Internal Properties Of Galaxies Research Articles

Lopsided galaxies in a cosmological context: a new galaxy–halo connection

ABSTRACT Disc galaxies commonly show asymmetric features in their morphology, such as warps and lopsidedness. These features can provide key information regarding the recent evolution of a given disc galaxy. In the nearby Universe, up to ∼30 per cent of late-type galaxies display a global non-axisymmetric lopsided mass distribution. However, the origin of this perturbation is not well understood. In this work, we study the origin of lopsided perturbations in simulated disc galaxies extracted from the TNG50 simulation of the IllustrisTNG project. We statistically explore different excitation mechanisms for this perturbation, such as direct satellite tidal interactions and distortions of the underlying dark matter distributions. We also characterize the main physical conditions that lead to lopsided perturbations. 50 per cent of our sample galaxy have lopsided modes m = 1 greater than ∼0.12. We find a strong correlation between internal galaxy properties, such as central stellar surface density and disc radial extension with the strength of lopsided modes. The majority of lopsided galaxies have lower central surface densities and more extended discs than symmetric galaxies. As a result, such lopsided galaxies are less self-gravitationally cohesive, and their outer disc region is more susceptible to different types of external perturbations. However, we do not find strong evidence that tidal interactions with satellite galaxies are the main driving agent of lopsided modes. Lopsided galaxies tend to live in asymmetric dark matter haloes with high spin, indicating strong galaxy–halo connections in late-type lopsided galaxies.

The origin of star–gas misalignments in simulated galaxies

ABSTRACT We study the origin of misalignments between the stellar and star-forming gas components of simulated galaxies in the eagle simulations. We focus on galaxies with stellar masses ≥109 M⊙ at 0 ≤ z ≤ 1. We compare the frequency of misalignments with observational results from the SAMI survey and find that overall, eagle can reproduce the incidence of misalignments in the field and clusters, as well as the dependence on stellar mass and optical colour within the uncertainties. We study the dependence on kinematic misalignments with internal galaxy properties and different processes related to galaxy mergers and sudden changes in stellar and star-forming gas mass. We find that galaxy mergers happen in similar frequency in mis- and aligned galaxies, with the main difference being misaligned galaxies showing a higher tidal field strength and fraction of ex situ stars. We find that despite the environment being relevant in setting the conditions to misalign the star-forming gas, the properties internal to galaxies play a crucial role in determining whether the gas quickly aligns with the stellar component or not. Hence, galaxies that are more triaxial and more dispersion dominated display more misalignments because they are inefficient at realigning the star-forming gas towards the stellar angular momentum vector.

Retrieving Internal Kinematics of Galaxies with Deep Learning Using Single-band Optical Images

Abstract Using deep machine learning we show that the internal velocities of galaxies can be retrieved from optical images trained using 4596 systems observed with the SDSS-MaNGA survey. Using only i-band images we show that the velocity dispersions and the rotational velocities of galaxies can be measured to an accuracy of 29 km s−1 and 69 km s−1 respectively, close to the resolution limit of the spectroscopic data. This shows that galaxy structures in the optical holds important information concerning the internal properties of galaxies, and that the internal kinematics of galaxies are quantitatively reflected in their stellar light distributions.

The neutral hydrogen properties of galaxies in gas-rich groups

We present an analysis of the integrated neutral hydrogen (HI) properties for 27 galaxies within nine low mass, gas-rich, late-type dominated groups which we denote "Choirs". We find that majority of the central Choir galaxies have average HI content: they have a normal gas-mass fraction with respect to isolated galaxies of the same stellar mass. In contrast, we find more satellite galaxies with a lower gas-mass fraction than isolated galaxies of the same stellar mass. A likely reason for the lower gas content in these galaxies is tidal stripping. Both the specific star formation rate and the star formation efficiency of the central group galaxies are similar to galaxies in isolation. The Choir satellite galaxies have similar specific star formation rate as galaxies in isolation, therefore satellites that exhibit a higher star formation efficiency simply owe it to their lower gas-mass fractions. We find that the most HI massive galaxies have the largest HI discs and fall neatly onto the HI size-mass relation, while outliers are galaxies that are experiencing interactions. We find that high specific angular momentum could be a reason for galaxies to retain the large fraction of HI gas in their discs. This shows that for the Choir groups with no evidence of interactions, as well as those with traces of minor mergers, the internal galaxy properties dominate over the effects of residing in a group. The probed galaxy properties strengthen evidence that the Choir groups represent the early stages of group assembly.

Investigating the star formation histories of the brightest cluster galaxies

This article is devoted to the study of the central stellar populations of the brightest cluster galaxies (BCGs). High signal-to-noise ratio, long-slit spectra for a sample of 39 galaxies were fitted against two stellar population models, Pegase.HR (P.HR) and Vazdekis/MILES, to determine the star formation histories of the galaxies using full spectrum fitting, to investigate, in particular, whether a single stellar population (SSP) or composite stellar population model provides the better description. Monte Carlo simulations and χ2 maps were used to check the reliability of the solutions. The ages and [Fe/H] were (i) compared with those derived with the Lick indices and (ii) tested against the internal galaxy properties (the velocity dispersions and absolute K-band magnitudes) and the properties of the host cluster environment (X-ray temperatures, luminosities, offsets and the presence of cooling flows (CFs)), to determine whether any statistically significant correlations existed. The results indicate that 79 per cent of the BCG sample could be represented by an SSP fit, while the remaining 21 per cent likely experienced more than one star formation epoch. The correlations showed that the BCGs hosted in CF clusters are generally found closer to the centre of the clusters, while the BCGs in non-CF clusters are generally found further away. The main results suggested that at least some of the galaxies in the BCG sample had a more complex star formation history than first assumed and that the presence of CFs in the clusters could account for some, but not all, of the star formation activity of BCGs.

On the orbital and internal evolution of cluster galaxies

Galaxies inhabiting a cluster environment experience significant evolution in their orbital motions throughout time; this is accompanied by changes in the anisotropy parameter, measuring the relative importance of radial and tangential motions for a given class of objects. Along with orbital changes, galaxies in clusters are well known to undergo severe alteration in their hot/cold gas content and star formation properties. Understanding the link between the changes in the internal properties of galaxies and their orbital motion is of crucial importance in the study of galaxy evolution, as it could unveil the primary mechanism responsible for its environmental dependence. Do the changes in the internal properties happen in parallel with those in the orbital motion? Or are the orbital features at the time of infall that determines the fate of the member galaxies? Alternatively: are the properties of galaxies at a given time related to the coeval orbital anisotropy or are they better related to the anisotropy at infall? In order to answer these questions, we studied the orbital evolution of different galaxy populations in the semi-analytic models of Guo et al. applied on to the Millennium Simulation. For each class of objects, characterized by different internal properties (such as age, star formation rate and colour), we studied the anisotropy profile at redshift zero and its evolution by tracing the progenitors back in time. We conclude that the orbital properties at infall strongly influence the subsequent evolution of the internal features of galaxies and that the overall anisotropy of the galaxy population tends to increase with time.

Star formation quenching in galaxies

We investigate the correlation of star formation quenching with internal galaxy properties and environment by comparing observation and theoretical models. We first classify galaxies as the most massive “central” or “satellite” in each halo in order to investigate the environmental effect. For observed central galaxies, we are unable to determine whether star formation quenching is primarily connected with halo mass or stellar mass, because these two quantities are strongly correlated. For satellite galaxies, a nearly equal dependence on halo mass and stellar mass is seen. We find that theoretical models with AGN feedback reproduce the dependence on colours and specific star formation rates for central galaxies reasonably. However, the same models seriously fail to reproduce the star formation rates of satellite galaxies by over-quenching star formation via strong strangulation: satellite over-quenching problem .

THE EFFECT OF GALACTIC PROPERTIES ON THE ESCAPE FRACTION OF IONIZING PHOTONS

The escape fraction, fesc, of ionizing photons from early galaxies is a crucial parameter for determining whether the observed galaxies at z ⩾ 6 are able to reionize the high-redshift intergalactic medium. Previous attempts to measure fesc have found a wide range of values, varying from less than 0.01 to nearly 1. Rather than finding a single value of fesc, we clarify through modeling how internal properties of galaxies affect fesc through the density and distribution of neutral hydrogen within the galaxy, along with the rate of ionizing photons' production. We find that the escape fraction depends sensitively on the covering factor of clumps, along with the density of the clumped and interclump medium. One must therefore be cautious when dealing with an inhomogeneous medium. Fewer high-density clumps lead to a greater escape fraction than more numerous low-density clumps. When more ionizing photons are produced in a starburst, fesc increases, as photons escape more readily from the gas layers. Large variations in the predicted escape fraction, caused by differences in the hydrogen distribution, may explain the large observed differences in fesc among galaxies. Values of fesc must also be consistent with the reionization history. High-mass galaxies alone are unable to reionize the universe, because fesc >1 would be required. Small galaxies are needed to achieve reionization, with greater mean escape fraction in the past.

Galaxy Zoo: the properties of merging galaxies in the nearby Universe - local environments, colours, masses, star formation rates and AGN activity

Following the study of Darg et al., we explore the environments, optical colours, stellar masses, star formation and active galactic nucleus activity in a sample of 3003 pairs of merging galaxies drawn from the Sloan Digital Sky Survey using visual classifications from the Galaxy Zoo project. While Darg et al. found that the spiral-to-elliptical ratio in (major) mergers appeared higher than that of the global galaxy population, no significant differences are found between the environmental distributions of mergers and a randomly selected control sample. This makes the high occurrence of spirals in mergers unlikely to be an environmental effect and must therefore arise from differing time-scales of detectability for spirals and ellipticals. We find that merging galaxies have a wider spread in colour than the global galaxy population, with a significant blue tail resulting from intense star formation in spiral mergers. Galaxies classed as star-forming using their emission-line properties have average star formation rates approximately doubled by the merger process though star formation is negligibly enhanced in merging elliptical galaxies. We conclude that the internal properties of galaxies significantly affect the time-scales over which merging systems can be detected (as suggested by recent theoretical studies) which leads to spirals being ‘over-observed’ in mergers. We also suggest that the transition mass 3 × 1010 M⊙, noted by Kauffmann et al., below which ellipticals are rare could be linked to disc survival/destruction in mergers.

The correlation of star formation quenching with internal galaxy properties and environment

We investigate the correlation of star formation quenching with internal galaxy properties and large-scale environment (halo mass) in empirical data and theoretical models. We make use of the halo-based group catalogue of Yang and collaborators, which is based on the Sloan Digital Sky Survey. Data from the Galaxy evolution explorer are also used to extract the recent star formation rate. In order to investigate the environmental effects, we examine the properties of 'central' and 'satellite' galaxies separately. For central galaxies, we are unable to conclude whether star formation quenching is primarily connected with halo mass or stellar mass, because these two quantities are themselves strongly correlated. For satellite galaxies, a nearly equally strong dependence on halo mass and stellar mass is seen. We make the same comparison for five different semi-analytic models based on three independently developed codes. We find that the models with active galactic nuclei feedback reproduce reasonably well the dependence of the fraction of central red and passive galaxies on halo mass and stellar mass. However, for satellite galaxies, the same models badly overproduce the fraction of red/passive galaxies and do not reproduce the empirical trends with stellar mass or halo mass. This satellite overquenching problem is caused by the too-rapid stripping of the satellites' hot gas haloes, which leads to rapid strangulation of star formation.

Galaxy pairs in the 2dF survey - II. Effects of interactions on star formation in groups and clusters

We assess the effects of galaxy–galaxy interactions on star formation (SF) by analysing pairs in groups and clusters of galaxies with virial masses in the range 1013–1015 M⊙ extracted from the 2dF Galaxy Redshift Survey (2dFGRS). We find that galaxy pairs with relative separation rp < 15 h−1 kpc show significant SF activity in comparison with other group member galaxies. Similar results were found for galaxy pairs in the field (rp < 25 h−1 kpc) so that the nature of SF driven by galaxy interactions is nearly independent of environment, in spite of the general lower level of SF activity in systems of galaxies. The above results reflect, on the one hand, the local nature of SF induced by tidal interactions and, on the other, the role played by the internal properties of galaxies. By using a 2dFGRS mock catalogue we estimate the contamination by spurious pairs, finding that our statistics are dominated by real pairs, in particular for close relative separations, for which our study indicates significant SF activity. We obtain a similar radial and relative velocity distribution of the pairs with respect to the group centre compared with those of other typical group members, so that galaxy pairs have no particular location and dynamics within groups. Except for pairs with small relative separations, which show significant SF activity, we find that pairs in groups are systematically redder and have a lower present‐day SF activity than other group members.

The Seyfert Population in the Local Universe

The magnitude-limited catalog of the Southern Sky Redshift Survey (SSRS2) is used to characterize the properties of galaxies hosting active galactic nuclei (AGNs). Using emission-line ratios, we identify a total of 162 (3%) Seyfert galaxies out of the parent sample with 5399 galaxies. The sample contains 121 Seyfert 2 galaxies and 41 Seyfert 1 galaxies. The SSRS2 Seyfert galaxies are predominantly in spirals of types Sb and earlier or in galaxies with perturbed appearance as the result of strong interactions or mergers. Seyfert galaxies in this sample are twice as common in barred hosts as the non-Seyfert galaxies. By assigning galaxies to groups using a percolation algorithm, we find that the Seyfert galaxies in the SSRS2 are more likely to be found in binary systems when compared with galaxies in the SSRS2 parent sample. However, there is no statistically significant difference between the Seyfert and SSRS2 parent sample when systems with more than two galaxies are considered. The analysis of the present sample suggests that there is a stronger correlation between the presence of the AGN phenomenon with internal properties of galaxies (morphology, presence of bar, luminosity) than with environmental effects (local galaxy density, group velocity dispersion, nearest neighbor distance).

SECULAR EVOLUTION OF SPIRAL GALAXIES

It is now a well established fact that galaxies undergo significant morphological transformation during their lifetimes, manifesting as an evolution along the Hubble sequence from the late to the early Hubble types. The physical processes commonly believed to be responsible for this observed evolution trend, i.e. the major and minor mergers, as well as gas accretion under a barred potential, though demonstrated applicability to selected types of galaxies, on the whole have failed to reproduce the most important statistical and internal properties of galaxies. The secular evolution mechanism reviewed in this paper has the potential to overcome most of the known difficulties of the existing theories to provide a natural and coherent explanation of the properties of present day as well as high-redshift galaxies.

Southern Sky Redshift Survey: Clustering of Local Galaxies

We use the two-point correlation function to calculate the clustering properties of the recently completed SSRS2 survey. The redshift space correlation function for the magnitude-limited SSRS2 is given by xi(s)=(s/5.85 h-1 Mpc)^{-1.60} for separations between 2 < s < 11 h-1 Mpc, while our best estimate for the real space correlation function is xi(r) = (r/5.36 h-1 Mpc)^{-1.86}. Both are comparable to previous measurements using surveys of optical galaxies over much larger and independent volumes. By comparing the correlation function calculated in redshift and real space we find that the redshift distortion on intermediate scales is small. This result implies that the observed redshift-space distribution of galaxies is close to that in real space, and that beta = Omega^{0.6}/b < 1, where Omega is the cosmological density parameter and b is the linear biasing factor for optical galaxies. We also use the SSRS2 to study the dependence of xi on the internal properties of galaxies. We confirm earlier results that luminous galaxies (L>L*) are more clustered than sub-L* galaxies and that the luminosity segregation is scale-independent. We find that early types are more clustered than late types, but that in the absence of rich clusters, the relative bias between early and late types in real space, is not as strong as previously estimated. Furthermore, both morphologies present a luminosity-dependent bias, with the early types showing a slightly stronger dependence on luminosity. We also find that red galaxies are significantly more clustered than blue ones, with a mean relative bias stronger than that seen for morphology. Finally, we find that the relative bias between optical and iras galaxies in real space is b_o/b_I $\sim$ 1.4.