Luminous Compact Galaxies at Intermediate Redshifts: Progenitors of Bulges of Massive Spirals?

Abstract

VLT spectra of 14 luminous compact galaxies (LCGs) reveal strong metallic absorption line sytems as well as narrow and intense emission lines. Their gas extinction is found to be large (A V ∼ 1.5 mag) leading to an upward revision of their star formation rate (SFR) to an average value of ∼ 40 M ⊙ yr −1 . Large extinction values are also supported by the large rate of detection in one field observed by ISO. Gas metal abundances in LCGs have about half the solar value. LCG absorption spectra can be synthesized with a mix of a few Gyr old and relatively metal-rich (generally solar to over-solar values) stellar population and a younger stellar population (< 5×10 8 years) having a metal abundance similar to that of the gas.We argue that LCGs are the progenitors of present-day spiral bulges. LCGs have masses and light concentrations similar to those of present-day bulges. They could have been formed entirely during a period of a few Gyr prior to the epoch of their observations if the star formation has been sustained at the observed rate. As in present-day galactic bulges, LCG stars show a wide range of abundances. Thus, observing LCGs allows us to directly witness an important stage in the formation of a massive galaxy, the building of the bulge prior to that of the disk. The gas needed to feed the observed star formation is likely to be falling in from the outskirts of the galaxy, being tidally pulled out from interacting companion galaxies. An infall scenario naturally explains the gas metal abundance which is generally lower than that of the older stellar component. At least for the strongest star-forming LCGs, there is clear imaging evidence for the presence of companions. Some LCGs also show evidence for the beginning of a disk formation. If the above scenario holds for most LCGs, we estimate that at least 20% of present-day spiral galaxies have formed the bulk of their stars at relatively recent epochs, during the last 8-9 Gyr, at redshifts less than ∼ 1. Since they are heavily extincted, we predict their IR luminosities to be relatively large, around L IR =10 11 L ⊙ , i.e. near or slightly below the luminosities of the galaxies detected by ISO in the same redshift range. Taking into account the integrated IR luminosity of the LCG galaxy population can lead to a significant upward revision of the cosmic SFR density in the redshift range from 0.5 to 1.

A Multiwavelength Approach to the Star Formation Rate Estimation in Galaxies at Intermediate Redshifts

“…It shows the lowest metallicity of the galaxy sample, and its value matches that of the sample of LCGs at 0:5dzd0:7 (Fig. 11, asterisks) of Hammer et al (2001). Its mass ($4 Â 10 10 M , as read from Fig.…”

“…In this sense, hd2-264.1, hd4-656.1, hd4-795.111, and GSS 084_4521 are perfectly compatible with the locus spanned by local SBN galaxies (open circles). In addition, GSS 084_4515 overlaps with the luminous compact galaxies (LCGs) of Hammer et al (2001), whereas hd2-264.2 is close to the location of the z $ 0:1 0:5 emission-line objects of Kobulnicky & Zaritsky (1999). Finally, since the positions of the two interacting galaxies GSS 073_1810 in Figure 11 indicate that their metallicities are likely to be in the range covered by the remaining spiral galaxies of the sample, and considering their rest-frame absolute B magnitude, their location in Figure 11 should also be compatible with the SBN objects.…”

“…Note that in order to perform the comparison, the rest-frame absolute B magnitudes of the galaxies of our paper have been transformed to a cosmology with H 0 ¼ 100 km s À1 Mpc À1 and q 0 ¼ 0:5. The comparison samples include nonlocal emission-line galaxies from Kobulnicky & Zaritsky (1999; four CNELGs of their sample are represented with filled stars) and 14 LCGs from Hammer et al (2001). The solid line is the least-squares fit to local irregular and spiral galaxies from Kobulnicky & Zaritsky (1999; Fig.…”

A giant bar induced by a merger event at z = 0.4?

“…Those were converted to SFR using the method described in Kennicutt (1998). Luminosities have been estimated using interpolated relations between observed photometric points provided by HST/ACS, EIS and Spitzer/MIPS, using the method described in Hammer et al (2001Hammer et al ( , 2005. The ratio of the SFR IR to the SFR 2800 is consistent with Av = 0.22 (standard extinction curve), which is low.…”

The FALCON Concept: Multi-Object Spectroscopy Combined with MCAO in Near-IR

“…The deepest past and current galaxy surveys (CFRS, HDF, DEEP) all show that galaxies beyond z = 0.5 were smaller, more irregular and had higher star formation rate than present-day galaxies (Lilly et al, 1998;Hammer et al, 2001). Merging rates were also much higher than today, being proportional to (1 + z) 4 (Yee & Ellingston, 1995;Le Fèvre et al, 2000).…”