Ana Campos

Galaxy clustering in the Herschel Deep Field

ABSTRA C T We present a study of the angular correlation function as measured in the William Herschel Deep Field, a high galactic latitude field which has been the subject of an extensive observing campaign from optical to infrared wavelengths. It covers 50 arcmin 2 and with it we are able to investigate the scaling of the angular correlation function to B , 28; R; I , 26, K , 20 and H , 22:5: We compare our measurements to results obtained from the smaller Hubble Deep Field. To interpret our results, we use a model which correctly predicts colours, number counts and redshift distributions for the faint galaxy population. We find that at fixed separation the amplitude of v (u ) measured in BRI bandpasses is lower than the predictions of a model containing no luminosity evolution and stable clustering growth in proper coordinates. However, in the near-infrared bandpasses, our measurements are consistent with the predictions of an essentially non-evolving K-selected galaxy redshift distribution. In the range B , 27‐28 we find that our correlation amplitudes are independent of magnitude, which is consistent with the observed flattening of the number count slope and correspondingly slower increase of the cosmological volume element expected at these magnitudes. If our luminosity evolution models provide a correct description of the underlying redshift distributions (and comparisons to available observations at brighter magnitudes suggest they do), then our measurements in all bandpasses are consistent with a rapid growth of galaxy clustering O0 , e , 2 in the normal parametrization) on the sub-Mpc scales which our survey probes. We demonstrate that this rapid growth of clustering is consistent with the predictions of biased models of galaxy formation, which indicate that a rapid rate of clustering growth is expected for the intrinsically faint galaxies which dominate our survey.

On the Use of Scaling Relations for the Tolman Test

The use of relations between structural parameters of early-type galaxies to perform the Tolman test for the expansion of the universe is reconsidered. Scaling relations such as the fundamental plane or the Kormendy relation require the transformation from angular to metric sizes, to compare the relation at different z-values. This transformation depends on the assumed world model: galaxies of a given angular size, at a given z, are larger (in units of kiloparsecs) in a nonexpanding universe than in an expanding one. Furthermore, the luminosities of galaxies are expected to evolve with z in an expanding model. These effects are shown to conspire to reduce the difference between the predicted surface brightness (SB) change with redshift in the expanding and nonexpanding cases. We have considered expanding models with passive luminosity evolution. We find that their predictions for the visible photometric bands are very similar to those of the static model until z~1, and therefore the test cannot distinguish between the two world models. Recent good-quality data on the Kormendy relation and the fundamental plane at intermediate redshifts are consistent with the predictions from both models. In the K band, where the expected (model) luminosity evolutionary corrections are smaller, the differences between the expanding and static models amount to ~0.4 (0.8) mag at z=0.4 (1). It is shown that, owing to that small difference between the predictions in the covered z-range, and to the paucity and uncertainties of the relevant SB photometry, the existing K-band data are not adequate to distinguish between the different world metrics and cannot yet be used to discard the static case. It is pointed out that the scaling relations could still be used to rule out the nonevolving case if it could be shown that the coefficients change with the redshift.

Dwarf Galaxies with Gentle Star Formation and the Counts of Galaxies from the Hubble Deep Field

In this paper the counts and colors of the faint galaxies observed in the Hubble Deep Field are fitted by means of simple luminosity evolution models that incorporate a numerous population of fading dwarfs. The observed color distribution of the very faint galaxies now allows us to put constraints on the star formation history in dwarfs. It is shown that the star-forming activity in these small systems has to proceed in a gentle way, i.e., through episodes where each one lasts much longer than a simple instantaneous burst of star formation. By allowing dwarfs to form stars in this gentle way, the number of predicted red remnants is severely reduced, in good agreement with the observations. Then, if the faint counts are to be fitted by means of dwarfs, the simple model for dwarfs forming stars in single, very short episodes is challenged, and a more complex star formation history has to be invoked. Recent observational evidence supporting this new dwarf model is also discussed.

Modelling the deep counts: luminosity evolution, dust and faint galaxies

In this paper we analyse the deep number counts problem, taking account of new observational and theoretical developments. First we show that the new Bruzual and Charlot (1993) models allow a new class of spiral dominated luminosity evolution (LE) model where significant amounts of the luminosity evolution needed to fit faint count data are due to spiral rather than early-type galaxies. Second we show that the inclusion of dust may be a vital ingredient for obtaining fits with any LE model. Third we compare the quality of fit of both the spiral and early-type LE models, including dust, for a wide variety of observational data. We find that parameters can be found for both LE models which allow a good fit to all data with the exception of the faintest B>25 counts in the case of q0=0.5 cosmologies, where some luminosity dependent evolution may be needed (see also Metcalfe et al 1995). Otherwise both these classes of LE model, with the inclusion of dust, provide an excellent foundation for understanding the B<25 galaxy counts and galaxy counts and redshift distributions in a variety of other wavebands.

Evidence for Galaxy Formation at High Redshift

We have shown previously that galaxy counts from the UV to the near-IR are well-fitted by simple evolutionary models where the space density of galaxies remains constant with look-back time while the star-formation rate rises exponentially. We now extend these results, first by using data from the Herschel Deep Field to show that these same models give detailed fits to the faint galaxy r-i:b-r colour-colour diagram. We then use these models to predict the number counts of high redshift galaxies detected by the Lyman break technique. At z~3 there is almost exact agreement between ou r prediction and the data, suggesting that the space density of galaxies at z~3 may be close to its local value. At z~4 the space density of bright galaxies remains unchanged; however, the space density of dwarf galaxies is significantly lower than it is locally, suggesting that we have detected an epoch of dwarf galaxy formation at z~4. Finally, significant numbers of Lyman-break galaxy candidates are also detected at z~6 in the Hubble and Herschel Deep Fields; taking this observation together with a number of recent detections of spectroscopically confirmed z~6 galaxies suggests that the space density of bright galaxies at z~6 remains comparable to the local space density and thus that the epoch of formation of bright galaxies may lie at yet higher redshift.

Photometric study of the improved GRB 970815 error box

An improved error box of GRB 970815 has been reported very recently (Smith et al.1999). This error box is consistent with the decaying X-ray source detected by ROSAT, which strongly supports the source as the GRB X-ray afterglow. We report a re-analysis of the follow up observations performed in 1997 containing the improved RXTE error box. No variable optical/IR source has been found neither within the entire RXTE error box nor within the ROSAT and ASCA X-ray circles. Upper limits for any transient object are K′⩾18 (Aug 17 1997), B⩾21.5, R⩾22 (20 Aug 1997), B⩾22.5 and R⩾23 (23 Aug 1997). Also we report additional BVRI optical observations performed in 1999 of the ROSAT X-ray source aimed to detect the host galaxy of GRB 970815.