Spiral-like Galaxies Research Articles

Observations of the initial formation and evolution of spiral galaxies at 1 < z < 3 in the CANDELS fields

ABSTRACT Many aspects concerning the formation of spiral and disc galaxies remain unresolved, despite their discovery and detailed study over the past 150 yr. As such, we present the results of an observational search for proto-spiral galaxies and their earliest formation, including the discovery of a significant population of spiral-like and clumpy galaxies at z > 1 in deep Hubble Space Telescope CANDELS imaging. We carry out a detailed analysis of this population, characterizing their number density evolution, masses, star formation rates (SFR), and sizes. Overall, we find a surprisingly high overall number density of massive $M_{*} \gt 10^{10}\, \mathrm{M}_{\odot }$ spiral-like galaxies (including clumpy spirals) at z > 1 of $0.18\, {\rm per}\, \mathrm{arcmin}^{-2}$. We measure and characterize the decline in the number of these systems at higher redshift using simulations to correct for redshift effects in identifications, finding that the true fraction of spiral-like galaxies grows at lower redshifts as ∼ (1 + z)−1.1. This is such that the absolute numbers of spirals increases by a factor of ∼10 between z = 2.5 and z = 0.5. We also demonstrate that these spiral-like systems have large sizes at z > 2, and high SFRs, above the main-sequence, These galaxies represent a major mode of galaxy formation in the early Universe, perhaps driven by the spiral structure itself. We finally discuss the origin of these systems, including their likely formation through gas accretion and minor mergers, but conclude that major mergers are an unlikely cause.

Populations of accreting white dwarfs

AbstractUsing a hybrid binary population synthesis approach, we modelled the formation and evolution of populations of accreting white dwarfs (WDs) for differing star formation histories. We found that the delay time distribution of SNe Ia in the single degenerate scenario is inconsistent with observations. Additionally, we found that our predicted X-ray and UV emission of populations of accreting WDs are consistent with the X-ray luminosities of early-type galaxies observed by Chandra and the HeII 4686 Å/Hβ line ratio measured in stacked SDSS spectra of passively evolving galaxies. Moreover, we found that the majority of current novae in elliptical-like galaxies have low-mass WDs, long decay times, long recurrence periods and are relatively faint. In contrast, the majority of current novae in spiral-like galaxies have massive WDs, short decay times, short recurrence periods and are relatively bright. Our predicted distribution of mass-loss timescales in an M31-like galaxy is consistent with observations for Andromeda.

Modelling nova populations in galaxies

Theoretical modelling of the evolution of classical and recurrent novae plays an important role in studies of binary evolution, nucleosynthesis and accretion physics. However, from a theoretical perspective the observed statistical properties of novae remain poorly understood. In this paper, we have produced model populations of novae using a hybrid binary population synthesis approach for differing star formation histories (SFHs): a starburst case (elliptical-like galaxies), a constant star formation rate case (spiral-like galaxies) and a composite case (in line with the inferred SFH for M31). We found that the nova rate at 10\;Gyr in an elliptical-like galaxy is $\sim 10-20$ times smaller than a spiral-like galaxy with the same mass. The majority of novae in elliptical-like galaxies at the present epoch are characterized by low mass white dwarfs (WDs), long decay times, relatively faint absolute magnitudes and long recurrence periods. In contrast, the majority of novae in spiral-like galaxies at 10\;Gyr have massive WDs, short decay times, are relatively bright and have short recurrence periods. The mass loss time distribution for novae in our M31-like galaxy is in agreement with observational data for Andromeda. However, it is possible that we underestimate the number of bright novae in our model. This may arise in part due to the present uncertainties in the appropriate bolometric correction for novae.

Limitations of model-fitting methods for lensing shear estimation

Gravitational lensing shear has the potential to be the most powerful tool for constraining the nature of dark energy. However, accurate measurement of galaxy shear is crucial and has been shown to be non-trivial by the Shear TEsting Programme. Here we demonstrate a fundamental limit to the accuracy achievable by model-fitting techniques, if oversimplistic models are used. We show that even if galaxies have elliptical isophotes, model-fitting methods which assume elliptical isophotes can have significant biases if they use the wrong profile. We use noise-free simulations to show that on allowing sufficient flexibility in the profile the biases can be made negligible. This is no longer the case if elliptical isophote models are used to fit galaxies made up of a bulge plus a disk, if these two components have different ellipticities. The limiting accuracy is dependent on the galaxy shape but we find the most significant biases for simple spiral-like galaxies. The implications for a given cosmic shear survey will depend on the actual distribution of galaxy morphologies in the universe, taking into account the survey selection function and the point spread function. However our results suggest that the impact on cosmic shear results from current and near future surveys may be negligible. Meanwhile, these results should encourage the development of existing approaches which are less sensitive to morphology, as well as methods which use priors on galaxy shapes learnt from deep surveys.

RATES AND DELAY TIMES OF TYPE Ia SUPERNOVAE

We analyze the evolution of binary stars to calculate synthetic rates and delay times of the most promising Type Ia Supernovae (SNe Ia) progenitors. We present and discuss evolutionary scenarios in which a white dwarf (WD) reaches the Chandrasekhar mass and potentially explodes in a SNe Ia. We consider Double Degenerate (DDS; merger of two WDs), Single Degenerate (SDS; WD accreting from H-rich companion), and AM Canum Venaticorum (AM CVn; WD accreting from He-rich companion) scenarios. The results are presented for two different star formation histories: burst (elliptical-like galaxies) and continuous (spiral-like galaxies). It is found that delay times for the DDS in our standard model (with common envelope efficiency αCE = 1) follow a power-law distribution. For the SDS we note a wide range of delay times, while AM CVn progenitors produce a short burst of SNe Ia at early times. The DDS median delay time falls between ∼0.5 and 1 Gyr; the SDS between ∼2 and 3 Gyr; and the AM CVn between ∼0.8 and 0.6 Gyr depending on the assumed αCE. For a Milky-Way-like (MW-like) galaxy, we estimate the rates of SNe Ia arising from different progenitors as: ∼10−4 yr−1 for the SDS and AM CVn, and ∼10−3 yr−1 for the DDS. We point out that only the rates for two merging carbon–oxygen WDs, the only systems found in the DDS, are consistent with the observed rates for typical MW-like spirals. We also note that DDS progenitors are the dominant population in elliptical galaxies. The fact that the delay time distribution for the DDS follows a power law implies more SNe Ia (per unit mass) in young rather than in aged populations. Our results do not exclude other scenarios, but strongly indicate that the DDS is the dominant channel generating SNe Ia in spiral galaxies, at least in the framework of our adopted evolutionary models. Since it is believed that WD mergers cannot produce a thermonuclear explosion given the current understanding of accreting WDs, either the evolutionary calculations along with accretion physics are incorrect, or the explosion calculations are inaccurate and need to be revisited.

SCALING RELATIONS IN DISSIPATIONLESS SPIRAL-LIKE GALAXY MERGERS

We determine both representations of the Fundamental Plane [Re Sig^a Ie^b and Re (Sig^2/Ie)^lambda] and the luminosity-effective phase space density [L fe^(-gamma)] scaling relation for N-body remnants of binary mergers of spiral-like galaxies. The main set of merger simulations involves a mass-ratio of the progenitors in the range of about 1:1 to 1:5, harboring or not a bulge-like component, and are constructed using a cosmological motivated model. Equal-mass mergers are also considered. Remnants lead to average values for the scaling indices of <a>~1.6, <b>~0.6, <lambda>~0.7, and <gamma>~0.65. These values are consistent with those of K--band observations (Mobasher et al. 1999) of ellipticals: <a>~1.5, <b>~0.8, <lambda>~0.7, and <gamma>~0.60. The b index is, however, not well reproduced. This study does not allow us to establish a conclusive preference for models with or without a bulge as progenitors. Our results indicate that the L--fe and FP scalings might be determined to a large extent by dissipationless processes, a result that appears to be in contradiction to other dissipationless results.

Infrared spectroscopy of faint 15 μm sources in the Hubble Deep Field South: First hints at the properties of the sources of the IR background

We present a spectroscopic analysis of a sample of 21 galaxies with z = 0.2-1.5 drawn from a 25 square arcmin ultra-deep ISOCAM survey at lambdaeff =15 mu m centered in the WFPC-2 Hubble Deep Field South. Near-infrared spectra are reported for 18 ISO sources, carried out with ISAAC on the VLT, aimed at detecting the redshifted Halpha +[NII]. Additional optical data come from the ESO VLT/FORS2 and NTT/EMMI, primarily targeting [OII], [OIII] and Hbeta for further physical insight. Although not numerous in terms of areal density in the sky, this population of very luminous IR sources has been recently found to be responsible for a substantial fraction of the extragalactic background light energy density. Halpha line emission is detected in virtually all the observed objects down to a flux limit of 7 x 10-17 erg cm-2 s-1 (corresponding to LH_alpha > 1041 erg s-1 at z = 0.6 for H0 = 65, OmegaLambda=0.7 and Omegam = 0.3). Our analysis (including emission line, morphology, and SED properties) shows clear evidence for AGN activity in only two of these sources: one type-I (with broadened Halpha at z=1.57) and one type-II quasars (with inverted [NII]/Halpha ratio at z=1.39), while we suspect the presence of an AGN in two further sources (an Ultra-Luminous IR Galaxy, ULIRG, at z=1.27 and a luminous galaxy at z=0.69). The Halpha luminosities indicate star formation rates (SFR) in the remaining sources between 0.5 and 20 Msun/yr, assuming a Salpeter IMF between 0.1 and 100 Msun and without extinction corrections. We find good correlations between the mid-IR, the radio and Halpha luminosities, confirming the mid-IR light as a good tracer of star formation (while the SFR based on Halpha flux show some large scatter and offset, which are still to be understood). We have estimated the baryonic masses in stars with a newly-developed tool fitting the overall optical-IR continuum, and found that the host galaxies of ISO sources are massive members of groups with typically high rates of SF (SFR ~ 10 to 300 Msun/yr). We have finally compared this ongoing SF activity with the already formed stellar masses to estimate the timescales tSF for the stellar build-up, which turn-out to be widely spread in these objects between 0.1 Gyrs to more than 10 Gyr. The faint ISOCAM galaxies appear to form a composite population, including moderately active but very massive spiral-like galaxies, and very luminous ongoing starbursts, in a continuous sequence. From the observed tSF and assuming typical starburst timescales, we infer that, with few exceptions, only a fraction of the galactic stars can be formed in any single starburst event, while several of such episodes during a protracted SF history are required for the whole galactic build-up.

Differential population synthesis of S0 galaxies. II - Methods and results

The integrated spectra of the disk and nuclei of a sample of S0 galaxies are compared to those of normal elliptical galaxies using a differential technique to search for and analyze any possible differences between the two galaxy types. A fiducial elliptical sequence representing stellar populations of uniform age but variable metallicity is constructed. Comparison of the S0 disk spectra to this sequence at a location appropriate for their line strengths reveals the presence, in roughly half of the S0 disks studied, of hot subpopulations which are not present in normal ellipticals. These subpopulations can contribute from 5 percent to as much as 50 percent or more of the V light in a particular S0 disk. It is argued that the subpopulations can be divided into two categories: those that are similar to the solar neighborhood population and are actively forming stars; and those that are older, intermediate-age populations. This supports the view that S0 galaxies have evolved from spiral or spiral-like galaxies. 45 references.