Studies of the Faint Galaxy Population
| Main Author: | Roukema, Boudewijn F. |
|---|---|
| Format: | info publication-thesis Journal |
| Terbitan: |
, 1993
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| Subjects: | |
| Online Access: |
https://zenodo.org/record/4294185 |
Daftar Isi:
- Recent observations of faint galaxies to $b_J \approx 28$ (e.g., Tyson & Seitzer, 1988) show an excess in number density with respect to simple flat universe models which incorporate K- and E-corrections but unevolving luminosity functions. Low $q_0,$ high $z_f$ models are unfavoured by recent redshift measurements, but merging dominated models and models involving differential evolution between bright and faint galaxies or a new population of faint galaxies remain consistent with the data and a flat universe. In this thesis, observations and theory which contribute to our understanding of these faint galaxy populations are described. In chapter 2 it is shown that $d_L,$ $dV/dz,$ $q_0,$ $z_f,$ the K- and E- corrections, $\phi^*,$ $M^*,$ $\alpha,$ and $\eta$ all affect the faint number counts significantly, though not independently, while the effect of $H_0$ is small. A preliminary search for low surface brightness galaxies described in Chapter 3 gave unpromising results, with a number density to $z \approx 0.05$ of $n \approx (9\pm5) \times 10^{-3} h^3 Mpc^{-3},$ which is about $7\pm 4\%$ of the number density for normal galaxies in the corresponding magnitude range of $-14\ge M_B \ge -20$ represented in a Schechter (1976) luminosity function with Efstathiou et al.'s (1988) parametrisation. Only about half of this low surface brightness galaxy population is likely to be excess to that represented in the Schechter function. The diameters of the population observed are inconsistent with the hypothesis that they are the low-redshift counterparts of the excess faint galaxies if the latter are assumed to have a typical redshift of $z=0.25$ at $B \approx 24$ (as in Cowie et al., 1991), though their magnitudes are consistent. The angular two-point correlation function has been measured for a field of faint galaxies to $v \approx 26.5$ at the South Galactic Pole. The clustering of these faint galaxies is shown to be as low as that found by Efstathiou et al. (1991), but Neuschaefer et al.'s (1991) rising correlation function amplitudes as a function of median sample magnitude are not found. The former implies that clustering growth is faster than it would be if clustering were fixed in proper coordinates, i.e., $\epsilon > 0$ (eqn (4.25)). If for some reason we have overestimated the uncertainties in our measurements, this result would be even stronger. Efstathiou et al. feel that $\epsilon > 0$ is unlikely, so their favoured explanation is that the weakness in clustering is due to the excess faint galaxies being an intrinsically faint, low redshift, more weakly clustered than normal population. N-body models used in this thesis do in fact predict $\epsilon <0$ in agreement with Efstathiou et al. (Sect. 6.4), but they also have a spatial correlation function amplitude which is far lower than cosmological amplitudes, so this does not seriously overrule the N-body results of Melott (1992) or Yoshii et al. (1993) or the observational data of Warren et al. (1993), which all indicate that $\epsilon > 0.$ Instead, it provides a constraint with which to check future N-body simulations which are normalised with the intention of having correlation functions at a cosmological scale. Merger-induced evolutionary population synthesis (MIEPS) models are defined and results shown in Chapters 5 and 6. Apart from two caveats on spatial correlation function normalisation and the size of the time interval between time stages used, these models look like a good candidate for explaining the faint counts, as expected. Burst-only star formation rate models are found to be necessary, as exponentially decaying star formation rates do not flatten the faint end of the mass function enough in converting it into a luminosity function. The burst-only models with initial perturbation spectra as power law spectra with indices of $n=0$ and $n=-2$ and detection thresholds of $r_{thresh}=5$ and $r_{thresh}=1000$ were run. The model with the most expected parameters ($n=-2, r_{thresh}=1000$) gives a luminosity function which roughly fits a Schechter function at $t \approx t_0,$ but gives number counts which clearly don't fit the observations; while a model with less likely parameters ($n=0, r_{thresh}=5$) gives a luminosity function which has the slope of a Schechter function and fits a Schechter function overall if the compensatory factor $A$ is allowed, in which case the number counts fit reasonably well to the observations apart from the faint end. An increase in time resolution of the N-body output is likely to improve the fit of the latter model more than that of the former. Hence, these models favour a white-noise-like initial perturbation spectrum ($n \approx 0$) with a low detection threshold ($r_{thresh} \approx 5$) and a correction factor $A=7$ as a candidate for explaining the excess of faint galaxies; while a CDM-like spectrum on these scales ($n \approx -2$) appears less likely. An additional result from the N-body galaxy evolutionary modelling is that the individual merger rates can be very different from the average merger rates and that the fraction of mass coming from accretion can be quite high. For example, for the $n=0, r_{thresh} = 5$ model, the mean number of peaks which collapse from the intergalactic medium at any time stage and end up in a peak at the final time stage is $7.4$, while the standard deviation in this quantity is $20.7$. While this result is likely to quantitatively change with the new N-body simulations, qualitatively it is unlikely to.
- The postscript file is version 2, produced on 1993-04-15, following the referees' reports (21 January 1993 is the date of version 1, the original submission). The pdf file was produced from the postscript file in 2020 using ps2pdf. The degree of Doctor of Philosophy in Astronomy and Astrophysics was formally awarded on 1 October 1993.