Mergers Of Early-type Galaxies Research Articles

Abundance patterns in early-type galaxies: is there a “knee” in the [Fe/H] vs. [α/Fe] relation?

Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in accordance with their old ages and short formation timescales. In this contribution we aim to resolve the enrichment histories of ETGs. This means we study the abundance of Fe ([Fe/H]) and the alpha-element groups ([alpha/Fe]) separately for stars older than 9.5 Gyr ([Fe/H]o, [alpha/Fe]o) and for stars between 1.5 and 9.5 Gyr ([Fe/H]i, [alpha/Fe]i). Through extensive simulation we show that we can indeed recover the enrichment history per galaxy. We then analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS selected to be ETGs. We separate out those galaxies for which the abundance of iron in stars grows throughout the lifetime of the galaxy, i.e. in which [Fe/H]o < [Fe/H]i. We confirm earlier work where the [Fe/H] and [alpha/Fe] parameters are correlated with the mass and velocity dispersion of ETGs. We emphasize that the strongest relation is between [alpha/Fe] and age. This relation falls into two regimes, one with a steep slope for old galaxies and one with a shallow slope for younger ETGs. The vast majority of ETGs in our sample do not show the 'knee' in the plot of [Fe/H] vs. [alpha/Fe] commonly observed in local group galaxies. This implies that for the vast majority of ETGs, the stars younger than 9.5 Gyrs are likely to have been accreted or formed from accreted gas. The properties of the intermediate-age stars in accretion-dominated ETGs indicate that mass growth through late (minor) mergers in ETGs is dominated by galaxies with low [Fe/H] and low [alpha/Fe]. The method of reconstructing the stellar enrichment histories of ETGs introduced in this paper promises to constrain the star formation and mass assembly histories of large samples of galaxies in a unique way.

Equal and Unequal-Mass Mergers of Disk and Elliptical Galaxies with Black Holes

We present binary galaxy merger simulations of gas-rich disks (Sp-Sp), of early-type galaxies and disks (E-Sp, mixed mergers), and mergers of early-type galaxies (E-E, dry mergers) including radiative cooling, star formation, black hole (BH) accretion, and the associated feedback processes. The numerical simulations include cooling, star formation, supernova feedback, and BH accretion modeled following a Bondi–Hoyle accretion parameterization. The maximum accretion rate is limited to the Eddington rate, with a total of 0.5% of the accreted rest-mass energy distributed as thermal energy to the surrounding gas.

EQUAL- AND UNEQUAL-MASS MERGERS OF DISK AND ELLIPTICAL GALAXIES WITH BLACK HOLES

We present binary galaxy merger simulations with varying mass ratios and different progenitor morphologies. The simulations include mergers of gas-rich disks (Sp-Sp), of early-type galaxies and disks (E-Sp, mixed mergers), and mergers of early-type galaxies (E-E, dry mergers). We follow the dynamics of gas, stars, and dark matter, and include radiative cooling, star formation and black hole (BH) accretion, and the associated feedback processes as in Springel et al. We study the mass assembly of the BHs and discuss technical issues of the implemented model in detail. For Sp-Sp mergers, the peak star formation rate and BH accretion rate decrease and the growth timescales of the central BHs and newly formed stars increase with higher progenitor mass ratios. The peak BH accretion rate typically occurs shortly after the time of BH merging for low progenitor mass ratios (e.g., 3:1 and lower), whereas for higher progenitor mass ratios there is no clear correlation between the peak BH accretion rate and the BH merging time. The termination of star formation by BH feedback in disk mergers is significantly less important for higher progenitor mass ratios (e.g., 3:1 and higher). In addition, the inclusion of BH feedback efficiently suppresses star formation in dry E-E mergers and mixed E-Sp mergers. All merger remnants, independent of their progenitors, follow the observed relations between the central BH mass and the stellar velocity dispersion (MBH − σ), the bulge mass (MBH − M*), and the bulge binding energy (MBH − M*σ2), with the dominant source of scatter arising from variations in the initial gas mass fraction. The normalizations for all relations and the simulated slope of the MBH − σ and MBH − M*σ2 relations are in good agreement with the observations, whereas the simulated slope of the MBH − M* relation is slightly steeper compared to the observations. This indicates that the simple BH feedback model self-regulates the mass growth of the BHs in accordance with the observed relations for a very wide range of merger progenitors.

Why Are Radio Galaxies Prolific Producers of Type Ia Supernovae?

An analysis of SN Ia events in early-type galaxies from the database of Cappellaro and coworkers provides conclusive evidence that the rate of SNe Ia in radio-loud galaxies is about 4 times higher than the rate measured in radio-quiet galaxies, i.e., SN Ia rate(radio-loud galaxies) = 0.43 h SNu as compared to SN Ia rate(radio-quiet galaxies) = 0.11 h SNu. The actual value of the enhancement is likely to be in the range ~2-7 (P ~ 10-4). This finding puts on robust empirical grounds the results obtained by Della Valle & Panagia on the basis of a smaller sample of SNe. We analyze the possible causes of this result and conclude that the enhancement of the SN Ia explosion rate in radio-loud galaxies has the same origin as their being strong radio sources, but there is no causal link between the two phenomena. We argue that repeated episodes of interaction and/or mergers of early-type galaxies with dwarf companions, on timescales of about 1 Gyr, are responsible for both inducing strong radio activity observed in ~14% of early-type galaxies and supplying an adequate number of SN Ia progenitors to the stellar population of elliptical galaxies.

The Rate and the Origin of Type Ia SNe in Radio Galaxies

SummaryAn analysis of type Ia supernova (SNIa) events in early type galaxies from the Cappellaro et al. [6] database provides strong evidence that the rate of type Ia supernovae (SNe) in radio-loud galaxies is about 4 times higher than the rate measured in radio-quiet galaxies, i.e. SNIa-rate(radio-loud) SNe per century and per 1010 (SNU) as compared to SNIa-rate(radio-quiet)= 0.11 ± 0.03 SNU. The exact value of the enhancement is still rather uncertain, but is likely to be in the range ~ 2 – 7. We discuss the possible causes of this result and we conclude that the enhancement of the SNIa explosion rate in radio-loud galaxies has the same common origin as their being strong radio sources, but that there is no causality link between the two phenomena. We argue that repeated episodes of interaction and/or mergers of early type galaxies with dwarf companions are responsible for inducing both strong radio activity in ~14% of early type galaxies, and the ~ 1 Gyr old stellar population needed to supply an adequate number SNIa progenitors.

The growth history of giant black holes and the radio dichotomy of quasars

The radio jets’ energy source is still controversial. Blandford and Znajek [MNRAS 179 (1977) 433] proposed that this source is the rotational kinetic energy of a supermassive black hole (SMBH). This proposal begs the question of whether SMBHs have the appropriate spin distribution to account for the observed radio property distribution. If the BZ process is the correct model for the radio-loud/quiet dichotomy and if SMBHs only acquire spin through gas accretion, then it is very difficult to reproduce a bimodal radio property distribution. Instead, a bimodal radio property distribution can be reproduced, if BHs also grow by coalescing with other SMBHs. The attractive point of this model is that it links the radio properties of quasars to SMBHs’ merging histories, and therefore to the host galaxy’s environment and morphology. I have used semi-analytic simulations of galaxy formation to investigate the growth histories of SMBHs and their environmental dependence. I present a scenario in which mergers of late type galaxies form radio-quiet quasars (RQQs) and mergers of early-type galaxies, which already contain SMBHs, are more likely to produce radio-loud quasars (RLQs). This scenario predicts that RLQs have a longer clustering length than RQQs.

The Rate and the Origin of Type Ia Supernovae in Radio Galaxies

An analysis of type Ia supernova (SNIa) events in early type galaxies from Evans et al (1989) database provides strong evidence that the rate of type Ia supernovae (SNe) in radio-loud galaxies is about 6 times higher than the rate measured in radio-quiet galaxies, i.e. SNIa-rate$(radio-loud galaxies)=0.47^{+0.23}_{-0.15} h^2_{50}$ SNe per century and per 10$^{10}$L$^{B}_\odot$ (SNU) as compared to SNIa-rate$(radio-quiet galaxies)\lsim 0.080 h^2_{50}$ SNU. The exact value of the enhancement is still rather uncertain, but is likely to be in the range $\sim 4-15$. We discuss the possible causes of this result and we conclude that that the enhancement of SNIa explosion rate in radio-loud galaxies has the same common origin as their being strong radio sources, but that there is no causality link between the two phenomena. We argue that repeated episodes of interaction and/or mergers of early type galaxies with dwarf companions are responsible for inducing both strong radio activity in $\sim$14% of early type galaxies, and the $\sim1$ Gyr old stellar population needed to supply an adequate number SNIa progenitors. Within this scenario we predict that the probability of detecting a core-collapse SN event in radio-loud elliptical galaxies amounts to about 4% of their SNIa events.