arXiv subjects

S. Khochfar

Publications and source records attributed to S. Khochfar.

42 records · Page 3Linked to original sources

arXiv2007

The Evolving Faint-End of the Luminosity Function

We investigate the evolution of the faint-end slope of the luminosity function, $α$, using semi-analytical modeling of galaxy formation. In agreement with observations, we find that the slope can be fitted well by $α(z) =a+b z$, with a=-1.13 and b=-0.1. The main driver for the evolution in $α$ is the evolution in the underlying dark matter mass function. Sub-L_* galaxies reside in dark matter halos that occupy a different part of the mass function. At high redshifts, this part of the mass function is steeper than at low redshifts and hence $α$ is steeper. Supernova feedback in general causes the same relative flattening with respect to the dark matter mass function. The faint-end slope at low redshifts is dominated by field galaxies and at high redshifts by cluster galaxies. The evolution of $α(z)$ in each of these environments is different, with field galaxies having a slope b=-0.14 and cluster galaxies b=-0.05. The transition from cluster-dominated to field-dominated faint-end slope occurs roughly at a redshift $z_* \sim 2$, and suggests that a single linear fit to the overall evolution of $α(z)$ might not be appropriate. Furthermore, this result indicates that tidal disruption of dwarf galaxies in clusters cannot play a significant role in explaining the evolution of $α(z)$ at z< z_*. In addition we find that different star formation efficiencies a_* in the Schmidt-Kennicutt-law and supernovae-feedback efficiencies $ε$ generally do not strongly influence the evolution of $α(z)$.

astro-ph↗

arXiv2006

On the Origin of Stars in Bulges and Elliptical Galaxies

We investigate the stellar composition of bulges and elliptical galaxies as predicted by the CDM paradigm using semi-analytical modelling. We argue that spheroid stars are built up of two main components, {\it merger} and {\it quiescent}, according to the origin of the stars. The merger component is formed during major mergers by gas driven to the centre, while the quiescent component is formed in gaseous discs and added later to the spheroid during major mergers. Galaxies more massive than $M_C=3 \times 10^{10}$ M$_{\odot}$ have on average only a 15% merger component in their spheroids, while smaller galaxies can have up to 30%. The merger component increases with redshift due to mergers involving more gas. However we do not find mergers with gas fraction above $ \sim 40 %$ of the remnants mass. Generally the gas fraction is a decreasing function of the redshift at which the merger occurs and the mass of the remnant, with more massive remnants having smaller gas fraction and hence smaller merger components. This trend is independent of the environment of the galaxy with the only impact of the environment being that galaxies less massive than $M_C$ have slightly larger merger components in dense environments. The fraction of stars in bulges for galaxies more massive than $M_C$ is larger than 50%. We find that the majority of stars in galaxies more massive than $M_C$ reside within bulges and ellipticals independent of redshift and that the fraction increases with redshift. The most massive galaxies at each redshift are elliptical galaxies.

astro-ph↗

arXiv2006

The effect of environment on the UV colour-magnitude relation of early-type galaxies

We use \textit{GALEX} (Galaxy Evolution Explorer) near-UV (NUV) photometry of a sample of early-type galaxies selected in \textit{SDSS} (Sloan Digital Sky Survey) to study the UV color-magnitude relation (CMR). $NUV-r$ color is an excellent tracer of even small amounts ($\sim 1$% mass fraction) of recent ($\la 1$ Gyr) star formation and so the $NUV-r$ CMR allows us to study the effect of environment on the recent star formation history. We analyze a volume-limited sample of 839 visually-inspected early-type galaxies in the redshift range $0.05 < z < 0.10$ brighter than $M_{r}$ of -21.5 with any possible emission-line or radio-selected AGN removed to avoid contamination. We find that contamination by AGN candidates and late-type interlopers highly bias any study of recent star formation in early-type galaxies and that, after removing those, our lower limit to the fraction of massive early-type galaxies showing signs of recent star formation is roughly $30 \pm 3%$ This suggests that residual star formation is common even amongst the present day early-type galaxy population. We find that the fraction of UV-bright early-type galaxies is 25% higher in low-density environments. However, the density effect is clear only in the lowest density bin. The blue galaxy fraction for the subsample of the brightest early-type galaxies however shows a very strong density dependence, in the sense that the blue galaxy fraction is lower in a higher density region.

astro-ph↗

arXiv2006

How galaxies lose their angular momentum

The processes are investigated by which gas loses its angular momentum during the protogalactic collapse phase, leading to disk galaxies that are too compact with respect to the observations. High-resolution N-body/SPH simulations in a cosmological context are presented including cold gas and dark matter. A halo with quiet merging activity since z~3.8 and with a high spin parameter is analysed that should be an ideal candidate for the formation of an extended galactic disk. We show that the gas and the dark matter have similar specific angular momenta until a merger event occurs at z~2 with a mass ratio of 5:1. All the gas involved in the merger loses a substantial fraction of its specific angular momentum due to tidal torques and falls quickly into the center. Dynamical friction plays a minor role,in contrast to previous claims. In fact, after this event a new extended disk begins to form from gas that was not involved in the 5:1 merger event and that falls in subsequently. We argue that the angular momentum problem of disk galaxy formation is a merger problem: in cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1 are expected to occur in almost all galaxies. We suggest that energetic feedback processes could in principle solve this problem, however only if the heating occurs at the time or shortly before the last substantial merger event. Good candidates for such a coordinated feedback would be a merger-triggered star burst or central black hole heating. If a large fraction of the low angular momentum gas would be ejected as a result of these processes, late-type galaxies could form with a dominant extended disk component, resulting from late infall, a small bulge-to-disk ratio and a low baryon fraction, in agreement with observations.

astro-ph↗

arXiv2005

Orbital Parameters of Merging Dark Matter Halos

In order to specify cosmologically motivated initial conditions for major galaxy mergers (mass ratios $\leq$ 4:1) that are supposed to explain the formation of elliptical galaxies we study the orbital parameters of major mergers of cold dark matter halos using a high-resolution cosmological simulation. Almost half of all encounters are nearly parabolic with eccentricities $e \approx 1$ and no correlations between the halo spin planes or the orbital planes. The pericentric argument $ω$ shows no correlation with the other orbital parameters and is distributed randomly. In addition we find that 50 % of typical pericenter distances are larger than half the halo's virial radii which is much larger than typically assumed in numerical simulations of galaxy mergers. In contrast to the usual assumption made in semi-analytic models of galaxy formation the circularities of major mergers are found to be not randomly distributed but to peak around a value of $ε\approx 0.5$. Additionally all results are independent of the minimum progenitor mass and major merger definitions (i.e. mass ratios $\leq$ 4:1; 3:1; 2:1).

astro-ph↗

arXiv2001

Redshift Evolution of the Merger Fraction of Galaxies in CDM Cosmologies

We use semi-analytical modelling of galaxy formation to study the redshift evolution of the galaxy merger fractions and merger ratesin Lambda CDM and quintessence (QCDM) cosmologies, their dependence on physical parameters as the environment, the merger timescale, the way major mergers are defined, and the minimum mass of objects taken into account. We find that for a given final halo mass the redshift dependence of the merger fraction F_mg and the resulting merger rate can be fitted well by a power law for redshifts z <= 1. The normalization F_{mg}(0) and the slope m depend on the final halo mass. For a given merger timescale t_merg and an assumed maximum mass ratio R_major for major mergers, F_mg(0) and m depend exponentially on each other. The slope m depends logarithmically on the ratio of the final halo mass and the minimum halo mass taken into account. In addition, the local normalization F_{mg}(0) increases for larger R_{major} while m decreases. We compare the predicted merger fractionwith recent observations and find that the model cannot reproduce both the merger index and the normalization at the same time. In general the model underestimates F_{mg}(0) and m by a factor of 2.

astro-ph↗