GOODS–Herschel: an infrared main sequence for star-forming galaxies

Abstract

We present the deepest 100 to 500 μm far-infrared observations obtained with the Herschel Space Observatory as part of the GOODS-Herschel key program, and examine the infrared (IR) 3-500 μm spectral energy distributions (SEDs) of galaxies at 0 < z < 2.5, supplemented by a local reference sample from IRAS, ISO, Spitzer, and AKARI data. We determine the projected star formation densities of local galaxies from their radio and mid-IR continuum sizes. We find that the ratio of total IR luminosity to rest-frame 8 μm luminosity, IR8 (≡L tot IR /L 8 ), follows a Gaussian distribution centered on IR8 = 4 (σ = 1.6) and defines an IR main sequence for star-forming galaxies independent of redshift and luminosity. Outliers from this main sequence produce a tail skewed toward higher values of IR8. This minority population (<20%) is shown to consist of starbursts with compact projected star formation densities. IR8 can be used to separate galaxies with normal and extended modes of star formation from compact starbursts with high-IR8, high projected IR surface brightness (Σ IR > 3 × 10 10 L kpc −2 ) and a high specific star formation rate (i.e., starbursts). The rest-frame, UV-2700 Å size of these distant starbursts is typically half that of main sequence galaxies, supporting the correlation between star formation density and starburst activity that is measured for the local sample. Locally, luminous and ultraluminous IR galaxies, (U)LIRGs (L tot IR ≥ 10 11 L ), are systematically in the starburst mode, whereas most distant (U)LIRGs form stars in the "normal" main sequence mode. This confusion between two modes of star formation is the cause of the so-called "mid-IR excess" population of galaxies found at z > 1.5 by previous studies. Main sequence galaxies have strong polycyclic aromatic hydrocarbon (PAH) emission line features, a broad far-IR bump resulting from a combination of dust temperatures (T dust ∼ 15-50 K), and an effective T dust ∼ 31 K, as derived from the peak wavelength of their infrared SED. Galaxies in the starburst regime instead exhibit weak PAH equivalent widths and a sharper far-IR bump with an effective T dust ∼ 40 K. Finally, we present evidence that the mid-to-far IR emission of X-ray active galactic nuclei (AGN) is predominantly produced by star formation and that candidate dusty AGNs with a power-law emission in the mid-IR systematically occur in compact, dusty starbursts. After correcting for the effect of starbursts on IR8, we identify new candidates for extremely obscured AGNs.

A galaxy rapidly forming stars 700 million years after the Big Bang at redshift 7.51

“…66 The Chandra imaging reaches Lx = 10 42 erg s −1 at z = 1.78, sufficient to detect weak AGNs. To see if an obscured AGN interpretation matches the available data, we examined the Spitzer/MIPS 24 µm, Herschel/PACS 100 and 160 µm, Herschel/SPIRE 250, 350 and 500 µm and the JVLA 1.4 GHz data [67][68][69] . We found no counterpart to z8 GND 5296 at any of these wavelengths.…”

Inefficient star formation in extremely metal poor galaxies

“…Our estimated errors are quite reasonable compared to the expected point source flux errors from HSPOT (also listed in Extended Table 3). Note that although in the SPIRE bands the confusion noise is 2-3 times higher than the photon noise, this can be mitigated by using a PACS 160-micron prior on the position 39 . We can further compare our noise estimates to those from other Herschel observations of similar depth.…”

Akari-Nep : Effects of Agn Presence on SFR Estimates of Galaxies

“…In particular, many recent studies have been arguing the role played by AGN and BH accretion in galaxy formation and evo-lution and how these are connected with the star formation activity in galaxies (e.g. Delvecchio et al 2014;Elbaz et al 2011). Starburst sources characterised by a high SFR are likely to happen simultaneously or coexist with the presence of an AGN which has been demonstrated both theoretically and by the observations (e.g., Bower et al 2006;Farrah et al 2003;Brusa et al 2014) .…”

“…For this reason it has become more and more important to characterise the interplay between the star formation activity and the nuclear gravitational accretion activity. Moreover, many authors have shown that exists a correlation between M * and SFR that defines the so called Galaxy Main Sequence (Elbaz et al 2011) which appears to evolve with cosmic time. How galaxies are distributed in this parameter space is still debated and e.g., ?, have shown that many of the off-main sequence objects require the presence of an AGN to justify their properties.…”