Evolution in Motion: Evidence of an Environmental

EVOLUTION IN MOTION: EVIDENCE OF AN ENVIRONMENTAL TRANSITION OF A LOCAL DWARF GALAXY FROM ITS NEUTRAL HYDROGEN STRUCTURE Luca Beale Astronomy Department, University of Virginia, Charlottesville, VA 22904

Abstract

Dedicated neutral hydrogen (HI) surveys have recently led to a growing category of exciting low- mass galaxies found in the Local Volume. We present synthesis imaging of one such galaxy, Pisces A, a low-mass dwarf originally confirmed via optical imaging and spectroscopy of HI sources in the Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey. Using HI observations taken with the Karl G. Jansky Very Large Array (JVLA), we characterize the kinematic structure of the gas and connect it to the galaxy’s environment and evolutionary history. While the galaxy shows overall ordered rotation, a number of kinematic features indicate a disturbed gas morphology. These features are suggestive of a tumultuous recent history and represent ∼ 28% of the total baryonic mass. We find a total baryon fraction fbary ∼ 0.3 if we include these features. We also quantify the cosmic environment of Pisces A, finding an apparent alignment of the disturbed gas with a local filament. We consider several scenarios for the origin of the disturbed gas. While we cannot rule out stellar feedback from a recent starburst, our observations support the suggestion from previous work that Pisces A has moved from the Local Void to a higher-density filament.

1. Introduction ing galaxies to funnel gas from large distances. Possible evidence for such an accretion may be The survival of gas in dwarf galaxies observable in the gas kinematics (possible ex- throughout cosmic history, especially in the amples can be found in Stanonik et al., 2009; low-mass regime, implies that dark matter Kreckel et al., 2011b; Ott et al., 2012). This haloes must shield the gas inside them (e.g., mechanism is also expected to be more signif- Penny et al., 2009). This relationship is cru- icant in low density regions, and a handful of cial to determining the overall growth of dwarf individual systems from the Void Galaxy Sur- galaxies, and how their star formation and ISM vey (VGS; Kreckel et al., 2014), in which the are coupled (Dekel & Silk, 1986; Kennicutt, morphology and kinematics of 59 galaxies se- 1998; Brooks & Zolotov, 2014). The mech- lected to reside within the deepest voids identi- anisms by which dwarfs consume their fuel ﬁed in the SDSS were studied, reveal kinematic (and the timescales involved) are complex and warps and low column density envelopes con- poorly studied, especially when HI persists in sistent with gas accretion (see especially Fig- extended structures around these galaxies (e.g., ures 9 and 10 in Kreckel et al., 2012). Roychowdhury et al., 2011). Here we present resolved HI imaging of one The cosmic environment of dwarf galaxies such low-mass dwarf galaxy taken with the can also provide insights into the evolution Karl G. Jansky Very Large Array (JVLA1). of their gas content. Low-mass galaxies are Pisces A, a Local Volume dwarf galaxy, was thought to accrete their gas mainly through the so-called “cold mode” (Keresˇ et al., 2005, 1The National Radio Astronomy Observatory is a fa- 2009). The low temperature gas is expected to cility of the National Science Foundation operated under be transported along cosmic ﬁlaments, allow- cooperative agreement by Associated Universities, Inc.

Beale 1 14 continuum 12 10 8 52 6 Table 1. Key Parameters of Pisces A

4 50 J bm mJy 2

− Parameter Value Reference 48 1 Dec (J2000) RA (J2000) 00h14m46s 46 Dec. (J2000) +10◦4847.01 0 +0.15 3kpc . ◦ 5 64 +10 44 Distance (Mpc) −0.13 (3) 15m00s 50s 40s 0h14m30s b/a q . ± . RA (J2000) Optical (= ) 0 66 0 01 (3) Inclination (◦) 59 ± 9 (1) +5 reff,major (pc) 145−6 (3) M / . +0.4 Figure 1 Continuum emission in a 5 × 5 region log( M)70−1.7 (3) v −1 ± around Pisces A. Light orange contours highlight sys,opt (km s ) 240 34 (2) Arecibo −1 continuum sources at the depth of our imaging. The vsys,HI (km s ) 236 ± 0.5 (2) − − −1 σ . 1 · 1 W 50HI (km s ) 22.5 ± 1.3 (2) black 2 rms =568mJybm km s contour 6 MHI (10 M) 8.9 ± 0.8 (2, 3) highlighting the extent of the HI emission is from KAT-7 Array −1 the foundation-derived integrated intensity map (see vsys,HI (km s ) 233 ± 0.5 (4) −1 W 50HI (km s ) 28 ± 3 (4) Section 3.1). The 44 ×40 inset color image shows 6 MHI (10 M) 13 ± 4 (3, 4) the optical extent of the galaxy. The colors approxi- JVLA −1 vsys,HI (km s ) 236.0 ± 0.8 (1) mate the human eye response to the F606W/F814W −1 W 50HI (km s ) 26 ± 2 (1) 6 HST ACS imaging. See Tollerud et al. (2016) for MHI (10 M) 7.1 ± 1.6 (1) more details. a(1) This work. (2) Tollerud et al. (2015). (3) Tollerud et al. (2016). (4) Carignan et al. (2016) originally identified in the GALFA-HI DR1 catalog (Peek et al., 2011) as an HI cloud with a and conclude in Section 5. size < 20 and a velocity FWHM < 35 km s−1 (Saul et al., 2012; Tollerud et al., 2015). Fol- 2. Data & Methods lowup optical spectroscopy (Tollerud et al., 2016) confirmed the coincidence of the HI Pisces A was observed in December 2015 emission with a stellar component, cementing with the JVLA in D-configuration for 2h, ∼ . its status as a galaxy. corresponding to 1 3h of on-source inte- From the location of Pisces A within the gration time (JVLA/15B-309; PI Donovan Local Volume, and its star formation history Meyer). This is the most compact configu- (SFH), Tollerud et al. (2016) suggest that the ration available for the JVLA, which gives galaxy has recently fallen into a higher-density increased sensitivity to extended emission. region from a void. If this is indeed the case, the Observations were centered on the HI line (rest subsequent gas accretion would trigger delayed frequency: 1.42040580 GHz) and spanned 8 (recent) evolution and leave an imprint on the MHz subdivided into 2048 channels. Standard gas content observable through the kinematics calibration of the visibility data was done and morphology. We explore this possibility within the Common Astronomy Software here by examining the resolved gas kinematics Applications (CASA) environment. The of Pisces A. calibrated data were then split off into their This paper is organized as follows: In Sec- own MeasurementSet for further analysis. The tion 2, we describe the JVLA observations as galaxy is shown in Figure 1 and its parameters well as our techniques for extracting kinematic are summarized in Table 1. and morphological information. We highlight the velocity structure and kinematics in Section 2.1. Data Cubes 3. We place Pisces A in context in Section 4 A clean HI data cube is produced by binning to a velocity resolution of 2 km s−1 and interac-

Beale 2 foundation 52 185 187 189 191 193 195 197 199 201 203 51

50 40 49

48 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc +10◦47 52 30 205 207 209 211 213 215 217 219 221 223 51

49 20 48 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc +10◦47 52 225 227 229 231 233 235 237 239 241 243 J bm mJy 51

50 10 49 −

48 1 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc +10◦47 52 245 247 249 251 253 255 257 259 261 263 51

49 48 0 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc +10◦47 52 265 267 269 271 273 275 277 279 281 283 51

48 Dec (J2000) 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc 1kpc +10◦47 56s 48s 56s 48s 56s 48s 56s 48s 56s 48s 56s 48s 56s 48s 56s 48s 56s 48s 56s 48s 0h14m40s RA (J2000)

Figure 2 Channel maps from the foundation data cube of Pisces A. Contour levels are at (-2, 2, 3, 5, 10, −1 −1 15)σrms where σrms =1.6mJybm . The velocity (in km s ) is marked in the upper right corner of each panel. The JVLA beam is given in the lower left panel.

noise ratio) in the narchan data cube is achieved Table 2. Properties of the Data Cubes by binning to 1 km s−1 velocity resolution. Fi- nally, the widechan data cube is binned to 4 Cube Δ v Beam σrms −1 (km s−1)(×) (mJy bm−1) km s channels for increased sensitivity to extended structure, at the cost of spectral resolu- foundation 2 62.31 × 44.68 1.6 narbeam 2 48.57 × 34.05 2.4 tion. narchan 1 61.81 × 45.01 1.9 We derive moment maps for Pisces A from widechan 4 62.24 × 44.70 1.4 the cleaned data cubes using the immoments tively masking real emission until the residuals task in CASA. Velocity fields (moment 1) and appear noiselike. Imaging is restricted to a 100 dispersion maps (moment 2) are clipped to the −1 ≥ 3σ level. km s range centered on the systemic HI ve- rms locity (Tollerud et al., 2016) as no significant emission is observed outside this region. 2.2. PV Diagrams To maximize our sensitivity to both small Position-velocity (PV) diagrams are con- scale structure as well as extended emission, we structed by placing slices across the galaxy produce multiple versions of the data cube with and extracting the velocity information at each different imaging parameters, as summarized in pixel along the slice, while encompassing the Table 2. In the narbeam data cube, we increase total spatial extent of emission. An approx- spatial resolution by flagging data in the Fourier imate “major” axis is defined using the HI domain, which in principle increases our sensi- isovelocity contours, and an “extension” axis tivity to the most compact emission. Improved is defined to cross the northeast extension (see spectral resolution (at the cost of signal-to- Section 3.2). Both axes are centered on the

Beale 3 700 −1 foundation 8 (μ, σ) = (236.0, 11.3) km s 51 51 F606W+mom0 (foundation) foundation −1 −1 600 2kms ch inner 99.7%

50 500 50 )

1 6 − 400 bm mJy

49 49 − 1 · 300 s km Dec (J2000) − Dec (J2000) 1 4 48 200 48

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◦ +10◦47 +10 47 Flux (mJy bm

0 2 1kpc 1kpc s s s s s h m s 56s 52s 48s 44s 40s 0h14m36s 56 52 48 44 40 0 14 36 RA (J2000) RA (J2000)

20 250 0 foundation (3σ) foundation (3σ) 51 1 51 18 245

50 16 240 0 50

14 Residuals 235 s km 49 s km 12 − 49 − − 1 1 1 Dec (J2000) Dec (J2000) 230 10 185 193 201 209 217 225 233 241 249 257 265 273 281 48 48 −1 8 Velocity (km s ) 225

+10◦47 +10◦47 6 1kpc 1kpc 220 56s 52s 48s 44s 0h14m40s s s s s s h m 4s RA (J2000) 56 52 48 44 40 0 14 36 RA (J2000) Figure 4 Global HI spectrum extracted from the foundation cube. The blue solid curve is the best fit Figure 3 Moment analysis of the foundation data Gaussian. In both panels, the shaded region (con- cube. The JVLA beam is given in the bottom left taining 99.7% of the emission) denotes the range of corner of each panel. Top Left: Integrated inten- emission used in further calculations. sity map. Contour levels are at (-2, 2, 3, 5, 10, 15, σ σ =1.6mJybm−1 20) rms where rms . Top Right: the ≥ 2σrms level and removing outlier bright HST ACS F606W imaging of Pisces A. Contours pixels believed not to follow the galaxy rota- are the same as in the top left. Bottom Left: Veloc- tion, we rebin the data to only sample every ity field of the galaxy, as described in Section 2.1. half-beam. To estimate the associated uncer- Isovelocity contours are spaced every 2 km s−1, and tainties, we fit a Gaussian to the intensities at the thick black contour denotes the systemic veloc- each pixel bin, and treat the resulting estimate ity. Bottom Right: Dispersion map with contours v spaced every 2 km s−1. of rot as the “true” solution. We then draw ran- dom samples from a distribution with the same coordinates of Pisces A as given in Table 1 parameters as the initial solution and re-fit, re- so that a 0 offset corresponds to the optical peating this 1000 times. The errors are esti- (v , − v , ) /2 center of the galaxy. mated as median rot fit rot true , added in quadrature with the channel width for that cube. 2.3. Rotation Curves A full analysis of the rotation curve requires both high spectral and spatial resolution in or- 3. Results der to fit the velocity field (typically via tilted- ring models). The extent of the HI in Pisces A (∼ 2 across, corresponding to 2-3x the beam 3.1. Overall HI Content and Kinematics size) prohibits us from taking advantage of this Channel maps for the foundation cube are method. Instead, we use a variation of the peak presented in Figure 2. In general, the channel intensity method (Mathewson et al., 1992) to maps reveal regular emission across the main estimate the rotation curve. body of the galaxy, which is also seen in other From the major-axis PV diagram, we select data cubes. However, multiple features are ob- the pixels with maximum intensity across the served which are inconsistent with the general spatial direction. After masking to emission at rotation, which we discuss in detail in Section 3.2.

Beale 4 that although both maps only include emission 700 51 foundation major extension 600 Tollerud+2015 ≥ 3σrms, there may still be unphysical features 50 500 J bm mJy

400 present, which can be especially seen in the ve- 49 − 1 · 300 s km Dec (J2000) − 1 locity ﬁeld. 48 200

100 +10◦47 The global HI spectrum of the galaxy is pre- 1kpc 0

56s 52s 48s 44s 40s 0h14m36s RA (J2000) sented in Figure 4. We ﬁnd a systemic ve- −1 30 30 v . ± . 280 280 foundation (major) locity of sys,H = 236 0 0 8 km s ,in foundation (extension) I 25 25 260 260 20 20 agreement with previous single dish and com- ) ) 1 1 − − J bm mJy bm mJy 240 15 240 15 − −

1 1 pact array observations. The 50% profile width 10 10 220 220 Velocity (km s Velocity (km s 5 5 (uncorrected for inclination or broadening) is 200 200 0 0 W 50 =26± 2 −1 − − − − − km s . We also find an HI 100 50 0 50 100 150 100 50 0 50 100 150 HI Offset (arcsec) Offset (arcsec) M . ± . × 6 mass of HI =(71 1 6) 10 M. PV diagrams for Pisces A are shown in Figure 5 Top: Moment 0 map of Pisces A demon- Figure 5, along with an integrated intensity strating the construction of the position velocity map demonstrating the selection process. The slices. Integrated intensity contours are the same as in Figure 3. Also overlaid are the coordi- overall rotation appears shallow, with the nates of the galaxy as given by Tollerud et al. faintest contours contributing little to the bulk (2015). Bottom Left and Right: Corresponding motion. In both the major- and extension-axis major- and extension-axis PV diagrams. Contours PV diagrams, we observe fragmented emission are at (-2, 2, 3, 5, 10, 15)σrms, where σrms = at multiple velocities and spatial offsets, which 1.6mJybm−1. The northeast extension is approx- we next discuss in turn. imately near −100 in the extension-axis PV diagram. 3.2. Beyond Bulk Motion The Northeast Extension In Figure 3 we show the moment analy- One of the most striking features we detect is sis for Pisces A, as applied to the foundation a ‘filament’ of gas extending toward the north- data cube. The overall HI distribution (top left east. We call this the “northeast extension” panel) is centered on the optical component of (NEe) when referring to all spatially coincident the galaxy and exhibits extensions toward the emission in this region. In Figure 2, it is most northeast and southwest. Both features are also visible across a few channels near 233 km s−1, seen in the channel maps, although the south- but it also appears in the moment maps (Figure western feature is mostly bright in a few non- 3), and the extension-axis PV diagram in Figure contiguous channels. These features are dis- 5 suggests that there are multiple kinematically cussed further in Section 3.2. distinct features in that region. The velocity field of Pisces A is shown in To assess the significance of the brightest the bottom left panel of Figure 3. Pisces A ex- components, we extract a global spectrum of hibits approximately solid-body rotation out to the NEe which we show in the right panels of the extent of the HI, consistent with typical ex- Figure 6. In the narbeam cube, we observe pectations of dwarf galaxies. There is a slight three kinematically distinct components at 227 asymmetry on the approaching side (marked km s−1, 243 km s−1, and 277 km s−1, which we by the arrow in Figure 3), which is approxi- label as NElow, NEmid, and NEhigh, respec- mately at the edge of the optical disk, indicat- tively. Per channel estimates of the rms noise ing a possible warp. The dispersion map in and peak intensity suggest all three components the bottom right panel also suggests an approx- are detected at the > 3σrms level, and all have imately rotation-dominated body. We stress approximately Gaussian profiles.

Beale 5 −1 narbeam 800 280 (μ, σ) = (227.5, 1.7) km s 52 25 narbeam −1 4 −1 −1 (μ, σ) = (243.4, 2.2) km s narbeam (extension) 2kms ch (μ, σ) = (276.8, 2.1) km s−1 700 ) 20 1 − 3 51 600 260 ) J bm mJy

1 15 500 2 − J bm mJy 50 240 −

400 Flux (mJy bm

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ms km 1 − 1

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280 30 3.0 −1 widechan (μ, σ) = (202.6, 2.9) km s 52 700 widechan −1 −1 −1 (μ, σ) = (234.0, 2.2) km s widechan (extension) 2.5 2kms ch (μ, σ) = (246.9, 2.6) km s−1 25 (μ, σ) = (278.2, 2.4) km s−1 )

600 1 260 − 2.0 51 500 20

) .

J bm mJy 1 5 1 − 240 bm mJy 400 . 50 15 1 0 − Flux (mJy bm 1 · ms km −

300 1 0.5 Dec (J2000)

− 10 1 220 49 Velocity (km s 0.0 200 0.4 5 100 200 0.0 +10◦48

0 Residuals 0 1kpc −0.4 56s 52s 48s 44s 40s 0h14m36s −150 −100 −50 0 50 100 150 185 193 201 209 217 225 233 241 249 257 265 273 281 RA (J2000) Oﬀset (arcsec) Velocity (km s−1)

Figure 6 Kinematic decomposition of the NEe in the narbeam (top) and widechan (bottom) data cubes. Left: Selection process based on moment 0 maps. The black trapezium outlines the region used to extract −1 the global spectrum. Contours are at (-2, 2, 3, 5, 10)σrms, where σrms =2.4(1.4) mJy bm for the narbeam (widechan) cube. Center: Extension-axis PV diagram with contours similar to Figure 5. The colored rectangles bound the spatial and spectral extent of the best ﬁt Gaussians. Right: Corresponding global spectrum of the NEe. Thick colored lines represent Gaussian ﬁts to the brightest components. Colors are the same as the rectangles in the center panels.

20 narchan 52 20 15 narchan

+2 −1 18 σ =6.5−1.8 km s 10 51 16 ) 1 − 5 14 50 ms km

(km s 0 10 20 30 40 −1 c σ (km s ) v 12 −

0 1

Dec (J2000) 49 10 −5 8 +10◦48 −10 6 −1.5 −1.0 −0.5 0.0 0.5 1.0 1.5 1kpc R (kpc) 4 52s 48s 44s 40s 0h14m36s RA (J2000)

Figure 7 Rotation curve of Pisces A from the narchan data cube. Open circles denote the > 2σrms Figure 8 Dispersion map from the narchan data oversampled pixels. The ﬁnal resampled rotation cube. Contours are spaced at 1 km s−1 intervals. curve is marked by the blue ﬁlled circles. Nega- Inset: Corresponding distribution of velocity disper- tive galactocentric distances indicate the approach- sions. The NEe is visible at ∼ 28 km s−1. ing side.

Beale 6 LG Dwarfs > 300 kpc (Mc12) 8 1.0 Void Galaxies (Kr11) Starburst Dwarfs (Le14) Pisces A

0.8 4

0.6

0 bary (Mpc) f Z 0.4 −4

0.2 nearby galaxies −8 Pisces A NEe 0.0 −8 −4 0 4 8 5 6 7 8 9 X (Mpc) log(MHI/M)

3 Low-mass dwarfs in α.40 (Hu12) Low-mass dwarfs in α.40+DR7 (Br15) Figure 9 The cosmic density ﬁeld around Pisces Leo P (BC14) 2 Isolated UDGs (Pa17) A. Black points are a sample of nearby galaxies Void Galaxies (Kr11) Starburst Dwarfs (Le14) (following the prescription in Tollerud et al., 2016). Pisces A ) Pisces A is shown as a red star. The Delaunay tri- 1 /M

angulation is given in blue, with the corresponding HI M density ﬁeld in greyscale. In this coordinate system, 0 log( the Z-axis is pointed toward the Local Void.

−1 A similar analysis of the NEe with the widechan data cube also reveals multiple dis- −2 5 6 7 8 9 10 11 tinct components, but at slightly different log(M/M) velocities and signal-to-noise ratios (SNRs). Here, the NEe is most signiﬁcant at ∼ 233 km Figure 10 Top: The log M − f plane. Pisces s−1 (corresponding approximately to NElow), HI bary A is plotted as a red star. Bottom: The log M − similar to what is seen in the foundation cube. M /M log ( HI ) plane. See text for description of all The disagreement in spectral location may in- samples. dicate that the most compact emission exists at lower velocities (to which the narbeam cube The Northern Clump would be most sensitive), while fainter emis- A small feature appears in the channel map sion (which the widechan cube could detect) is (Figure 2) at 219 km s−1 approximately 2 at higher velocities. above Pisces A, which we dub the “northern The Southwest Emission clump” (NC). Recurring across multiple chan- The moment analysis presented in Figure 3 nels at the 3σrms level, it appears to be separated reveals a peanut-shaped feature which we refer from the main body of the galaxy. However, the to as the “southwest emission” (SWe). This fea- shape of Pisces A at 221 km s−1 suggests a pos- ture, which appears aligned with the NEe, is de- sible spatial connection. tected in all data cubes except for narbeam (see Table 3 summarizes the HI properties of all the top left panel of Figure 6), suggesting that anomalous HI features. We indicate which it is mostly diffuse gas. We perform a similar data cube is used to calculate all parameters in kinematical decomposition for this feature. It parentheses. Separations are calculated with persists across multiple channels, and the peak respect to the optical center of Pisces A, and σ intensity is detected at the 3 rms level. both separations and sizes assume the clump is

Beale 7 velocity dispersion, and R is the galactocentric Table 3. Derived HI Clump Properties distance. This functional form gives weight to both the rotation and dispersion components in (narchan) dwarf galaxies (see Hoffman et al., 1996, and Parameter SWe NC references therein). −1 vfit (km s ) 213.1 ± 0.6 219.2 ± 0.8 −1 a Figure 8 shows the dispersion map and cor- vpeak (km s ) 214 220 Separation (kpc)b 3.0 3.8 responding distribution of values, also derived Size (kpc)b,c 1.5 1.1 −1 e from narchan. Note that the distribution in- Δvfit (km s ) −22.9 ± 1.0 −16.8 ± 1.1 −1 e ∼ Δvpeak (km s ) −22 −16 cludes the NEe (the small bump near 28 km W −1 . ± . . ± . −1 50HI (km s ) 5 9 1 955 2 2 5 s ), but excludes the SWe. We find a velocity MH 5.9 ± 1.42.6 ± 1.3 I (10 M ) σ . +2 −1 (narbeam) dispersion of =65−1.8 km s , where the NElow NEmid NEhigh errors represent the inner 68% of the values. −1 −1 vfit (km s ) 228 ± 1243.4 ± 0.3276.8 ± 0.3 v =12.8 R =1.3 −1 a Assuming rot km s and vpeak (km s ) 233 243 277 Separation (kpc)b 2.8 2.5 3.0 kpc from Figure 7, we find a total dynamical b,d 7 Size (kpc) 1.5 1.1 1.7 M =(9.0±3.6)×10 M −1 e mass of dyn .Itisim- Δvfit (km s ) −8.5 ± 1.37.4 ± 0.940.8 ± 0.8 −1 e Δvpeak (km s ) −3 741portant to stress that this is a lower limit, since W −1 . ± . . ± . . ± . 50HI (km s ) 3 9 3 052 1 049 0 7 5 we have not captured the full rotation curve. MH 6.3 ± 2.615.7 ± 3.19.0 ± 3.1 I (10 M ) Additionally, the effects of smoothing act to aaChannel of brightest emission. b flatten the rotation curve, further underestimat- Measured in the vpeak channel. c Based on the 2σrms contour. ing vrot. Assuming the total baryonic mass is d Based on the 3σrms contour. M M . M e v − v bary = +14 HI (for a discussion of the Defined as sys,HI (see Table 1). factor of 1.4, see McGaugh, 2012), we find a Mbary baryon fraction of fbary = =0.22±0.28. at the same distance as the galaxy itself. Mdyn Note that we exclude the HI clumps (discussed in Section 3.2) from the total baryonic mass. 3.4. Dynamics & Baryon Fraction Including these in the baryonic mass budget In Figure 7 we present the rotation curve of raises f by ∼ 28% to f =0.28 ± 0.29. Pisces A, derived from the narchan data cube bary bary in order to best characterize the rotation. It is 4. Pisces A in Context clear that the rotation curve is still rising out to the last measured data point, indicating that we 4.1. Cosmic Environment are only detecting the solid body rotation of the Tollerud et al. (2016) highlighted the fact galaxy. This is expected considering the beam that Pisces A appears to lie near the boundary size relative to Pisces A. of local filamentary structure. Using a sample Because we do not have a complete knowl- of galaxies within the Local Volume (see their edge of the matter distribution in the galaxy, Section 6 for a full discussion of the data), they i.e., the detailed interplay between rotation and showed that Pisces A is located in an overden- dispersion, we do not have a direct method of sity, and that the overdensity points toward the calculating the total dynamical mass. Instead, h m ◦ Local Void (18 38 18 ; Karachentsev et al., we use the following approximation: 2002). We investigate this idea further by character- v2 σ2 R 5 rot +3 izing the underlying density field. We employ M =2.325×10 M , dyn km2 s−2 kpc the basic method of Sousbie (2011), which (1) takes advantage of the fact that the structures of where vrot is the rotational velocity, σ is the the cosmic web (voids, walls, filaments) have

Beale 8 well-defined analogues in computational topol- text, we include low-mass dwarfs in the AL- ogy. A full description of this method is be- FALFA catalog (grey thin diamonds and small yond the scope of this paper, but we summarize black dots; Giovanelli et al., 2013; Huang et al., it as follows: given the positions and distances 2012; Bradford et al., 2015), Leo P (purple tri- of some catalog of galaxies, we can construct angle; Bernstein-Cooper et al., 2014), isolated the Delaunay triangulation (and thus, the un- ultra-faint dwarfs (yellow triangles; Papastergis derling density field) by assuming the locations et al., 2017), as well as the void galaxies and of the galaxies accurately sample the local den- starburst dwarfs previously described. While sity. We refer the reader to Sousbie (2011) and Pisces A is similar to other low-mass dwarfs references therein for a more complete descrip- observed by ALFALFA, it is somewhat gas- tion. poor for its stellar mass, with an HI fraction of M /M < The Delaunay triangulation of galaxies in log ( HI ) 0. However, including the HI M ∼ . × 7 the Local Void (and the estimated underlying clumps yields an HI mass of HI,tot 1 1 10 density field) is presented in Figure 9. As M, making it somewhat gas-rich. this is a simplified version of the method of 5. Discussion and Conclusions Sousbie (2011), our reconstructed cosmic web has artificially sharp boundaries, and has The variety of spatially and kinematically not been smoothed to eliminate noise spikes. distinct components in the HI distribution of However, it is immediately clear that there is Pisces A suggests that it is not yet in equilib- filamentary structure in the underlying density rium. In particular, our results in Table 3 sug- field, beyond the appearance of the data them- gest that there exist relatively large clumps (up selves. Pisces A is well within this filament, to 50% of the extent of Pisces A itself) harbor- supporting the idea of its current membership. ing almost 10% of the gas mass as in the main Additionally, the NEe appears to be roughly body of the galaxy. parallel to the filament, which is also observed It is difficult to explain the origin of these in other coordinate plane projections. clumps. One possibility is that they are markers of past mergers. Recent interactions or mergers 4.2. The Dwarf Galaxy Population with other dwarf galaxies have been invoked as In Figure 10, we place Pisces A in the possible triggers for recent star formation (e.g., M −f log HI bary plane. Also plotted are a sample Noeske et al., 2001). Nidever et al. (2013), in of Local Group galaxies from McConnachie particular, detected a long gas extension asso- (2012, orange squares), galaxies from the Void ciated with the nearby starburst galaxy IC 10. Galaxy Survey (blue pentagons; Kreckel et al., They argue that this feature is most likely the 2011a, 2014), and a sample of nearby starburst result of a recent interaction or merger, and that dwarf galaxies (light blue diamonds; Lelli et al., the interaction also triggered a recent (< 10 2014b). Note that starburst galaxies marked has Myr) starburst. having kinematically disturbed HI disks (out- Nidever et al. (2013) explicitly rule out a lined in red) do not have reliable rotation curves stellar feedback origin to the extension in IC – instead, the kinematical parameters are esti- 10 since this would require outflow velocities mated from the outermost parts. Pisces A has a ∼ 30 times the observed velocity offset. How- baryon fraction broadly similar to both the void ever, stellar feedback is not ruled out as an ori- galaxy population and the local starburst dwarf gin for the NEe. The SFH of Pisces A im- population. plies increased star formation within 300 Myr Figure 10 also shows the HI mass fraction (Tollerud et al., 2016). Assuming the fur- as a function of the stellar mass. For con- thest observed extent (3.5 kpc, measured from

Beale 9 NEmid), this would imply feedback-driven Dekel, A. et al. 1986, ApJ, 303, 39 outflow velocities of 3.5kpc/300 Myr ≈ Giovanelli, R. et al. 2013, AJ, 146, 15 11.7kms−1, slightly larger, but still compara- Grcevich, J. et al. 2009, ApJ, 696, 385 ble, to the observed offset of 7.4 km s−1. Pisces Hoffman, G. L. et al. 1996, ApJS, 105, 269 A is broadly similar to other local starbursting Huang, S. et al. 2012, ApJ, 756, 113 dwarf galaxies (Lelli et al., 2014a,b), lending Karachentsev, I. D. et al. 2002, A&A, 389, 812 further support to this idea. Kennicutt, Jr., R. C. 1998, ApJ, 498, 541 Interestingly, the NEe approximately points Keres,ˇ D. et al. 2009, MNRAS, 395, 160 in the direction of the filament, and therefore, is Keresˇ D. et al. 2005, MNRAS, 363, 2 roughly perpendicular to the Local Void. This Klypin, A. et al. 1999, ApJ, 522, 82 is what we would expect if Pisces A was accret- Kreckel, K. et al. 2011a, AJ, 141, 204 ing gas along filaments via the so-called “cold Kreckel, K. et al. 2011b, AJ, 141, 4 mode” (Keresˇ et al., 2005), and this may ex- Kreckel, K. et al. 2012, AJ, 144, 16 plain the negative velocity offset of NElow. Kreckel, K. et al. 2014, ArXiv e-prints, In summary: arXiv:1410.6597 Lelli, F. et al. 2014a, A&A, 566, A71 • With the resolution afforded by the JVLA, Lelli, F. et al. 2014b, MNRAS, 445, 1694 we detect multiple morphological and Mathewson, D. S. et al. 1992, ApJS, 81, 413 kinematic features in the HI distribution McConnachie, A. W. 2012, AJ, 144, 4 not previously observed. These features McGaugh, S. S. 2012, AJ, 143, 40 are indicative of non-equilibrium, and sug- Nidever, D. L. et al. 2013, ApJL, 779, L15 gest an active history. Noeske, K. G. et al. 2001, A&A, 371, 806 Ott, J. et al. 2012, AJ, 144, 123 • By further quantifying the environment in Papastergis, E. et al. 2017, A&A, 601, L10 which Pisces A resides, we confirm its ex- Peek, J. E. G. et al. 2011, ApJS, 194, 20 istence within a cosmic filament. Our ob- Penny, S. J. et al. 2009, Astron. Nach., 330, 991 servations provide strong support to the Roychowdhury, S. et al. 2011, MNRAS, 414, idea that the galaxy has recently transi- L5 tioned from a relatively under-dense envi- Saul, D. R. et al. 2012, ApJ, 758, 44 ronment to an over-dense environment. Sousbie, T. 2011, MNRAS, 414, 350 Stanonik et al. 2009, ApJL, 696, L6 Acknowledgments Tollerud, E. J. et al. 2015, ApJL, 798, L21 Tollerud, E. J. et al. 2016, ApJ, 827, 89 This work was partially supported by the National Radio Astronomy Observatory and the Virginia Space Grant Consortium under grant NNX15A120H. This research made use of NASA’s ADS Bibliographic Services.

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