to predict the median descendant mass of high-redshift progenitor galaxies. The main uncertainty in this study is that semi-analytical models do not reproduce the observed mass evolution of galaxies, which makes the quantitative aggregate effects of star formation, merging, and quenching on the rank order of galaxies somewhat uncertain.

Dwarf galaxies coherently orbit Andromeda

The Milky Way's nearby twin is a good place to study galaxy formation. The Milky Way's nearby twin is a good place to study galaxy formation.

GMASS ultradeep spectroscopy of galaxies atz ~ 2

The populations of both quiescent and actively star-forming galaxies at 1<z<2 are still under-represented in our spectroscopic census of galaxies throughout the history of the Universe. In the light of galaxy formation models, however, the evolution of galaxies at these redshifts is of pivotal importance and merits further investigation. We therefore designed a spectroscopic observing campaign of a sample of both massive, quiescent and star-forming galaxies at z>1.4, called Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS). To determine redshifts and physical properties, such as metallicity, dust content, dynamical masses, and star formation history, we performed ultra-deep spectroscopy with the red-sensitive optical spectrograph FORS2 at the VLT. Our sample consists of objects, within the CDFS/GOODS area, detected at 4.5 micron, to be sensitive to stellar mass rather than star formation intensity. The spectroscopic targets were selected with a photometric redshift constraint (z>1.4) and magnitude constraints (B(AB)<26, I(AB)<26.5), which should ensure that these are faint, distant, and fairly massive galaxies. We present the sample selection, survey design, observations, data reduction, and spectroscopic redshifts. Up to 30 hours of spectroscopy of 174 spectroscopic targets and 70 additional objects enabled us to determine 210 redshifts, of which 145 are at z>1.4. From the redshifts and photometry, we deduce that the BzK selection criteria are efficient (82%) and suffer low contamination (11%). Several papers based on the GMASS survey show its value for studies of galaxy formation and evolution. We publicly release the redshifts and reduced spectra. In combination with existing and on-going additional observations in CDFS/GOODS, this data set provides a legacy for future studies of distant galaxies.

Moving mesh cosmology: numerical techniques and global statistics

We present the first hydrodynamical simulations of structure formation using the new moving mesh code AREPO and compare the results with GADGET simulations based on a traditional smoothed particle hydrodynamics (SPH) technique. The two codes share the same Tree-PM gravity solver and include identical sub-resolution physics, but employ different methods to solve the equations of hydrodynamics. This allows us to assess the impact of hydro-solver uncertainties on the results of cosmological studies of galaxy formation. We focus on predictions for global baryon statistics, such as the cosmic star formation rate density, after we introduce our simulation suite and numerical methods. Properties of individual galaxies and haloes are examined by Keres et al. (2011), while a third paper by Sijacki et al. (2011) uses idealised simulations to analyse the differences between the hydrodynamical schemes. We find that the global baryon statistics differ significantly between the two simulation approaches. AREPO shows higher star formation rates at late times, lower mean temperatures, and different gas mass fractions in characteristic phases of the intergalactic medium, in particular a reduced amount of hot gas. Although both codes use the same implementation of cooling, more gas cools out of haloes in AREPO compared with GADGET towards low redshifts. We show that this is caused by a higher heating rate with SPH in the outer parts of haloes, owing to viscous dissipation of SPH's inherent sonic velocity noise and SPH's efficient damping of subsonic turbulence injected in the halo infall region, and because of a higher efficiency of gas stripping in AREPO. As a result of such differences, AREPO leads also to more disk-like morphologies compared to GADGET. Our results indicate that inaccuracies in hydrodynamic solvers can lead to comparatively large systematic differences.

Moving-mesh cosmology: characteristics of galaxies and haloes

We discuss cosmological hydrodynamic simulations of galaxy formation performed with the new moving-mesh code arepo, which promises higher accuracy compared with the traditional smoothed particle hydrodynamics (SPH) technique that has been widely employed for this problem. In this exploratory study, we deliberately limit the complexity of the physical processes followed by the code for ease of comparison with previous calculations, and include only cooling of gas with a primordial composition, heating by a spatially uniform ultraviolet background, and a simple subresolution model for regulating star formation in the dense interstellar medium. We use an identical set of physics in corresponding simulations carried out with the well-tested SPH code gadget, adopting also the same high-resolution gravity solver. We are thus able to compare both simulation sets on an object-by-object basis, allowing us to cleanly isolate the impact of different hydrodynamical methods on galaxy and halo properties. In accompanying papers, Vogelsberger et al. and Sijacki et al., we focus on an analysis of the global baryonic statistics predicted by the simulation codes, and complementary idealized simulations that highlight the differences between the hydrodynamical schemes. Here we investigate their influence on the baryonic properties of simulated galaxies and their surrounding haloes. We find that arepo leads to significantly higher star formation rates for galaxies in massive haloes and to more extended gaseous discs in galaxies, which also feature a thinner and smoother morphology than their gadget counterparts. Consequently, galaxies formed in arepo have larger sizes and higher specific angular momentum than their SPH correspondents. Interestingly, the more efficient cooling flows in arepo yield higher densities and lower entropies in halo centres compared to gadget, whereas the opposite trend is found in halo outskirts. The cooling differences leading to higher star formation rates of massive galaxies in arepo also slightly increase the baryon content within the virial radius of massive haloes. We show that these differences persist as a function of numerical resolution. While both codes agree to acceptable accuracy on a number of baryonic properties of cosmic structures, our results thus clearly demonstrate that galaxy formation simulations greatly benefit from the use of more accurate hydrodynamical techniques such as arepo and call into question the reliability of galaxy formation studies in a cosmological context using traditional standard formulations of SPH, such as the one implemented in gadget. Our new moving-mesh simulations demonstrate that a population of extended gaseous discs of galaxies in large volume cosmological simulations can be formed even without energetic feedback in the form of galactic winds, although such outflows appear required to obtain realistic stellar masses.