arXiv subjects

Zhizheng Pan

Publications and source records attributed to Zhizheng Pan.

At least 19 recordsLinked to original sources

arXiv2025

On the formation of star-forming galaxies having anomalously low-metallicity regions: the role of galaxy interaction and CGM/IGM accretion

Gas accretion from both the circum-galactic medium (CGM)/inter-galatic medium (IGM) and interacting companion galaxy can dilute the gas phase metallicity of a galaxy. However, their relative contribution to the chemical evolution of galaxies remains to be quantified. To this end, in this work we study a sample of 510 star-forming galaxies (SFGs) having anomalously low-metallicity (ALM) regions selected from the MaNGA data available in the Data Release 17 from the Sloan Digital Sky Survey. ALM regions are defined as those having gas phase metallicities that are at least $\sim 2σ$ lower than the emprical relation between stellar mass surface density ($Σ_{*}$) and gas phase metallicity, i.e., the $Σ_{*}-Z$ relation. We find that ALM galaxies have higher star formation rates and \Hi~gas fractions than normal SFGs at fixed $M_*$. $\sim$25\% of the ALM galaxies exhibit tidal features, while the tidal fraction is only $\sim$12\% for normal SFGs, indicating that galaxy interaction is an important factor responsible for the ALM phenomenon. To explore the origin of non-tidal ALM galaxies, we compare their morphologies and environments with those of mass-matched normal SFGs. We find that non-tidal ALM galaxies tend to have more disk-dominated morphologies and reside in less-dense environment. These findings suggest that cold gas accretion from the CGM/IGM is the primary cause for the ALM phenomenon, while galaxy interaction plays a minor but non-negligible role.

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arXiv2025

The H$α$ concentration of local star-forming galaxies: implications for galaxy structure formation

In this work, we present a study on the H$α$ emission line flux concentration of 3098 low-redshift star-forming galaxies (SFGs) using the MaNGA data available in the Data Release 17 from the Sloan Digital Sky Survey. We define the H$α$ flux concentration index ($C_{\rm Hα}$) as $C_{\rm Hα}=F_{\rm Hα,0.8~R_e}/F_{\rm Hα,1.5~R_e}$, where $F_{\rm Hα,0.8~R_e}$ and $F_{\rm Hα,1.5~R_e}$ are the cumulative H$α$ flux inside $0.8$ and $1.5$ $r-$band effective radius, respectively. We find that $C_{\rm Hα}$ is strongly correlated with the luminosity weighted stellar age gradient. $C_{\rm Hα}$ is also sensitive to environmental effects, in the sense that low-mass satellite galaxies below the star formation main sequence tend to have higher $C_{\rm Hα}$. For central galaxies, we find that massive disk galaxies with enhanced star formation rate tend to have higher $C_{\rm Hα}$, while such a phenomenon is not seen in the low-mass regime. We interpret this as evidence that compaction events more frequently occur in the high-mass regime, which eventually resulting in the buildup of prominent bulges in massive SFGs. Implications of these findings on galaxy structure formation are discussed.

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arXiv2025

A Glimpse of Satellite Galaxies in the Milky Way with the 2.5-meter Wide Field Survey Telescope (WFST): Bootes III and Draco

We carry out deep imaging of the Milky Way satellite galaxies, Bootes III and Draco, with WFST as one pilot observing program to demonstrate the capability of WFST. Combining catalogs with PS1 DR2 and Gaia DR3, we derive proper motions for candidate member stars in these two satellite galaxies over a 12-year time baseline, yielding uncertainties of ~1.8 mas/yr at 21 mag and ~3.0 mas/yr at 22 mag in the r band. The proper motions derived from bright and faint stars are consistent, indicating no significant variation in proper motion across stellar luminosity as these galaxies undergo tidal interactions with the MW. Meanwhile, we suggest that Bootes III represents the bound remnant of the progenitor galaxy that gave rise to the Styx stream, as evidenced by its elongated density profile and overdensity in both spatial and kinematic space. This is the first paper to use WFST to measure the proper motions of faint stars in Milky Way satellite galaxies. More detailed analyses will be presented in forthcoming papers from the wide field survey (WFS) program.

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arXiv2025

Balmer Decrement and IRX Break in Tracing Dust Attenuation at Scales of Individual Star-forming Regions in NGC 628

We investigate the relationships between infrared excess (IRX=$L_{\rm IR}/L_{\rm UV}$) and Balmer decrement (${\rm H}α/{\rm H}β$) as indicators of dust attenuation for 609 ${\rm {H\,{\small II}}}$ regions at scales of $\sim 50-200$ pc in NGC 628, utilizing data from AstroSat, James Webb Space Telescope (JWST) and Multi Unit Spectroscopic Explorer (MUSE). Our findings indicate that about three fifths of the sample ${\rm {H\,{\small II}}}$ regions reside within the regime occupied by local star-forming galaxies (SFGs) along the dust attenuation correlation described by their corresponding color excess parameters $E(B-V)_{\rm IRX} = 0.51\,E(B-V)_{{\rm H}α/{\rm H}β}$. Nearly 27$\%$ of the sample exhibits $E(B-V)_{\rm IRX}> E(B-V)_{{\rm H}α/{\rm H}β}$, while a small fraction ($\sim 13\%$) displays significantly lower $E(B-V)_{\rm IRX}$ compared to $E(B-V)_{{\rm H}α/{\rm H}β}$. These results suggest that the correlation between the two dust attenuation indicators no longer holds for spatially resolved ${\rm {H\,{\small II}}}$ regions. Furthermore, the ratio of $E(B-V)_{\rm IRX}$ to $E(B-V)_{{\rm H}α/{\rm H}β}$ remains unaffected by various physical parameters of the ${\rm {H\,{\small II}}}$ regions, including star formation rate (SFR), SFR surface density, infrared luminosity ($L_{\rm IR}$), $L_{\rm IR}$ surface density, stellar mass, gas-phase metallicity, circularized radius, and the distance to galactic center. We argue that the ratio is primarily influenced by the evolution of surrounding interstellar medium (ISM) of the star-forming regions, transitioning from an early dense and thick phase to the late blown-away stage.

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arXiv2025

Searching for Nearby Diffuse Dwarf Galaxies in the COSMOS Field

It remains challenging to systematically survey nearby diffuse dwarf galaxies and address the formation mechanism of this population distinguishing from regular ones. We carry out a pilot search for these galaxies in the COSMOS field using the deep \textit{HST}/F814W imaging data. We report three diffuse dwarf galaxies satisfying the criteria: (1) redshift $z<0.2$, (2) effective radius $r_{\rm e}>1.0''$, and (3) central surface brightness $μ_{\rm 0}>24$ mag arcsec$^{-2}$. Two of the three galaxies, COSMOS-UDG1 and COSMOS-UDG2, are recognized as ultra-diffuse galaxies (UDGs) with redshift $z=0.130$ and $0.049$, respectively. The third galaxy, COSMOS-dw1, is spectroscopically confirmed as a dwarf galaxy at $z=0.004$. We derive the physical properties through fitting their spectral energy distributions (SEDs) extracted from deep multiwavelength observations. COSMOS-dw1 has a stellar mass of $5.6_{-2.7}^{+2.5}\times10^{6}$ M$_{\odot}$, harboring neutral hydrogen gas of mass $4.90\pm0.90\times10^{6}$ M$_{\odot}$, hinting that this galaxy may be in the nascent stages of quenching. The estimated dynamical mass of $3.4\times10^{7}\,M_{\odot}$ further suggests that COSMOS-dw1 is predominantly of dark matter. COSMOS-UDG1 and COSMOS-UDG2 exhibit comparable stellar masses of $\sim 2\times10^{8}$ M$_{\odot}$. Notably, COSMOS-UDG1 is younger and more metal-rich than COSMOS-UDG2 and COSMOS-dw1. Conversely, COSMOS-UDG2 and COSMOS-dw1 have similar stellar metallicities, yet COSMOS-UDG2 is older than COSMOS-dw1. All three galaxies adhere to the stellar mass-metallicity relation (MZR) for dwarf galaxies in the local Universe, implying they belong to the dwarf galaxy population.

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arXiv2024

The Dust Attenuation Scaling Relation of Star-Forming Galaxies in the EAGLE Simulations

Dust attenuation in star-forming galaxies (SFGs), as parameterized by the infrared excess (IRX $\equiv L_{\rm IR}/L_{\rm UV}$), is found to be tightly correlated with star formation rate (SFR), metallicity and galaxy size, following a universal IRX relation up to $z=3$. This scaling relation can provide a fundamental constraint for theoretical models to reconcile galaxy star formation, chemical enrichment, and structural evolution across cosmic time. We attempt to reproduce the universal IRX relation over $0.1\leq z\leq 2.5$ using the EAGLE hydrodynamical simulations and examine sensitive parameters in determining galaxy dust attenuation. Our findings show that while the predicted universal IRX relation from EAGLE approximately aligns with observations at $z\leq 0.5$, noticeable disparities arise at different stellar masses and higher redshifts. Specifically, we investigate how modifying various galaxy parameters can affect the predicted universal IRX relation in comparison to the observed data. We demonstrate that the simulated gas-phase metallicity is the critical quantity for the shape of the predicted universal IRX relation. We find that the influence of the infrared luminosity and infrared excess is less important while galaxy size has virtually no significant effect. Overall, the EAGLE simulations are not able to replicate some of the observed characteristics between IRX and galaxy parameters of SFGs, emphasizing the need for further investigation and testing for our current state-of-the-art theoretical models.

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arXiv2023

Local star-forming galaxies build up central mass concentration most actively near $M_{*}=10^{10}M_{\odot}$

To understand in what mass regime star-forming galaxies (SFGs) build up central mass concentration most actively, we present a study on the luminosity-weighted stellar age radial gradient ($\nabla_{\rm age}$) distribution of $\sim3600$ low-redshift SFGs using the MaNGA Pipe3D data available in the SDSS DR17. The mean age gradient is negative, with $\nabla_{\rm age}=-0.14$log Gyr/$R_{\rm e}$, consistent with the inside-out disk formation scenario. Specifically, SFGs with positive $\nabla_{\rm age}$ consist of $\sim 28\%$ at log$(M_{*}/M_{\odot})<9.5$, while this fraction rises up to its peak ($\sim 40\%$) near log$(M_{*}/M_{\odot})=10$ and then decreases to $\sim 15\%$ at log$(M_{*}/M_{\odot})=11$. At fixed $M_{*}$, SFGs with positive $\nabla_{\rm age}$ typically have more compact sizes and more centrally concentrated star formation than their counterparts, indicative of recent central mass build-up events. These results suggest that the build-up of central stellar mass concentration in local SFGs is mostly active near $M_{*}=10^{10}M_{\odot}$. Our findings provide new insights on the origin of morphological differences between low-mass and high-mass SFGs.

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arXiv2023

Sciences with the 2.5-meter Wide Field Survey Telescope (WFST)

The Wide Field Survey Telescope (WFST) is a dedicated photometric surveying facility being built jointly by the University of Science and Technology of China and the Purple Mountain Observatory. It is equipped with a 2.5-meter diameter primary mirror, an active optics system, and a mosaic CCD camera with 0.73 gigapixels on the primary focal plane for high-quality image capture over an FOV of 6.5-square-degree. It is anticipated that WFST will be set up at the Lenghu site in the summer of 2023 and begin to observe the northern sky in four optical bands (u, g, r, and i) with a range of cadences, from hourly/daily in the Deep High-Cadence Survey (DHS) program to semiweekly in the Wide-Field Survey (WFS) program, three months later. During a photometric night, a nominal 30 s exposure in the WFS program will reach a depth of 22.27, 23.32, 22.84, and 22.31 (AB magnitudes) in these four bands, respectively, allowing for the detection of a tremendous amount of transients in the low-z universe and a systematic investigation of the variability of Galactic and extragalactic objects. In the DHS program, intranight 90 s exposures as deep as 23 (u) and 24 mag (g), in combination with target of opportunity follow-ups, will provide a unique opportunity to explore energetic transients in demand for high sensitivities, including the electromagnetic counterparts of gravitational wave events, supernovae within a few hours of their explosions, tidal disruption events and fast, luminous optical transients even beyond a redshift of unity. In addition, the final 6-year co-added images, anticipated to reach g=25.8 mag in WFS or 1.5 mags deeper in DHS, will be of fundamental importance to general Galactic and extragalactic science. The highly uniform legacy surveys of WFST will serve as an indispensable complement to those of LSST that monitor the southern sky.

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arXiv2023

What boost galaxy mergers in two massive galaxy protoclusters at z = 2.24

Characterizing the structural properties of galaxies in high-redshift protoclusters is key to our understanding of the environmental effects on galaxy evolution in the early stages of galaxy and structure formation. In this study, we assess the structural properties of 85 and 87 Halpha emission-line candidates (HAEs) in the densest regions of two massive protoclusters, BOSS1244 and BOSS1542, respectively, using HST H-band imaging data. Our results show a true pair fraction of 22+-5 (33+-6) percent in BOSS1244 (BOSS1542), which yields a merger rate of 0.41+-0.09 (0.52+-0.04) per Gyr for massive HAEs with log (M_*/M_sun) > 10.3. This rate is 1.8 (2.8) times higher than that of the general fields at the same epoch. Our sample of HAEs exhibits half-light radii and Sersic indices that cover a broader range than field star-forming galaxies. Additionally, about 15 percent of the HAEs are as compact as the most massive (log(M_*/M_sun) > 11) spheroid-dominated population. These results suggest that the high galaxy density and cold dynamical state (i.e., velocity dispersion of <400 km/s) are key factors that drive galaxy mergers and promote structural evolution in the two protoclusters. Our findings also indicate that both the local environment (on group scales) and the global environment play essential roles in shaping galaxy morphologies in protoclusters. This is evident in the systematic differences observed in the structural properties of galaxies between BOSS1244 and BOSS1542.

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arXiv2022

The Physical Properties of Star-Forming Galaxies with Strong [O III] Lines at z=3.25

We present an analysis of physical properties of 34 [O III] emission-line galaxies (ELGs) at z=3.254$\pm$0.029 in the Extended Chandra Deep Field South (ECDFS). These ELGs are selected from deep narrow H2S(1) and broad Ks imaging of 383 arcmin$^{2}$ obtained with CFHT/WIRCam. We construct spectral energy distributions (SEDs) from U to Ks to derive the physical properties of ELGs. These [O III] ELGs are identified as starburst galaxies with strong [O III] lines of L([O III]) ~ 10$^{42.6}$ - 10$^{44.2}$ erg s$^{-1}$, and have stellar masses of M* ~ 10$^{9.0}$-10$^{10.6}$ M$_\odot$ and star formation rates of ~ 10-210 M$_\odot$ yr$^{-1}$. Our results show that 24% of our sample galaxies are dusty with Av > 1 mag and EW(OIII)$_{rest}$ ~ 70-500 $Å$, which are often missed in optically selected [O III] ELG samples. Their rest-frame UV and optical morphologies from HST/ACS and HST/WFC3 deep imaging reveal that these [O III] ELGs are mostly multiple-component systems (likely mergers) or compact. And 20% of them are nearly invisible in the rest-frame UV owing to heavy dust attenuation. Interestingly, we find that our samples reside in an overdensity consisting of two components: one southeast (SE) with an overdensity factor of $δ_{gal}$ ~ 41 over a volume of 13$^{3}$ cMpc$^{3}$ and the other northwest (NW) with $δ_{gal}$ ~ 38 over a volume of 10$^{3}$ cMpc$^{3}$. The two overdense substructures are expected to be virialized at z=0 with a total mass of ~ 1.1 x 10$^{15}$ M$_\odot$ and ~ 4.8 x 10$^{14}$ M$_\odot$, and probably merge into a Coma-like galaxy cluster.

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arXiv2022

Submillimetre galaxies in two massive protoclusters at z = 2.24: witnessing the enrichment of extreme starbursts in the outskirts of HAE density peaks

Submillimetre galaxies represent a rapid growth phase of both star formation and massive galaxies. Mapping SMGs in galaxy protoclusters provides key insights into where and how these extreme starbursts take place in connections with the assembly of the large-scale structure in the early Universe. We search for SMGs at 850$\,μm$ using JCMT/SCUBA-2 in two massive protoclusters at $z=2.24$, BOSS1244 and BOSS1542, and detect 43 and 54 sources with $S_{850}>4\,$mJy at the $4σ$ level within an effective area of 264$\,$arcmin$^2$, respectively. We construct the intrinsic number counts and find that the abundance of SMGs is $2.0\pm0.3$ and $2.1\pm0.2$ times that of the general fields, confirming that BOSS1244 and BOSS1542 contain a higher fraction of dusty galaxies with strongly enhanced star formation. The volume densities of the SMGs are estimated to be $\sim15-$30 times the average, significantly higher than the overdensity factor ($\sim 6$) traced by H$α$ emission-line galaxies (HAEs). More importantly, we discover a prominent offset between the spatial distributions of the two populations in these two protoclusters -- SMGs are mostly located around the high-density regions of HAEs, and few are seen inside these regions. This finding may have revealed for the first time the occurrence of violent star formation enhancement in the outskirts of the HAE density peaks, likely driven by the boosting of gas supplies and/or starburst triggering events. Meanwhile, the lack of SMGs inside the most overdense regions at $z\sim2$ implies a transition to the environment disfavouring extreme starbursts.

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arXiv2022

Systematic biases in determining dust attenuation curves through galaxy SED fitting

While the slope of the dust attenuation curve ($δ$) is found to correlate with effective dust attenuation ($A_V$) as obtained through spectral energy distribution (SED) fitting, it remains unknown how the fitting degeneracies shape this relation. We examine the degeneracy effects by fitting SEDs of a sample of local star-forming galaxies (SFGs) selected from the Galaxy And Mass Assembly survey, in conjunction with mock galaxy SEDs of known attenuation parameters. A well-designed declining starburst star formation history is adopted to generate model SED templates with intrinsic UV slope ($β_0$) spanning over a reasonably wide range. The best-fitting $β_0$ for our sample SFGs shows a wide coverage, dramatically differing from the limited range of $β_0<-2.2$ for a starburst of constant star formation. Our results show that strong degeneracies between $β_0$, $δ$, and $A_V$ in the SED fitting induce systematic biases leading to a false $A_V$--$δ$ correlation. Our simulation tests reveal that this relationship can be well reproduced even when a flat $A_V$--$δ$ relation is taken to build the input model galaxy SEDs. The variations in best-fitting $δ$ are dominated by the fitting errors. We show that assuming a starburst with constant star formation in SED fitting will result in a steeper attenuation curve, smaller degeneracy errors, and a stronger $A_V$--$δ$ relation. Our findings confirm that the $A_V$--$δ$ relation obtained through SED fitting is likely driven by the systematic biases induced by the fitting degeneracies between $β_0$, $δ$, and $A_V$.

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arXiv2021

The cosmic environment overtakes the local density in shaping galaxy star formation

The gas supply from the cosmic web is the key to sustain star formation in galaxies. It remains to be explored how the cosmic large-scale structure (LSS) effects on galaxy evolution at given local environments. We examine galaxy specific star formation rate as a function of local density in a LSS at $z=0.735$ in the Extended Chandra Deep Field South. The LSS is mapped by 732 galaxies with $R<24$\,mag and redshift at $0.72\le z \le 0.75$ collected from the literature and our spectroscopic observations with Magellan/IMACS, consisting of five galaxy clusters/groups and surrounding filaments over an area of $23.9 \times22.7$\,co-moving\,Mpc$^2$. The spread of spectroscopic redshifts corresponds a velocity dispersion of 494\,km\,s$^{-1}$, indicating the LSS likely to be a thin sheet with a galaxy density $\gtrsim 3.9$ times that of the general field. These clusters/groups in this LSS mostly exhibit elongated morphologies and multiple components connected with surrounding filaments. Strikingly, we find that star-forming galaxies in the LSS keep star formation at the same level as field, and show no dependence on local density but stellar mass. Meanwhile, an increasing fraction of quiescent galaxies is detected at increasing local density in both the LSS and the field, consistent with the expectation that galaxy mass and local dense environment hold the key to quench star formation. Combined together, we conclude that the cosmic environment of the LSS overtakes the local environment in remaining galaxy star formation to the level of the field.

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arXiv2021

The role of HI in regulating size growth of local galaxies

We study the role of atomic hydrogen (HI) in regulating size growth of local galaxies. The size of a galaxy, $D_{\rm r,~25}$, is characterized by the diameter at which the $r-$band surface brightness reaches $μ_{\rm r}=25.0~\rm mag~arcsec^{-2}$. We find that the positions of galaxies in the size ($D_{\rm r,~25}$)$-$stellar mass ($M_{\ast}$) plane strongly depend on their HI-to-stellar mass ratio ($M_{\rm HI}/M_{\ast}$). In the HI-rich regime, galaxies that are more rich in HI tend to have larger sizes. Such a trend is not seen in the HI-poor regime, suggesting that size growth is barely affected by the HI content when it has declined to a sufficiently low level. An investigation of the relations between size, $M_{\rm HI}/M_{\ast}$ and star formation rate (SFR) suggests that size is more intrinsically linked with $M_{\rm HI}/M_{\ast}$, rather than SFR. We further examine the HI-to-stellar disk size ratio ($D_{\rm HI}/D_{\rm r,~25}$) of galaxies and find that at log($M_{\rm HI}/M_{\ast})>-0.7$, $D_{\rm HI}/D_{\rm r,~25}$ is weakly correlated with $M_{\ast}$. These findings support a picture in which the HI-rich galaxies live in an inside-out disk growing phase regulated by gas accretion and star formation. The angular momentum of the accreted materials is probably the key parameter in shaping the size of an HI-rich galaxy.

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arXiv2021

From haloes to galaxies. III. The gas cycle of local galaxy populations

In Dou et al. (2021), we introduced the Fundamental Formation Relation (FFR), a tight relation between specific SFR (sSFR), H$_2$ star formation efficiency (SFE$_{\rm H_2}$), and the ratio of H$_2$ to stellar mass. Here we show that atomic gas HI does not follow a similar FFR as H$_2$. The relation between SFE$_{\rm HI}$ and sSFR shows significant scatter and strong systematic dependence on all of the key galaxy properties that we have explored. The dramatic difference between HI and H$_2$ indicates that different processes (e.g., quenching by different mechanisms) may have very different effects on the HI in different galaxies and hence produce different SFE$_{\rm HI}$-sSFR relations, while the SFE$_{\rm H_2}$-sSFR relation remains unaffected. The facts that SFE$_{\rm H_2}$-sSFR relation is independent of other key galaxy properties, and that sSFR is directly related to the cosmic time and acts as the cosmic clock, make it natural and very simple to study how different galaxy populations (with different properties and undergoing different processes) evolve on the same SFE$_{\rm H_2}$-sSFR $\sim t$ relation. In the gas regulator model (GRM), the evolution of a galaxy on the SFE$_{\rm H_2}$-sSFR($t$) relation is uniquely set by a single mass-loading parameter $λ_{\rm net,H_2}$. This simplicity allows us to accurately derive the H$_2$ supply and removal rates of the local galaxy populations with different stellar masses, from star-forming galaxies to the galaxies in the process of being quenched. This combination of FFR and GRM, together with the stellar metallicity requirement, provide a new powerful tool to study galaxy formation and evolution.

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arXiv2020

Long Tidal Tails in Merging Galaxies and Their Implications

We investigate the properties of long tidal tails using the largest to date sample of 461 merging galaxies with $\log(M_\ast/\rm M_\odot)\geq9.5$ within $0.2 \leq z \leq 1$ from the COSMOS survey in combination with {\it Hubble Space Telescope} imaging data. Long tidal tails can be briefly divided into three shape types: straight (41\,per\,cent), curved (47\,per\,cent) and plume (12\,per\,cent). Their host galaxies are mostly at late stages of merging, although 31\,per\,cent are galaxy pairs with projected separations $d>20$\,kpc. The high formation rate of straight tidal tails needs to be understood as the projection of curved tidal tails accounts for only a small fraction of the straight tails. We identify 165 tidal dwarf galaxies (TDGs), yielding a TDG production rate of 0.36 per merger. Combined with a galaxy merger fraction and a TDG survival rate from the literature, we estimate that $\sim$\,5\,per\,cent of local dwarf galaxies are of tidal origin, suggesting the tidal formation is not an important formation channel for the dwarf galaxies. More than half of TDGs are located at the tip of their host tails. These TDGs have stellar masses in the range of $7.5\leq\log (M_\ast/\rm M_\odot)\leq9.5$ and appear compact with half-light radii following the $M_\ast$ - $R_{\rm e}$ relation of low-mass elliptical galaxies. However, their surface brightness profiles are generally flatter than those of local disc galaxies. Only 10 out of 165 TDGs have effective radii larger than 1.5 kpc and would qualify as unusually bright ultra-diffuse galaxies.

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arXiv2020

Understanding the Discrepancy between IRX and Balmer Decrement in Tracing Galaxy Dust Attenuation

We compare the infrared excess (IRX) and Balmer decrement (${\rm Hα/Hβ}$) as dust attenuation indicators in relation to other galaxy parameters using a sample of $\sim$32 000 local star-forming galaxies (SFGs) carefully selected from SDSS, GALEX and WISE. While at fixed ${\rm Hα/Hβ}$, IRX turns out to be independent on galaxy stellar mass, the Balmer decrement does show a strong mass dependence at fixed IRX. We find the discrepancy, parameterized by the color excess ratio $R_{\rm EBV} \equiv E(B-V)_{\rm IRX}/E(B-V)_{\rm Hα/Hβ}$, is not dependent on the gas-phase metallicity and axial ratio but on the specific star formation rate (SSFR) and galaxy size ($R_{\rm e}$) following $R_{\rm EBV}=0.79+0.15\log({\rm SSFR}/R_{\rm e}^{2})$. This finding reveals that the nebular attenuation as probed by the Balmer decrement becomes increasingly larger than the global (stellar) attenuation of SFGs with decreasing SSFR surface density. This can be understood in the context of an enhanced fraction of intermediate-age stellar populations that are less attenuated by dust than the HII region-traced young population, in conjunction with a decreasing dust opacity of the diffuse ISM when spreading over a larger spatial extent. Once the SSFR surface density of an SFG is known, the conversion between attenuation of nebular and stellar emission can be well estimated using our scaling relation.

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arXiv2020

xGASS: HI fueling of star formation in disk-dominated galaxies

We introduce a method to estimate the HI mass within the optical radius of disk galaxies from integrated HI spectra, with an uncertainty of 0.09 dex. We use these estimates to study how inner HI fuels star formation in late-type disk galaxies. We find that star formation rate (SFR) at a given stellar mass ($M_*$) is well correlated with the inner HI surface density ($Σ_{\rm HI,in}$) and inner HI mass-to-stellar mass ratio. For the massive ($M_*>10^{10} M_{\odot}$) disk galaxies, higher SFR at a given stellar mass is also related with higher efficiency of converting inner HI to molecular gas, but no such correlation is found for the total HI mass. The highest $Σ_{\rm HI,in}$ and the fastest depletion of the total neutral gas within the optical disks are found in the most compact and star-forming disk galaxies at a given stellar mass. These results highlight the important role of inner HI as an intermediate step of fueling star formation in disk galaxies.

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