Galaxy formation and evolution

Abstract:

We have used the EFAR sample of galaxies to investigate the light distributions of early-type galaxies. We decompose the two-dimensional light distribution of the galaxies in a flattened spheroidal component with a Sérsic radial light profile and an inclined disc component with an exponential light profile. We show that if we assume that all galaxies can have a spheroidal and a disc component, then the brightest, bulge-dominated elliptical galaxies have a fairly broad distribution in the Sérsic profile shape parameter nB, with a median of approximately 3.7 and with σ ∼ 0.9. Other galaxies have smaller nB values. This means that spheroids are in general less concentrated than the de Vaucouleurs R1/4-law profile, which has nB = 4. While the result of our light decomposition is robust, we cannot prove without kinematic information that these components are spheroids and discs, in the usual sense of pressure- and rotation-supported stellar systems. However, we show that the distribution of disc inclination angles is consistent with a random orientation if we take our selection effects into account. If we assume that the detected spheroids and discs are indeed separate components, we can draw the following conclusions: (1) the spheroid and disc scale sizes are correlated; (2) bulge-tototal luminosity ratios, bulge effective radii and bulge nB values are all positively correlated; (3) the bivariate space density distribution of elliptical galaxies in the (luminosity, scale size)plane is well described by a Schechter luminosity function in the luminosity dimension and a lognormal scale-size distribution at a given luminosity; (4) at the brightest luminosities, the scale size distribution of elliptical galaxies is similar to those of bright spiral galaxies, but extending to brighter magnitudes; at fainter luminosities the scale size distribution of elliptical galaxies peaks at distinctly smaller sizes than the size distribution of spiral galaxies; and (5) bulge components of early-type galaxies are typically a factor of 1.5-2.5 smaller than the discs of spiral galaxies with a slight luminosity dependence, while disc components of early-type galaxies are typically twice as large as the discs of spiral galaxies at all luminosities.

Abstract:

We determine the abundance of i′-band dropouts in the recently released HST/ACS Hubble Ultra-Deep Field (UDF). Because the majority of these sources are likely to be z ≈ 6 galaxies whose flux decrement between the F775W i′-band and F850LP z′-band arises from Lyman-α absorption, the number of detected candidates provides a valuable upper limit to the unextincted star formation rate at this redshift. We demonstrate that the increased depth of UDF enables us to reach an 8 σ limiting magnitude of z′AB = 28.5 (equivalent to 1.5 h10-2 M⊙ yr-1 at z = 6.1, or 0.1 LUV* for the z ≈ 3 U-drop population), permitting us to address earlier ambiguities arising from the unobserved form of the luminosity function. We identify 54 galaxies (and only one star) at zAB′ < 28.5 with (i′ - z′)AB > 1-3 over the deepest 11-arcmin2 portion of the UDF. The characteristic luminosity (L*) is consistent with values observed at z ≈ 3. The faint end slope (α) is less well constrained, but is consistent with only modest evolution. The main change appears to be in the number density (φ*). Specifically, and regardless of possible contamination from cool stars and lower-redshift sources, the UDF data support our previous result that the star formation rate at z ≈ 6 was approximately six times less than at z ≈ 3. This declining comoving star formation rate [0.005 h70 M⊙ yr-1 Mpc-3 at z ≈ 6 at LUV > 0. 1 L* for a Salpeter initial mass function (IMF)] poses an interesting challenge for models which suggest that LUV > 0.1 L* star-forming galaxies at z ≃ 6 reionized the Universe. The short-fall in ionizing photons might be alleviated by galaxies fainter than our limit, or a radically different IMF. Alternatively, the bulk of reionization might have occurred at z ≫ 6.