Early Hierarchical Formation of Massive Galaxies Triggered by Interactions (original) (raw)

The interaction-driven starburst contribution to the cosmic star formation rate density

Astronomy & Astrophysics, 2013

An increasing amount of observational evidence supports the notion that there are two modes of star formation: a quiescent mode in disk-like galaxies, and a starburst mode, which is generally interpreted as driven by merging. Using a semi-analytic model of galaxy formation, we derive the relative contribution to the cosmic star formation rate density of quiescently starforming and starburst galaxies, predicted under the assumption that starburst events are triggered by galaxy encounters (merging and fly-by kind) during their merging histories. We show that, within this framework, quiescently starforming galaxies dominate the cosmic star formation rate density at all redshifts. The contribution of the burst-dominated starforming galaxies increases with redshift, rising from 5 % at low redshift (z 0.1) to ∼ 20% at z ≥ 5. We estimated that the fraction of the final (z=0) galaxy stellar mass which is formed through the burst component of star formation is ∼10% for 10 10 M ≤M * ≤ 10 11.5 M . Starburst galaxies, selected according to their distance from the galaxy main sequence, account for ∼10% of the star formation rate density in the redshift interval 1.5< z <2.5, i.e. at the cosmic peak of the star formation activity.

THE LESSER ROLE OF STARBURSTS IN STAR FORMATION AT z = 2

The Astrophysical Journal, 2011

Two main modes of star formation are know to control the growth of galaxies: a relatively steady one in disk-like galaxies, defining a tight star formation rate (SFR)-stellar mass sequence, and a starburst mode in outliers to such a sequence which is generally interpreted as driven by merging. Such starburst galaxies are rare but have much higher SFRs, and it is of interest to establish the relative importance of these two modes. PACS/Herschel observations over the whole COSMOS and GOODS-South fields, in conjunction with previous optical/near-IR data, have allowed us to accurately quantify for the first time the relative contribution of the two modes to the global SFR density in the redshift interval 1.5 < z < 2.5, i.e., at the cosmic peak of the star formation activity. The logarithmic distributions of galaxy SFRs at fixed stellar mass are well described by Gaussians, with starburst galaxies representing only a relatively minor deviation that becomes apparent for SFRs more than 4 times higher than on the main sequence. Such starburst galaxies represent only 2% of mass-selected star forming galaxies and account for only 10% of the cosmic SFR density at z ∼ 2. Only when limited to SFR> 1000M ⊙ /yr, off-sequence sources significantly contribute to the SFR density (46 ± 20%). We conclude that merger-driven starbursts play a relatively minor role for the formation of stars in galaxies, whereas they may represent a critical phase towards the quenching of star formation and morphological transformation in galaxies.

Starburst galaxies in the COSMOS field: clumpy star-formation at redshift 0 <z< 0.5

Astronomy & Astrophysics, 2016

Context. At high redshift, starburst galaxies present irregular morphologies with 10−20% of their star formation occurring in giant clumps. These clumpy galaxies are considered the progenitors of local disk galaxies. To understand the properties of starbursts at intermediate and low redshift, it is fundamental to track their evolution and the possible link with the systems at higher z. Aims. We present an extensive, systematic, and multiband search and analysis of the starburst galaxies at redshift (0 < z < 0.5) in the COSMOS field, as well as detailed characteristics of their star-forming clumps by using Hubble Space Telescope/Advance Camera for Surveys (HST/ACS) images. Methods. The starburst galaxies are identified using a tailor-made intermediate-band color excess selection, tracing the simultaneous presence of Hα and [OIII] emission lines in the galaxies. Our methodology uses previous information from the zCOSMOS spectral database to calibrate the color excess as a function of the equivalent width of both spectral lines. This technique allows us to identify 220 starburst galaxies at redshift 0 < z < 0.5 using the SUBARU intermediate-band filters. Combining the high spatial resolution images from the HST/ACS with ground-based multi-wavelength photometry, we identify and parametrize the star-forming clumps in every galaxy. Their principal properties, sizes, masses, and star formation rates are provided. Results. The mass distribution of the starburst galaxies is remarkably similar to that of the whole galaxy sample with a peak around M/M ∼ 2 × 10 8 and only a few galaxies with M/M > 10 10. We classify galaxies into three main types, depending on their HST morphology: single knot (Sknot), single star-forming knot plus diffuse light (Sknot+diffuse), and multiple star-forming knots (Mknots/clumpy) galaxy. We found a fraction of Mknots/clumpy galaxy f clumpy = 0.24 considering out total sample of starburst galaxies up to z ∼ 0.5. The individual star-forming knots in our sample follows the same L(Hα) vs. size scaling relation as local giant HII regions. However, they slightly differ from the one provided using samples at high redshift. This result highlights the importance of spatially resolving the star-forming regions for this kind of study. Star-forming clumps in the central regions of Mknots galaxies are more massive, and present higher star formation rates, than those in the outskirts. This trend is less clear when we consider either the mass surface density or surface star formation rate. Sknot galaxies do show properties similar to both dwarf elliptical and irregulars in the surface brightness (µ) versus M host diagram in the B-band, and to spheroidals and ellipticals in the µ versus M host diagram in the V-band. Conclusions. The properties of our star-forming knots in Sknot+diffuse and Mknots/clumpy galaxies support the predictions of recent numerical simulations claiming that they have been produced by violent disk instabilities. We suggest that the evolution of these knots means that large and massive clumps at the galaxy centers represent the end product of the coalescence of surviving smaller clumps from the outskirts. Our results support this mechanism and make it unlikely that mergers are the reason behind the observed starburst knots. Sknot galaxies might be transitional phases of the Blue Compact Dwarfs (BCD) class, with their properties consistent with spheroidal-like, but blue structures.

Galaxy Interactions and Starbursts at High Redshift

1999

Using high resolution N-body simulations with hydrodynamics and star formation, we investigate interactions and the resulting starbursts in galaxies with properties typical of zsim3z\sim 3zsim3. We apply spectral population models to produce mock-HST images, and discuss the observed magnitude, color, and morphological appearance of our simulated galaxies in both the rest-UV and rest-visual bands.

Star formation in distant starburst galaxies

Astronomy and Astrophysics

This paper discusses the stellar population content of distant (5 000 km s −1 ≤ V R ≤ 16 000 km s −1 ) galaxies with enhanced star-formation activity. Distinction is made between isolated galaxies and galaxies morphologically disturbed, with clear signs of interaction such as mergers. In these galaxies the International Ultraviolet Explorer (IUE) large aperture samples most of the galaxy's body. Consequently, the resulting integrated spectra arise primarily from blue stellar populations of different ages together with significant contributions from intermediate and old age components, subject to varying reddening amounts. Instead of analysing individual, usually low Signal-to-Noise ratio (S/N) spectra, our approach is to coadd the spectra of objects with similar spectral properties in the UV, considering as well their properties in the visible/near-infrared ranges. Consequently, the resulting high (S/N) template spectra contain the average properties of a rather uniform class of objects, and information on spectral features can now be analysed with more precision. Three groups have been found for the interacting galaxies, corresponding to a red, blue and very blue continuum. Isolated galaxies have been separated into two groups, one with a flat/red continuum and the other with a blue continuum. For comparison, we also include in the present analysis two groups of nearby disturbed galaxies. Stellar populations are analysed by means of a synthesis algorithm based on star cluster spectral components of different ages which fit the observed spectra both in terms of continuum distribution and spectral features. Flux fractions of the different age groups found in the synthesis have been transformed into mass fractions, allowing inferences on the star formation histories. Young stellar populations (age < 500 Myr) are the main flux contributors, except for the groups with a red spectrum not due to extinction, arising from the intermediate (age ≈ 1 − 2 Gyr) and old age populations. We also study the reddening values and the extinction law: a Small Magellanic Cloud-like extinction law is appropriate for all cases. As compared to nearby galaxies with enhanced star-formation, the distant starburst galaxy spectral groups exhibit larger contributions from the intermediate and old age populations. This effect is mainly accounted for by the larger spatial area sampled Send offprint requests to: C. Bonatto

Breaking the hierarchy of galaxy formation

Monthly Notices of the Royal Astronomical Society, 2006

Recent observations of the distant Universe suggest that much of the stellar mass of bright galaxies was already in place at z > 1. This presents a challenge for models of galaxy formation because massive halos are assembled late in the hierarchical clustering process intrinsic to the cold dark matter (CDM) cosmology. In this paper, we discuss a new implementation of the Durham semi-analytic model of galaxy formation in which feedback due to active galactic nuclei (AGN) is assumed to quench cooling flows in massive halos. This mechanism naturally creates a break in the local galaxy luminosity function at bright magnitudes. The model is implemented within the Millennium N-body simulation. The accurate dark matter merger trees and large number of realisations of the galaxy formation process enabled by this simulation result in highly accurate statistics. After adjusting the values of the physical parameters in the model by reference to the properties of the local galaxy population, we investigate the evolution of the K-band luminosity and galaxy stellar mass functions. We calculate the volume-averaged star formation rate density of the Universe as a function of redshift and the way in which this is apportioned amongst galaxies of different mass. The model robustly predicts a substantial population of massive galaxies out to redshift z ∼ 5 and a star formation rate density which rises at least out to z ∼ 2 in objects of all masses. Although observational data on these properties have been cited as evidence for "anti-hierarchical" galaxy formation, we find that when AGN feedback is taken into account, the fundamentally hierarchical CDM model provides a very good match to these observations.

The evolution of the star-forming sequence in hierarchical galaxy formation models

It has been argued that the specific star formation rates of star forming galaxies inferred from observational data decline more rapidly below z = 2 than is predicted by hierarchical galaxy formation models. We present a detailed analysis of this problem by comparing predictions from the GALFORM semi-analytic model with an extensive compilation of data on the average star formation rates of star-forming galaxies. We also use this data to infer the form of the stellar mass assembly histories of star forming galaxies. Our analysis reveals that the currently available data favours a scenario where the stellar mass assembly histories of star forming galaxies rise at early times and then fall towards the present day. In contrast, our model predicts stellar mass assembly histories that are almost flat below z = 2 for star forming galaxies, such that the predicted star formation rates can be offset with respect to the observational data by factors of up to 2 − 3. This disagreement can be explained by the level of coevolution between stellar and halo mass assembly that exists in contemporary galaxy formation models. In turn, this arises because the standard implementations of star formation and supernova feedback used in the models result in the efficiencies of these process remaining approximately constant over the lifetime of a given star forming galaxy. We demonstrate how a modification to the timescale for gas ejected by feedback to be reincorporated into galaxy haloes can help to reconcile the model predictions with the data.

Cosmological evolution and hierarchical galaxy formation

Monthly Notices of the Royal Astronomical Society, 1999

We calculate the rate at which dark matter halos merge to form higher mass systems. Two complementary derivations using Press-Schechter theory are given, both of which result in the same equation for the formation rate. First, a derivation using the properties of the Brownian random walks within the framework of Press-Schechter theory is presented. We then use Bayes' theorem to obtain the same result from the standard Press-Schechter mass function. The rate obtained is shown to be in good agreement with results from Monte-Carlo and N-body simulations. We illustrate the usefulness of this formula by calculating the expected cosmological evolution in the rate of star formation that is due to short-lived, merger-induced starbursts. The calculated evolution is well-matched to the observed evolution in ultraviolet luminosity density, in contrast to the lower rates of evolution that are derived from semi-analytic models that do not include a dominant contribution from starbursts. Hence we suggest that the bulk of the observed ultraviolet starlight at z > 1 arises from short-lived, merger-induced starbursts. Finally, we show that a simple merging-halo model can also account for the bulk of the observed evolution in the comoving quasar space density.

Structure and Evolution of Starburst and Normal Galaxies

The Astrophysical Journal, 2004

A comparative study of the rest-frame morphology and structural properties of optically selected starburst galaxies at redshift z 1 is carried out using multi-waveband (BV iz) high resolution images taken by the Advanced Camera for Surveys (ACS) as part of the Great Observatories Origins Deep Survey (GOODS). We classify galaxies into starburst, early and late types by comparing their observed spectral energy distributions (SEDs) with local templates. We find that early-type systems have significantly higher rest-frame B-band concentration indices and AGN fraction (>25 %) than late-type spirals and optically-selected starbursts. These results are consistent with the scenario that early-epoch (z ≫ 1) gas-rich dissipative processes (e.g., major mergers) have played an important role in developing large central concentrations in early-type E/Sa galaxies and that a concurrent growth of central black holes and bulges occur in some of these early merger events. The lower AGN fraction and concentration indices in the majority of the optically-selected starbursts at z 1 suggest that either the starbursts and early types are different in nature (being respectively disk and bulge dominated), or/and are in different evolutionary phases such that some of the starbursts in major mergers evolve into early-types as the dynamical phase of the merger evolves and the spectral signature of the starburst fades out. The starbursts have, on average, larger asymmetries than our control sample of normal galaxies in both restframe B and R-bands, suggesting that a significant fraction of the starburst activity is tidally triggered.

Starbursts and galaxy evolution

1987

Color, far-IR, and multiwavelength data on starbursts in galaxies with a variety of properties are considered. The hypothesis that sporadic star formation is the principal mode of evolution in normal galaxies is shown to explain such phenomena as the variety of starburst galaxies, the range in current star formation rates per unit area or mass for a given morphological type, and the existence of red yet H-I-rich spiral galaxies. Three independent estimates of the star formation history of the Milky Way confirm this hypothesis.