THE SPATIALLY RESOLVED STAR FORMATION LAW FROM INTEGRAL FIELD SPECTROSCOPY: VIRUS-P OBSERVATIONS OF NGC 5194

By G. Blanc, A. Heiderman, K. Gebhardt, N. Evans II, J. Adams. 2009

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We investigate the relation between the star formation rate (SFR) surface density (ΣSFR) and the mass surface density of gas (Σgas) in NGC 5194 (a.k.a. M51a, Whirlpool Galaxy). Visible Integral field Replicable Unit Spectrograph Prototype (VIRUS-P) integral field spectroscopy of the central 4.1 × 4.1 kpc2 of the galaxy is used to measure Hα, Hβ, [O iii]λ5007, [N ii]λλ6548,6584, and [S ii]λλ6717,6731 emission line fluxes for 735 regions ∼170 pc in diameter. We use the Balmer decrement to calculate nebular dust extinctions, and correct the observed fluxes in order to accurately measure ΣSFR in each region. Archival H i 21 cm and CO maps with spatial resolution similar to that of VIRUS-P are used to measure the atomic and molecular gas surface density for each region. We present a new method for fitting the star formation law (SFL), which includes the intrinsic scatter in the relation as a free parameter, allows the inclusion of non-detections in both Σgas and ΣSFR, and is free of the systematics involved in performing linear regressions over incomplete data in logarithmic space. After rejecting regions whose nebular spectrum is affected by the central active galactic nucleus in NGC 5194, we use the [S ii]/Hα ratio to separate spectroscopically the contribution from the diffuse ionized gas (DIG) in the galaxy, which has a different temperature and ionization state from those of H ii regions in the disk. The DIG only accounts for 11% of the total Hα luminosity integrated over the whole central region, but on local scales it can account for up to a 100% of the Hα emission, especially in the inter-arm regions. After removing the DIG contribution from the Hα fluxes, we measure a slope N = 0.82 ± 0.05, and an intrinsic scatter ϵ = 0.43 ± 0.02 dex for the molecular gas SFL. We also measure a typical depletion timescale Gyr, in good agreement with recent measurements by Bigiel et al. The atomic gas density shows no correlation with the SFR, and the total gas SFL in the sampled density range closely follows the molecular gas SFL. Integral field spectroscopy allows a much cleaner measurement of Hα emission line fluxes than narrow-band imaging, since it is free of the systematics introduced by continuum subtraction, underlying photospheric absorption, and contamination by the [N ii] doublet. We assess the validity of different corrections usually applied in narrow-band measurements to overcome these issues and find that while systematics are introduced by these corrections, they are only dominant in the low surface brightness regime. The disagreement with the previous measurement of a super-linear molecular SFL by Kennicutt et al. is most likely due to differences in the fitting method. Our results support the recent evidence for a low, and close to constant, star formation efficiency (SFE =τ−1) in the molecular component of the interstellar medium. The data show an excellent agreement with the recently proposed model of the SFL by Krumholz et al. The large intrinsic scatter observed may imply the existence of other parameters, beyond the availability of gas, which are important in setting the SFR.

Subfields: Astronomy and Astrophysics

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ESTIMATING THE STAR FORMATION RATE AT 1 kpc SCALES IN NEARBY GALAXIES (2012, background): “In theory this emission should thus be added back to the likely parent region (e.g., Blanc et al. 2009) and should not be entirely removed from the map.”
Dusty Star Forming Galaxies at High Redshift (2014, background): “On the other hand, Blanc et al. (2009); Shetty et al. (2013a,b) suggest that due to issues related to bisector fitting methods (which have been used in many of the resolved KS relation studies), the true KS slope should be sub-linear.”
Metallicity gradients in local field star-forming galaxies: insights on inflows, outflows, and the coevolution of gas, stars and metals (2015, methodology): “To quantify the fractional contributions from the DIG and HII regions in a given spaxel, we adopt a similar approach as Blanc et al. (2009).”
An uncertainty principle for star formation - I. Why galactic star formation relations break down below a certain spatial scale (2014, background): “Although a detailed comparison with observations is deferred to K14,Figure 3 does show which part of parameter space is covered by the observed nearby galaxies of Leroy et al. (2013), as well as M33 (Schruba et al. 2010) and M51 (Blanc et al. 2009), indicatinghat our model agrees very well with the ”
A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES (2011, background): “…d’Hères, France 9 Observatorio Astronómico Nacional, C/ Alfonso XII, 3, 28014, Madrid, Spain 10 MPIfR, Auf dem Hügel 69, 53121 Bonn, Germany 11 Hubble Fellow 2007; Bigiel et al. 2008; Leroy et al. 2008; Wilson et al. 2008; Blanc et al. 2009; see also the recent review by Bigiel et al. 2011).”
SDSS-IV MaNGA : the impact of diffuse ionized gas on emission-line ratios, interpretation of diagnostic diagrams and gas metallicity measurements (2016, methodology): “Observed relations between line ratios and ΣHα provide an empirical method to separate DIG dominated and H ii region dominated regions (Blanc et al. 2009; Kreckel et al. 2016; Kaplan et al. 2016).”
A CONSTANT MOLECULAR GAS DEPLETION TIME IN NEARBY DISK GALAXIES (2011, background): “It is also challenging practically, because of, e.g., issues of completeness and upper limits (see Blanc et al. 2009), zero point uncertainties (compare Rahman et al. 2010) or a correct treatment of the uncertainties associated with physical parameter estimation.”
VALIDATION OF THE EQUILIBRIUM MODEL FOR GALAXY EVOLUTION TO z ∼ 3 THROUGH MOLECULAR GAS AND DUST OBSERVATIONS OF LENSED STAR-FORMING GALAXIES (2013, background): “Since most recent studies suggest that the KS relation is near-linear (e.g. Leroy et al. 2008; Blanc et al. 2009; Genzel et al. 2010; Daddi et al. 2010b; Bigiel et al. 2011; Rahman et al. 2012; Saintonge et al. 2012; Shetty et al. 2013; Feldmann 2013), the problem is further simplified and αCO can…”
A UNIVERSAL, LOCAL STAR FORMATION LAW IN GALACTIC CLOUDS, NEARBY GALAXIES, HIGH-REDSHIFT DISKS, AND STARBURSTS (2011, unknown): “More recently, a number of authors have extended this analysis to ∼kpc-scale regions within several Local Group galaxies (Kennicutt et al. 2007; Bigiel et al. 2008; Leroy et al. 2008; Blanc et al. 2009).”
BARYONS MATTER: WHY LUMINOUS SATELLITE GALAXIES HAVE REDUCED CENTRAL MASSES (2012, result): “…formation in the new model is tied directly to the local H2 abundance, which is regulated by the gas metallicity and the ability of the gas to self-shield, in accordance with observational results (Leroy et al. 2008; Bigiel et al. 2008; Blanc et al. 2009; Bigiel et al. 2010; Schruba et al. 2011).”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “There is some uncertainty as to whether this value is in fact constant (Bigiel et al. 2008; Blanc et al. 2009; Rahman et al. 2011, 2012; Leroy et al. 2013), or varies slightly with the gas surface density or other large-scale properties of a galaxy (Ken- nicutt et al. 2007; Liu et al. 2011;…”
THE STATE OF THE GAS AND THE RELATION BETWEEN GAS AND STAR FORMATION AT LOW METALLICITY: THE SMALL MAGELLANIC CLOUD (2011, result): “Schruba et al. (2011) and Bigiel et al. (2011) extended these findings to a larger sample of 30 disk galaxies, while Blanc et al. (2009) and Rahman et al. (2011) arrived at similar conclusions in very detailed studies of individual targets.”
A COMPARISON OF METHODS FOR DETERMINING THE MOLECULAR CONTENT OF MODEL GALAXIES (2010, background): “…nearby galaxies in the last decade have unequivocally established that the star formation rate in a galaxy correlates much more strongly with its molecular content than with its total gas content (Wong & Blitz 2002; Kennicutt et al. 2007; Bigiel et al. 2008; Leroy et al. 2008; Blanc et al. 2009).”
THE STAR FORMATION RATE AND GAS SURFACE DENSITY RELATION IN THE MILKY WAY: IMPLICATIONS FOR EXTRAGALACTIC STUDIES (2010, background): “Current “spatially resolved” extragalactic measurements are still limited to scales of ∼0.2-2 kpc (Kennicutt et al. 2007; Bigiel et al. 2008; Blanc et al. 2009); therefore, we cannot directly measure extragalactic SFRs on scales of galactic star forming regions.”
THE IMPACT OF INTERACTIONS, BARS, BULGES, AND ACTIVE GALACTIC NUCLEI ON STAR FORMATION EFFICIENCY IN LOCAL MASSIVE GALAXIES (2012, result): “These values are in agreement with other derivations of the star formation law at the kpc-scale (Leroy et al. 2008; Blanc et al. 2009; Bigiel et al. 2011; Rahman et al. 2012) and at high redshift (Genzel et al. 2010; Daddi et al. 2010).”
Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation (2012, background): “…the SFR toH2 at various spatial scales and across a wide sample of galaxies also found that the SFR correlates most strongly withH2 (Rownd & Young 1999; Murgia et al. 2002; Heyer et al. 2004; Gao & Solomon 2004; Kennicutt et al. 2007; Blanc et al. 2009; Warren et al. 2010; Bigiel et al. 2010).”
The physics driving the cosmic star formation history (2009, methodology): “The normalization (A) and slope (n) are constrained by observations, but remain controversial (e.g. Blanc et al. 2009).”
The SAURON project – XVII. Stellar population analysis of the absorption line strength maps of 48 early-type galaxies (2010, background): “…et al. 2007; Spolaor et al. 2009) or radially averaging the data from integral-field-units (Rawle et al. 2008); spatially resolved stellar population maps of early-type galaxies remain the exception (e.g., McDermid et al. 2006; Chilingarian 2009; Pracy et al. 2009; Blanc et al. 2009).”
Star Formation in the Milky Way and Nearby Galaxies (2012, methodology): “For observations of normal galaxies where local attenuations are modest, integral-field mapping of galaxies offers the means to produce high-quality Hα maps corrected for dust using the Balmer decrement (e.g., Blanc et al. 2009, 2010; Sánchez et al. 2012).”
MAXIMALLY STAR-FORMING GALACTIC DISKS. I. STARBURST REGULATION VIA FEEDBACK-DRIVEN TURBULENCE (2011, methodology): “In main-disk regions where Σ <∼ 100 M⊙ pc−2, high-resolution observations have found that ΣSFR is proportional to Σmol, the mean surface density of molecular gas, with a timescale tSF,mol ≡ Σmol/ΣSFR ≈ 2 Gyr (Bigiel et al. 2008; Blanc et al. 2009).”