We report the identification of an extreme protocluster of galaxies in the early universe whose core (nicknamed Distant Red Core, DRC, because of its very red color in Herschel SPIRE bands) is formed ...by at least 10 dusty star-forming galaxies (DSFGs), spectroscopically confirmed to lie at z spec = 4.002 via detection of C i(1-0), 12CO(6-5), 12CO(4-3), 12CO(2-1), and H 2 O ( 2 11 - 2 02 ) emission lines with ALMA and ATCA. These DSFGs are distributed over a 260 kpc × 310 kpc region and have a collective obscured star formation rate (SFR) of ∼ 6500 M yr − 1 , considerably higher than those seen before in any protocluster at z 4 . Most of the star formation is taking place in luminous DSFGs since no Ly emitters are detected in the protocluster core, apart from a Ly blob located next to one of the DRC components, extending over 60 kpc . The total obscured SFR of the protocluster could rise to SFR ∼ 14,400 M yr − 1 if all the members of an overdensity of bright DSFGs discovered around DRC in a wide-field Large APEX BOlometer CAmera 870 m image are part of the same structure. C i(1-0) emission reveals that DRC has a total molecular gas mass of at least M H 2 ∼ 6.6 × 10 11 M , and its total halo mass could be as high as ∼ 4.4 × 10 13 M , indicating that it is the likely progenitor of a cluster at least as massive as Coma at z = 0.
Starburst galaxies at the peak of cosmic star formation are among the most extreme star-forming engines in the Universe, producing stars over about 100 million years (ref. 2). The star-formation ...rates of these galaxies, which exceed 100 solar masses per year, require large reservoirs of cold molecular gas to be delivered to their cores, despite strong feedback from stars or active galactic nuclei. Consequently, starburst galaxies are ideal for studying the interplay between this feedback and the growth of a galaxy. The methylidyne cation, CH
, is a most useful molecule for such studies because it cannot form in cold gas without suprathermal energy input, so its presence indicates dissipation of mechanical energy or strong ultraviolet irradiation. Here we report the detection of CH
(J = 1-0) emission and absorption lines in the spectra of six lensed starburst galaxies at redshifts near 2.5. This line has such a high critical density for excitation that it is emitted only in very dense gas, and is absorbed in low-density gas. We find that the CH
emission lines, which are broader than 1,000 kilometres per second, originate in dense shock waves powered by hot galactic winds. The CH
absorption lines reveal highly turbulent reservoirs of cool (about 100 kelvin), low-density gas, extending far (more than 10 kiloparsecs) outside the starburst galaxies (which have radii of less than 1 kiloparsec). We show that the galactic winds sustain turbulence in the 10-kiloparsec-scale environments of the galaxies, processing these environments into multiphase, gravitationally bound reservoirs. However, the mass outflow rates are found to be insufficient to balance the star-formation rates. Another mass input is therefore required for these reservoirs, which could be provided by ongoing mergers or cold-stream accretion. Our results suggest that galactic feedback, coupled jointly to turbulence and gravity, extends the starburst phase of a galaxy instead of quenching it.
The high interstellar abundances of polycyclic aromatic hydrocarbons (PAHs) and their size distribution are the result of complex chemical processes implying dust, UV radiation, and the main gaseous ...components (H, C
+
, and O). These processes must explain the high abundance of relatively small PAHs in the diffuse interstellar medium (ISM) and imply the continuous formation of some PAHs that are small enough (number of carbon atoms
N
C
< ~35–50) to be completely dehydrogenated by interstellar UV radiation. The carbon clusters C
n
thus formed are constantly exposed to the absorption of ~10–13.6 eV UV photons, allowing isomerization and favoring the formation of the most stable isomers. They might tend to form irregular carbon cages. The frequent accretion of interstellar C
+
ions could favor further cage isomerization, as is known in the laboratory for C
60
, possibly yielding most stable fullerenes, such as C
40
, C
44
, and C
50
. These fullerenes are expected to be very stable in the diffuse ISM because C
2
ejection is not possible by single UV photon absorption, but could need rare two-photon absorption. It is possible that at least one of these fullerenes or its cation is as abundant as C
60
or C
60
+
in the diffuse ISM, although this abundance is limited by the lack of observed matching features in observed mid-infrared spectra. B3LYP calculations of the visible spectrum for a number of fullerene isomers with 40 ≤
N
C
≤ 50 show that they generally have a few spectral bands in the visible range, with f-values in the range of a few 10
−2
. This could make such fullerenes interesting candidates for the carriers of some diffuse interstellar bands.
ABSTRACT Until recently, only a handful of dusty, star-forming galaxies (DSFGs) were known at z > 4, most of them significantly amplified by gravitational lensing. Here, we have increased the number ...of such DSFGs substantially, selecting galaxies from the uniquely wide 250, 350, and 500 m Herschel-ATLAS imaging survey on the basis of their extremely red far-infrared colors and faint 350 and 500 m flux densities, based on which, they are expected to be largely unlensed, luminous, rare, and very distant. The addition of ground-based continuum photometry at longer wavelengths from the James Clerk Maxwell Telescope and the Atacama Pathfinder Experiment allows us to identify the dust peak in their spectral energy distributions (SEDs), with which we can better constrain their redshifts. We select the SED templates that are best able to determine photometric redshifts using a sample of 69 high-redshift, lensed DSFGs, then perform checks to assess the impact of the CMB on our technique, and to quantify the systematic uncertainty associated with our photometric redshifts, = 0.14 (1 + z), using a sample of 25 galaxies with spectroscopic redshifts, each consistent with our color selection. For Herschel-selected ultrared galaxies with typical colors of S500/S250 ∼ 2.2 and S500/S350 ∼ 1.3 and flux densities, S500 ∼ 50 mJy, we determine a median redshift, , an interquartile redshift range, 3.30-4.27, with a median rest-frame 8-1000 m luminosity, , of 1.3 × 1013 L . A third of the galaxies lie at z > 4, suggesting a space density, z > 4, of 6 × 10−7 Mpc−3. Our sample contains the most luminous known star-forming galaxies, and the most overdense cluster of starbursting proto-ellipticals found to date.
We use the first systematic data sets of CO molecular line emission in z∼ 1–3 normal star-forming galaxies (SFGs) for a comparison of the dependence of galaxy-averaged star formation rates on ...molecular gas masses at low and high redshifts, and in different galactic environments. Although the current high-z samples are still small and biased towards the luminous and massive tail of the actively star-forming ‘main-sequence’, a fairly clear picture is emerging. Independent of whether galaxy-integrated quantities or surface densities are considered, low- and high-z SFG populations appear to follow similar molecular gas–star formation relations with slopes 1.1 to 1.2, over three orders of magnitude in gas mass or surface density. The gas-depletion time-scale in these SFGs grows from 0.5 Gyr at z∼ 2 to 1.5 Gyr at z∼ 0. The average corresponds to a fairly low star formation efficiency of 2 per cent per dynamical time. Because star formation depletion times are significantly smaller than the Hubble time at all redshifts sampled, star formation rates and gas fractions are set by the balance between gas accretion from the halo and stellar feedback. In contrast, very luminous and ultraluminous, gas-rich major mergers at both low and high z produce on average four to 10 times more far-infrared luminosity per unit gas mass. We show that only some fraction of this difference can be explained by uncertainties in gas mass or luminosity estimators; much of it must be intrinsic. A possible explanation is a top-heavy stellar mass function in the merging systems but the most likely interpretation is that the star formation relation is driven by global dynamical effects. For a given mass, the more compact merger systems produce stars more rapidly because their gas clouds are more compressed with shorter dynamical times, so that they churn more quickly through the available gas reservoir than the typical normal disc galaxies. When the dependence on galactic dynamical time-scale is explicitly included, disc galaxies and mergers appear to follow similar gas-to-star formation relations. The mergers may be forming stars at slightly higher efficiencies than the discs.
Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the ...distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars, and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today’s massive spiral galaxies. The slow decrease between z 2 and z 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.
The identification of the carriers of the diffuse interstellar bands (DIBs) remains to be established, with the exception of five bands attributed to C 60+ $_{60}^+$ 60+ , although it is generally ...agreed that DIB carriers should be large carbon-based molecules (with ~10–100 atoms) in the gas phase, such as polycyclic aromatic hydrocarbons (PAHs), long carbon chains or fullerenes. The aim of this paper is to investigate more specific possible carriers among PAHs, namely elongated molecules, which could explain a correlation between the DIB wavelength and the apparent UV resilience of their carriers. More specifically, we address the case of polyacenes, C4N+2H2N+4, with N ~ 10–18 fused rectilinear aligned hexagons. Polyacenes are attractive DIB carrier candidates because their high symmetry and large linear size allow them to form regular series of bands in the visible range with strengths larger than most other PAHs, as confirmed by recent laboratory results up to undecacene (C46H26). Those with very strong bands in the DIB spectral domain are just at the limit of stability against UV photodissociation. They are part of the prominent PAH family of interstellar carbon compounds, meaning that only ~10−5 of the total PAH abundance is enough to account for a medium-strength DIB. After summarizing the limited current knowledge about the complex properties of polyacenes and recent laboratory results, the likelihood that they might meet the criteria for being carriers of some DIBs is addressed by reviewing the following properties: wavelength and strength of their series of visible bands; interstellar stability and abundances, charge state and hydrogenation; and DIB rotation profiles. No definite inconsistency has been identified that precludes polyacenes from being the carriers of some DIBs with medium or weak strength, including the so-called C2 DIBs. But, despite their many interesting properties, additional experimental data about long acenes and their visible bands are needed to make robust conclusions.
Using the IRAM NOrthern Extended Millimeter Array (NOEMA), we conducted a program to measure redshifts for 13 bright galaxies detected in the
Herschel
Astrophysical Large Area Survey with
S
500
μ
m
...≥ 80 mJy. We report reliable spectroscopic redshifts for 12 individual sources, which are derived from scans of the 3 and 2 mm bands, covering up to 31 GHz in each band, and are based on the detection of at least two emission lines. The spectroscopic redshifts are in the range 2.08 <
z
< 4.05 with a median value of
z
= 2.9 ± 0.6. The sources are unresolved or barely resolved on scales of 10 kpc. In one field, two galaxies with different redshifts were detected. In two cases the sources are found to be binary galaxies with projected distances of ∼140 kpc. The linewidths of the sources are large, with a mean value for the full width at half maximum of 700 ± 300 km s
−1
and a median of 800 km s
−1
. We analyze the nature of the sources with currently available ancillary data to determine if they are lensed or hyper-luminous (
L
FIR
> 10
13
L
⊙
) galaxies. We also present a reanalysis of the spectral energy distributions including the continuum flux densities measured at 3 and 2 mm to derive the overall properties of the sources. Future prospects based on these efficient measurements of redshifts of high-
z
galaxies using NOEMA are outlined, including a comprehensive survey of all the brightest
Herschel
galaxies.
Abstract
We present 1.3- and/or 3-mm continuum images and 3-mm spectral scans, obtained using Northern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter Array (ALMA), of 21 distant, ...dusty, star-forming galaxies. Our sample is a subset of the galaxies selected by Ivison et al. on the basis of their extremely red far-infrared (far-IR) colours and low Herschel flux densities; most are thus expected to be unlensed, extraordinarily luminous starbursts at z ≳ 4, modulo the considerable cross-section to gravitational lensing implied by their redshift. We observed 17 of these galaxies with NOEMA and four with ALMA, scanning through the 3-mm atmospheric window. We have obtained secure redshifts for seven galaxies via detection of multiple CO lines, one of them a lensed system at z = 6.027 (two others are also found to be lensed); a single emission line was detected in another four galaxies, one of which has been shown elsewhere to lie at z = 4.002. Where we find no spectroscopic redshifts, the galaxies are generally less luminous by 0.3–0.4 dex, which goes some way to explaining our failure to detect line emission. We show that this sample contains the most luminous known star-forming galaxies. Due to their extreme star-formation activity, these galaxies will consume their molecular gas in ≲ 100 Myr, despite their high molecular gas masses, and are therefore plausible progenitors of the massive, ‘red-and-dead’ elliptical galaxies at z ≈ 3.
We present the IRAM-30 m observations of multiple-J CO (Jup mostly from 3 up to 8) and C I(3P2 → 3P1) (C I(2–1) hereafter) line emission in a sample of redshift ~2–4 submillimeter galaxies (SMGs). ...These SMGs are selected among the brightest-lensed galaxies discovered in the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS). Forty-seven CO lines and 7 C I(2–1) lines have been detected in 15 lensed SMGs. A non-negligible effect of differential lensing is found for the CO emission lines, which could have caused significant underestimations of the linewidths, and hence of the dynamical masses. The CO spectral line energy distributions (SLEDs), peaking around Jup ~ 5–7, are found to be similar to those of the local starburst-dominated ultra-luminous infrared galaxies and of the previously studied SMGs. After correcting for lensing amplification, we derived the global properties of the bulk of molecular gas in the SMGs using non-LTE radiative transfer modelling, such as the molecular gas density nH2 ~ 102.5–104.1 cm-3 and the kinetic temperature Tk ~ 20–750 K. The gas thermal pressure Pth ranging from~105 K cm-3 to 106 K cm-3 is found to be correlated with star formation efficiency. Further decomposing the CO SLEDs into two excitation components, we find a low-excitation component with nH2 ~ 102.8–104.6 cm-3 and Tk ~ 20–30 K, which is less correlated with star formation, and a high-excitation one (nH2 ~ 102.7–104.2 cm-3, Tk ~ 60–400 K) which is tightly related to the on-going star-forming activity. Additionally, tight linear correlations between the far-infrared and CO line luminosities have been confirmed for the Jup ≥ 5 CO lines of these SMGs, implying that these CO lines are good tracers of star formation. The C I(2–1) lines follow the tight linear correlation between the luminosities of the C I(2–1) and the CO(1–0) line found in local starbursts, indicating that C I lines could serve as good total molecular gas mass tracers for high-redshift SMGs as well. The total mass of the molecular gas reservoir, (1–30) × 1010M⊙, derived based on the CO(3–2) fluxes and αCO(1–0) = 0.8 M⊙ ( K km s-1 pc2)-1, suggests a typical molecular gas depletion time tdep ~ 20–100 Myr and a gas to dust mass ratio δGDR ~ 30–100 with ~20%–60% uncertainty for the SMGs. The ratio between CO line luminosity and the dust mass L′CO/Mdust appears to be slowly increasing with redshift for high-redshift SMGs, which need to be further confirmed by a more complete SMG sample at various redshifts. Finally, through comparing the linewidth of CO and H2O lines, we find that they agree well in almost all our SMGs, confirming that the emitting regions of the CO and H2O lines are co-spatially located.
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