The Kaleidoscopic Universe

Posted on by chemadiegor

In very few scientific fields, color matters more than in Astronomy. First photographic plates, and later CCDs, capture the light of distant objects by integrating during long exposures. When photons hit the CCD, small electric currents record the light intensity. Current CCDs working in the optical, UV and NIR range are basically insensitive to the “color” of the arriving light and filters need to be placed in front of the CCD to select the spectral range, or color, that wants to be studied. Often, these filters are “wide” in the sense that they allow a generous amount of light to go through (since distant objects tend to be faint), and the entire optical spectrum can be covered with 3 filters. However, their are situations where much narrower filters would be desirable. Astronomical objects tend to emit light in a combination of continuum (free-electrons being captured by the atoms) and spectral lines (electrons jumping between atomic levels). The spectral lines carry very valuable information about the conditions (like temperature and metallicity or abundance of elements heavier than Helium) that get diluted when observing the objects with wide bands. On the other hand, sufficiently narrow bands can resolve these spectral features.

Today (July 7th 2020) we published the first results of the J-PAS survey pathfinder (or miniJPAS). J-PAS is a narrow band survey of 15% of the sky, with an unprecedented number of narrow band filters (54). The image below shows the 54 narrow-band filters, plus 5 of the wide-band filters.

Using the narrow-band filters, J-PAS can identify features as the H-alpha line, or the 4000 Angstrom break, allowing for precise SED fittings and photometric redshifts of all galaxies in the catalog.

J-PAS will be particularly powerful at detecting rare objects with string emission lines. Chief among these, high redshift quasars, where the bright Lyman-alpha gets redshifted into the J-PAS spectral range, allowing for unambiguous estimation of the quasar redshifts. Earlier estimations suggest that J-PAS will be capable of detecting half a million quasars in the surveyed area.

The superior power of J-PAS to estimate photometric redshifts will open the door also to do tomographic studies and unveil the 3D structure of the galaxy distribution to unprecedented detail. Stay tunned for more. First light of the full scientific survey is expected to take place in late 2020 or early 2021. You can also find the first data release in J-PAS website and explore the images directly from your browser using the Sky Navigator

You can find the paper below and in this link https://arxiv.org/pdf/2007.01910.pdf

Click to access 2007.01910.pdf

Is LIGO really seeing Mass Gap events?

Posted on by chemadiegor

The recent publication by LIGO of two events (GW190412 and GW190814) with high mass ratios, and with one of the masses close to the mass gap (that is, a mass between 3 and 5 solar masses which are difficult to explain with standard models) has created an intense debate on the nature of these objects. If confirmed, the implications of these observations are important since they can give us information about the equation of state of neutron stars (where one can study exotic forms of matter, such as axions or hyperons), or reveal a new type of black hole, including a leading dark matter candidate, primordial black holes.

However, a simpler alternative could be that these events are strongly lensed. Gravitational lensing can amplify the signal of observed gravitational waves, allowing their observation from much farther distances (and hence much larger volumes). In earlier work, we showed that if the rate of mergers (that produce gravitational waves) at redshift z>1 is sufficently high, observation of these distant events by LIGO is not only possible, but unavoidable. On fact, for rates larger than a few times 10^4 mergers per year and Gpc^3, lensed events will dominate over not-lensed events, in a similar fashion as lensed gravitational lensed IR galaxies dominated over not-lensed IR galaxies in the bright end of Herschel observations.

The left plot shows the prediction from our lensing model (colored circles) compared with the observations (squares and diamonds with error bars). All events concentrate around two locus regions. BBH and NSBH. Note how observations match perfectly the prediction.

 

Lensed gravitational waves get stretched due to cosmic expansion as they travel from their originating source to the detector. The farther the source is, the lager the stretch. The stretch is proportional to (1+z), where z is the redshift if the source. Higher z translates into gravitational waves that, when observed, appear as having a longer wavelength. If the gravitational wave is being magnified by strong lensing, and this magnification goes unnoticed (there is no way a priori to know if a gravitational wave is being magnified), the longer wavelength will be missinterpreted as being due to a larger mass of the two compact objects that ar causing the gravitational wave. For instance, if a neutron star with a mass of 1.3 solar masses is merging with a black hole of 12 solar masses (these are the typical masses found in our Galaxy for these objects) at redhift z=1, the observed gravitational wave will appear identical as the one from a much closer merger (z =0) with a neutron star of 2.6 solar masses and a blackhole with 24 solar masses. This example is not arbitrary since it was chosen to match the observed masses of the latest published gravitational wave event , GW190814, interpreted as being a local event (z=0), but which based on our interpretation could be also a lensed event at z=1 or z>1.

The lensing model interpretauon makes a series of interesting predictions, but among these, it is interesting to pay attention to the predicted mass ratio for binary black holes and neutron star black hole mergers. As shown in the figure illustrating this blog, lensing predicts these type of events will appear in the M1-M2 plane in two well defined regions. Interestingly, all observed data points so far agree remarkably well with this prediction. The two possible mass gap events are marked with a big yellow circle and follow well the predicted locus for lensed NSBH events. If confirmed, our lensing model would offer a simple solution to the mass gap problem, and would imply a much higher rate of events at z>1 that previously thought. 

You can see our full work below. A Fun Fact about this paper is that it was put “On Hold” by arxiv moderators after being submitted to arxiv on June 19 (Friday). After requesting an explanation from arxiv, none was given. At the time of our original submission, we where unaware of the upcoming publication, by the LIGO team, of the GW190814 “Mass Gap” event. The following week, in June 24 we saw in arxiv the LIGO paper with the values of M1 and M2 for GW190814. The same day, the “Hold” on our paper was lifted and we where able to just add the new data point to our figure (see point marked GW190814 in the figure above), before it appeared on arxiv the following day (June 25). I do understand (and support) the need for moderation in arxiv, but this process is far from transparent. The lack of communication and explanation of why papers are being put on hold, inhebitably leads to one suspect foul play, which is something that should be avoided at all cost, specially in portals such as arxiv, that makes research freely available. 

 

Link to paper https://arxiv.org/pdf/2006.13219.pdf

Click to access 2006.13219.pdf