Rotational Dynamics and Star Formation in the Nearby Dwarf
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Accepted for publication in the Astronomical Journal A Preprint typeset using LTEX style emulateapj v. 5/2/11 ROTATIONAL DYNAMICS AND STAR FORMATION IN THE NEARBY DWARF GALAXY NGC 5238 John M. Cannon1, Andrew T. McNichols1,2, Yaron G. Teich1,3, Catherine Ball1, John Banovetz1,4,5, Annika Brock1, Brian A. Eisner1, Kathleen Fitzgibbon1, Masao Miazzo1, Asra Nizami1, Bridget Reilly1, Elizabeth Ruvolo1, Quinton Singer1 Accepted for publication in the Astronomical Journal Abstract We present new H I spectral line images of the nearby low-mass galaxy NGC 5238, acquired with the Karl G. Jansky Very Large Array (VLAa). Located at a distance of 4.51 ± 0.04 Mpc, NGC5238 is an actively star-forming galaxy with widespread Hα and UV continuum emission. The source is included in many ongoing and recent nearby galaxy surveys, but until this work the spatially resolved qualities of its neutral interstellar medium have remained unstudied. Our H I images resolve the disk on physical scales of ∼400 pc, allowing us to undertake a detailed comparative study of the gaseous and stellar components. The H I disk is asymmetric in the outer regions, and the areas of high H I mass surface density display a crescent-shaped morphology that is slightly offset from the center of the stellar populations. The H I column density exceeds 1021 cm−2 in much of the disk. We quantify the degree of co-spatiality of dense H I gas and sites of ongoing star formation as traced by far-UV and Hα emission. The neutral gas kinematics are complex; using a spatially-resolved position- velocity analysis, we infer a rotational velocity of 31 ± 5 kms−1. We place NGC 5238 on the baryonic Tully-Fisher relation and contextualize the system amongst other low-mass galaxies. Subject headings: galaxies: evolution — galaxies: dwarf — galaxies: irregular — galaxies: individual (NGC 5238) 1. INTRODUCTION to feedback to radiation balance in the ISM. Nearby, star-forming dwarf galaxies are important lab- From the H I perspective, the nearby galaxy popu- lation has been studied in significant detail in multiple oratories for probing our understanding of the internal I processes that govern low-mass halos. Systems within major interferometric surveys: WHISP (Westerbork H ∼10 Mpc can be resolved into individual stars, allowing Survey of Irregular and Spiral Galaxies; Swaters 2002), detailed study of the interplay between recently-formed FIGGS (Faint Irregular Galaxies GMRT Survey; Begum massive stars and the surrounding interstellar medium et al. 2008), SHIELD (the Survey of H I in Extremely (ISM). With multi-wavelength supporting data and suf- Low-mass Dwarfs; Cannon etal. 2011, Teich etal. 2016, ficient angular resolution of the H I 21-cm spectral line, McNichols et al. 2016), VLA-ANGST (Very Large Array we can also probe the dynamics of the sources and con- Survey of ACS Nearby Galaxy Survey Treasury Galax- ies; Ott etal. 2012), LITTLE-THINGS (Local Irregu- strain the ratios of dark to baryonic matter. I The dwarf galaxies in the local volume have received lars That Trace Luminosity Extremes in The H Nearby Galaxy Survey; Hunter et al. 2012), and LVHIS (the Lo- significant attention in recent major surveys that span I the electromagnetic spectrum. An incomplete list in- cal Volume H Survey; Kirby 2012). These surveys have cludes the GALEX ultraviolet (UV) imaging survey by provided a nearly complete observational census of the Lee et al. (2011), the Hubble Space Telescope (HST) LE- neutral hydrogen properties of the star-forming dwarfs GUS UV survey by Calzetti et al. (2015), the HST opti- in the local universe. cal survey ANGST by Dalcanton et al. (2009), and the Since each of the aforementioned H I surveys has dif- SINGS, LVL, and KINGFISH surveys in the infrared ferent selection criteria, there exist multiple local volume arXiv:1610.00495v1 [astro-ph.GA] 3 Oct 2016 (Kennicutt et al. 2003; Dale et al. 2009; Kennicutt et al. dwarf irregular galaxies that have not yet been stud- 2011). Taken as a whole, these surveys have revolu- ied in detail in the H I spectral line. Some of these tionized our understanding of low-mass galaxies; the systems also have extensive multi-wavelength observa- panchromatic approach touches on multiple and diverse tions from the UV to the infrared (e.g., from the afore- aspects of galaxy evolution, from massive star formation mentioned nearby galaxy surveys). One such system is NGC5238 (also known as UGC8565, VV828, Mrk1479, 1 Department of Physics & Astronomy, Macalester College, SBS 1332+518, I Zw 64). Originally cataloged in Dreyer 1600 Grand Avenue, Saint Paul, MN 55105, USA (1888), this nearby, actively star-forming galaxy has ap- 2 NRAO Charlottesville, 520 Edgemont Road, Charlottesville, peared in more than 100 peer-reviewed manuscripts. Re- VA 22903-2475, USA 3 markably, interferometric measurements of its neutral Science Department, Somerville High School, 81 Highland Avenue, Somerville, MA 02143 hydrogen have not been published until the present work. 4 Department of Physics, MS-B1807, Hamline University, 1536 The proximity, low mass, and active star formation make Hewitt Avenue, Saint Paul, MN 55104, USA 5 it an interesting target for detailed observations of the Department of Physics and Astronomy, Purdue University, neutral ISM. 525 Northwestern Avenue, West Lafayette, IN 47907, USA a The National Radio Astronomy Observatory is a facility of the NGC 5238 is resolved into stars in Sloan Digitized National Science Foundation operated under cooperative agree- Sky Survey (SDSS) images, which reveal a blue stel- ment by Associated Universities, Inc. lar population and numerous prominent stellar clusters; 2 2.1. HI Observations Table 1 Basic Characteristics of NGC 5238 HI spectral-line observations of NGC5238 were ac- quired with the Karl G. Jansky Very Large Array (VLA) Parameter Value in the C configuration on February 1, 2016 for program Right ascension (J2000) 13h 34m 42.s5 TDEM0021. This program was executed under the aus- Declination (J2000) +51◦36′49′′ pices of the “Observing for University Classes” program, Adopted distance (Mpc) 4.51 ± 0.04a a service provided by the NRAO as an opportunity for b MB (Mag.) −14.61 7c courses teaching radio astronomy theory to acquire new M⋆ (M⊙) 8.9 × 10 −1 d observations to be analyzed by students. All of the re- Single-dish SHI (Jy km s ) 5.77 ± 0.61 −1 e sults in this manuscript stem from the efforts of under- Interferometric SHI (Jy km s ) 4.56 ± 0.46 7d graduate students at Macalester College. Single-dish H I mass MHI (M⊙) (2.8± 0.3) × 10 7e Interferometric H I mass MHI (M⊙) (2.2± 0.3) × 10 The WIDAR correlator divides a 4.0 MHz bandwidth into 1,024 channels, delivering a native spectral resolu- −1 −1 a Tully et al. (2009) tion of 0.86 km s ch . The primary and phase cal- b Modifying MB from Cook et al. (2014a) for the adopted ibrators were J1331+305 (3C286) and J1400+621, re- distance. spectively. The total on-source integration time was ap- c Modifying M⋆ from Cook et al. (2014b) for the adopted proximately 1.6 hours. The VLA data were calibrated distance. 6 7 d using standard prescriptions in the AIPS and CASA Thuan et al. (1999); note that those authors apply a +16% increase in the observed flux integral (4.98 ± 0.53 environments. Imaging of the J1400+6210 phase cali- Jy km s−1) for beam effects. brator field yielded a flux density Sν = 4.24 ± 0.01 Jy. e This work. Continuum subtraction of the NGC 5238 field was per- the central cluster shows very strong nebular emission formed in the uv plane using a first-order fit to line-free lines. Broad-band HST images are used to derive a dis- channels bracketing the galaxy in the central 50% of the tance of 4.51 ± 0.04 Mpc in Tully et al. (2009); we adopt bandpass. this distance throughout the present work, and show Imaging of the calibrated uv visibilties followed the resulting HST images below. NGC 5238 is included standard prescriptions similar to those described in in the aforementioned LVL (Dale et al. 2009), GALEX Cannon et al. (2015) and in Teich et al. (2016). In brief, UV (Lee et al. 2011), and LEGUS (Calzetti et al. 2015) the input continuum-subtracted database was spectrally smoothed by a factor of two, to produce a velocity res- surveys. Moustakas & Kennicutt (2006) use strong- − − − line methods to derive a gas-phase oxygen abundance olution of 7.812 kHz ch 1 (1.56 km s 1 ch 1). The cube 12 + log(O/H) = 7.96 ±0.20 (see also Marble et al. 2010), was then inverted and cleaned using the IMAGR task corresponding to Z ≃ 19%Z⊙ using the Solar oxygen in AIPS; the ROBUST factor is 0.5. Cleaning was per- abundance of Asplund et al. (2009). Modifying the rele- formed to 2.5 times the rms noise per channel in line- vant values derived in those works for the updated dis- free channels. Residual flux rescaling was enforced dur- ing the imaging process (Jorsater & van Moorsel 1995). tance, we collect global physical properties of NGC 5238 ′′ ′′ in Table 1. The resulting synthesized beam size of 17.69 × 15.40 was smoothed to a circular 18′′ beam. The rms noise in The neutral hydrogen properties of NGC5238 have ′′ − only been studied with single-dish observations. Using the final 18 cube is 1.4 mJy Bm 1. the Nan¸cay 300-m telescope, Thuan et al. (1999) find Two-dimensional moment maps were produced from −1 the three-dimensional datacube as follows. The original W50 = 32 ± 4 kms and an observed H I line inte- ′′ ′′ −1 17.69 × 15.40 cube was first spatially smoothed by a gral SHI = 4.98 ± 0.53 Jy km s (corrected to 5.77 ± 0.61 ′′ Jykms−1 for beam effects).