arXiv subjects

Lihwai Lin

Publications and source records attributed to Lihwai Lin.

At least 37 records · Page 2Linked to original sources

arXiv2022

The Molecular-Gas Main Sequence and Schmidt-Kennicutt relation are fundamental, the Star-Forming Main Sequence is a (useful) byproduct

We investigate the relationship between the star formation rate (SFR), stellar mass ($M_*$) and molecular gas mass ($M_{H_2}$) for local star-forming galaxies. We further investigate these relationships for high-z (z=1-3) galaxies and for the hosts of a local sample of Active Galactic Nuclei (AGN). We explore which of these dependencies are intrinsic and which are an indirect by-product by employing partial correlation coefficients and random forest regression. We find that for local star-forming galaxies, high-z galaxies, and AGN host galaxies, the Schmidt-Kennicutt relation (SK, between $M_{H_2}$ and SFR), and the Molecular Gas Main Sequence (MGMS, between $M_{H_2}$ and $M_*$) are intrinsic primary relations, while the relationship between $M_*$ and SFR, i.e. the Star-Forming Main Sequence (SFMS), is an indirect by-product of the former two. Hence the Star-Forming Main Sequence is not a fundamental scaling relation for local or high-redshift galaxies. We find evidence for both the evolution of the MGMS and SK relation over cosmic time, where, at a given stellar mass, the higher the redshift, the greater the molecular gas mass and the star formation efficiency. We offer a parameterisation of both the MGMS and SK relation's evolution with redshift, showing how they combine to form the observed evolution of the SFMS. In addition, we find that the local AGN host galaxies follow an AGN-MGMS relation (as well as a AGN-SK relation), where the MGMS is offset to lower $M_{H_2}$ for a given $M_*$ compared to local SF galaxies.

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arXiv2022

LoTSS Jellyfish Galaxies IV: Enhanced Star Formation on the Leading Half of Cluster Galaxies and Gas Compression in IC3949

With MaNGA integral field spectroscopy, we present a resolved analysis of star formation for 29 jellyfish galaxies in nearby clusters, identified from radio continuum imaging taken by the Low Frequency Array. Simulations predict enhanced star formation on the "leading half" of galaxies undergoing ram pressure stripping, and in this work we report observational evidence for this elevated star formation. The dividing line (through the galaxy center) that maximizes this star formation enhancement is systematically tied to the observed direction of the ram pressure stripped tail, suggesting a physical connection between ram pressure and this star formation enhancement. We also present a case study on the distribution of molecular gas in one jellyfish galaxy from our sample, IC3949, using ALMA CO J=1-0, HCN J=1-0, and HCO$^+$ J=1-0 observations from the ALMaQUEST survey. The $\mathrm{H_2}$ depletion time (as traced by CO) in IC3949 ranges from $\sim\!1\,\mathrm{Gyr}$ in the outskirts of the molecular gas disk to $\sim\!11\,\mathrm{Gyr}$ near the galaxy center. IC3949 shows a clear region of enhanced star formation on the leading half of the galaxy where the average depletion time is $\sim\!2.7\,\mathrm{Gyr}$, in line with the median value for the galaxy on the whole. Dense gas tracers, HCN and HCO$^+$, are only detected at the galaxy center and on the leading half of IC3949. Our results favour a scenario in which ram pressure compresses the interstellar medium, promoting the formation of molecular gas that in turn fuels a localized increase of star formation.

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arXiv2022

Resolved Molecular Gas Observations of MaNGA Post-starbursts Reveal a Tumultuous Past

Post-starburst galaxies (PSBs) have recently and rapidly quenched their star-formation, thus they are an important way to understand how galaxies transition from star-forming late-types to quiescent early-types. The recent discovery of large cold gas reservoirs in PSBs calls into question the theory that galaxies must lose their gas to become quiescent. Optical Integral Field Spectroscopy (IFS) surveys have revealed two classes of PSBs: central PSBs with central quenching regions and ring PSBs with quenching in their outskirts. We analyze a sample of 13 nearby (z < 0.1) PSBs with spatially resolved optical IFS data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey and matched resolution Atacama Large (sub-)Millimeter Array (ALMA) observations of $^{12}$CO(1-0). Disturbed stellar kinematics in 7/13 of our PSBs and centrally concentrated molecular gas is consistent with a recent merger for most of our sample. In galaxies without merger evidence, alternate processes may funnel gas inwards and suppress star-formation, which may include outflows, stellar bars, and minor mergers or interactions. The star-formation efficiencies of the post-starburst regions in nearly half our galaxies are suppressed while the gas fractions are consistent with star-forming galaxies. AGN feedback may drive this stabilization, and we observe AGN-consistent emission in the centers of 5/13 galaxies. Finally, our central and ring PSBs have similar properties except the ionized and molecular gas in central PSBs is more disturbed. Overall, the molecular gas in our PSBs tends to be compact and highly disturbed, resulting in concentrated gas reservoirs unable to form stars efficiently.

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arXiv2022

The cold gas and dust properties of red star-forming galaxies

We study the cold gas and dust properties for a sample of red star forming galaxies called "red misfits." We collect single-dish CO observations and HI observations from representative samples of low-redshift galaxies, as well as our own JCMT CO observations of red misfits. We also obtain SCUBA-2 850 um observations for a subset of these galaxies. With these data we compare the molecular gas, total cold gas, and dust properties of red misfits against those of their blue counterparts ("blue actives") taking non-detections into account using a survival analysis technique. We compare these properties at fixed position in the log SFR-log M* plane, as well as versus offset from the star-forming main sequence. Compared to blue actives, red misfits have slightly longer molecular gas depletion times, similar total gas depletion times, significantly lower molecular- and total-gas mass fractions, lower dust-to-stellar mass ratios, similar dust-to-gas ratios, and a significantly flatter slope in the $\log M_\mathrm{mol}$-$\log M_\star$ plane. Our results suggest that red misfits as a population are likely quenching due to a shortage in gas supply.

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arXiv2022

SDSS-IV MaNGA: Unveiling Galaxy Interaction by Merger Stages with Machine Learning

We use machine learning techniques to classify galaxy merger stages, which can unveil physical processes that drive the star formation and active galactic nucleus (AGN) activities during galaxy interaction. The sample contains 4,690 galaxies from the integral field spectroscopy survey SDSS-IV MaNGA, and can be separated to 1,060 merging galaxies and 3630 non-merging or unclassified galaxies. For the merger sample, there are 468, 125, 293, and 174 galaxies in (1) incoming pair phase, (2) first pericentric passage phase, (3) aproaching or just passing the apocenter, and (4) final coalescence phase or post-mergers. With the information of projected separation, line-of-sight velocity difference, SDSS gri images, and MaNGA Ha velocity map, we are able to classify the mergers and their stages with good precision, which is the most important score to identify interacting galaxies. For the 2-phase classification (binary; non-merger and merger), the performance can be high (precision>0.90) with LGBMClassifier. We find that sample size can be increased by rotation, so the 5-phase classification (non-merger, 1, 2, 3, and 4 merger stages) can be also good (precision>0.85). The most important features come from SDSS gri images. The contribution from MaNGA Ha velocity map, projected separation, and line-of-sight velocity difference can further improve the performance by 0-20%. In other words, the image and the velocity information are sufficient to capture important features of galaxy interactions, and our results can apply to the entire MaNGA data as well as future all-sky surveys.

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arXiv2022

The ALMaQUEST Survey X: What powers merger induced star formation?

Galaxy mergers are known to trigger both extended and central star formation. However, what remains to be understood is whether this triggered star formation is facilitated by enhanced star formation efficiencies, or an abundance of molecular gas fuel. This work presents spatially resolved measurements of CO emission collected with the Atacama Large Millimetre Array (ALMA) for 20 merging galaxies (either pairs or post-mergers) selected from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. Eleven additional merging galaxies are selected from the ALMA MaNGA QUEnching and STar formation (ALMaQUEST) survey, resulting in a set of 31 mergers at various stages of interaction and covering a broad range of star formation rates (SFR). We investigate galaxy-to-galaxy variations in the resolved Kennicutt-Schmidt relation (rKS: $Σ_{H_2}$ vs. $Σ_{SFR}$), the resolved molecular gas main sequence (rMGMS: $Σ_{\star}$ vs. $Σ_{H_2}$), and the resolved star-forming main sequence (rSFMS: $Σ_{\star}$ vs. $Σ_{SFR}$). We quantify offsets from these resolved relations to determine if star formation rate, molecular gas fraction, and/or star formation efficiency (SFE) is enhanced in different regions of an individual galaxy. By comparing offsets in all three parameters we can discern whether gas fraction or SFE powers an enhanced $Σ_{SFR}$. We find that merger-induced star formation can be driven by a variety of mechanisms, both within a galaxy and between different mergers, regardless of interaction stage.

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arXiv2022

Exploring stellar and ionized gas non--circular motions in barred galaxies with MUSE

We present MUSE integral field stellar and ionized velocity maps for a sample of 14 barred galaxies. Most of these objects exhibit "S"-shape iso-velocities in the bar region indicative of the presence of streaming motions in the velocity fields. % By applying circular rotation models we observe that bars leave symmetric structures in the residual maps of the stellar velocity. %which demonstrates the capabilities of the MUSE instrument for detecting kinematic bar signatures. % We built non-circular rotation models using the \xs~tool to characterize the observed velocity fields. In particular we adopt bisymmetric models and a harmonic decomposition for a bar potential for describing the non-axisymmetric velocities. We find that both models reproduce the observed kinematic features. % The position angle of the oval distortion estimated from the bisymmetric model correlates with the photometric bar position angle $(ρ_{pearson} = 0.95)$, which suggest that non-circular velocities are caused by the bar. However because of the low amplitudes of the $s_3$ harmonic we can not rule out radial flows as possible source. % Because of the weak detection of \ha~in our objects we are not able to compare gas to stellar non-circular motions in our sample, although we show that when galaxies are gas rich the oval distortion is also observed but with larger amplitudes. % Finally, we do not find evidence that the amplitude of the non-circular motions is dependent on the bar size, stellar mass or the global SFR.

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arXiv2022

The ALMaQUEST survey. VIII. What causes the velocity discrepancy between CO and H$α$ rotation curves in galaxies?

We compare the CO(1-0) and H$α$ kinematics in 34 nearby galaxies, selected from the ALMaQUEST and EDGE-CALIFA surveys. We use 3-D Barolo, a 3D tilted ring model, to derive the CO and H$α$ rotation curves. Before comparing rotation curves in the 34 nearby galaxies, we found systematics between the MaNGA and the CALIFA data using eight MaNGA-CALIFA overlapping galaxies. We assume the rotation curves based on the MaNGA data are accurate and made the corresponding correction to the CALIFA data. Our result shows that $\sim$56% (19/34) of our galaxies present slower H$α$ rotation curves compared to the CO rotation curves, with a median value of 6.5 km/s. The remaining galaxies (15/34) show consistent CO-H$α$ rotation velocity within uncertainties. As a result, the Ha rotation may underestimate the total dynamical mass by ~6% for a circular velocity of 200 km/s (the median value in our sample). Furthermore, the velocity difference between the CO and H$α$ rotation velocity is found to correlate with the velocity dispersion difference between CO and H$α$, suggesting that the gas pressure plays a role in the velocity discrepancy. After incorporating the effect of pressure support due to the turbulent gas motion to our sample, the median value of the velocity differences decreases to 1.9 km/s, which in turn reduces the underestimation of dynamical mass to $\sim$2%. Finally, we also investigate the role that the extra-planar diffuse ionized gas (eDIG) plays in the CO-H$α$ velocity discrepancy.

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arXiv2022

MUSUBI (MegaCam Ultra-deep Survey: $u^\ast$-Band Imaging)-Data for the COSMOS and SXDS Fields

The Subaru Hyper Suprime-Cam (HSC) Strategic Survey is the latest-generation multi-band optical imaging survey for galaxy evolution and structure formation. The "Ultra-Deep" component of the HSC survey provides $grizy$ broad-band images over $\sim3.4$ deg$^2$ to detection limits of $\sim26$-28 AB, along with narrow-band images, in the COSMOS and the SXDS fields. These images provide an unprecedented combination of depths and area coverage, for the studies galaxies up to $z\sim7$. However, the lack of coverage at $<4000$ Ang implies incomplete sampling of the rest-frame UV at $z\lesssim 3$, which is critically needed for understanding the buildup of stellar mass in the later cosmic time. We conducted a multi-year CFHT $u^\ast$-band imaging campaign in the two HSC Ultra-Deep fields with CFHT MegaCam. By including shallower archival data, we reach 5-$σ$ depths of $u^\ast=28.1$ and 28.4 (AB) at the centers of the COSMOS and SXDS fields, respectively, and $u^\ast=27.7$ and 27.8 in the central 1 deg$^2$ fields. The image quality is $\gtrsim0.90$ arcsec, fairly good for the $u^\ast$ band. Both the photometric and astrometric quality of our data are excellent. We show that the combination of our $u^\ast$-band and HSC data can lead to high-quality photometric redshifts at $z=0$-3, and robust measurements of rest-frame UV on galaxies at $0.4<z<0.6$ for distinguishing green-valley galaxies from star-forming and quiescent galaxies. We publicly release our reduced $u^\ast$-band images and reference catalogs that can be used readily for scientific studies.

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arXiv2022

ALMaQUEST -- VII: Star Formation Scaling Relations of Green Valley Galaxies

We utilize the ALMA-MaNGA QUEnch and STar formation (ALMaQUEST) survey to investigate the kpc-scale scaling relations, presented as the resolved star forming main sequence (rSFMS: $Σ_{\rm SFR}$ vs. $Σ_{*}$), the resolved Schmidt-Kennicutt relation (rSK: $Σ_{\rm SFR}$ vs. $Σ_{\rm H_{2}}$), and the resolved molecular gas main sequence (rMGMS: $Σ_{\rm H_{2}}$ vs. $Σ_{*}$), for 11478 star-forming and 1414 retired spaxels (oversampled by a factor of $\sim20$) located in 22 green valley (GV) and 12 main sequence (MS) galaxies. For a given galaxy type (MS or GV), the retired spaxels are found to be offset from the sequences formed by the star-forming spaxels on the rSFMS, rSK, and rMGMS planes, toward lower absolute values of sSFR, SFE, and $f_{\rm H_{2}}$ by $\sim$ 1.1, 0.6, and 0.5 dex. The scaling relations for GV galaxies are found to be distinct from that of the MS galaxies, even if the analyses are restricted to the star-forming spaxels only. It is found that for star-forming spaxels, sSFR, SFE, and $f_{\rm H_{2}}$ in GV galaxies are reduced by $\sim$0.36, 0.14, and 0.21 dex, respectively, compared to those in MS galaxies. Therefore, the suppressed sSFR/SFE/$f_{\rm H_{2}}$ in GV galaxies are associated with not only an increased proportion of retired regions in GV galaxies but also a depletion of these quantities in star-forming regions. Finally, the reduction of SFE and $f_{\rm H_{2}}$ in GV galaxies relative to MS galaxies is seen in both bulge and disk regions (albeit with larger uncertainties), suggesting that statistically, quenching in the GV population may persist from the inner to the outer regions.

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arXiv2022

The ALMaQUEST Survey IX: The nature of the resolved star forming main sequence

We investigate the nature of the scaling relations between the surface density of star formation rate ($Σ_{\rm SFR}$), stellar mass ($Σ_*$), and molecular gas mass ($Σ_{\rm H_2}$), aiming at distinguishing between the relations that are primary, i.e. more fundamental, and those which are instead an indirect by-product of the other relations. We use the ALMaQUEST survey and analyse the data by using both partial correlations and Random Forest regression techniques. We unambiguously find that the strongest intrinsic correlation is between $Σ_{\rm SFR}$ and $Σ_{\rm H_2}$ (i.e. the resolved Schmidt-Kennicutt relation), followed by the correlation between $Σ_{\rm H_2}$ and $Σ_*$ (resolved Molecular Gas Main Sequence, rMGMS). Once these two correlations are taken into account, we find that there is no evidence for any intrinsic correlation between $Σ_{\rm SFR}$ and $Σ_*$, implying that SFR is entirely driven by the amount of molecular gas, while its dependence on stellar mass (i.e. the resolved Star Forming Main Sequence, rSFMS) simply emerges as a consequence of the relationship between molecular gas and stellar mass.

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arXiv2022

Star Formation Properties of Sloan Digital Sky Survey BOSS Void Galaxies in the Hyper Suprime-Cam Survey

We utilize the Hyper Suprime-Cam (HSC) Wide Survey to explore the properties of galaxies located in the voids identified from the Baryon Oscillation Spectroscopic Survey (BOSS) up to z~0.7. The HSC reaches i~25, allowing us to characterize the void galaxies down to 10$^{9.2}$ solar mass. We find that the revised void galaxy densities, when including faint galaxies in voids defined by bright galaxies, are still underdense compared to the mean density from the entire field. In addition, we classify galaxies into star-forming, quiescent, and green valley populations, and find that void galaxies tend to have slightly higher fractions of star-forming galaxies under the mass and redshift control, although the significance of this result is only moderate (2$σ$). However, when we focus on the star-forming population, the distribution of the specific star formation rate (sSFR) of void galaxies shows little difference from that of the control galaxies. Similarly, the median sSFR of star-forming void galaxies is also in good agreement with that of the star-forming control galaxies. Moreover, the effective green valley fraction of void galaxies, defined as the number of green valley galaxies over the number of nonquiescent galaxies, is comparable to that of the control ones, supporting the suggestion that void and control galaxies evolve under similar physical processes and quenching frequencies. Our results thus favor a scenario of the galaxy assembly bias.

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arXiv2021

An IFU View of the Active Galactic Nuclei in MaNGA Galaxy Pairs

The role of active galactic nuclei (AGNs) during galaxy interactions and how they influence the star formation in the system are still under debate. We use a sample of 1156 galaxies in galaxy pairs or mergers (hereafter `pairs') from the MaNGA survey. This pair sample is selected by the velocity offset, projected separation, and morphology, and is further classified into four cases along the merger sequence based on morphological signatures. We then identify a total of 61 (5.5%) AGNs in pairs based on the emission-line diagnostics. No evolution of the AGN fraction is found, either along the merger sequence or compared to isolated galaxies (5.0%). We observe a higher fraction of passive galaxies in galaxy pairs, especially in the pre-merging cases, and associate the higher fraction to their environmental dependence. The isolated AGN and AGN in pairs show similar distributions in their global stellar mass, star formation rate (SFR), and central [OIII] surface brightness. AGNs in pairs show radial profiles of increasing specific SFR and declining Dn4000 from center to outskirts, and no significant difference from the isolated AGNs. This is clearly different from star-forming galaxies (SFGs) in our pair sample, which show enhanced central star formation, as reported before. AGNs in pairs have lower Balmer decrements at outer regions, possibly indicating less dust attenuation. Our findings suggest that AGNs likely follow an inside-out quenching and the merger impact on the star formation in AGNs is less prominent than in SFGs.

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arXiv2021

SDSS-IV MaNGA: the physical origin of off-galaxy H$α$ blobs in the local Universe

H$α$ blobs are off-galaxy emission-line regions with weak or no optical counterparts. They are mostly visible in H$α$ line, appearing as concentrated blobs. Such unusual objects have been rarely observed and studied, and their physical origin is still unclear. We have identified 13 H$α$ blobs in the public data of MaNGA survey, by visually inspecting both the optical images and the spatially resolved maps of H$α$ line for $\sim 4600$ galaxy systems. Among the 13 H$α$ blobs, 2 were reported in previously MaNGA-based studies and 11 are newly discovered. This sample, though still small in size, is by far the largest sample with both deep imaging and integral field spectroscopy. Therefore, for the first time we are able to perform statistical studies to investigate the physical origin of H$α$ blobs. We examine the physical properties of these H$α$ blobs and their associated galaxies, including their morphology, environments, gas-phase metallicity, kinematics of ionized gas, and ionizing sources. We find that the H$α$ blobs in our sample can be broadly divided into two groups. One is associated with interacting/merging galaxy systems, of which the ionization is dominated by shocks or diffuse ionized gas. It is likely that these H$α$ blobs used to be part of their nearby galaxies, but were stripped away at some point due to tidal interactions. The other group is found in gas-rich systems, appearing as low-metallicity star-forming regions that are visually detached from the main galaxy. These H$α$ blobs could be associated with faint disks, spiral arms, or dwarf galaxies.

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arXiv2021

SITELLE Hα Imaging Spectroscopy of z~0.25 Clusters: Emission Line Galaxy Detection and Ionized Gas Offset in Abell 2390 & Abell 2465

Environmental effects are crucial to the understanding of the evolution of galaxies in dense environments, such as galaxy clusters. Using the large field-of-view of SITELLE, the unique imaging fourier transform spectrograph at CFHT, we are able to obtain 2D spectral information for a large and complete sample of cluster galaxies out to the infall region. We describe a pipeline developed to identify emission line galaxies (ELGs) from the datacube using cross-correlation techniques. We present results based on the spatial offsets between the emission-line regions and stellar continua in ELGs from two z$\sim$0.25 galaxy clusters, Abell 2390 and Abell 2465. We find a preference in the offsets being pointed away from the cluster center. Combining the two clusters, there is a 3$σ$ excess for high-velocity galaxies within the virial radius having the offsets to be pointed away from the cluster center. Assuming the offset being a proxy for the velocity vector of a galaxy, as expected from ram pressure stripping, this excess indicates that ram pressure stripping occurs most effectively during the first passage of an infalling galaxy, leading to the quenching of its star formation. We also find that, outside the virial region, the continuum-normalized H$α$ line flux for infalling galaxies with large offsets are on average lower than those with small or no measurable offset, further supporting ram pressure as a dominant quenching mechanism during the initial infall stages.

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arXiv2020

The ALMaQUEST Survey: V. The non-universality of kpc-scale star formation relations and the factors that drive them

Using a sample of ~15,000 kpc-scale star-forming spaxels in 28 galaxies drawn from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we investigate the galaxy-to-galaxy variation of the `resolved' Schmidt-Kennicutt relation (rSK; Sigma_H2 - Sigma_SFR), the `resolved' star forming main sequence (rSFMS; Sigma_* - Sigma_SFR) and the `resolved' molecular gas main sequence (rMGMS; Sigma_* - Sigma_H2). The rSK relation, rSFMS and rMGMS all show significant galaxy-to-galaxy variation in both shape and normalization, indicating that none of these relations is universal between galaxies. The rSFMS shows the largest galaxy-to-galaxy variation and the rMGMS the least. By defining an `offset' from the average relations, we compute a Delta_rSK, Delta_rSFMS, Delta_rMGMS for each galaxy, to investigate correlations with global properties. We find the following correlations with at least 2 sigma significance: the rSK is lower (i.e. lower star formation efficiency) in galaxies with higher M_*, larger Sersic index and lower specific SFR (sSFR); the rSFMS is lower (i.e. lower sSFR) in galaxies with higher M_* and larger Sersic index; the rMGMS is lower (i.e. lower gas fraction) in galaxies with lower sSFR. In the ensemble of all 15,000 data points, the rSK relation and rMGMS show equally tight scatters and strong correlation coefficients, compared with a larger scatter and weaker correlation in the rSFMS. Moreover, whilst there is no correlation between Delta_rSK and Delta_rMGMS in the sample, the offset of a galaxy's rSFMS does correlate with both of the other two offsets. Our results therefore indicate that the rSK and rMGMS are independent relations, whereas the rSFMS is a result of their combination.

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arXiv2020

The ALMaQUEST Survey: VI. The molecular gas main sequence of `retired' regions in galaxies

In order to investigate the role of gas in the demise of star formation on kpc-scales, we compare the resolved molecular gas main sequence (rMGMS: Sigma_* vs Sigma_H2) of star-forming regions to the sequence of `retired' regions that have ceased to form new stars. Using data from the ALMaQUEST survey, we find that retired spaxels form a rMGMS that is distinct from that of star-forming spaxels, offset to lower Sigma_H2 at fixed Sigma_* by a factor of ~5. We study the rMGMS of star-forming and retired spaxels on a galaxy-by-galaxy basis for eight individual ALMaQUEST galaxies. Six of these galaxies have their retired spaxels concentrated within the central few kpc. Molecular gas is detected in 40-100% of retired spaxels in the eight galaxies in our sample. Both the star-forming and retired rMGMS show a diversity in normalization from galaxy-to-galaxy. However, in any given galaxy, the rMGMS for retired regions is found to be distinct from the star-forming sequence and gas fractions of retired spaxels are up to an order of magnitude lower than the star-forming spaxels. We conclude that quenching is associated with a depletion (but not absence) of molecular gas via a mechanism that typically begins in the centre of the galaxy.

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arXiv2020

ALMaQUEST -- IV. The ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) Survey

The ALMaQUEST (ALMA-MaNGA QUEnching and STar formation) survey is a program with spatially-resolved $^{12}$CO(1-0) measurements obtained with the Atacama Large Millimeter Array (ALMA) for 46 galaxies selected from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) DR15 optical integral-field spectroscopic survey. The aim of the ALMaQUEST survey is to investigate the dependence of star formation activity on the cold molecular gas content at kpc scales in nearby galaxies. The sample consists of galaxies spanning a wide range in specific star formation rate (sSFR), including starburst (SB), main-sequence (MS), and green valley (GV) galaxies. In this paper, we present the sample selection and characteristics of the ALMA observations, and showcase some of the key results enabled by the combination of spatially-matched stellar populations and gas measurements. Considering the global (aperture-matched) stellar mass, molecular gas mass, and star formation rate of the sample, we find that the sSFR depends on both the star formation efficiency (SFE) and the molecular gas fraction ($f_{\rm H_{2}}$), although the correlation with the latter is slightly weaker. Furthermore, the dependence of sSFR on the molecular gas content (SFE or $f_{\rm H_{2}}$) is stronger than that on either the atomic gas fraction or the molecular-to-atomic gas fraction, albeit with the small HI sample size. On kpc scales, the variations in both SFE and $f_{\rm H_{2}}$ within individual galaxies can be as large as 1-2 dex thereby demonstrating that the availability of spatially-resolved observations is essential to understand the details of both star formation and quenching processes.

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