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Register a new userALMA [CII] 158um imaging of an HI-Selected Major Merger at z~4
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Added by
J. Xavier Prochaska
Publication date
2020
fields
Physics
and research's language is
English
Authors
J. Xavier Prochaska
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We present high spatial-resolution (~2kpc) Atacama Large Millimeter/submillimeter Array (ALMA) observations of [CII] 158um and dust-continuum emission from a galaxy at z=3.7978 selected by its strong HI absorption (a damped Lya absorber, DLA) against a background QSO. Our ALMA images reveal a pair of star-forming galaxies separated by ~6kpc (projected) undergoing a major merger. Between these galaxies is a third emission component with highly elevated (2x) [CII] 158um emission relative to the dust continuum, which is likely to arise from stripped gas associated with the merger. This merger of two otherwise-normal galaxies is not accompanied by enhanced star-formation, contrary to mergers detected in most luminosity-selected samples. The DLA associated with the merger exhibits extreme kinematics, with a velocity width for the low-ionization metal lines of Dv90~470km/s that spans the velocity spread revealed in the [CII] 158um emission. We propose that DLAs with high Dv90 values are a signpost of major mergers in normal galaxies at high redshifts, and use the distribution of the velocity widths of metal lines in high-z DLAs to provide a rough estimate the fraction of z>3 galaxies that are undergoing a major merger.
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We report on a search for the [CII] 158 micron emission line from galaxies associated with four high-metallicity damped Ly-alpha absorbers (DLAs) at z ~ 4 using the Atacama Large Millimeter/sub-millimeter Array (ALMA). We detect [CII] 158 micron emission from galaxies at the DLA redshift in three fields, with one field showing two [CII] emitters. Combined with previous results, we now have detected [CII] 158 micron emission from five of six galaxies associated with targeted high-metallicity DLAs at z ~ 4. The galaxies have relatively large impact parameters, ~16 - 45 kpc, [CII] 158 micron line luminosities of (0.36 - 30) x 10^8 Lsun, and rest-frame far-infrared properties similar to those of luminous Lyman-break galaxies, with star-formation rates of ~7 - 110 Msun yr-1. Comparing the absorption and emission line profiles yields a remarkable agreement between the line centroids, indicating that the DLA traces gas at velocities similar to that of the [CII] 158 micron emission. This disfavors a scenario where the DLA arises from gas in a companion galaxy. These observations highlight ALMAs unique ability to uncover a high redshift galaxy population that has largely eluded detection for decades.
We report the detection of [CII]158um emission from a system of three closely-separated sources in the COSMOS field at z~4.56, as part of the ALMA Large Program to INvestigate CII at Early times (ALPINE). The two dominant sources are closely associated, both spatially (1.6~11kpc) and in velocity (~100km/s), while the third source is slightly more distant (2.8~18kpc, ~300km/s). The second strongest source features a slight velocity gradient, while no significant velocity gradient is seen in the other two sources. Using the observed [CII] luminosities, we derive a total log(SFR_[CII]/[Msol/year])=2.8+/-0.2, which may be split into contributions of 59%, 31%, and 10% from the central, east, and west sources, respectively. Comparison of these [CII] detections to recent zoom-in cosmological simulations suggests an ongoing major merger. We are thus witnessing a system in a major phase of mass build-up by merging, including an on-going major merger and an upcoming minor merger, which is expected to end up in a single massive galaxy by z~2.5.
We present a survey of the [CII] 158 $mu$m line and underlying far-infrared (FIR) dust continuum emission in a sample of 27 z>6 quasars using the Atacama Large Millimeter Array (ALMA) at ~1 resolution. The [CII] line was significantly detected (at >5-sigma) in 23 sources (85%). We find typical line luminosities of $L_{rm [CII]}=10^{9-10}$ L$_odot$, and an average line width of ~385 km/s. The [CII]-to-far-infrared luminosity ratio ([CII]/FIR) in our sources span one order of magnitude, highlighting a variety of conditions in the star-forming medium. Four quasar host galaxies are clearly resolved in their [CII] emission on a few kpc scales. Basic estimates of the dynamical masses of the host galaxies give masses between $2times10^{10}$ and $2times10^{11}$ M$_odot$, i.e., more than an order of magnitude below what is expected from local scaling relations, given the available limits on the masses of the central black holes ($>3times10^8$ M$_odot$, assuming Eddington-limited accretion). In stacked ALMA [CII] spectra of individual sources in our sample, we find no evidence of a deviation from a single Gaussian profile. The quasar luminosity does not strongly correlate with either the [CII] luminosity or equivalent width. This survey (with typical on-source integration times of 8 min) showcases the unparalleled sensitivity of ALMA at millimeter wavelengths, and offers a unique reference sample for the study of the first massive galaxies in the universe.
We present a study of the HI gas content of a large K-band selected sample of 88 close major-merger pairs of galaxies (H-KPAIR) which were observed by $it Herschel$. We obtained the 21 cm HI fine-structure emission line data for a total of 70 pairs from this sample, by observing 58 pairs using the Green Bank Telescope (GBT) and retrieving the HI data for an addition 12 pairs from the literature. In this HI sample, 34 pairs are spiral-spiral (S+S) pairs, and 36 are spiral-elliptical (S+E). Based on these data, we studied the HI-to-stellar mass ratio, the HI gas fraction and the HI star formation efficiency (SFE$_{mathrm{HI}}$ = star formation rate/$M_{mathrm{HI}}$) and searched for differences between S+S and S+E pairs, as well as between pairs with and without signs for merger/interaction. Our results showed that the mean HI-to-stellar mass ratio of spirals in these pairs is $=7.6pm1.0 %$, consistent with the average HI gas fraction of spiral galaxies in general. The differences in the HI gas fraction between spirals in S+S and in S+E pairs, and between spirals in pairs with and without signs of merger/interaction are insignificant ($< 1 sigma$). On the other hand, the mean SFE$_{mathrm{HI}}$ of S+S pairs is $sim4.6times$ higher than that of S+E pairs. This difference is very significant ($sim 4sigma$) and is the main result of our study. There is no significant difference in the mean SFE$_{mathrm{HI}}$ between galaxies with and without signs of merger/interaction. The mean SFE$_{mathrm{HI}}$ of the whole pair sample is $10^{-9.55pm 0.09} mathrm{yr}^{-1}$, corresponding to a HI consumption time of $3.5pm0.7$ Gyrs.
Galaxy mergers are thought to be one of the main mechanisms of the mass assembly of galaxies. Recently, many works have suggested a possible increase in the fraction of major mergers in the early Universe, reviving the debate on which processes (e.g., cold accretion, star formation, mergers) most contribute to the mass build-up of galaxies through cosmic time. To estimate the importance of major mergers in this context, we make use of the new data collected by the ALMA Large Program to INvestigate [CII] at Early times (ALPINE), which observed the [CII] 158 $mu$m emission line from a sample of 75 main-sequence star-forming galaxies at 4.4 < z < 5.9. We used, for the first time, the morpho-kinematic information provided by the [CII] emission to obtain the fraction of major mergers ($f_{MM}$) at z~5. By adopting different prescriptions for the merger timescales ($T_{MM}$), we converted this fraction into the merger rate per galaxy ($R_{MM}$) and per volume ($Gamma_{MM}$). We then combined our results with those at lower redshifts from the literature, computing the cosmic evolution of the merger fraction. This is described by a rapid increase from z~0 to higher redshifts, a peak at z~3, and a slow decrease towards earlier epochs. Depending on the timescale prescription used, this fraction translates into a merger rate ranging between ~0.1 and ~4.0 Gyr$^{-1}$ at z~5. Finally, we compare the specific star formation and star-formation rate density with the analogous quantities from major mergers. Our new ALPINE data reveal the presence of a significant merging activity in the early Universe. However, whether this population of mergers can provide a relevant contribution to the galaxy mass assembly at these redshifts and through the cosmic epochs is strongly dependent on the assumption of the merger timescale.
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