A HIGHER EFFICIENCY OF CONVERTING GAS TO STARS PUSHES GALAXIES AT z ∼ 1.6 WELL ABOVE THE STAR-FORMING MAIN SEQUENCE

It has recently come to light that the growth of galaxies may be less erratic than previously thought. The existence of a tight relation between the stellar mass (M_*) and star formation rate (SFR) of star-forming galaxies, termed the main sequence (MS; Daddi et al. 2007; Elbaz et al. 2007; Noeske et al. 2007; Whitaker et al. 2012; Kashino et al. 2013; Speagle et al. 2014), is indicative of a quasi-equilibrium being maintained between gas supply, SFR, and gas expulsion from galaxies (e.g., Bouché et al. 2010; Lilly et al. 2013a). This SFR–M_* relation evolves strongly with look-back time, paralleling the global evolution of the cosmic SFR density from the present to z ∼ 2 (Madau & Dickinson 2014). It is becoming clear that such a decline in SFR is in response to diminishing gas reservoirs (e.g., Tacconi et al. 2013; Santini et al. 2014; Scoville et al. 2014). The efficiency of star formation ($\mathrm{SFE}\equiv \mathrm{SFR}/{M}_{\mathrm{gas}}$), i.e., the efficiency at which gas is being converted to stars, is remarkably similar across a wide range of cosmic time within the global star-forming population, usually with gas mass M_gas inferred from the CO molecular line luminosity L^'_CO (e.g., Daddi et al. 2010b; Tacconi et al. 2010, 2013).

However, outliers from the MS are known to exist, with very high specific SFR (sSFR) compared to normal MS galaxies, such as local ultra-luminous infrared galaxies (ULIRGs; Sanders & Mirabel 1996). At z ∼ 2, these MS outliers represent ∼2% of the star-forming population and contribute only a moderate fraction (∼10%) of the global comoving SFR density (Rodighiero et al. 2011). Yet, they are still likely to play an important role in galaxy evolution, as a high fraction of galaxies may experience such a starbursting event. Therefore, it is important to understand the physical conditions that can lead to such a surge in SFR.

Local starbursts have a higher SFE compared to MS galaxies. Estimates of their molecular gas content from their CO luminosity indicate that a given gas mass is capable of producing higher SFRs (Solomon et al. 1997), and a bi-modal distribution of SFEs has been proposed for the general MS and outlier population (Daddi et al. 2010b; Genzel et al. 2010). While there are uncertainties with respect to the appropriate conversion of ${L}_{{\rm{CO}}}^{\prime }$ to H₂ gas mass (α_CO), an offset with respect to MS galaxies is already apparent when considering the observed quantities ${L}_{{\rm{CO}}}^{\prime }$ and total infrared luminosity (L_TIR). The physical mechanism responsible for this enhanced SFE is not entirely clear and one cannot exclude that sample selection may be responsible for the lack of galaxies with intermediate SFEs. While CO detections have been obtained for high-redshift submillimeter-selected galaxies (SMGs; e.g., Greve et al. 2005; Bothwell et al. 2013) and star-forming radio galaxies (SFRGs; Casey et al. 2011), these samples do not cleanly distinguish starburst outliers from the MS population and submillimeter-selected samples could be biased toward galaxies with lower dust temperatures (Magnelli et al. 2014). Therefore, it is imperative to study the CO emission of starbursts securely elevated above the MS at their respective stellar mass and near the peak epoch of star formation (z ∼ 2). Rodighiero et al. (2011) achieve a clear distinction between MS and MS-outlier galaxies using large photometric samples with multi-wavelength coverage, including the bulk of the FIR emission. Our targets then result in a homogeneous sample that does not have contaminants such as those inherent with other selections.

To address this issue, we report on interferometric observations of the molecular transition ¹²CO 2-1 and ¹²CO 3-2 for a sample of seven MS outliers using both the Atacama Large Millimeter/submillimeter Array (ALMA) and IRAM Plateau de Bure Interferometer. Our sample is extracted from the 246 MS outliers (with sSFRs ≳4× above the mean of the MS; Figure 1) at 1.4 < z < 2.5 identified with Herschel observations over the COSMOS field (Rodighiero et al. 2011) that effectively cover the peak FIR emission. Among these Herschel-detected galaxies, those with photometric redshifts between 1.4 and 1.7 are observed through our Subaru Intensive Project (Silverman et al. 2015) with the FMOS near-IR multi-object spectrograph. These spectra provide a detection of key diagnostic emission lines (i.e., Hα, Hβ, [N ii]λ6585, [O iii]λ5008) used to measure accurate spectroscopic redshifts, SFRs, and metallicities. Six targets, labeled in Figure 1, are extracted from this sample with an additional galaxy at a slightly higher redshift (PACS 282; z = 2.19) where the redshift comes from the zCOSMOS-deep program (Lilly et al. 2013b).

Zoom In Zoom Out Reset image size

Our confidence in these galaxies being "outliers" in the SFR $-{M}_{*}$ plane is based on the exquisite multi-wavelength coverage of the COSMOS field. Stellar masses are derived from SED-fitting using Hyperz with stellar population synthesis models (Bruzual & Charlot 2003) at the respective spectroscopic redshift. The deblending of detections in Herschel (or 70 μm Spitzer) images relies upon Spitzer MIPS 24 μm priors. SFRs are derived from L_TIR, an integral of the best-fit model SED (Draine & Li 2007) from 8 to 1000 μm using the Spitzer (24 μm), Herschel PACS (100 and 160 μm), and SPIRE (250, 350 and 500 μm) bands, thus accounting for the obscured star formation as in Magdis et al. (2012). Moreover, four of the seven galaxies have 1.4 GHz radio detections at >5σ level (Schinnerer et al. 2010) and radio-based SFRs are consistent with being above the MS. The presence of strong AGNs within our sample is ruled out by the lack of individual X-ray detections by Chandra. Even if an obscured AGN were present, the contribution to L_TIR would be negligible (Rodighiero et al. 2011; Pozzi et al. 2012). Throughout this work, we assume H₀ = 70 km s⁻¹ Mpc⁻¹, Ω_Λ = 0.7, Ω_M = 0.3, and use a Salpeter IMF for SFRs and stellar masses.

ALMA observations of five galaxies were carried out in Cycle 1 (Project 2012.1.00952.S) using 25–30 12 m antennas and the Band 3 receiver tuned to intermediate frequencies between 86.395 and 94.888 GHz with a resolution of 488 kHz and spectral bandpass of 1.875 GHz. Two "scheduling blocks" (SB1 and SB2) were defined to accommodate the requirements to detect CO 2-1 within a single sideband for each of our targets (SB1: PACS 819, 830; SB2: PACS 325, 299, and 867). On-source exposure times were 32 minutes (819 and 830), 2.3 hr (299 and 325), and 3.8 hr (867) that met our request based on predictions of CO emission from their IR luminosity and an assumed ratio ${L}_{{\rm{CO}}}/$L_TIR (3× lower than MS galaxies). The antenna configurations had baselines up to ∼400 m (∼200 m) for SB1 (SB2), thus resulting in a beam size (FWHM) of 13 × 10 (45 × 20; 54 × 27 for PACS 325 only). The images of PACS 819 (830) have a flux sensitivity level of 0.12 (0.10) mJy beam⁻¹ over a bandwidth of 400 km s⁻¹, while the deeper observations reach 0.073–0.14 mJy beam⁻¹ for the same spectral window. Standard targets were used for flux (Mars, Ganymede), bandpass, and phase calibration.

We measure the CO flux by fitting the data in UV space with the GILDAS task "uvfit" available in the MAPPING package. The fit was performed using a point-source model or a circular Gaussian for resolved sources that returns the centroid, deconvolved source size, and integrated flux. We find good agreement between fluxes returned from GILDAS with those using "imfit" in the image plane with CASA. Additionally, we resolve the emission for PACS 819 and 830; therefore, a measure of the source extent is reported.

To increase the sample size, interferometric measurements were obtained with IRAM/PdBI for PACS 282 (CO 3-2) and PACS 164 (CO 2-1). For PACS 282 (PACS 164), an integration time of 6.2 (12.9) hr, using the 3 mm band in configuration D, was achieved, which resulted in a limiting 1σ sensitivity level of 0.14 (0.11) mJy beam⁻¹ for a bandwidth of 240 (180) MHz. CO fluxes are measured with GILDAS. In Table 1, we list the source properties and CO measurements.

Table 1.  Sample Data and CO Measurements^a

ID	R.A.	Decl.	zCOb	zspecc	log M*d	SFRe	ICOf	${L}_{{\rm{CO}}}^{\prime }$ g	Δvh	CO	Gasi
	(CO)	(CO)			M⊙		Jy km s−1			size ('')	Fraction
299	09:59:41.31	02:14:42.91	1.6483	1.6467	10.44	554 ± 40	0.67 ± 0.08	10.44	590	<2.4	0.52
325	10:00:05.47	02:19:42.61	⋯	1.6557	10.29	139 ± 10	0.28 ± 0.06	10.07	764	⋯	0.32
819	09:59:55.55	02:15:11.70	1.4451	1.4449	10.61	783 ± 18	1.10 ± 0.07	10.55	592	0.34 ± 0.08	0.34
830	10:00:08.75	02:19:01.90	1.4631	1.4610	10.86	517 ± 12	1.18 ± 0.10	10.59	436	0.97 ± 0.17	0.46
867	09:59:38.12	02:28:56.56	1.5656	1.5673	10.67	358 ± 34	0.46 ± 0.04	10.24	472	<1.5	0.29
282	10:00:01.54	02:11:24.27	2.1869	2.1924	10.88	581 ± 42	0.75 ± 0.12	10.44	660	<3.4	0.28
164	10:01:30.53	01:54:12.96	1.6481	1.6489	>10.28	358 ± 8	0.61 ± 0.11	10.40	894	<4.8	0.52

Notes.

^aThe first five targets are observed with ALMA, while the remaining two with PdBI. ^bσ_z = 0.0003–0.0004. An accurate CO centroid was not obtained for 325, thus the Hα redshift was used for the CO flux measurement. ^cSpectroscopic redshifts are based on Hα with the exception of $\#282$ and have errors ${\sigma }_{{\rm{\Delta }}z/(1+z)}=1.8\times {10}^{-4}.$ ^d ${\sigma }_{{\rm{M}}}\sim 0.07$ dex (Ilbert et al. 2015). ^eUnits of M_⊙ yr⁻¹. ^fAll are based on the CO 2-1 transition with the exception of 282 (3-2). ^gLog base 10; units of K km s⁻¹ pc². ^hFull width of the CO line at zero intensity in units of velocity (km s⁻¹). ⁱ ${f}_{\mathrm{gas}}={M}_{\mathrm{gas}}/({M}_{*}+{M}_{\mathrm{gas}}$).

Download table as:  ASCIITypeset image

We detect CO emission in all seven targeted galaxies with integrated flux densities (I_CO) ranging from 0.28 to 1.18 Jy km s⁻¹. All but one have a high level of significance (>5σ) and those resulting from ALMA observations are above the 8σ level with the exception of PACS 325 (S/N = 4.7). In Figure 2, we display the optical Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) F814W image cutouts (Koekemoer et al. 2007) with CO and 3.6 μm emission overlaid as contours. Using a large beam for the majority of the sample, the CO emission is essentially unresolved with the exception of two observations taken at higher resolution (∼1''). PACS 819 has half of the emission coming from a region of 3.2 ± 0.8 kpc, while PACS 830 is more extended (9.0 ± 1.6 kpc). This may suggest that there is diversity in the size distribution of molecular gas at high redshift (e.g., Ivison et al. 2011; Riechers et al. 2011; Hodge et al. 2012), dissimilar to local starbursts (e.g., Scoville et al. 1989).

Zoom In Zoom Out Reset image size

Upon close inspection of the maps in Figure 2, it is evident that the centroid of the CO emission is not always coincident with the brightest regions of UV emission as seen in the HST/ACS image (i.e., PACS 164, PACS 830, PACS 867). This is likely evidence that a fair fraction of the star formation is obscured, as supported by the improved alignment of the CO emission with the peak infrared emission detected by Spitzer in both the IRAC and MIPS channels and clear association with highly significant radio emission at 1.4 GHz (Schinnerer et al. 2010).

We note that the Herschel far-IR (and possibly the CO) emission of PACS 164 is likely the sum of two separate components seen in the IRAC image. The peak of the CO emission, detected by IRAM, is located somewhat between two IR sources, as is made particularly clear by the K-band image also shown in Figure 2, and is slightly elongated, possibly indicating a contribution from both sources. The FMOS spectrum that provides the redshift (z = 1.650) refers to the western IR component that is co-spatial with the UV-bright source seen in the HST image. With a redshift of the CO emission (z = 1.647) very close to that of the FMOS source, this system appears to be in the early stages of a merger given the projected separation of 18.7 kpc. The lower limit for the stellar mass of the system as reported in Table 1 refers to the mass of the object observed with FMOS and is encircled in Figure 2.

3.1. CO-to-IR Ratio

Our primary interest is to determine whether high-redshift starbursts convert gas into stars with a higher efficiency than MS galaxies at these epochs. To begin with, we use the observed CO-to-IR luminosity ratio ${L}_{{\rm{CO}}}^{\prime }/{L}_{{\rm{TIR}}}$ rather than derived quantities, e.g., the gas mass, that depends on the CO-to-H₂ conversion factor (${\alpha }_{{\rm{CO}}}$), or gas mass surface density, which would further require CO-size information for which we only have for 2/7 galaxies in our sample. However, we note that ${L}_{{\rm{CO}}}^{\prime }/{L}_{{\rm{TIR}}}\propto {M}_{{\rm{gas}}}/$SFR (modulo α_CO), hence this luminosity ratio is a fair proxy for the gas depletion time that in turn is the inverse of the SFE.

In Figure 3(a), we plot ${L}_{{\rm{CO}}}^{\prime }$ as a function of L_TIR, indicative of the obscured SFR, and include both low- and high-redshift galaxies with measurements available in the literature and compiled in Sargent et al. (2014). All line luminosities are converted to CO (1-0) using values of 0.85 and 0.7, respectively, for CO (2-1) and CO (3-2) (Daddi et al. 2015). Different excitation corrections than those applied here would not impact our results since changes would either be insignificant (5%–10% if CO (2-1) transitions were generally more excited than assumed here) or affect only one of our sources (PACS 282, for which applying the CO(3-2)/CO(1-0) ratio of Bothwell et al. 2013 would result in a 35% difference).

Zoom In Zoom Out Reset image size

All galaxies in our sample have ${L}_{{\rm{CO}}}^{\prime }/{L}_{{\rm{TIR}}}$ below the well-defined correlation for MS galaxies. These observations indicate the existence of an offset in the ${L}_{{\rm{CO}}}^{\prime }/{L}_{{\rm{TIR}}}$ ratio for high-redshift starbursts as found by Solomon et al. (1997) for local ULIRGs. In Figure 3(b), we plot these quantities, normalized to the mean value of galaxies on the MS. In addition, on the abscissa we replace L_TIR with its implied specific SFR (sSFR = SFR/M_*), also normalized to the MS value. We measure a median value of ${L}_{{\rm{CO}}}^{\prime }/\langle {L}_{{\rm{CO}}}^{\prime }{\rangle }_{{\rm{MS}}}=0.60\pm 0.04$ for our sample. This represents an offset (1.7 ± 0.1×) from the MS that is smaller than in local samples of starbursts (∼3×; dashed line in Figure 3(a)). Along these lines, the starbursts in our sample have higher ${L}_{{\rm{CO}}}^{\prime }$ than expected by ∼0.2 dex (given their excess sSFR relative to MS galaxies) based on the empirical model of Sargent et al. (2014), shown in Figure 3(b).

These results may be indicative of a more continuous range in SFE, following the trend for MS galaxies (Figure 3(b)) and opposed to the notion of a second mode of star formation operating at a higher efficiency distinct from MS galaxies (Daddi et al. 2010b; Genzel et al. 2010, 2015). However, any statement on differences in SFE requires a factor α_CO as further addressed below. Even with continuity in these parameters, there could still exist an underlying bi-modal physical framework for star formation as demonstrated with empirical models shown in Figure 3(b) and fully explained in Sargent et al. (2014).

3.2. Gas Masses, ${\alpha }_{{\rm{CO}}},$ and SFE

With PACS 819 and PACS 830 having marginally resolved CO emission, we can estimate the total gas mass using the dynamical mass method (e.g., Tan et al. 2014). The dynamical mass (M_dyn) within the half-light radius (r_1/2) is given for the spherically symmetric case by

$$\begin{eqnarray}&&{M}_{{\rm{dyn}}}(r\lt {r}_{{\rm{1/2}}})\simeq \displaystyle \frac{5{\sigma }^{2}{r}_{{\rm{1/2}}}}{G}\end{eqnarray} \tag{ 1 }$$

where σ = ${\rm{\Delta }}{\upsilon }_{{\rm{CO}}}/2.35$ is the velocity dispersion, ${\rm{\Delta }}{\upsilon }_{{\rm{CO}}}$ is the line width—FWHM, and G is the gravitational constant. The gas mass is then derived by subtracting from the dynamical-mass half of the stellar mass and a dark matter component (M_DM = 0.25 × M_dyn; Daddi et al. 2010a) as follows:

$$\begin{eqnarray}&&{M}_{{\rm{dyn}}}=0.5\times ({M}_{*}+{M}_{{\rm{gas}}})+{M}_{{\rm{DM}}}(r\lt {r}_{{\rm{1/2}}}).\end{eqnarray} \tag{ 2 }$$

With this method, the gas mass for PACS 819 (PACS 830) is estimated to be 2.0 ± 1.2 (6.2 ± 2.2) × 10¹⁰ M_⊙, based on their half-light radii, respectively, 1.4 ± 0.3 kpc (4.1 ± 0.7 kpc) and velocity dispersion σ, respectively, 168.0 ± 20.2 km s⁻¹ (148.7 ± 20.8 km s⁻¹), that corresponds to a gas fraction ${f}_{{\rm{gas}}}={M}_{{\rm{gas}}}/({M}_{{\rm{gas}}}+{M}_{*}$) of 0.34 (0.46). Based on these values, we estimate ${\alpha }_{{\rm{CO}}}=0.6\pm 0.3$ for PACS 819 and 1.6 ± 0.6 for PACS 830, both not far from those found for the high-redshift submillimeter galaxies in the protocluster GN20 (Tan et al. 2014) and consistent with lower conversion factors for galaxies in mergers (Narayanan et al. 2011) as compared to isolated disk galaxies (see also Genzel et al. 2015).

We roughly estimate the H₂ gas masses from the CO line luminosity for the galaxies with unresolved CO emission using the average (${\alpha }_{{\rm{CO}}}=1.1$ ${M}_{\odot }$ pc⁻² (K km s⁻¹)⁻¹) value of the two estimates of α_CO given above, which is in very close agreement to that found for local nuclear starbursts (α_CO = 0.8; Downes & Solomon 1998). Based on this conversion factor and keeping in mind the uncertainty, the gas masses are in the range ∼(1.3–6.2) × 10¹⁰ M_⊙. This results in gas fractions ${f}_{{\rm{gas}}}={M}_{{\rm{gas}}}/({M}_{{\rm{gas}}}+{M}_{*}$) between 28% and 52% (see Table 1 for individual estimates), similar to those of MS galaxies at these redshifts (Tacconi et al. 2013; Béthermin et al. 2015) and in line with cosmological galaxy formation models (Narayanan et al. 2012).

Based on these results, an increase in the SFE is likely responsible for their elevation above the MS. In Figure 3(c), we illustrate this scenario by plotting the gas depletion timescale (${\tau }_{\mathrm{del}}\equiv {\mathrm{SFE}}^{-1};$ in units of Gyr) as a function of SFR. As described above, the lower CO luminosities at a given L_TIR, compared to MS galaxies, are indicative of a lower gas mass, hence shorter gas depletion timescales and higher SFEs, in agreement with previous studies (Daddi et al. 2010b; Genzel et al. 2010). Despite the remaining uncertainties on the assumptions used to derive H₂ masses, we see evidence that starbursts in our sample have a smaller contrast (i.e., less extreme) in SFE, relative to MS galaxies, as compared to local ULIRGs and the strongest starbursts at high-z.

As a final check, we can assess whether the values of α_CO derived above are appropriate for our starbursts by using the metallicity of the ISM as a proxy, given that it is well established that α_CO is anti-correlated with metallicity (e.g., Arimoto et al. 1996; Genzel et al. 2012; Schruba et al. 2012). In Figure 4(a), we show the mass–metallicity relation at z ∼ 1.6 (Zahid et al. 2014) based on [N ii]λ6585/Hα, with our sample shown in red. We find that four out of six galaxies (having errors <±0.2 on the ratio) have high metallicities, similar to those of local massive galaxies. Therefore, our derived values of α_CO are consistent with local galaxies of a similar mass and metallicity, although with the caveat that we are not certain whether the metallicity of the line-emitting gas is representative of the molecular gas producing the CO emission.

Zoom In Zoom Out Reset image size

We also examine whether AGN photoionization is contributing to the high [N ii]/Hα ratios seen in some of our galaxies. Figure 4(b) shows the FMOS-COSMOS version of the BPT diagram at z ∼ 1.6. Four galaxies with CO measurements have all key diagnostic lines detected by FMOS. None of our galaxies fall in the region where strong AGN contribution is expected, above the line separating AGNs from star-forming galaxies at z ∼ 1.6 (Kewley et al. 2013). While PACS 819 approaches this line, it is not detected in X-rays with Chandra. Nevertheless, we cannot rule out the presence of a low-to-moderate luminosity AGN, especially because its strong [N ii] emission also drives this galaxy above the local M–Z relation. For reference, in Figure 4(b), we plot the data for the larger star-forming galaxy population from FMOS-COSMOS (gray data points). It is worth highlighting that both PACS 830 and PACS 299 fall within the locus of the star-forming population, clearly offset toward a higher ionization state of the ISM compared to local galaxies (Steidel et al. 2014; Kartaltepe et al. 2015).

Our observations of the molecular CO gas content of seven galaxies well above the MS at z ∼ 1.6 with ALMA in Cycle 1 and IRAM/PdBI establish an offset in the ${L}_{{\rm{CO}}}^{\prime }-{L}_{{\rm{TIR}}}$ relation for starbursts at high redshift compared to MS galaxies, although smaller than expected from previous studies of low-z starburst outliers. These results may be indicative of a continuous distribution in SFE at high redshift, as a function of distance from the star-forming MS, as opposed to a bi-modal distribution.

An appealing physical explanation for a decrease in the ${L}_{{\rm{CO}}}^{\prime }/$ ${L}_{{\rm{TIR}}}$ ratio for galaxies well above the MS is galaxy mergers that can lead to rapid gas compression, hence effectively boosting star formation resulting in shorter gas depletion timescales. Several galaxies well above the MS are indeed in a merger phase (Rodighiero et al. 2011; Wuyts et al. 2011). Within the present sample, hints of merging come from the presence of multiple UV/optical emitting regions (e.g., in PACS 819, PACS 830, PACS 867), even being in a kinematically linked system (PACS 164).

We are grateful for the support from the regional ALMA ARCs, in particular Akiko Kawamura, Kazuya Saigo, Gaelle Dumas, and Sergio Martin. J.D.S. was supported by the ALMA Japan Research Grant of NAOJ Chile Observatory, NAOJ-ALMA-0127. This work was supported by World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan. C.M. and A.R. acknowledge support from an INAF PRIN 2012 grant. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2012.1.00952.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.