---
title: Pisces VII Ultra-Faint Dwarf
url: https://www.emergentmind.com/topics/pisces-vii
type: topic
---

# Pisces VII Ultra-Faint Dwarf

Pisces VII, also referred to as Triangulum III (Tri III), is an ultra-faint dwarf galaxy discovered in public imaging from the DESI Legacy Imaging Surveys in the outskirts of Messier 33 (M33). Its observational interpretation evolved from a candidate system that could either be an isolated ultra-faint or the second known satellite of M33 into a likely M33 satellite once the horizontal branch was resolved with deeper imaging. Subsequent analyses have constrained its distance, structural parameters, luminosity, neutral-gas content, and star-formation history, placing it among the faintest known galaxies in the Local Group and making it relevant to the satellite census of M33, the low-mass end of galaxy formation, and the effects of cosmic reionization on ultra-faint dwarfs [2104.03859] [2305.13966] [2508.00984].

## 1. Discovery and observational basis

Pisces VII was first noted by visual inspection of DESI Legacy Imaging Survey images around M33, outside the PAndAS footprint, using the Legacy Survey Sky Viewer. The detection was confirmed on SDSS DR9 and then followed up with deep imaging on the 3.58 m Telescopio Nazionale Galileo at La Palma using the DOLoRes focal-reducer spectro-photometer, which has a pixel scale of $0.252^{\prime\prime}\,\mathrm{px}^{-1}$. The survey material consisted of public images from the DESI Legacy Imaging Surveys, including DECaLS using DECam on the Blanco 4 m and complementary BASS/MzLS imaging, reaching surface-brightness limits $\gtrsim 29\,\mathrm{mag\,arcsec^{-2}}$ [2104.03859].

The TNG follow-up obtained $41\times180\,\mathrm{s}$ in $g^\prime$ with median seeing $\simeq1.15^{\prime\prime}$ and $20\times180\,\mathrm{s}$ in $r^\prime$ with median seeing $\simeq0.85^{\prime\prime}$. Data reduction used standard IRAF preprocessing for bias subtraction and flat-fielding, followed by PSF photometry with DAOPHOT/ALLFRAME, including source detection, aperture and PSF fitting, image registration with DAOMASTER, stacked-image creation, and refined ALLFRAME runs. Morphology cuts based on sharpness and visual inspection were used to remove spurious detections. Photometric calibration used approximately 121 stars in common with Pan-STARRS1, with rms $\lesssim0.03$ mag, and extinction was corrected using Schlafly & Finkbeiner (2011) [2104.03859].

The deeper distance and structural re-analysis was based on Gemini/GMOS-N imaging totaling $4\times450\,\mathrm{s}$ in $g$ and $4\times240\,\mathrm{s}$ in $r$, which resolved the horizontal branch. Later Hubble Space Telescope imaging and Jansky Very Large Array observations extended the characterization from detection and morphology to star-formation history and neutral-gas limits [2305.13966] [2508.00984].

## 2. Position, morphology, and structural characterization

In the discovery data, Pisces VII appeared as a round, diffuse over-density of unresolved light with central surface brightness $\mu_0 \simeq 27.7\,\mathrm{mag\,arcsec^{-2}}$, overlapping a small $\sim1$--$2^\prime$ clump of faint stars. Its J2000 center was measured as $\mathrm{RA}=1^\mathrm{h}21^\mathrm{m}40.5^\mathrm{s}\pm0.8^\mathrm{s}$ and $\mathrm{Dec}=+26^\circ23^\prime24^{\prime\prime}\pm4^{\prime\prime}$, with a projected separation from M33 of approximately $72\,\mathrm{kpc}$. MCMC fits to the spatial distribution of RGB candidates gave an angular half-light radius $r_h=0.5^\prime\pm0.2^\prime$, ellipticity $\epsilon=0.02^{+0.04}_{-0.02}$, position angle $\theta=61^\circ{}^{+58^\circ}_{-29^\circ}$, and number of member RGB stars $N_*=18^{+4}_{-4}$ [2104.03859].

The Gemini analysis remeasured the structure using an exponential profile,
$$
\rho_{\rm dwarf}(r)=\frac{1.68^2}{2\pi\,r_{\rm h}^2\,(1-\epsilon)}\,N_*\,\exp\!\left[-1.68\,\frac{r}{r_{\rm h}}\right],
$$
with a uniform background $\Sigma_b$ added so that $\rho_{\rm model}(r)=\rho_{\rm dwarf}(r)+\Sigma_b$, sampled with {\sc emcee}. This yielded $r_{\rm h}=0.67^{+0.20}_{-0.10}$ arcmin, corresponding to $186^{+58}_{-32}\,\mathrm{pc}$, ellipticity $\epsilon<0.1$ as an 84% upper limit, position angle $\mathrm{PA}=96^{+32}_{-36}$ degrees, and central coordinates $\mathrm{RA}=01{:}21{:}40.6$, $\mathrm{Dec}=+26{:}23{:}28$ (J2000) [2305.13966].

Taken together, the morphology is that of a compact but extremely low-surface-brightness stellar system in the ultra-faint dwarf regime rather than a globular cluster. The round appearance and low ellipticity are consistent across the discovery and Gemini-based remeasurement, while the deeper data revised the physical scale upward without changing its basic classification [2104.03859] [2305.13966].

## 3. Distance determinations and satellite status

The initial distance estimate used the tip of the red giant branch (TRGB). The luminosity function was constructed from stars within $2\,r_h$, with a colour cut $0.3<g-r<1.1$ and background subtraction from an annulus at $2.5$--$3.5^\prime$. Sobel edge detection identified a peak at $r_{\rm TRGB}\simeq22.1$ mag, roughly tracing the three brightest RGB candidates. A Bayesian broken-power-law fit following Tollerud et al. modeled the luminosity function as
$$
\phi_{\rm TRGB}(m)=
\begin{cases}
\alpha\,e^{\alpha(m-m_{\rm TRGB})}, & m>m_{\rm TRGB},\\[4pt]
f\,\beta\,e^{\beta(m-m_{\rm TRGB})}, & m\le m_{\rm TRGB},
\end{cases}
$$
convolved with photometric errors and sampled with {\sc emcee}, giving $m_{\rm TRGB}=22.0^{+0.6}_{-0.5}$ mag. With an adopted $M_{{\rm TRGB},r}=-3.01\pm0.10$ mag, this implied $\mu \simeq 25.01$ mag and $D\simeq1.0^{+0.3}_{-0.2}\,\mathrm{Mpc}$. Because the TRGB was sparsely populated, this distance was only marginally consistent with both the M33 halo at $820\,\mathrm{kpc}$ and a truly isolated field ultra-faint at $\gtrsim0.9\,\mathrm{Mpc}$, so membership to M33 could not be confirmed [2104.03859].

That ambiguity was reduced by the Gemini detection of a clear horizontal branch at $g\simeq25.5$ mag. In that analysis, SDSS $g,r$ magnitudes were converted to Johnson $V$ using the colour relations of Jordi et al. (2006), and an MCMC fit to the $V$-band luminosity function around the HB used
$$
\mathcal{L}=\exp\!\left[A\,(V_i-V_{\rm HB})\right]+\exp\!\left[-\frac{1}{2}\left(\frac{V_i-V_{\rm HB}}{C}\right)^2\right].
$$
Marginalization yielded $V_{\rm HB}=25.30\pm0.10$ mag. Adopting
$$
M_{V,{\rm HB}}=(0.13\pm0.09)\times([\mathrm{Fe/H}]+1.5)+(0.54\pm0.07)
$$
with $[\mathrm{Fe/H}]=-1.9\pm0.2$ gave $M_{V,{\rm HB}}=0.49^{+0.08}_{-0.04}$ mag and therefore $\mu_0=24.81\pm0.20$ mag, corresponding to $D=916^{+65}_{-53}\,\mathrm{kpc}$. This placed Pisces VII at a three-dimensional separation of only $97\pm6\,\mathrm{kpc}$ from M33, compared with $\sim285^{+21}_{-18}\,\mathrm{kpc}$ from M31, and thus made it almost certainly bound to M33 [2305.13966].

HST imaging later provided an independent HB-based determination by fitting the observed F606W magnitudes of HB stars to the fiducial HB locus of the metal-poor globular cluster M92, adopting $(m-M)_{\rm M92}=14.62\pm0.06$. The result, $\mu_{\rm Pis\,VII}=24.84\pm0.11$ mag, corresponds to $D=929\pm48\,\mathrm{kpc}$. This is consistent with the Gemini value and places Pisces VII within the range $0.7$--$1\,\mathrm{Mpc}$ derived for the three studied ultra-faint dwarfs. The convergence of the Gemini and HST distance scales strongly favors the interpretation of Pisces VII as M33’s second dwarf satellite galaxy rather than an isolated field system [2508.00984].

## 4. Luminosity, physical scale, and placement among ultra-faint dwarfs

The discovery analysis estimated the luminosity through Monte Carlo sampling of a 12 Gyr, $[\mathrm{Fe/H}]=-2.0$ isochrone. Under the isolated-distance assumption, the galaxy would have $M_V=-6.8\pm0.2$ and $L=3.7^{+0.8}_{-0.6}\times10^4\,L_\odot$; if placed at the distance of M33, it would have $M_V=-6.1\pm0.2$ and $L=2.1\pm0.4\times10^4\,L_\odot$. Its physical half-light radius would be $131\pm61\,\mathrm{pc}$ at $D=1.0\,\mathrm{Mpc}$, or approximately $119\pm48\,\mathrm{pc}$ if at M33’s distance, with $\mu_0(r)=27.7\pm0.3\,\mathrm{mag\,arcsec^{-2}}$ [2104.03859].

The deeper Gemini data remeasured these quantities using Monte Carlo sampling of a 12.5 Gyr, $[\mathrm{Fe/H}]=-1.9$ PARSEC luminosity function. In each trial, stars were drawn until $N_*$ fell within the CMD-selection box, their $g,r$ fluxes were summed, and the result was converted to $V$ over the distance and $N_*$ posterior. This yielded $M_g=-5.7\pm0.3$, $M_r=-6.2\pm0.3$, and $M_V=-6.0\pm0.3$, corresponding to
$$
L=10^{-0.4(M_V-M_{V,\odot})}L_\odot,\qquad M_{V,\odot}=4.83,
$$
and therefore $L=2.2^{+0.7}_{-0.5}\times10^4\,L_\odot$ [2305.13966].

In the size-luminosity plane, Pisces VII lies firmly among known ultra-faint dwarfs and is well separated from globular clusters. The discovery paper likewise placed it squarely among the Milky Way’s ultra-faint dwarfs, with $r_h\sim100$--$150\,\mathrm{pc}$ and $M_V\lesssim-6$, and noted similarity to And XXII at $M_V\approx-6.6$. If the original isolated interpretation had held, it would have been the faintest known field dwarf detected to date; the later distance measurements instead anchor it in the M33 halo [2104.03859] [2305.13966].

## 5. Stellar populations, neutral gas, and quenching history

The Gemini colour-magnitude diagram showed a metal-poor RGB with $[\mathrm{Fe/H}]\approx-1.9$ and $[\alpha/\mathrm{Fe}]=+0.4$, together with a clearly resolved HB at $g\sim25.5$ mag. It also showed a blue plume of stars just fainter than the HB and a small population of putative core He-burning stars above the HB. BaSTI isochrones indicated that the blue plume is consistent with a $\sim1.5\,\mathrm{Gyr}$ population, while the bulk RGB and HB agree with an old $12.5\,\mathrm{Gyr}$ population. The interpretation was explicitly conditional: if these stars are truly members of the galaxy, they imply star formation approximately $10\,\mathrm{Gyr}$ after reionization and would require either retained or accreted cold gas, or a revision of how reionization feedback is understood in the faintest galaxies [2305.13966].

The later HST-based star-formation-history analysis used a new Python-based fitting code based on StarFISH. In that framework, the present-day Hess diagram was modeled as a linear combination of single-stellar-population Hess templates spanning age and metallicity, with SSPs generated from PARSEC, MIST, and BaSTI over $\log t(\mathrm{yr})=6.5$ to $10.13$ and $[\mathrm{M/H}]=-2.2\ldots-1.3$. Photometric completeness and uncertainty from $10^6$ artificial stars were folded into the templates, a Poisson maximum-likelihood fit solved for the weights $w_{kl}$ of each age-metallicity bin under a weakly enforced metallicity evolution, and the recovered SFH was rebinned into broader intervals of $\Delta\log t=0.2$ dex. For Pisces VII, the PARSEC solution gave a star-formation rate of $1.7\pm0.5\times10^{-5}\,M_\odot\,\mathrm{yr}^{-1}$ in the oldest $10$--$13.8\,\mathrm{Gyr}$ bin and $<0.1\times10^{-5}\,M_\odot\,\mathrm{yr}^{-1}$ in all younger bins. After applying a recycling fraction of 43%, the present-day stellar mass is $M_*=10^{4.57}\,M_\odot$ [2508.00984].

In the same analysis, cosmic reionization was taken to end at $z\approx6$, corresponding to a lookback time of $t_{\rm reion}\simeq12.8\,\mathrm{Gyr}$ for a $\Lambda$CDM cosmology with $H_0=70\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ and $\Omega_m=0.3$. Pisces VII formed virtually all of its stars, $\gtrsim90\%$, in the oldest bin and shows no significant star formation at later times; within the time resolution, its SFH is therefore consistent with complete quenching by, and indeed shortly after, reionization. This result differs in emphasis from the earlier blue-plume interpretation and suggests that any intermediate-age component remains contingent on confirming whether the candidate $\sim1.5\,\mathrm{Gyr}$ stars are truly members [2305.13966] [2508.00984].

Neutral-gas observations with the VLA detected no H I line. Assuming a top-hat profile of width $\Delta v=25\,\mathrm{km\,s^{-1}}$ and a $3\sigma$ threshold with $\sigma_{\rm rms}=1.3\,\mathrm{mJy\,beam^{-1}}$ per $5\,\mathrm{km\,s^{-1}}$ channel gives $S_{\rm int,lim}\simeq0.0975\,\mathrm{Jy\,km\,s^{-1}}$. Using
$$
M_{\rm HI}=2.36\times10^5\,D^2\,S_{\rm int}\quad [M_\odot],
$$
with $D=0.929\,\mathrm{Mpc}$ yields $M_{\rm HI}<8\times10^3\,M_\odot$, or $\log M_{\rm HI}<3.9$. More generally, the VLA observations place stringent upper limits on the H I content of all three studied systems, with all having $M_{\rm HI}<10^4\,M_\odot$. For Pisces VII, the extremely low H I fraction, $M_{\rm HI}/M_*\lesssim0.2$, supports its categorization as a fossil of the reionization era [2508.00984].

## 6. Significance for the M33 satellite census and Local Group evolution

Pisces VII is significant because M33 has long lacked a substantial observed satellite population. Before its identification, only one potential satellite, Andromeda XXII/Tri I, had been detected around M33. If Pisces VII is included, the known count doubles, but the system still falls far short of the $\Lambda$CDM expectation: simulations cited in the discovery work predict approximately $9$--$25$ satellites with $M_*>10^3\,M_\odot$ around an M33-mass halo, including at least one with $M_*>10^6\,M_\odot$. This discrepancy was framed in terms of either a true dearth of low-mass subhalos around M33, possibly stripped by interactions with M31, or a population of satellites with surface brightness $\gtrsim28$--$30\,\mathrm{mag\,arcsec^{-2}}$ that had remained below prior survey limits [2104.03859].

The later geometric placement of Pisces VII inside the M33 halo sharpened the dynamical implications. Its three-dimensional separation from M33 is $97\pm6\,\mathrm{kpc}$, well inside the expected $r_{200}\sim160\,\mathrm{kpc}$, while And XXII lies at $\sim113^{+31}_{-30}\,\mathrm{kpc}$ from M33 but remains kinematically ambiguous. The existence of two distant satellites at $>100\,\mathrm{kpc}$ was argued to favor a first-infall scenario for M33 into the Andromeda system: if M33 had undergone a close passage of less than $100\,\mathrm{kpc}$ with M31 approximately $3\,\mathrm{Gyr}$ ago, most of its outer halo and satellites would have been heavily stripped. In that sense, Pisces VII is relevant not only to the census of low-mass galaxies, but also to M33’s orbital history within the Local Group [2305.13966].

The follow-up agenda remains correspondingly broad. Earlier work suggested Hubble Space Telescope imaging to reach the horizontal branch or main-sequence turn-off for a precise distance, and HB imaging has since delivered such a refinement. Remaining priorities include $8$--$10$ m-class spectroscopy of the brightest RGB stars to measure systemic velocity and metallicity, deep wide-field imaging in the outskirts of M33 to uncover fainter and more diffuse satellites, and space-based proper motions to test whether the satellites of M33 have been perturbed by M31. This suggests that Pisces VII will continue to function as a benchmark object for connecting ultra-faint dwarf demographics, reionization-era quenching, and Local Group assembly [2104.03859] [2305.13966] [2508.00984].

Source: https://www.emergentmind.com/topics/pisces-vii