"Dark Disc galaxy" in Virgo cluster +50 Mly

[Widdekind]

"Optically Cloaked" dark cloud in Virgo

 

Virgo-HI-21 (VH21) seems to be a "dark spiral galaxy", with a "dark disc" spanning some 50 thousand light-years across. VH21 resides in the Virgo galaxy cluster, about 50 million light-years away:

 

VH21 appears to possess a flattened, rotating galactic disc, typical of spiral galaxies (jb). Seemingly seen from the side, or 'edge-on', that disc is full of the diffuse, cold-and-un-ionized Hydrogen gas, otherwise common in the discs of ordinary, luminous, star-spangled spiral galaxies (register). But, sans stars, VH21 is only observable, from the faint, radio-frequency emissions (with a wave-length of 21 cm), of those cold clouds of neutral Hydrogen:

 

HI halo surrounding VH21

(with unassociated foreground dwarf galaxy at left)

, whose "dark disk", seen edge-on, is

50 thousand light-years

across

(

cf

.

UGC 10043

)

Now, those emissions imply, firstly, that the total mass, of Hydrogen gas, in VH21, is over 100 million solar masses; and, secondly, that the Hydrogen gas -- and so, assumedly, the dark disk in which they reside -- is rotating rather rapidly, spinning through space, with a speed, relative to the apparent center-of-mass, of ~100 km/s. And, such swift spin speeds imply, in turn, that VH21 must mass nearly 100 billion solar masses, nearly 1000x more than the emission-observable Hydrogen alone (or else self-gravity could not keep the galaxy together).

 

Moreover, that amount of mass is also required, to account for the 'bridge' of cold-and-neutral Hydrogen, stretching half-a-million light-years, from VH21, to NGC 4254, resulting from a close encounter a quarter billion years ago (Minchin 2008).

 

HI bridge between VH21

(upper left)

& NGC 4254

(lower right)

,

half-a-million-light-years

away

(

cf

.

Milky-Way-to-Fornax

(local dwarf galaxy, not remote galaxy cluster, seen in the same stretch of sky, from earth)

)

 

For comparison, VH21 is half as wide; spins half as fast; and, has one-tenth of the mass (both of gas alone, and overall, including DM), as our own Milky Way galaxy (astronomy). Also, these radio-band observations, of VH21, are most consistent, with a 'barred spiral' classification (SBc), similar to our own Milky Way (see below).

 

Now, anomalously, VH21 betrays the presence of precisely zero stars. Again, no stars have been detected, within the VH21 "dark spiral galaxy". Instead, HST images, of a square patch of sky, 50,000 light years on a side, and centered on VH21's position, detected only 119 old red giant stars -- "the number found in a typical region, of the same size, in intergalactic space" (NS). Meanwhile, VH21 itself "contains no starlight down to a very low surface-brightness level" (NAIC). Again, "not a single star is shining from this massive region of space" (UT). Thus, at optical frequencies, "this object remains in-visible" (phs). VH21's Hydrogen gas also emits no currently-human-detectable x-rays, consistent with completely cold-and-neutral conditions (T < 1000 K) (Bonamente 2008)*.

*

VH21's Hydrogen 'ISM' is cold, comparable in

temperature

, to the 'Cold Neutral Medium' (CNM) phase, of standard spiral

ISMs

. Yet, that Hydrogen is diffuse, comparable in calculated

density

(

M = 100 M*; D = 50 Kly, H = 1-10 Kly

), to the typical Super-Novae-shock-heated 'Hot Ionized Medium' (HIM) phase.

Thus, VH21 apparently possesses the mass, of a major galaxy; and, even, the stable spinning structure, of a spiral galaxy. Yet, VH21 is optically non-visible, emitting no currently-human-detectable star light. And, VH21 appears to be completely cold, as if neither star formation, nor ensuing super-novae, have heated its Hydrogen 'ISM' in eons.

 

Seeing a dark galaxy —- a galaxy without any stars -— is like seeing a city without any people. We want to know why nobody lives there.

Prof. Robert Minchin (Cardiff University, UK)

 

VH21 "dark spiral galaxy", range

50 million light-years

Optical-band image

 

VH21 "dark spiral galaxy", range

50 million light-years

Optical-band image, over-lain with Radio-band (

21 cm

) detected intensity contours

 

NGC 4945

, barred spiral galaxy (SBc), range

13 million light-years

disc

, seen edge-on, spans

75 thousand light-years

"the type of galaxy that astronomers would have expected to see, based on the measurements taken"

--

JB

 

 

 

References:

• http://arxiv.org/PS_cache/arxiv/pdf/0706/0706.1586v1.pdf
  
• http://arxiv.org/PS_cache/astro-ph/pdf/0502/0502312v1.pdf
  
• A Dark Hydrogen Cloud in the Virgo Cluster
  
• Swift XRT Observations of VIRGOHI 21
  

[Widdekind]

VH21 is optically thick, so self-absorption masks most of its mass ?

 

For radiative transfer, 'inbound' along a line-of-sight, 'back' towards the observer, through some emitting-and-absorbing medium, the light intensity (energy per time, per emitting-area-perpendicular-to-LOS, per solid-angle-towards-observer, per frequency band, measured in Jansky's [Jy]) increases by emission, and decreases by self-absorption:

 

[math]\frac{dI_{\nu}}{ds} = \epsilon_{\nu} - \kappa_{\nu} \, I_{\nu}[/math]

Generally, the emission & self-absorption coefficients, are functions of the light frequency, as well s the emitting-and-absorbing medium's particle-number-density & temperature. Simplistically, then, if the medium is assumed to be uniform in both density & temperature, then the radiative transfer equation can be easily integrated, to calculate the 'observer-bound' light intensity, emerging from the 'observer-facing' surface, of the medium:

 

[math]\frac{dI_{\nu}}{ds} = \kappa_{\nu} \left( \frac{\epsilon_{\nu}}{\kappa_{\nu}} - I_{\nu} \right)[/math]

Letting [math]B_{\nu} \equiv \epsilon_{\nu} / \kappa_{\nu}[/math], and assuming that the medium is not 'back-lit' at the frequencies of interest, w.h.t.:

 

[math]\int_{I_0=0}^{I_L} \frac{dI_{\nu}}{B_{\nu} - I_{\nu}} = \int_0^L \kappa_{\nu} ds[/math]

From a few further, quick calculations, w.h.t.:

 

[math]I_L = B_{\nu} \left( 1 - e^{-\kappa_{\nu} L} \right)[/math]

Now, the afore-going formula displays two important 'regimes' of behavior. First, if the medium is 'optically thin' ([math]\kappa_{\nu} L \ll 1[/math]), then [math]I_L \approx \epsilon_{\nu} L[/math]. Second, if the medium is 'optically thick' ([math]\kappa_{\nu} L \gg 1[/math]), then [math]I_L \approx B_{\nu} = \epsilon_{\nu} \kappa_{\nu}^{-1}[/math]. Thus, when the medium is transparent, then emissions along the entire 'inbound' LOS, 'back' towards the observer, contribute to the detected flux. But, when the medium is opaque, then only emissions from close to the observer-facing surface contribute to the detected flux. So, an opaque medium has a 'skin depth' ([math]\kappa_{\nu}^{-1} \ll L[/math]), whose length must replace that of the full LOS length through the medium, in the afore-going forumla [math]\left( I_L = \epsilon_{\nu} L \rightarrow \epsilon_{\nu} \kappa_{\nu}^{-1} \right)[/math].

 

Consider, then, an isolated (non-back-lit) HI cloud, with the afore-assumed simplistic structure (uniform, isothermal), emitting 21cm radio-frequency radiation. Imagine, too, that the cloud has a cross-sectional area [math]A[/math], total depth-thickness [math]L[/math], and emits, across this Cosmos, ultimately into a human radio antenna, at a distance [math]D[/math], having an effective cross-sectional area [math]a[/math], and so sub-tending a 'receiving solid-angle' of [math]\Omega = a/D^2[/math]. If the emitting cloud is diffuse, and so radio transparent, then the human radio detector will receive a flux of:

 

[math]F = \epsilon_{\nu} \times A \times L \times \Omega[/math]

Now, the emission coefficient is quickly, and comprehendably, calculated, as:

 

[math]\epsilon_{\nu} = \frac{h \nu_{21}}{4 \pi} \times \Gamma_{21} \times \left( \frac{3 n_H}{4} \times \phi(\nu) \right)[/math]

interpreted as the photon energy [math]h \nu_{21}[/math], emitted isotropically into [math]4 \pi[/math] steradians, at the 21cm spontaneous emission rate-per-particle [math]\Gamma_{21}[/math], by the [math]3/4[/math] of the hydrogen-atoms-per-volume [math]n_H[/math] that are expected to be in the excited-and-emission-capable state (according to the Boltzman Distribution, in the Radio Limit), and reduced by a further factor -- the 'frequency line-width' [math]\phi(\nu) \approx 1/ \Delta \nu[/math] -- characterizing the 'Doppler-induced de-synchronization', of the emitting-and-self-absorbing hydrogen atoms, by virtue of their various relative motions (thermal & bulk, e.g. rotational), which shift their rest-frame frequency responses (both emission & absorption), 'out of reach' of each other, so 'optically thinning apart' the cloud (even for the same physical density).

 

Therefore, if the emitting cloud is optically thin, then the eventually-human-detected flux is proportional to the total cloud Hydrogen mass:

 

[math]F \propto \left( n_H \times A \times L \right)[/math]

 

[math]\propto \left( n_H \times Volume \right) [/math]

 

[math]\propto M_H[/math]

Conversely, if the emitting cloud is optically thick, then the eventually-human-detected flux is only proportional to a fraction of the total cloud Hydrogen mass:

 

[math]F \propto \left( n_H \times A \times \kappa_{\nu}^{-1} \right)[/math]

 

[math]\propto \left( n_H \times Volume \times \frac{\kappa_{\nu}^{-1}}{L} \right) [/math]

 

[math]\propto M_H / (\kappa_{\nu} L)[/math]

Now, in practice, human radio Astronomers prefer to convert their 'line widths', from frequency to velocity, according to the Doppler Shift formula [math]v_r = c \left( 1 - \nu / \nu_{21} \right)[/math], the differentiation of which yields [math]1/\Delta \nu = \lambda_{21} / \Delta v[/math], recalling that [math]c = \nu \lambda[/math], to convert from frequency to wave-length. And, by Kirchoff's Law, in the Radio limit, w.h.t.:

 

[math]B_{\nu} \approx \frac{2 h \nu^3}{c^2} \frac{k T}{h \nu}[/math]

s.t., again converting from frequency to wave-length:

 

[math]\kappa_{\nu} = \frac{\lambda_{21}^2}{8 \pi} \Gamma_{21} \frac{3 n_H}{4} \frac{\lambda_{21}}{\Delta v} \left( \frac{h \nu_{21}}{k T} \right)[/math]

Therefore, the 'skin depth', for HI clouds, is:

 

[math]\mathcal{L} \equiv \kappa_{\nu}^{-1} \approx 20 \; thousand \; lightyears \times \left( \frac{\Delta v}{100 \; km/s} \right) \left( \frac{n_H}{1 \; atom/cm^3} \right)^{-1}[/math]

In particular, the "dark disc" of "dark spiral galaxy" VH21 has [math]\Delta v \approx 200 \; km/s[/math]. And, its cold-and-neutral Hydrogen gas has temperatures conceivably comparable, to the CNM phase, of standard spiral galaxies' ISMs. So, if VH21's Hydrogen gas is also similarly dense, [math]n_H \approx 20-50 cm^{-3}[/math], then VH21 could be very optically thick, with a skin-depth [math]\mathcal{L} < 1 \; thousand \; lightyears[/math]*. And if so, then human-received 21cm emissions, from VH21, arising only from the earth-facing surface layers of the HI halo, would have been dramatically dimmed, by internal self-absorption. That, in turn, would lead to large mathematical under-estimates, of VH21's Hydrogen gas mass, along the lines of those recently reported. Could, then, VH21 be optically opaque, to 21cm emissions, so masking most of its mass, of Hydrogen gas ?

*

If our own Milky Way galactic disc were similarly 'foggy', at optical frequencies, then the Orion Nebula might not be visible from earth.

Edited July 7, 2011 by Widdekind