The nitrogen-to-oxygen (N/O) abundance ratio is an important diagnostic of galaxy evolution because the ratio is closely tied to the growth of metallicity and the star formation history in galaxies. ...Estimates for the N/O are traditionally made with optical lines that could suffer from extinction and excitation effects, so the N/O is arguably measured better through far-infrared (far-IR) fine-structure lines. Here we show that the N iii57 m/O iii52 m line ratio, denoted N3O3, is a physically robust probe of N/O. This parameter is insensitive to gas temperature and only weakly dependent on electron density. Although it has a dependence on the hardness of the ionizing radiation field, we show that it is well corrected when the Ne iii15.5 m/Ne ii12.8 m line ratio is included. We verify the method, and characterize its intrinsic uncertainties by comparing the results to photoionization models. We then apply our method to a sample of nearby galaxies using new observations obtained with SOFIA/FIFI-LS in combination with available Herschel/PACS data, and the results are compared with optical N/O estimates. We find evidence for a systematic offset between the far-IR and optically derived N/O. We argue that the likely reason is that our far-IR method is biased toward younger and denser H ii regions, while the optical methods are biased toward older H ii regions as well as diffuse ionized gas. This work provides a local template for studies of the abundance of interstellar medium in the early Universe.
Abstract
We investigate the properties of the interstellar medium, star formation, and the current-day stellar population in the strongly lensed star-forming galaxy H-ATLAS J091043.1-000321 (SDP.11), ...at
z
= 1.7830, using new Herschel and Atacama Large Millimeter/submillimeter Array (ALMA) observations of far-infrared fine-structure lines of carbon, oxygen, and nitrogen. We report detections of the O
iii
52
μ
m, N
iii
57
μ
m, and O
i
63
μ
m lines from Herschel/PACS, and present high-resolution imaging of the C
ii
158
μ
m line, and underlying continuum, using ALMA. We resolve the C
ii
line emission into two spatially offset Einstein rings, tracing the red and blue velocity components of the line, in the ALMA/Band 9 observations at 0.″2 resolution. The values seen in the C
ii
/far-infrared (FIR) ratio map, as low as ∼0.02% at the peak of the dust continuum, are similar to those of local ULIRGs, suggesting an intense starburst in this source. This is consistent with the high intrinsic FIR luminosity (∼3 × 10
12
L
⊙
), ∼16 Myr gas depletion timescale, and ≲8 Myr timescale since the last starburst episode, estimated from the hardness of the UV radiation field. By applying gravitational lensing models to the visibilities in the uv-plane, we find that the lensing magnification factor varies by a factor of two across SDP.11, affecting the observed line profiles. After correcting for the effects of differential lensing, a symmetric line profile is recovered, suggesting that the starburst present here may not be the result of a major merger, as is the case for local ULIRGs, but instead could be powered by star formation activity spread across a 3–5 kpc rotating disk.
Context. Submillimetre galaxies (SMGs) represent an important source population in the origin and cosmic evolution of the most massive galaxies. Hence, it is imperative to place firm constraints on ...the fundamental physical properties of large samples of SMGs. Aims. We determine the physical properties of a sample of SMGs in the COSMOS field that were pre-selected at the observed-frame wavelength of λobs = 1.1 mm, and followed up at λobs = 1.3 mm with the Atacama Large Millimetre/submillimetre Array (ALMA). Methods. We used the MAGPHYS model package to fit the panchromatic (ultraviolet to radio) spectral energy distributions (SEDs) of 124 of the target SMGs, which lie at a median redshift of z = 2.30 (19.4% are spectroscopically confirmed). The SED analysis was complemented by estimating the gas masses of the SMGs by using the λobs = 1.3 mm dust emission as a tracer of the molecular gas component. Results. The sample median and 16th–84th percentile ranges of the stellar masses, obscured star formation rates, dust temperatures, and dust and gas masses were derived to be log(M⋆/M⊙) = 11.09+0.41-0.53, SFR = 402+661-233 M⊙ yr-1, Tdust = 39.7+9.7-7.4 K, log(Mdust/M⊙) = 9.01+0.20-0.31, and log(Mgas/M⊙ = 11.34+0.20-0.23, respectively. The Mdust/M⋆ ratio was found to decrease as a function of redshift, while the Mgas/Mdust ratio shows the opposite, positive correlation with redshift. The derived median gas-to-dust ratio of 120+73-30 agrees well with the canonical expectation. The gas fraction (Mgas/ (Mgas + M⋆)) was found to range from 0.10 to 0.98 with a median of 0.62+0.27-0.23. We found that 57.3% of our SMGs populate the main sequence (MS) of star-forming galaxies, while 41.9% of the sources lie above the MS by a factor of greater than three (one source lies below the MS). These super-MS objects, or starbursts, are preferentially found at z ≳ 3, which likely reflects the sensitivity limit of our source selection. We estimated that the median gas consumption timescale for our SMGs is ~535 Myr, and the super-MS sources appear to consume their gas reservoir faster than their MS counterparts. We found no obvious stellar mass–size correlations for our SMGs, where the sizes were measured in the observed-frame 3 GHz radio emission and rest-frame UV. However, the largest 3 GHz radio sizes are found among the MS sources. Those SMGs that appear irregular in the rest-frame UV are predominantly starbursts, while the MS SMGs are mostly disk-like. Conclusions. The physical parameter distributions of our SMGs and those of the equally bright, 870 μm selected SMGs in the ECDFS field (the so-called ALESS SMGs) are unlikely to be drawn from common parent distributions. This might reflect the difference in the pre-selection wavelength. Albeit being partly a selection bias, the abrupt jump in specific SFR and the offset from the MS of our SMGs at z ≳ 3 might also reflect a more efficient accretion from the cosmic gas streams, higher incidence of gas-rich major mergers, or higher star formation efficiency at z ≳ 3. We found a rather flat average trend between the SFR and dust mass, but a positive SFR−Mgas correlation. However, to address the questions of which star formation law(s) our SMGs follow, and how they compare with the Kennicutt-Schmidt law, the dust-emitting sizes of our sources need to be measured. Nonetheless, the larger radio-emitting sizes of the MS SMGs compared to starbursts is a likely indication of their more widespread, less intense star formation activity. The irregular rest-frame UV morphologies of the starburst SMGs are likely to echo their merger nature. The current stellar mass content of the studied SMGs is very high, so they must quench to form the so-called red-and-dead massive ellipticals. Our results suggest that the transition from high-z SMGs to local ellipticals via compact, quiescent galaxies (cQGs) at z ~ 2 might not be universal, and the latter population might also descend from the so-called blue nuggets. However, z ≳ 4 SMGs could be the progenitors of higher redshift, z ≳ 3 cQGs, while our results are also consistent with the possibility that ultra-massive early-type galaxies found at 1.2 ≲ z ≲ 2 experienced an SMG phase at z ≤ 3.
ABSTRACT
High-redshift observations are often biased towards massive and bright galaxies that are not necessarily representative of the full population. In order to accurately study galaxy evolution ...and mass assembly at these redshifts, observations of ‘normal’ main sequence galaxies are required. Here we present Atacama Large Millimeter/Submillimeter Array (ALMA) 0.3 arcsec resolution observations of the C ii emission line at 158 μm of HZ7, a main sequence galaxy at z = 5.25. Comparing to archival rest-frame UV observations taken by the Hubble Space Telescope (HST), we find strong evidence of the existence of extended C ii emission, which we estimate to be twice the size of the rest-frame UV emission, yielding one of the first high-redshift objects where a clear signature of a C ii ‘Halo’ has been detected to date. For a matched Sérsic profile with n = 1, we measured a C ii effective radius of 0.50 ± 0.04 arcsec (3.07 ± 0.25 kpc) and an average rest-frame UV effective radius of 0.2 ± 0.04 arcsec (1.48 ± 0.16 kpc). The C ii morphology and kinematics of the system suggest a merging event resulting in a non-rotating disc system. This event could be responsible for the extended CII emission. Alternatively, some potential obscured emission could also explain the C ii to UV size ratio. These results contribute to the growing consensus with respect to the existence of extended C ii emission around galaxies.
Context. The star formation rate (SFR) per unit area correlates well with the gas surface density for different types of galaxies. However, this Kennicutt-Schmidt (K-S) law has not yet been examined ...for a large, homogeneously selected sample of submillimetre galaxies (SMGs), which could provide useful SF implementation information for models of massive galaxy formation and evolution. Aims. We aim at determining the K-S law parameters for the first time for a well-selected, statistical sample of SMGs. Methods. We used the Atacama Large Millimetre/submillimetre Array (ALMA) to conduct a high resolution (0.̋2), 870 μm continuum imaging survey of 40 SMGs, which were initially selected at 1.1 mm in the COSMOS field. We analysed a sample of 32 out of the 40 target SMGs, for which our new ALMA 870 μm data provide information about the spatial extent of dust emission, and all of which have dust-obscured SFR and dust-based gas mass estimates available from our previous study. Results. We divided our sample into equally large subsamples of main-sequence (MS) objects and starbursts (factor of > 3 above the MS), and found their K-S relations to be of the form ΣSFR ∝Σgas0.81±0.01 and ΣSFR ∝Σgas0.84±0.39, respectively. Conclusions. The slightly sub-linear K-S slopes we derived suggest that the SF efficiency (SFE) is nearly constant across the Σgas range probed. Under the assumption of a Galactic CO-to-H2 conversion factor (αCO) for the whole sample, the MS SMGs obey a constant global SFE of about 21% per 100 Myr, while that of starburst SMGs is about 27% per 100 Myr. The corresponding gas depletion times are ~ 480 Myr and 370 Myr. On average, our SMGs have Σgas ≳ 103.9 M⊙ pc-2, which suggests that they are Eddington-limited. This is consistent with the theoretical expectation of a linear K-S relation for such systems. However, size measurements of the CO-emitting regions of SMGs, and the αCO values of SMGs are needed to further constrain their Σgas values.
Aims. We exploit deep observations of the GOODS-N field taken with PACS, the Photodetector Array Camera and Spectrometer, onboard of Herschel, as part of the PACS evolutionary probe guaranteed time ...(PEP), to study the link between star formation and stellar mass in galaxies to z ~ 2. Methods. Starting from a stellar mass – selected sample of ~4500 galaxies with mag4.5 µm < 23.0 (AB), we identify ~350 objects with a PACS detection at 100 or 160 ~1500 with only Spitzer 24 μm counterpart. Stellar masses and total IR luminosities (LIR) are estimated by fitting the spectral energy distributions (SEDs). Results. Consistently with other Herschel results, we find that LIR based only on 24 μm data is overestimated by a median factor ~1.8 at z ~ 2, whereas it is underestimated (with our approach) up to a factor ~1.6 at 0.5 < z < 1.0. We then exploit this calibration to correct LIR based on the MIPS/Spitzer fluxes. These results clearly show how Herschel is fundamental to constrain LIR, and hence the star formation rate (SFR), of high redshift galaxies. Using the galaxies detected with PACS (and/or MIPS), we investigate the existence and evolution of the relations between the SFR, the specific star formation rate (SSFR=SFR/mass) and the stellar mass. Moreover, in order to avoid selection effects, we also repeat this study through a stacking analysis on the PACS images to fully exploit the far-IR information also for the Herschel and Spitzer undetected subsamples. We find that the SSFR-mass relation steepens with redshift, being almost flat at z < 1.0 and reaching a slope of α = -0.50+0.13-0.16 at z ~ 2, at odds with recent works based on radio-stacking analysis at the same redshift. The mean SSFR of galaxies increases with redshift, by a factor ~15 for massive M > 1011 $M_{\odot}$ galaxies from z = 0 to z = 2, and seems to flatten at z > 1.5 in this mass range. Moreover, the most massive galaxies have the lowest SSFR at any z, implying that they have formed their stars earlier and more rapidly than their low mass counterparts (downsizing).
ABSTRACT
We present Herschel–PACS spectroscopy of four main-sequence star-forming galaxies at z ∼ 1.5. We detect OI63 μm line emission in BzK-21000 at z = 1.5213, and measure a line luminosity, ...$L_{\rm O\, {\small I}63\, \mu m} = (3.9\pm 0.7)\times 10^9$ L⊙. Our PDR modelling of the interstellar medium in BzK-21000 suggests a UV radiation field strength, G ∼ 320G0, and gas density, n ∼ 1800 cm−3, consistent with previous LVG modelling of the molecular CO line excitation. The other three targets in our sample are individually undetected in these data, and we perform a spectral stacking analysis which yields a detection of their average emission and an O i63 μm line luminosity, $L_{\rm O\, {\small I}63\, \mu m} = (1.1\pm 0.2)\times 10^9$ L⊙. We find that the implied luminosity ratio, $L_{\rm O\, {\small I}63\, \mu m}/L_{\rm IR}$, of the undetected BzK-selected star-forming galaxies broadly agrees with that of low-redshift star-forming galaxies, while BzK-21000 has a similar ratio to that of a dusty star-forming galaxy at z ∼ 6. The high O i63 μm line luminosities observed in BzK-21000 and the z ∼ 1−3 dusty and sub-mm luminous star-forming galaxies may be associated with extended reservoirs of low density, cool neutral gas.
We present detections of the CO(4-3) and C i 609 m spectral lines, as well as the dust continuum at 480.5 GHz (rest frame), in 3C 368, a Fanaroff-Riley class II (FR-II) galaxy at redshift (z) 1.131. ...3C 368 has a large stellar mass, ∼ 3.6 × 1011 M , and is undergoing an episode of vigorous star formation, at a rate of ∼ 350 M yr−1, and active galactic nucleus activity, with radio-emitting lobes extended over ∼ 73 kpc. Our observations allow us to inventory the molecular-gas reservoirs in 3C 368 by applying three independent methods: (1) using the CO(4-3)-line luminosity, excitation state of the gas, and an CO conversion factor, (2) scaling from the C i-line luminosity, and (3) adopting a gas-to-dust conversion factor. We also present gas-phase metallicity estimates in this source, both using far-infrared fine-structure lines together with radio free-free continuum emission and independently employing the optical O iii 5007 and O ii 3727 lines (R23 method). Both methods agree on a subsolar gas-phase metallicity of ∼ 0.3 Z . Intriguingly, comparing the molecular-gas mass estimated using this subsolar metallicity, Mgas ∼ 6.4 × 1010 M , to dust-mass estimates from multicomponent spectral energy distribution modeling, Mdust ∼ 1.4 × 108 M , yields a gas-to-dust ratio within ∼ 15% of the accepted value for a metallicity of 0.3 Z . The derived gas mass puts 3C 368 on a par with other galaxies at z ∼ 1 in terms of specific star formation rate and gas fraction. However, it does not explain how a galaxy can amass such a large stellar population while maintaining such a low gas-phase metallicity. Perhaps 3C 368 has recently undergone a merger, accreting pristine molecular gas from an external source.
Context. The observed spatial scale of the radio continuum emission from star-forming galaxies can be used to investigate the spatial extent of active star formation, constrain the importance of ...cosmic-ray transport, and examine the effects of galaxy interactions. Aims. We determine the radio size distribution of a large sample of 152 submillimetre galaxies (SMGs) in the COSMOS field that were pre-selected at 1.1 mm, and later detected with the Atacama Large Millimetre/submillimetre Array (ALMA) in the observed-frame 1.3 mm dust continuum emission at a signal-to-noise ratio (S/N) of ≥5. Methods. We used the deep, subarcsecond-resolution (1σ = 2.3μJy beam-1; .̋ 75) centimetre radio continuum observations taken by the Karl G. Jansky Very Large Array (VLA)-COSMOS 3 GHz Large Project. Results. One hundred and fifteen of the 152 target SMGs (76% ± 7%) were found to have a 3 GHz counterpart (≥ 4.2σ), which renders the radio detection rate notably high. The median value of the deconvolved major axis full width at half maximum (FWHM) size at 3 GHz is derived to be 0.̋59 ± 0.̋05 , or 4.6 ± 0.4 kpc in physical units, where the median redshift of the sources is z = 2.23 ± 0.13 (23% are spectroscopic and 77% are photometric values). The radio sizes are roughly log-normally distributed, and they show no evolutionary trend with redshift, or difference between different galaxy morphologies. We also derived the spectral indices between 1.4 and 3 GHz, and 3 GHz brightness temperatures for the sources, and the median values were found to be α1.4 GHz3 GHz = -0.67 (Sν ∝ να) and TB = 12.6 ± 2 K. Three of the target SMGs, which are also detected with the Very Long Baseline Array (VLBA) at 1.4 GHz (AzTEC/C24b, 61, and 77a), show clearly higher brightness temperatures than the typical values, reaching TB(3 GHz) > 104.03 K for AzTEC/C61. Conclusions. The derived median radio spectral index agrees with a value expected for optically thin non-thermal synchrotron radiation, and the low median 3 GHz brightness temperature shows that the observed radio emission is predominantly powered by star formation and supernova activity. However, our results provide a strong indication of the presence of an active galactic nucleus in the VLBA and X-ray-detected SMG AzTEC/C61 (high TB and an inverted radio spectrum). The median radio-emitting size we have derived is ~ 1.5–3 times larger than the typical far-infrared dust-emitting sizes of SMGs, but similar to that of the SMGs’ molecular gas component traced through mid-J line emission of carbon monoxide. The physical conditions of SMGs probably render the diffusion of cosmic-ray electrons inefficient, and hence an unlikely process to lead to the observed extended radio sizes. Instead, our results point towards a scenario where SMGs are driven by galaxy interactions and mergers. Besides triggering vigorous starbursts, galaxy collisions can also pull out the magnetised fluids from the interacting disks, and give rise to a taffy-like synchrotron-emitting bridge. This provides an explanation for the spatially extended radio emission of SMGs, and can also cause a deviation from the well-known infrared-radio correlation owing to an excess radio emission. Nevertheless, further high-resolution observations are required to examine the other potential reasons for the very compact dust-emitting sizes of SMGs, such as the radial dust temperature and metallicity gradients.
Abstract
We present the detection of four far-infrared fine-structure oxygen lines, as well as strong upper limits for the CO(2–1) and N
ii
205
μ
m lines, in 3C 368, a well-studied radio-loud galaxy ...at
z
= 1.131. These new oxygen lines, taken in conjunction with previously observed neon and carbon fine-structure lines, suggest a powerful active galactic nucleus (AGN), accompanied by vigorous and extended star formation. A starburst dominated by O8 stars, with an age of ∼6.5 Myr, provides a good fit to the fine-structure line data. This estimated age of the starburst makes it nearly concurrent with the latest episode of AGN activity, suggesting a link between the growth of the supermassive black hole and stellar population in this source. We do not detect the CO(2–1) line, down to a level twelve times lower than the expected value for star-forming galaxies. This lack of CO line emission is consistent with recent star formation activity if the star-forming molecular gas has low metallicity, is highly fractionated (such that CO is photodissociated throughout much of the clouds), or is chemically very young (such that CO has not yet had time to form). It is also possible, although we argue it is unlikely, that the ensemble of fine-structure lines is emitted from the region heated by the AGN.
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![An extended [C ii ] halo a...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0035-8711/small "An extended [C ii ] halo around a massive star-forming galaxy at z = 5.3")