arXiv subjects

Rieko Momose

Publications and source records attributed to Rieko Momose.

At least 55 records · Page 3Linked to original sources

arXiv2017

Surface density: a new parameter in the fundamental metallicity relation of star-forming galaxies

Star-forming galaxies display a close relation among stellar mass, metallicity and star-formation rate (or molecular-gas mass). This is known as the fundamental metallicity relation (FMR) (or molecular-gas FMR), and it has a profound implication on models of galaxy evolution. However, there still remains a significant residual scatter around the FMR. We show here that a fourth parameter, the surface density of stellar mass, reduces the dispersion around the molecular-gas FMR. In a principal component analysis of 29 physical parameters of 41,338 star-forming galaxies, the surface density of stellar mass is found to be the fourth most important parameter. The new four-dimensional (4D) fundamental relation forms a tighter hypersurface that reduces the metallicity dispersion to 50% of that of the molecular-gas FMR. We suggest that future analyses and models of galaxy evolution should consider the FMR in a 4D space that includes surface density. The dilution time scale of gas inflow and the star-formation efficiency could explain the observational dependence on surface density of stellar mass. AKARI is expected to play an important role in shedding light on the infrared properties of the new 4D FMR.

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arXiv2017

SILVERRUSH. II. First Catalogs and Properties of ~2,000 Lya Emitters and Blobs at z~6-7 Identified over the 14-21 deg2 Sky

We present an unprecedentedly large catalog consisting of 2,230 > L* Lya emitters (LAEs) at z=5.7 and 6.6 on the 13.8 and 21.2 deg2 sky, respectively, that are identified by the SILVERRUSH program with the first narrowband imaging data of the Hyper Suprime-Cam (HSC) survey. We confirm that the LAE catalog is reliable on the basis of 96 LAEs whose spectroscopic redshifts are already determined by this program and the previous studies. This catalogue is also available on-line. Based on this catalogue, we derive the rest-frame Lya equivalent-width distributions of LAEs at z~5.7-6.6 that are reasonably explained by the exponential profiles with the scale lengths of ~120-170A, showing no significant evolution from z~5.7 to z~6.6. We find that 275 LAEs with a large equivalent width (LEW) of >240A are candidates of young-metal poor galaxies and AGNs. We also find that the fraction of LEW LAEs to all ones is 4% and 21% at z~5.7 and z~6.6, respectively. Our LAE catalog includes 11 Lya blobs (LABs) that are LAEs with spatially extended Lya emission whose profile is clearly distinguished from those of stellar objects at the >~ 3sigma level. The number density of the LABs at z=6-7 is ~ 10^-7-10^-6 Mpc^-3, being ~ 10-100 times lower than those claimed for LABs at z~ 2-3, suggestive of disappearing LABs at z>~6, albeit with the different selection methods and criteria for the low and high-z LABs.

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arXiv2017

SILVERRUSH. III. Deep Optical and Near-Infrared Spectroscopy for Lya and UV-Nebular Lines of Bright Lya Emitters at z=6-7

We present Lya and UV-nebular emission line properties of bright Lya emitters (LAEs) at z=6-7 with a luminosity of log L_Lya/[erg s-1] = 43-44 identified in the 21-deg2 area of the SILVERRUSH early sample developed with the Subaru Hyper Suprime-Cam (HSC) survey data. Our optical spectroscopy newly confirm 21 bright LAEs with clear Lya emission, and contribute to make a spectroscopic sample of 96 LAEs at z=6-7 in SILVERRUSH. From the spectroscopic sample, we select 7 remarkable LAEs as bright as Himiko and CR7 objects, and perform deep Keck/MOSFIRE and Subaru/nuMOIRCS near-infrared spectroscopy reaching the 3sigma-flux limit of ~ 2x10^{-18} erg s-1 for the UV-nebular emission lines of He II1640, C IV1548,1550, and O III]1661,1666. Except for one tentative detection of C IV, we find no strong UV-nebular lines down to the flux limit, placing the upper limits of the rest-frame equivalent widths (EW_0) of ~2-4 A for He II, C IV, and O III] lines. Here we also investigate the VLT/X-SHOOTER spectrum of CR7 whose 6 sigma detection of He II is claimed by Sobral et al. Although two individuals and the ESO-archive service carefully re-analyze the X-SHOOTER data that are used in the study of Sobral et al., no He II signal of CR7 is detected, supportive of weak UV-nebular lines of the bright LAEs even for CR7. Spectral properties of these bright LAEs are thus clearly different from those of faint dropouts at z~7 that have strong UV-nebular lines shown in the various studies. Comparing these bright LAEs and the faint dropouts, we find anti-correlations between the UV-nebular line EW_0 and UV-continuum luminosity, which are similar to those found at z~2-3.

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arXiv2017

A 16 deg$^2$ survey of emission-line galaxies at z<1.5 in HSC-SSP PDR1

We present initial results from the Subaru Strategic Program (SSP) with Hyper Suprime-Cam (HSC) on a comprehensive survey of emission-line galaxies at z<1.5 based on narrowband (NB) imaging. The first Public Data Release (PDR1) provides us with data from two NB filters, specifically NB816 and NB921 over 5.7 deg$^2$ and 16.2 deg$^2$ respectively. The $5 σ$ limiting magnitudes are 25.2 (UDeep layer, 1.4 deg$^2$) and 24.8 (Deep layer, 4.3 deg$^2$) mag in NB816, and 25.1 (UDeep, 2.9 deg$^2$) and 24.6--24.8 (Deep, 13.3 deg$^2$) mag in NB921. The wide-field imaging allows us to construct unprecedentedly large samples of 8,054 H$α$ emitters at z ~ 0.25 and 0.40, 8,656 [OIII] emitters at z ~ 0.63 and 0.84, and 16,877 [OII] emitters at z ~ 1.19 and 1.47. We map the cosmic web on scales out to about 50 comoving Mpc that includes galaxy clusters, identified by red sequence galaxies, located at the intersection of filamentary structures of star-forming galaxies. The luminosity functions of emission-line galaxies are measured with precision and consistent with published studies. The wide field coverage of the data enables us to measure the luminosity functions up to brighter luminosities than previous studies. The comparison of the luminosity functions between the different HSC-SSP fields suggests that a survey volume of $>5\times10^5$ Mpc$^3$ is essential to overcome cosmic variance. Since the current data have not reached the full depth expected for the HSC-SSP, the color cut in i-NB816 or z-NB921 induces a bias towards star-forming galaxies with large equivalent widths, primarily seen in the stellar mass functions for the H$α$ emitters at z ~ 0.25--0.40. Even so, the emission-line galaxies clearly cover a wide range of luminosity, stellar mass, and environment, thus demonstrating the usefulness of the NB data from the HSC-SSP to investigate star-forming galaxies at z<1.5.

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arXiv2017

First Data Release of the Hyper Suprime-Cam Subaru Strategic Program

The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is a three-layered imaging survey aimed at addressing some of the most outstanding questions in astronomy today, including the nature of dark matter and dark energy. The survey has been awarded 300 nights of observing time at the Subaru Telescope and it started in March 2014. This paper presents the first public data release of HSC-SSP. This release includes data taken in the first 1.7 years of observations (61.5 nights) and each of the Wide, Deep, and UltraDeep layers covers about 108, 26, and 4 square degrees down to depths of i~26.4, ~26.5, and ~27.0 mag, respectively (5sigma for point sources). All the layers are observed in five broad bands (grizy), and the Deep and UltraDeep layers are observed in narrow bands as well. We achieve an impressive image quality of 0.6 arcsec in the i-band in the Wide layer. We show that we achieve 1-2 per cent PSF photometry (rms) both internally and externally (against Pan-STARRS1), and ~10 mas and 40 mas internal and external astrometric accuracy, respectively. Both the calibrated images and catalogs are made available to the community through dedicated user interfaces and database servers. In addition to the pipeline products, we also provide value-added products such as photometric redshifts and a collection of public spectroscopic redshifts. Detailed descriptions of all the data can be found online. The data release website is https://hsc-release.mtk.nao.ac.jp/.

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arXiv2017

Small-scale Intensity Mapping: Extended Ly$α$, H$α$ and Continuum emission as a Probe of Halo Star Formation in High-redshift Galaxies

Lyman alpha halos are observed ubiquitously around star-forming galaxies at high redshift, but their origin is still a matter of debate. We demonstrate that the emission from faint unresolved satellite sources, $M_{\rm UV} \gtrsim -17$, clustered around the central galaxies may play a major role in generating spatially extended Ly$α$, continuum (${\rm UV + VIS}$) and H$α$ halos. We apply the analytic formalism developed in Mas-Ribas & Dijkstra (2016) to model the halos around Lyman Alpha Emitters (LAEs) at $z=3.1$, for several different satellite clustering prescriptions. In general, our UV and Ly$α$ surface brightness profiles match the observations well at $20\lesssim r \lesssim 40$ physical kpc from the centers of LAEs. We discuss how our profiles depend on various model assumptions and how these can be tested and constrained with future H$α$ observations by the James Webb Space Telescope (JWST). Our analysis shows how spatially extended halos constrain (i) the presence of otherwise undetectable satellite sources, (ii) the integrated, volumetric production rates of Ly$α$ and LyC photons, and (iii) their population-averaged escape fractions. These quantities are all directly relevant for understanding galaxy formation and evolution and, for high enough redshifts, cosmic reionization.

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arXiv2016

Formation, Evolution, and Revolution of Galaxies by SKA: Activities of SKA-Japan Galaxy Evolution Sub-SWG

Formation and evolution of galaxies have been a central driving force in the studies of galaxies and cosmology. Recent studies provided a global picture of cosmic star formation history. However, what drives the evolution of star formation activities in galaxies has long been a matter of debate. The key factor of the star formation is the transition of hydrogen from atomic to molecular state, since the star formation is associated with the molecular phase. This transition is also strongly coupled with chemical evolution, because dust grains, i.e., tiny solid particles of heavy elements, play a critical role in molecular formation. Therefore, a comprehensive understanding of neutral-molecular gas transition, star formation and chemical enrichment is necessary to clarify the galaxy formation and evolution. Here we present the activity of SKA-JP galaxy evolution sub-science working group (subSWG) Our activity is focused on three epochs: z \sim 0, 1, and z > 3. At z \sim 0, we try to construct a unified picture of atomic and molecular hydrogen through nearby galaxies in terms of metallicity and other various ISM properties. Up to intermediate redshifts z \sim 1, we explore scaling relations including gas and star formation properties, like the main sequence and the Kennicutt-Schmidt law of star forming galaxies. To connect the global studies with spatially-resolved investigations, such relations will be plausibly a viable way. For high redshift objects, the absorption lines of HI 21-cm line will be a very promising observable to explore the properties of gas in galaxies. By these studies, we will surely witness a real revolution in the studies of galaxies by SKA.

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arXiv2015

ALMA Census of Faint 1.2 mm Sources Down to ~0.02 mJy: Extragalactic Background Light and Dust-Poor High-z Galaxies

We present statistics of 133 faint 1.2-mm continuum sources detected in about 120 deep ALMA pointing data that include all the archival deep data available by 2015 June. We derive number counts of 1.2 mm continuum sources down to 0.02 mJy partly with the assistance of gravitational lensing, and find that the total integrated 1.2 mm flux of the securely identified sources is 22.9^(+6.7)_(-5.6) Jy deg^(-2) that corresponds to 104^(+31)_(-25) % of the extragalactic background light (EBL) measured by COBE observations. These results suggest that the major 1.2 mm EBL contributors are sources with 0.02 mJy, and that very faint 1.2 mm sources with ~< 0.02 mJy contribute negligibly to the EBL with the possible flattening and/or truncation of number counts in this very faint flux regime. To understand the physical origin of our faint ALMA sources, we measure the galaxy bias bg by the counts-in-cells technique, and place a stringent upper limit of bg < 3.5 that is not similar to bg values of massive DRGs and SMGs but comparable to those of UV-bright sBzKs and LBGs. Moreover, in optical and near-infrared (NIR) deep fields, we identify optical-NIR counterparts for 59% of our faint ALMA sources, majority of which have luminosities, colors, and the IRX-beta relation same as sBzKs and LBGs. We thus conclude that about a half of our faint ALMA sources are dust-poor high-z galaxies as known as sBzKs and LBGs in optical studies, and that these faint ALMA sources are not miniature (U)LIRGs simply scaled down with the infrared brightness.

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arXiv2015

Statistical Properties of Diffuse Lyman-alpha Halos around Star-forming Galaxies at z~2

We present statistical properties of diffuse Lyman-alpha halos (LAHs) around high-$z$ star-forming galaxies with large Subaru samples of Lyman-alpha emitters (LAEs) at $z=2.2$. We make subsamples defined by the physical quantities of LAEs' central Lyman-alpha luminosities, UV magnitudes, Lyman-alpha equivalent widths, and UV slopes, and investigate LAHs' radial surface brightness (SB) profiles and scale lengths $r_n$ as a function of these physical quantities. We find that there exist prominent LAHs around LAEs with faint Lyman-alpha luminosities, bright UV luminosities, and small Lyman-alpha equivalent widths in cumulative radial Lyman-alpha SB profiles. We confirm this trend with the anti-correlation between $r_n$ and Lyman-alpha luminosities (equivalent widths) based on the Spearman's rank correlation coefficient that is $ρ=-0.9$ ($-0.7$) corresponding to the $96\%$ ($93\%$) confidence level, although the correlation between $r_n$ and UV magnitudes is not clearly found in the rank correlation coefficient. Our results suggest that LAEs with properties similar to typical Lyman-break galaxies (with faint Lyman-alpha luminosities and small equivalent widths) possess more prominent LAHs. We investigate scenarios for the major physical origins of LAHs with our results, and find that the cold stream scenario is not preferred, due to the relatively small equivalent widths up to $77$Å in LAHs that include LAEs' central components. There remain two possible scenarios of Lyman-alpha scattering in circum-galactic medium and satellite galaxies that cannot be tested with our observational data.

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arXiv2015

Sensitivity for 21cm Bispectrum from Epoch of Reionization

The 21cm line brightness temperature brings rich information about Epoch of Reionizaton (EoR) and high-$z$ universe (Cosmic Dawn and Dark Age). While the power spectrum is a useful tool to investigate the EoR signal statistically, higher-order statistics such as bispectrum are also valuable because the EoR signal is expected to be highly non-Gaussian. In this paper, we develop a formalism to calculate the bispectrum contributed from the thermal noise taking array configularion of telescopes into account, by extending a formalism for the power spectrum \cite{2006ApJ...653..815M}. We apply our formalism to the ongoing and future telescopes such as expanded Murchison Widefield Array (MWA), LOw Frequency ARray (LOFAR) and Square Kilometre Array (SKA). We find that expanded MWA does not have enough sensitivity to detect the bispectrum signal. On the other hand, LOFAR has better sensitivity and will be able to detect the peaks of the bispectrum as a function of redshift at large scales with comoving wavenumber $k \lesssim 0.03 {\rm Mpc}^{-1}$. The SKA has enough sensitivity to detect the bispectrum for much smaller scales $k \lesssim 0.3 {\rm Mpc}^{-1}$ and redshift $z \lesssim 20$

astro-ph.CO↗

arXiv2015

On the Diffuse Lyman-alpha Halo Around Lyman-alpha Emitting Galaxies

Ly$α$ photons scattered by neutral hydrogen atoms in the circumgalactic media or produced in the halos of star-forming galaxies are expected to lead to extended Ly$α$ emission around galaxies. Such low surface brightness Ly$α$ halos (LAHs) have been detected by stacking Ly$α$ images of high-redshift star-forming galaxies. We study the origin of LAHs by performing radiative transfer modeling of nine $z=3.1$ Lyman-Alpha Emitters (LAEs) in a high resolution hydrodynamic cosmological galaxy formation simulation. We develop a method of computing the mean Ly$α$ surface brightness profile of each LAE by effectively integrating over many different observing directions. Without adjusting any parameters, our model yields an average Ly$α$ surface brightness profile in remarkable agreement with observations. We find that observed LAHs cannot be accounted for solely by photons originating from the central LAE and scattered to large radii by hydrogen atoms in the circumgalactic gas. Instead, Ly$α$ emission from regions in the outer halo is primarily responsible for producing the extended LAHs seen in observations, which potentially includes both star-forming and cooling radiation. With the limit on the star formation contribution set by the ultra-violet (UV) halo measurement, we find that cooling radiation can play an important role in forming the extended LAHs. We discuss the implications and caveats of such a picture.

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arXiv2014

Faint Submillimeter Galaxies Revealed by Multifield Deep ALMA Observations: Number Counts, Spatial Clustering, and A Dark Submillimeter Line Emitter

We present the statistics of faint submillimeter/millimeter galaxies (SMGs) and serendipitous detections of a submillimeter/millimeter line emitter (SLE) with no multi-wavelength continuum counterpart revealed by the deep ALMA observations. We identify faint SMGs with flux densities of 0.1-1.0 mJy in the deep Band 6 and Band 7 maps of 10 independent fields that reduce cosmic variance effects. The differential number counts at 1.2 mm are found to increase with decreasing flux density down to 0.1 mJy. Our number counts indicate that the faint (0.1-1.0 mJy, or SFR_IR ~ 30-300 Msun/yr) SMGs contribute nearly a half of the extragalactic background light (EBL), while the remaining half of the EBL is mostly contributed by very faint sources with flux densities of <0.1 mJy (SFR_IR <~ 30 Msun/yr). We conduct counts-in-cells analysis with the multifield ALMA data for the faint SMGs, and obtain a coarse estimate of galaxy bias, b_g <4. The galaxy bias suggests that the dark halo masses of the faint SMGs are <~ 7x10^12 Msun, which is smaller than those of bright (>1 mJy) SMGs, but consistent with abundant high-z star-forming populations such as sBzKs, LBGs, and LAEs. Finally, we report the serendipitous detection of SLE-1 with continuum counterparts neither in our 1.2 mm-band nor multi-wavelength images including ultra deep HST/WFC3 and Spitzer data. The SLE has a significant line at 249.9 GHz with a signal-to-noise ratio of 7.1. If the SLE is not a spurious source made by unknown systematic noise of ALMA, the strong upper limits of our multi-wavelength data suggest that the SLE would be a faint galaxy at z >~ 6.

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arXiv2014

Accelerated Evolution of Ly$α$ Luminosity Function at $\textit{z} \gtrsim 7$ Revealed by the Subaru Ultra-Deep Survey for Ly$α$ Emitters at $\textit{z}=7.3$

We present the ultra-deep Subaru narrowband imaging survey for Lya emitters (LAEs) at $z=7.3$ in SXDS and COSMOS fields with a total integration time of 106 hours. Exploiting our new sharp bandwidth filter, NB101, installed on Suprime-Cam, we have reached $L(Lya)=2.4\times10^{42} \ erg \ s^{-1}$ ($5σ$) for $z=7.3$ LAEs, about 4 times deeper than previous Subaru $z \gtrsim 7$ studies, which allows us to reliably investigate evolution of Lya luminosity function (LF), for the first time, down to the luminosity limit same as those of Subaru $z=3.1-6.6$ LAE samples. Surprisingly, we only find three and four LAEs in SXDS and COSMOS fields, respectively, while one expects a total of $\sim 65$ LAEs by our survey in the case of no Lya LF evolution from $z=6.6$ to $7.3$.We identify a decrease of Lya LF from $z=6.6$ to $7.3$ at the $>90\%$ confidence level from our $z=7.3$ Lya LF.Moreover, the evolution of Lya LF is clearly accelerated at $z>6.6$ beyond the measurement uncertainties including cosmic variance. Because no such accelerated evolution of UV-continuum LF or cosmic star-formation rate (SFR) is found at $z\sim 7$, but suggested only at $z>8$ (Oesch et al. 2013, Bouwens et al. 2014), this accelerated Lya LF evolution is explained by physical mechanisms different from a pure SFR decrease but related to Lya production and escape in the process of cosmic reionization. Because a simple accelerating increase of IGM neutral hydrogen absorbing Lya would not reconcile with Thomson scattering optical depth measurements from WMAP and Planck, our findings may support new physical pictures suggested by recent theoretical studies, such as the existence of HI clumpy clouds within cosmic ionized bubbles selectively absorbing Lya and the large ionizing photon escape fraction of galaxies making weak Lya emission.

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arXiv2014

Diffuse Lyman-alpha Halos around Galaxies at z=2.2-6.6: Implications for Galaxy Formation and Cosmic Reionization

We present diffuse Lyman-alpha halos (LAHs) identified in the composite Subaru narrowband images of 100-3600 Lyman-alpha emitters (LAEs) at z=2.2, 3.1, 3.7, 5.7, and 6.6. First, we carefully examine potential artifacts mimicking LAHs that include a large-scale point-spread function (PSF) made by instrumental and atmospheric effects. Based on our critical test with composite images of non-LAE samples whose narrowband-magnitude and source-size distributions are the same as our LAE samples, we confirm that no artifacts can produce a diffuse extended feature similar to our LAHs. After this test, we measure the scale lengths of exponential profile for the LAHs estimated from our z=2.2-6.6 LAE samples of L(Lyman-alpha) > 2 x 10^42 erg s^-1. We obtain the scale lengths of ~ 5-10 kpc at z=2.2-5.7, and find no evolution of scale lengths in this redshift range beyond our measurement uncertainties. Combining this result and the previously-known UV-continuum size evolution, we infer that the ratio of LAH to UV-continuum sizes is nearly constant at z=2.2-5.7. The scale length of our z=6.6 LAH is larger than 5-10 kpc just beyond the error bar, which is a hint that the scale lengths of LAHs would increase from z=5.7 to 6.6. If this increase is confirmed by future large surveys with significant improvements of statistical and systematical errors, this scale length change at z > 6 would be a signature of increasing fraction of neutral hydrogen scattering Lyman-alpha photons, due to cosmic reionization.

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arXiv2013

First Systematic Search for Oxygen-Line Blobs at High Redshift: Uncovering AGN Feedback and Star-Formation Quenching

We present the first systematic search for extended metal-line [OII]λλ3726,3729 nebulae, or [OII] blobs (OIIBs), at z=1.2 using deep narrowband imaging with a survey volume of 1.9x10^5 Mpc^3 on the 0.62 deg^2 sky of Subaru-XMM Deep Survey (SXDS) field. We discover a giant OIIB, dubbed 'OIIB 1', with a spatial extent over ~75 kpc at a spectroscopic redshift of z=1.18, and also identify a total of twelve OIIBs with a size of >30 kpc. Our optical spectrum of OIIB 1 presents [NeV]λ3426 line at the 6σ level, indicating that this object harbors an obscured type-2 AGN. The presence of gas outflows in this object is suggested by two marginal detections of FeIIλ2587 absorption and FeII*λ2613 emission lines both of which are blueshifted at as large as 500-600 km/s, indicating that the heating source of OIIB 1 is AGN or associated shock excitation rather than supernovae produced by starbursts. The number density of OIIB 1-type giant blobs is estimated to be ~5x10^{-6} Mpc^{-3} at z~1.2, which is comparable with that of AGNs driving outflow at a similar redshift, suggesting that giant OIIBs are produced only by AGN activity. On the other hand, the number density of small OIIBs, 6x10^{-5} Mpc^{-3}, compared to that of z~1 galaxies in the blue cloud in the same M_B range, may imply that 3% of star-forming galaxies at z~1 are quenching star formation through outflows involving extended [OII] emission.

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arXiv2013

An Intensely Star-Forming Galaxy at z~7 with Low Dust and Metal Content Revealed by Deep ALMA and HST Observations

We report deep ALMA observations complemented with associated HST imaging for a luminous (m_uv=25) galaxy, `Himiko', at a redshift z=6.595. The galaxy is remarkable for its high star formation rate, 100 Mo/yr, securely estimated from our deep HST and Spitzer photometry, and the absence of any evidence for strong AGN activity or gravitational lensing magnification. Our ALMA observations probe an order of magnitude deeper than previous IRAM observations, yet fail to detect a 1.2mm dust continuum, indicating a flux <52uJy comparable with or weaker than that of local dwarf irregulars with much lower star formation rates. We likewise provide a strong upper limit for the flux of [CII] 158um, L([CII]) < 5.4x10^7 Lo, a diagnostic of the hot interstellar gas often described as a valuable probe for early galaxies. In fact, our observations indicate Himiko lies off the local L([CII]) - star formation rate scaling relation by a factor of more than 30. Both aspects of our ALMA observations suggest Himiko is an unique object with a very low dust content and perhaps nearly primordial interstellar gas. Our HST images provide unique insight into the morphology of this remarkable source, highlighting an extremely blue core of activity and two less extreme associated clumps. Himiko is undergoing a triple major merger event whose extensive ionized nebula of Lyman alpha emitting gas, discovered in our earlier work with Subaru, is powered by star formation and the dense circum-galactic gas. We are likely witnessing an early massive galaxy during a key period of its mass assembly close to the end of the reionization era.

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arXiv2013

Star Formation On Sub-kpc Scale Triggered By Non-linear Processes In Nearby Spiral Galaxies

We report a super-linear correlation for the star formation law based on new CO($J$=1-0) data from the CARMA and NOBEYAMA Nearby-galaxies (CANON) CO survey. The sample includes 10 nearby spiral galaxies, in which structures at sub-kpc scales are spatially resolved. Combined with the star formation rate surface density traced by H$α$ and 24 $μ$m images, CO($J$=1-0) data provide a super-linear slope of $N$ = 1.3. The slope becomes even steeper ($N$ = 1.8) when the diffuse stellar and dust background emission is subtracted from the H$α$ and 24 $μ$m images. In contrast to the recent results with CO($J$=2-1) that found a constant star formation efficiency (SFE) in many spiral galaxies, these results suggest that the SFE is not independent of environment, but increases with molecular gas surface density. We suggest that the excitation of CO($J$=2-1) is likely enhanced in the regions with higher star formation and does not linearly trace the molecular gas mass. In addition, the diffuse emission contaminates the SFE measurement most in regions where star formation rate is law. These two effects can flatten the power law correlation and produce the apparent linear slope. The super linear slope from the CO($J$=1-0) analysis indicates that star formation is enhanced by non-linear processes in regions of high gas density, e.g., gravitational collapse and cloud-cloud collisions.

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arXiv2013

Resolved Giant Molecular Clouds in Nearby Spiral Galaxies: Insights from the CANON CO (1-0) Survey

We resolve 182 individual giant molecular clouds (GMCs) larger than 2.5 $\times$ 10$^{5}$ \Msun in the inner disks of five large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC 4826, and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and $^{12}$CO (J=1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H$_{2}$ mass (or \xcons) for each galaxy is 1-2 \xcounits, all within a factor of two of the Milky Way disk value ($\sim$2 \xcounits). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-\xco trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.

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