The Bright Galaxies that HST Can Barely See

The large number of galaxies that are clearly detected in the deep HST/CANDELS images enable us to carry out very exciting studies that we regularly report in this blog. Today, instead we will focus on a special type of galaxies that are very faint in all the CANDELS images, but at least 40 times brighter at longer wavelengths, in the so-called mid-infrared regime. Until very recently, only a few isolated cases of these galaxies were known, but thanks to the depth of the CANDELS data, and making use of mid-infrared Spitzer Space Telescope images, we have discovered 25 such galaxies within a single CANDELS field.

Since its launch in 2003, the Spitzer Space Telescope has allowed us to study, in a systematic way, the infrared emission of galaxies at different cosmic times. With respect to previous infrared observatories, Spitzer represented a major step in infrared astronomy, which was possible thanks to the fast progress of infrared detector technology over the last three decades.

Four examples of sources that are bright in the Spitzer Space Telescope mid-infrared
images, but very faint in the HST/CANDELS images. The multi-wavelength analysis of these
sources indicates that they are very likely massive galaxies formed in the first two billion years
of cosmic time. Image credit: Caputi et al. (2012), ApJ, 750, L20.

At low redshifts, the Spitzer mid-infrared images trace the dust emission of star forming galaxies, which occurs after the dust is heated by the UV photons produced by the new stars. The UV photons that are the consequence of accretion of matter onto a galaxy's central black hole can have a similar effect, namely heating any surrounding dust and making it emit at mid-infrared wavelengths. But for high redshift galaxies, the mid-infrared emission seen in the Spitzer maps has a rather different origin: it directly traces the redshifted light of the galaxy oldest stars. The CANDELS images, in turn, show the redshifted light of a (high-redshift) galaxy's young stars.

By comparing the multi-wavelength emission of our 25 Spitzer-bright galaxies with theoretical galaxy spectral models, we determined that the vast majority of these sources are very likely at high redshifts (z>3), which means that we are observing the light that was emitted by these galaxies when the Universe was less than two billion years old. Actually, nowadays we know many z>3 galaxies, but the properties of our newly discovered 25 galaxies are very special: the fact that they are bright in the Spitzer images, but much fainter in the CANDELS maps, indicates that these objects should be among the oldest and most massive galaxies to be found at such early cosmic times.

Finding massive galaxies in the early Universe has important implications for galaxy formation theories, which need to explain how such objects could have formed so quickly and efficiently only a few billion years after the Big Bang. According to our most-accepted cosmological model, the Cold Dark Matter model, galaxies are embedded in dark matter halos, and grow with them hierarchically, through mergers, from small to larger units, through mergers. In such a scenario, one would expect that massive galaxies are the last to form. But different astronomical observations  conducted over the last decade indicate that the most massive galaxies that we see in the Universe today basically finished their growth when the Universe was less than a half of its present age (this is 8 billion years ago), while less massive galaxies continued forming a significant amount of stars later. This phenomenon is what extragalactic astronomers call 'galaxy downsizing.' So, searching for massive galaxies further back in time is very important for understanding when galaxies could assemble a large mass for the first time in the history of the Universe, and thus constraining galaxy formation models.

Another exciting aspect of our new, massive galaxy candidates at high redshifts is that they potentially constitute the 'tip of the iceberg' of a  larger galaxy population that still remains to be discovered. To fully understand the importance of such a galaxy population we will have to wait for the advent of the HST successor, the James Webb Space Telescope (JWST), which is due for launch in 2018.  The JWST will provide us with much deeper images than current telescopes, thanks to its large collecting area  -- seven times larger than the HST, and almost 60 times larger than Spitzer. With these deeper images,  we will be able to search for fainter analogues of our galaxies at higher redshifts. In the meanwhile, we are trying to follow up our galaxies at far infrared wavelengths with the Atacama Large Millimetre Array (ALMA), which is the only instrument that, currently, can independently confirm the nature of our sources.